Method of storing carbon dioxide underground

Abstract

To provide a method of storing carbon dioxide underground capable of securing the strength of a carbon dioxide-shielding layer, surly sealing off carbon dioxide into an optimum depth region in the stratum, and efficiently storing a good deal of carbon dioxide.SOLUTION: Carbon dioxide is stored in the depth region of the stratum where a pressure requirement and temperature requirement, in which a density of the carbon dioxide in under sea bottom face F and a density of pore space water balance, meet by pressing carbon dioxide in the lower side than a carbon dioxide-sealed region S composed of a stratum existing from a sea bottom face F to a predetermined depth which meet a pressure requirement and a temperature requirement capable of generating carbon dioxide hydrate to form a carbon dioxide-storing layer G, and rising at least a part of carbon dioxide pressed into the carbon dioxide-storing layer G to the carbon dioxide-sealed region S side to generate carbon dioxide hydrate to form a carbon dioxide shield layer C in the carbon dioxide-sealed region S.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] The present invention relates to a method for underground storage of carbon dioxide. [Background technology]

[0002] Carbon dioxide Capture and Storage (hereinafter simply referred to as "CCS") has been attracting attention as an innovative technology that can reduce carbon dioxide (CO2) emissions while generating electricity using fossil fuels. In addition, demonstration tests and surveys of suitable storage locations are being actively conducted for methods of storing carbon dioxide underground using CCS technology.

[0003] Generally, carbon dioxide exists in one of four phases: gas, liquid, solid, or supercritical, depending on the pressure and temperature conditions. Carbon dioxide becomes solid dry ice at normal pressure and temperatures below 194 K (-79.15 °C), but when mixed with water, it is known to become hydrated (solid) under different temperature and pressure conditions. Carbon dioxide hydrate (CO2 hydrate) is generated when liquid carbon dioxide is mixed with water at temperatures below 10 °C and pressures above 4.5 MPa, and overlaps with part of the liquid carbon dioxide region (liquid phase region).

[0004] The carbon dioxide hydrate described above is a solid having a crystal structure similar to that of methane hydrate, and has a structure in which gas molecules are trapped in a three-dimensional lattice formed by water molecules. Such carbon dioxide hydrate is generally also called gas hydrate, and Non-Patent Document 1 is the first research report to confirm that carbon dioxide hydrate causes clogging in natural gas transport lines. Carbon dioxide hydrate has also been confirmed as a blocking substance in plant piping for oil and gas processing, and research into it is also progressing in the field of science related to the Earth planet.

[0005] Moreover, Non-Patent Document 2 reports that in 1990, natural carbon dioxide hydrate was discovered rising up from the seafloor in the form of columns.

[0006] The density of carbon dioxide changes due to a gas-liquid phase change when pressure increases, but because it becomes a supercritical state at temperatures of 31°C or higher and pressures of 7.4 MPa or higher, density changes due to pressure increases beyond this are relatively slow. Since the density of carbon dioxide in a liquid or supercritical state is smaller than that of seawater, for example, when attempting to store carbon dioxide in a geological layer under the seabed, buoyancy occurs due to the density difference, and some kind of sealing function is required to contain the carbon dioxide that rises to the surface.

[0007] Meanwhile, in Japan, the government and administrative agencies have been leading the search for suitable locations for deep saline aquifer storage (DSA) as a method of underground storage of carbon dioxide since the 1990s, but commercial storage sites and storage capacity have yet to be determined. This is thought to be due to the fact that Japan has a low production of oil and natural gas, and there is little natural cap rock (mudstone or other shielding layer (cap rock)) in Japan's geological strata, which is commonly found in the Middle East, Europe, the United States, and Asia.

[0008] For this reason, as an alternative method of storing carbon dioxide underground, there is hope for the practical application of carbon dioxide hydrate storage, which takes advantage of the above-mentioned property of carbon dioxide hydrating and becoming a solid, and uses hydrated carbon dioxide as a shielding layer with a sealing function to contain any carbon dioxide that rises to the surface. Active research is being conducted into this technology.

[0009] The hydrated carbon dioxide shielding layer described above has the advantage of not requiring a cap rock such as mudstone. In addition, the underground storage layer for carbon dioxide hydrate storage is, for example, a Quaternary sedimentary layer (geological age is from now to about 2.58 million years ago) in which sediment transported from rivers on land has accumulated on the acoustic bedrock to a thickness of about 400 m, and is widely distributed in the seas surrounding Japan.

[0010] In addition, the presence of methane hydrate in the seas surrounding Japan has revealed the presence of methane gas deep underground. However, this methane gas is not trapped in the cap rock as described above, but is trapped as methane hydrate in shallow strata several hundred meters below the seafloor, which is why it is thought to exist in large quantities in the seas surrounding Japan.

[0011] The basic concept of carbon dioxide hydrate storage as described above is shown, for example, in Non-Patent Documents 3 and 4, but research into specific mechanisms, etc. has not progressed. Meanwhile, in the UK (Scotland), as shown in Non-Patent Document 4, research results from indoor experiments have been reported as part of a nationally led project.

[0012] Here, Patent Document 1 proposes a method for simultaneously achieving carbon dioxide fixation and methane gas production without relying on a methane hydrate layer. Patent Document 1 describes a method including the steps of: forming a methane gas production layer 4 by adding a microbial group including at least methanogens to the gaps of a layer under temperature and pressure conditions in which the methanogens produce methane gas; forming a carbon dioxide hydrate seal layer by injecting an emulsion of liquid carbon dioxide particles dispersed in water that are smaller than the gaps into the gaps of the layer shallower than the methane gas production layer and under temperature and pressure conditions in which carbon dioxide becomes hydrate; injecting liquid carbon dioxide as a methane gas production raw material into the gaps of the layer between the methane gas production layer and the seal layer; and recovering methane gas 5 produced in the methane gas production layer to the ground.

[0013] In addition, as a concern in promoting carbon dioxide hydrate storage, it has been pointed out that the hydration of carbon dioxide as shown in Non-Patent Document 1 may cause blockage of pores in the geological formations, resulting in clogging, making it difficult to inject carbon dioxide into the underlying geological formations. In order to solve such problems, Patent Document 2 proposes a method and an apparatus for forming a carbon dioxide storage layer by injecting carbon dioxide below a carbon dioxide shielding layer that exists from the seabed to a specified depth and satisfies pressure and temperature conditions that can generate carbon dioxide hydrate. The technology described in Patent Document 2 makes it possible to prevent clogging when storing carbon dioxide underground and efficiently store large amounts of carbon dioxide. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] JP 2010-239962 A [Patent Document 2] JP 2019-126787 A [Non-patent literature]

[0015] [Non-Patent Document 1] Hammerschmidt, EG, “Formation of Gas Hydrates in Natural Gas Transmission Lines”, Industrial Engineering Chemistry, 26(8), (US), 1934, p851-855 [Non-Patent Document 2] Sakai H, Gamo T, Kim ES, Tsutsumi M, Tanaka T,Ishibashi J, Wakita H, Yamano M, Oomori T. “Re-ports : Venting of Carbon Dioxide-Rich Fluid and Hydrate Formation in Mid-Okinawa Trough Backarc Basin”, Science, Vol.248, (US), 1 June, 1990, p1093-1096 [Non-Patent Document 3] Hitoshi Koide, Manabu Takahashi, Hitoshi Tsukamoto, Yuji Shindo, “Self-trapping mechanisms of carbon dioxide in the aquifer disposal”, Energy Conversion and Management, Vol.36, Issues 6-9, (US), June-September 1995, p505-508 [Non-Patent Document 4] Research Institute of Innovative Technology for the Earth, "2003 Carbon Dioxide Fixation and Effective Utilization Technology Development Program (Development of Fundamental Technology for Chemical Fixation and Hydrate Fixation of CO2 in the Deep Underground and Seafloor Environments)," March 2004 Summary of the Invention [Problem to be solved by the invention]

[0016] On the other hand, when storing carbon dioxide underground using carbon dioxide hydrate, in order to store carbon dioxide underground efficiently and reliably, it is necessary to increase the strength of the shielding layer so that the injected carbon dioxide does not rise to the surface and leak due to destruction of the shielding layer formed by the hydration of carbon dioxide, etc. However, until now, there has been no knowledge regarding the conditions for ensuring the strength of the shielding layer, and the reality has been that it has been necessary to rely on trial and error at the storage site.

[0017] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a method for underground storage of carbon dioxide that ensures the strength of a carbon dioxide shielding layer formed by hydrated carbon dioxide, can reliably seal carbon dioxide in an optimal depth region in a geological layer, and can efficiently store large amounts of carbon dioxide. [Means for solving the problem]

[0018] In order to solve the above problems, the present invention employs the following configuration. [1] A method for storing carbon dioxide in a liquefied state in a seafloor substrata consisting of sediments on an acoustic bedrock, comprising the steps of: injecting liquid carbon dioxide to form a carbon dioxide storage layer below a carbon dioxide sealed area consisting of a stratum that exists from the seafloor surface to a predetermined depth and that satisfies pressure and temperature conditions capable of producing carbon dioxide hydrate; causing at least a portion of the carbon dioxide injected into the carbon dioxide storage layer to rise naturally toward the carbon dioxide sealed area due to the buoyancy of the carbon dioxide to produce carbon dioxide hydrate, thereby forming a carbon dioxide shielding layer in the carbon dioxide sealed area; adjusting the injection amount of the liquid carbon dioxide using a threshold pressure of 0.02 MPa by the carbon dioxide shielding layer consisting of carbon dioxide hydrate as a control standard; and storing the carbon dioxide in a depth region in the seafloor substrata that satisfies the pressure and temperature conditions under which the density of the carbon dioxide and the density of pore water are in equilibrium, [2] A method for underground storage of carbon dioxide described in [1] above, comprising calculating a depth of the stratum in the seabed substratum that satisfies pressure and temperature conditions under which the density of the carbon dioxide and the density of pore water are in equilibrium, and controlling the injection position of the carbon dioxide into the seabed substratum based on the calculated value. [3] The time-strength relationship of the carbon dioxide barrier layer consisting of carbon dioxide hydrate under the temperature and pressure conditions under which carbon dioxide hydrate can exist stably in the seafloor substrata, and the carbon dioxide density ρ in the seafloor substrata CO2 is the density of seawater, ρ 35swA method for exploring marine areas suitable for underground storage of carbon dioxide based on matching temperature and pressure conditions. [4] The time-strength relationship of the carbon dioxide barrier layer consisting of carbon dioxide hydrate under the temperature and pressure conditions under which carbon dioxide hydrate can exist stably in the seafloor substrata and the carbon dioxide density in the seafloor substrata, ρ CO2 is the density of seawater ρ 35sw A program to search for ocean areas suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above. Effect of the Invention

[0019] According to the method for underground storage of carbon dioxide of the present invention, by adopting the above-mentioned configuration, it is possible to reliably ensure the strength of the carbon dioxide shielding layer formed by hydrated carbon dioxide, and to reliably seal carbon dioxide in an optimal depth region in the geological layer, thereby making it possible to efficiently store a large amount of carbon dioxide. [Brief description of the drawings]

[0020] [Figure 1A] FIG. 1A is a diagram illustrating a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a schematic cross-sectional view showing the general structure when carbon dioxide is stored in a geological layer under the seabed, and is an image diagram showing each layer over a wide area. [Figure 1B] FIG. 1B is a diagram illustrating a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a schematic cross-sectional view showing a general structure when carbon dioxide is stored in a stratum beneath the seabed, and is an enlarged view of a key portion of FIG. 1A. [Diagram 2] FIG. 2 is a diagram for explaining a method for underground storage of carbon dioxide which is one embodiment to which the present invention is applied, and is a graph showing isolines of carbon dioxide density in the temperature-pressure relationship and a stable region of carbon dioxide hydrate. [Diagram 3] FIG. 3 is a diagram for explaining a method for underground storage of carbon dioxide, which is one embodiment to which the present invention is applied, and is a graph showing a temperature-pressure phase diagram of carbon dioxide hydrate. [Figure 4] FIG. 4 is a diagram illustrating a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a graph showing the pressure acting on a carbon dioxide shielding layer caused by the buoyancy of carbon dioxide injected into a stratum beneath the seabed. [Diagram 5] FIG. 5 is a diagram for explaining a method for underground storage of carbon dioxide, which is one embodiment to which the present invention is applied, and is a schematic diagram showing the general structure of a pressure cell used in a laboratory experiment to demonstrate the present invention. [Figure 6] 6(a) to (c) are diagrams illustrating a method for underground storage of carbon dioxide which is one embodiment of the present invention, and are graphs showing the strength of a membrane made of carbon dioxide hydrate obtained in an experiment using the pressure cell shown in FIG. [Figure 7] FIG. 7 is a diagram illustrating a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a graph showing the temperature rise due to the heat of formation of carbon dioxide hydrate when carbon dioxide permeates into a geological layer, obtained from an experiment using the pressure cell shown in FIG. [Figure 8] FIG. 8 is a diagram for explaining a method for underground storage of carbon dioxide which is one embodiment of the present invention, and is a graph showing the relationship between retention time and differential pressure when the pressure was maintained for 17 days under conditions where the differential pressure was approximately 2 m or less in terms of head, obtained in an experiment using the pressure cell shown in FIG. [Figure 9] FIG. 9 is a diagram for explaining a method for underground storage of carbon dioxide which is one embodiment of the present invention, and is a graph showing the strength of a carbon dioxide barrier layer made of a carbon dioxide hydrate membrane when a differential pressure of 1 MPa was maintained for 24 hours after maintaining the pressure difference for 17 days under conditions of a head of approximately 2 m or less, obtained in an experiment using the pressure cell shown in FIG. [Figure 10] FIG. 10 is a diagram for explaining a method for underground storage of carbon dioxide which is one embodiment of the present invention, and is a graph showing the strength of a carbon dioxide barrier layer made of a carbon dioxide hydrate membrane when the differential pressure is increased stepwise, obtained in an experiment using the pressure cell shown in FIG. 5. [Figure 11] FIG. 11 is a diagram for explaining a method for underground storage of carbon dioxide which is one embodiment of the present invention, and is a graph showing the strength of a carbon dioxide barrier layer made of a carbon dioxide hydrate membrane when a reverse pressure differential is applied, obtained from an experiment using the pressure cell shown in FIG. 5. [Figure 12] FIG. 12 is a diagram for explaining a method for underground storage of carbon dioxide which is one embodiment of the present invention, and is a graph showing contour lines of the optimal volume ratio of {carbon dioxide / water} in the relationship between temperature and pressure. [Figure 13] FIG. 13 is a diagram for explaining a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a diagram that considers the shielding mechanism of carbon dioxide hydrate by a membrane based on the experimental results using the pressure cell shown in FIG. [Figure 14] FIG. 14 is a diagram illustrating a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a graph showing the relationship between temperature and pressure conditions at a depth where the density of carbon dioxide and the density of seawater are in equilibrium in a stratum below the seabed. [Figure 15] FIG. 15 is a diagram for explaining a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a schematic diagram of a storage method assuming islands, seamounts, and ocean plateaus, and is a graph showing the relationship between horizontal distance and water depth. [Figure 16] FIG. 16 is a graph showing the stability region of CO2 hydrate in terms of temperature-pressure relationship. [Figure 17] Figure 17 is a graph showing the region in which CO2 hydrate exists stably in terms of the temperature-pressure relationship when the surface of the Pacific Ocean is 3,000 m and the geothermal gradient is 30°C / km. [Figure 18] Figure 18 is a graph showing the region in which CO2 hydrate exists stably in terms of the temperature-pressure relationship when the surface of the Pacific Ocean is 3,000 m and the geothermal gradient is 60°C / km. [Figure 19] Figure 19 is a graph showing the region in which CO2 hydrate exists stably in terms of the temperature-pressure relationship when the Pacific Ocean floor is 3,000 m above sea level and the geothermal gradient is 100°C / km. [Figure 20] Figure 20 is Fig. 2 of Reference 10 (House et al., 2006). [Figure 21] Figure 21 is a graph showing the xy-axis scale of the density-pressure relationship (2) the same as that of Figure 20. [Figure 22] Figure 22 is a graph showing the superposition of (2) in Figures 20 and 21. [Figure 23] Figure 23 shows the isotherms of the difference in CO2 density - seawater density in the temperature-pressure relationship. Fig. 1 of Reference 10 (House et al., 2006) is rotated so that the y-axis is pressure (water depth) and displayed. [Figure 24] Figure 24 is the superposition of the isotherms of seawater density based on TEOS-10 and Figure 23. [Diagram 25] Figure 25 is a diagram showing the geothermal gradient in the waters around Japan. [Figure 26] Figure 26 is a diagram showing the long-term change of the injected CO2. Fig. 4 of Reference 10 (House et al., 2006) is cited. [Figure 27] Figure 27 is a diagram showing the temporal change of physical properties in the relationship between the radial distance (m) from the injection point and the depth (mbsf) from the seafloor. Fig. 2 of Reference 2 (Teng and Zhang, 2018) is cited. [Figure 28A] Figure 28A is a diagram showing the CO2 storage model (ρCO2 = ρsw) when HFZ > NBZ. [Figure 28B] Figure 28B is a diagram showing the CO2 storage model (ρCO2 = ρsw) when HFZ < NBZ. [Figure 29] Figure 29 is a graph showing the time course of CO2 underground storage. The graph on p. 208 of Reference 15 (IPCC, 2005) is cited. [Diagram 30] Figure 30 is a diagram showing the water depth and potential sea areas around the U.S.A. Fig. 5 of Reference 10 (House, 2006) is cited. [Diagram 31] Figure 31 is a diagram showing the potential storage sea areas in the waters around Japan. [Diagram 32] FIG. 32 is a graph showing the region in which CO2 hydrate exists stably in terms of the temperature-pressure relationship when the ocean floor is at a depth of 3,000 m and the geothermal gradient is 30°C / km. [Diagram 33] Figure 33 shows contour lines of NBZ-HFZ values ​​in the case of storage potential ocean area (ρCO2 = ρsw) and GG = 100°C / km. [Diagram 34] FIG. 34 shows an example of calculation for the Pacific Ocean taking into account the cross section of the ocean floor topography. [Diagram 35] FIG. 35 shows an example of calculation for the Sea of ​​Japan taking into account the cross section of the ocean floor topography. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, a method for underground storage of carbon dioxide, which is one embodiment of the present invention, will be described in detail with reference to the drawings. Note that the drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality.

[0022] Figures 1A and 1B are diagrams for explaining a method for underground storage of carbon dioxide (hereinafter, sometimes simply referred to as the "storage method") which is one embodiment of the present invention, and are schematic cross-sectional diagrams showing the general structure when carbon dioxide is stored in a stratum U below the seabed surface F, with Figure 1A being a diagram showing a wide area of ​​the stratum U including each of the stratum, and Figure 1B being a diagram showing an enlarged view of a key portion of Figure 1A.

[0023] The storage method of this embodiment is a method of storing carbon dioxide (CO2) alone or in a liquefied state of a mixed gas mainly composed of carbon dioxide in a stratum U below the seabed surface F consisting of deposits on an acoustic basement B or in a stratum in a land area not shown. That is, in the storage method of this embodiment, carbon dioxide is injected to form a carbon dioxide reservoir G below a carbon dioxide sealed area S consisting of a stratum that satisfies pressure and temperature conditions capable of generating carbon dioxide hydrate and exists to a predetermined depth from the seabed (seabed surface) F or the ground of a land area not shown. Also, in the storage method of this embodiment, at least a part of the carbon dioxide injected into the carbon dioxide reservoir G naturally rises to the carbon dioxide sealed area S side by the buoyancy of the carbon dioxide to generate carbon dioxide hydrate, thereby forming a carbon dioxide shielding layer C in the carbon dioxide sealed area S.

[0024] As described above, the method for underground storage of carbon dioxide according to the present invention is a method in which the storage location is either the stratum U below the seabed surface F or a stratum below the ground surface of a land area, but in this embodiment, a case in which carbon dioxide is stored in the stratum U below the seabed surface F will be described as an example. Meanwhile, in this embodiment, it is also possible to use a stratum below the ground surface of a land area as the storage location if predetermined pressure and temperature conditions are satisfied.

[0025] In addition, the carbon dioxide described in this embodiment includes not only pure carbon dioxide (CO2) alone, but also a liquefied state of a mixed gas whose main component is carbon dioxide, as described above. That is, the carbon dioxide to be stored in this embodiment may contain water by using existing countermeasures technology in combination. Furthermore, the carbon dioxide to be stored in this embodiment includes, in addition to carbon dioxide itself, other components other than carbon dioxide, such as carbon monoxide (CO), hydrogen (H2), methane (CH4), water (H2O), and hydrogen sulfide (H2S).

[0026] <Geological storage device used for underground storage of carbon dioxide> First, an underground carbon dioxide storage device that can be used in a method for underground storage of carbon dioxide, which is one embodiment to which the present invention is applied, will be described. FIG. 1A shows an underground carbon dioxide storage device (hereinafter, sometimes simply referred to as "storage device") 1 of this embodiment (see also an enlarged view of a main part of FIG. 1B). The storage device 1 in the illustrated example injects and stores carbon dioxide alone or in a liquefied state of a mixed gas mainly composed of carbon dioxide into a stratum U below the seafloor surface F consisting of sediments on an acoustic bedrock B. The storage device 1 is generally configured to include an injection well 2 that penetrates a carbon dioxide sealed area S consisting of a stratum that exists from the seafloor surface F to a predetermined depth and satisfies pressure and temperature conditions capable of generating carbon dioxide hydrate, and extends to a carbon dioxide storage layer G below the carbon dioxide sealed area S, a carbon dioxide supply source that is loaded on a ship or the like (not shown) above sea level M, for example, and a pumping facility (not shown) that is loaded on the ship or the like like the carbon dioxide supply source and pumps the carbon dioxide generated in the carbon dioxide supply source to the injection well 2. The storage device 1 is also provided with a supply line (not shown) for supplying carbon dioxide from a pumping facility installed on a ship to an injection well 2 provided with the seabed surface F as its origin. Furthermore, the storage device 1 of the example shown in Figures 1A and 1B is provided with an artificial water seal facility 6 for increasing the pore pressure in the carbon dioxide sealed area S by injecting at least one of seawater and water as pore water into the carbon dioxide sealed area S when carbon dioxide is injected into the stratum U below the seabed surface F.

[0027] In the schematic cross-sectional view of Fig. 1A, the pressure in a stratum U below the seafloor surface F located below seawater W usually varies depending on the depth from the sea surface M to the seafloor surface F and on any depth from the seafloor surface F to the stratum U. The storage device 1 of the illustrated example injects and stores carbon dioxide into a carbon dioxide storage layer G below a carbon dioxide sealing area S that exists from the seafloor surface F to a predetermined depth and that satisfies the pressure and temperature conditions capable of producing carbon dioxide hydrate.

[0028] As described above, the carbon dioxide supply source (not shown) supplies carbon dioxide to be stored in the carbon dioxide reservoir G in the stratum U. Such a carbon dioxide supply source is not particularly limited, and examples thereof include devices for recovering carbon dioxide derived from coal, oil, and the like discharged from power generation facilities using fossil fuels in various facilities such as offshore plants for extracting oil and gas, and devices for temporarily receiving and storing the carbon dioxide recovered by these devices from ships or pipelines transporting them.

[0029] Furthermore, in the storage device 1, the carbon dioxide supply source (not shown) depends on the characteristics of the device, but is preferably a supply source exposed above sea level M from the viewpoints of supplyability and maintainability of materials such as fuel and carbon dioxide, device life, etc. As such, an example of a configuration in which the carbon dioxide supply source is exposed above sea level M is one in which an offshore platform similar to those installed to extract resources such as oil and gas from the seabed is constructed and the carbon dioxide supply source is installed on the platform.

[0030] The offshore platform may be, for example, a fixed type or a floating type. An example of a fixed offshore platform is one constructed by fixing a structure assembled from high-strength steel or the like directly to the seabed F. Furthermore, examples of floating offshore platforms include semi-submersible vessels.

[0031] The carbon dioxide supply source is not limited to a configuration installed on an offshore platform such as a ship, etc. For example, a configuration may be adopted in which carbon dioxide generated in a carbon dioxide supply source installed on land is transported to the injection well 2 via a pumping facility and piping (pipeline).

[0032] The pumping facility (not shown) pumps carbon dioxide (CO2) to the injection well 2, and as described above, is installed on an offshore platform such as a ship. Furthermore, the storage device 1 can be configured so that carbon dioxide is supplied from a pumping facility installed on a ship to an injection well 2 provided with the seabed surface F as its origin via a supply line (not shown).

[0033] As the pressure-feeding equipment, a pump for pressure-feeding liquid, which has been conventionally used in this field, can be used without any restrictions. Furthermore, as the supply line (not shown) that connects the pumping facility and the injection well 2, for example, piping members made of metal or resin materials that can be used in seawater can be used without any restrictions. It should be noted that the pumping equipment is not limited to that installed on an offshore platform such as a ship as described above, and it is also possible to install the pumping equipment on land or on the seabed F, for example.

[0034] As described above, the injection well 2 penetrates the carbon dioxide sealed area S that exists from the seabed surface F to a predetermined depth, and extends to the carbon dioxide reservoir G below the carbon dioxide sealed area S, and injects carbon dioxide into the carbon dioxide reservoir G. In this embodiment, the injection well 2 injects carbon dioxide into the carbon dioxide reservoir G based on the density (ρ CO2 ) and the density of pore water (ρ sw Carbon dioxide is injected and stored at a depth in stratum U that satisfies the pressure and temperature conditions for equilibrium between the gas and the base rock. The injection well 2 is, for example, a hole formed by drilling or the like, through which carbon dioxide is injected.

[0035] The injection well 2 in the illustrated example is provided as a vertical well, but it can be an inclined well as necessary, or a combination of a vertical well and a horizontal well (e.g., L-shaped cross section), or a structure in which a vertical well, a horizontal well, and an inclined well are appropriately combined. It can also be installed as an inclined well directly from land, in which case a carbon dioxide supply source and pumping equipment (not shown) will be installed on land.

[0036] As described above, in the illustrated example of the storage device 1, an artificial water sealing device 6 is provided for increasing the pore pressure in the carbon dioxide sealing area S, and in the illustrated example, the artificial water sealing device 6 is arranged so that a portion of it is exposed on the upper surface of the carbon dioxide sealing area S. As shown in the enlarged view of Fig. 1(b), the artificial water seal facility 6 is composed of multiple pipes arranged in an annular shape so as to surround the injection well 2. In the illustrated example, the artificial water seal facility 6 is composed of multiple pipes whose cross section is stepped from the injection well 2 side to the outside.

[0037] When carbon dioxide is injected into the stratum U below the seabed surface F, the artificial water sealing facility 6 injects at least one of seawater and water as pore water into the carbon dioxide sealed area S. This increases the pore pressure in the carbon dioxide sealed area S, thereby improving the strength of the entire carbon dioxide sealed area S including the carbon dioxide shielding layer C.

[0038] In addition to the above-mentioned configurations, the storage device 1 used in this embodiment may further be equipped with various monitoring devices for detecting the state of the carbon dioxide stored in the carbon dioxide storage layer G, or the state of the carbon dioxide sealing area S including the carbon dioxide shielding layer C in which carbon dioxide hydrate is generated.

[0039] <Methods for underground storage of carbon dioxide> Hereinafter, a method for underground storage of carbon dioxide, which is one embodiment to which the present invention is applied, will be described in detail with reference to the above-mentioned Figs. 1A and 1B as well as Figs. 2 to 15 as appropriate.

[0040] [The depth region in a geological formation where the density of carbon dioxide and the density of pore water are in equilibrium] As described above, the storage method of this embodiment is a method of storing carbon dioxide (CO2) alone or in a liquefied state of a mixed gas mainly composed of carbon dioxide in a stratum U below the seabed surface F consisting of deposits on an acoustic basement B or in a stratum in a land area not shown. In this embodiment, carbon dioxide is injected to form a carbon dioxide reservoir G below a carbon dioxide sealed area S consisting of a stratum that satisfies pressure and temperature conditions capable of generating carbon dioxide hydrate and exists to a predetermined depth from the seabed (seabed surface) F or the ground of a land area not shown. In addition, at least a part of the carbon dioxide injected into the carbon dioxide reservoir G naturally rises to the carbon dioxide sealed area S side by the buoyancy of the carbon dioxide to generate carbon dioxide hydrate, thereby forming a carbon dioxide shielding layer C in the carbon dioxide sealed area S. In the storage method of the present embodiment, the density of carbon dioxide (ρ CO2 ) and the density of pore water (ρ sw The method involves storing carbon dioxide at depths in geological formations that satisfy the pressure and temperature conditions for equilibrium between the gas and the rock.

[0041] In the storage method of this embodiment, carbon dioxide alone or a liquefied mixed gas containing carbon dioxide as the main component can be stored in a stratum U below the seabed surface F consisting of sediments on an acoustic base B using the storage device 1 shown in Figures 1A and 1B.

[0042] That is, in this embodiment, by carrying out experiments and field surveys in advance, for example, the density of carbon dioxide (ρ CO2 ) and the density of pore water (ρ sw ) and then storing carbon dioxide in that depth region.

[0043] In this embodiment, the density ρ of carbon dioxide in the stratum U under the seabed surface F or in the stratum on land is CO2 and the density of pore water ρ swIt is also possible to adopt a method of calculating the depth of the stratum that satisfies the pressure and temperature conditions for equilibrium between the above and the carbon dioxide, and controlling the injection position of the carbon dioxide into the stratum U below the seafloor surface F or into a stratum on land based on the calculated value.

[0044] Considering the geothermal gradient in layer U below the seafloor surface F, the density of liquid carbon dioxide (ρ CO2 ) and the density of pore water such as seawater (ρ sw ) coincides with (ρ CO2 =ρ sw In other words, it is believed that carbon dioxide can be stored by utilizing the fact that, in the geological layers at these depths, carbon dioxide injected as a liquid remains without sinking or rising.

[0045] The graph in Figure 14 shows the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) at the depth where equilibrium is reached, the results of calculations of the relationship between temperature and pressure conditions are shown. The calculations for the results shown in the graph in Figure 14 were performed under the conditions of seawater salinity: 35‰psu, geothermal gradient: 30℃ / km, and the seawater temperature was taken from an example of Argo data in the Pacific Ocean.

[0046] In the graph of FIG. 14, (1) is the region where carbon dioxide hydrate is generated (stable), and the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) is expressed by the following formula {ρ CO2 <ρ sw}, and deeper than this position is the temperature region where carbon dioxide liquefies. Also, (2) in the graph of FIG. 14 is the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) is expressed by the following formula {ρ CO2 =ρ sw}, and the density of these carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) is the depth / area that satisfies the pressure and temperature conditions at which equilibrium is reached. Also, (3) in the graph of FIG. 14 is the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) is expressed by the following formula {ρ CO2 >ρ sw} and acoustic bedrock: the area that appears in the reflection method used in geological surveys.

[0047] Then, in the layer U below the seafloor F, the following equation {ρ CO2 >ρ sw}, the water depth was calculated as follows: water depth = 3964.645 m, water depth at seafloor surface F = 3600.000 m, and depth from seafloor surface F = 364.645 mbsf.

[0048] The conditions for the application of the above calculation are the relationship between seawater temperature and pressure (depth) in the ocean area, and the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) is expressed by the following formula {ρ CO2 >ρ sw} and this is the stratum U below the seafloor F at a depth of 2,700 m or more (Pacific Ocean excluding the Sea of ​​Japan).

[0049] In the graph of FIG. 3 shown above, the hatched areas at the base points of the X and Y axes represent the density of carbon dioxide (ρ CO2 ) is the density of seawater (ρ sw ) are shown. Such a formula {ρ CO2 >ρ sw A method has also been proposed to store carbon dioxide in concave areas on the seafloor by utilizing an environment where the relationship is expressed as follows (Reference 1); however, in reality, this is not feasible due to the international law-based "Act on the Prevention of Marine Pollution, etc. and Maritime Disasters" (Marine Pollution Prevention Act). On the other hand, the geological layer U below the seafloor surface F is not within the scope of application of the above-mentioned Marine Pollution Prevention Act. Therefore, the results of a study on storing carbon dioxide in geological layer U below the seafloor surface F at a depth of 3,000 m or more have been reported (Reference 2), but so far the geothermal gradient has not been taken into consideration.

[0050] Therefore, in the graph showing the relationship between temperature and pressure in Figure 14, calculations were carried out taking into account the geothermal gradient of layer U below the seafloor surface F. As a result, the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) is expressed by the following formula {ρ CO2 =ρ sw We found that a density-matched depth emerges that satisfies the relationship expressed by {} and is in equilibrium. The solid line in the graph of Figure 14 shows the seawater temperature from depths of 0 to 3,500 m, and at depths greater than 3,500 m, the temperature rises due to the influence of the ground temperature of layer U below the seafloor surface F. The dashed line in the graph of Figure 14 shows the density of seawater (ρ sw ) is shown. The dashed line in the graph of Figure 14 shows the density of carbon dioxide (ρ CO2 ) is shown.

[0051] As shown in the graph in Figure 14, the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw On the other hand, at depths of 2700 to 3500 m, the density of seawater (ρ sw ) than the density of carbon dioxide (ρ CO2 In addition, at depths of 3,500 m or more, the temperature rises as the water enters layer U below the seafloor surface F, and the density of carbon dioxide (ρ CO2 ) decreases, while at a depth of around 4000m, the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ) coincides with the density of seawater at depths of 4000m or more. sw ) is the density of carbon dioxide (ρ CO2 ) is larger than

[0052] As mentioned above, the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw), under the above calculation conditions, it is believed that by injecting carbon dioxide into stratum U beneath the seafloor surface F at a depth of approximately 4,000 m, the carbon dioxide will remain at that location without rising or sinking and will spread horizontally. Therefore, it is believed possible to store carbon dioxide underground at the above location in stratum U beneath the seafloor surface F.

[0053] In Figure 14, the layer U below the seafloor surface F follows the notation of the transitional zone (2500-3000m depth) in seawater described in the 2005 IPCC report (15, p280, Fig. 6, 9 / p286). In Figure 14, the following equation {ρ CO2 =ρ sw Similarly, in the layer U below the seafloor surface F, the geothermal gradient varies from point to point, so the following equation {ρ CO2 =ρ sw}The depth from the seafloor F changes.

[0054] As mentioned above, the density of carbon dioxide (ρ CO2 ) and the density of seawater (ρ sw ), under the above calculation conditions, it is believed that by injecting carbon dioxide into stratum U beneath the seafloor surface F at a depth of approximately 4,000 m, the carbon dioxide will remain at that location without rising or sinking and will spread horizontally. Therefore, it is believed possible to store carbon dioxide underground at the above location in stratum U beneath the seafloor surface F.

[0055] As mentioned above, the density of carbon dioxide in the layer U below the seafloor surface F (ρ CO2 ) and the density of seawater (ρ sw ) will remain at that location without rising or sinking. It was previously believed that carbon dioxide would sink if stored at a depth of 3,000 m or deeper, but it was unclear to what depth in the strata it would sink. However, according to the findings of the present inventors, carbon dioxide will not sink or rise if stored at the above depth.

[0056] Furthermore, the density of the pore water with dissolved carbon dioxide (dissolved carbon dioxide water) becomes greater than the density of the pore water, so it sinks and when it comes into contact with bedrock rich in alkali metals such as calcium (Ca), such as the basalt present in the acoustic bedrock B below, it turns into carbonates such as calcium carbonate (CaCO3), ultimately isolating carbon dioxide (CO2).

[0057] A layer with equilibrium density, i.e., the following equation {ρ CO2 =ρ sw}, it is said that by using a geological layer that satisfies the relationship expressed by the formula: even if a highly permeable fault occurs due to an earthquake or other event, the risk of carbon dioxide leakage due to buoyancy is likely to be reduced, and the above-mentioned cap rock is not necessary.

[0058] When the strata are deformed by an earthquake or other cause, the pore volume generally increases (dilatancy). In this way, the pressure in the area where the pore volume has increased decreases, and carbon dioxide flows in from the surrounding area due to the pressure gradient, but inertia can cause the pressure to exceed hydrostatic pressure. In this case, there is a risk that an upward flow of carbon dioxide will occur and leak into the ocean, but in strata shallower than this depth, a carbon dioxide barrier layer C made of a carbon dioxide hydrate membrane is formed, providing a double defense against the above-mentioned risk of leakage. This double defense is defined by the above-mentioned following equation {ρ CO2 =ρ sw This refers to the utilization of a geological layer that satisfies the relationship expressed by the formula {} and the generation of a carbon dioxide barrier layer C consisting of a carbon dioxide hydrate membrane.

[0059] Here, at depths of 3000 m or more, the temperature of the seafloor surface F is generally less than 2°C. Also, taking into account the geothermal gradient underground, an isotherm can be drawn parallel to the seafloor surface F. When this isotherm is considered around islands, seamounts, and ocean plateaus, as shown in Figure 15, the following equation {ρ CO2 =ρ swIt is considered that the isopycnals satisfying the relationship expressed by { are inclined so as to be in equilibrium with the seafloor surface F. Here, the graph in Figure 15 shows a storage method assuming islands, seamounts, and ocean plateaus, and shows the relationship between horizontal distance and water depth with an aspect ratio of 1:10. In areas shallower than the above isopycnals, as shown in Figure 14, the following equation {ρ CO2 >ρ sw}, while the deep part is expressed by the following formula {ρ CO2 <ρ sw}, carbon dioxide is expressed by the following formula {ρ CO2 =ρ sw} and stays along the isopycnic line.

[0060] As mentioned above, in the layer U below the seafloor F at a depth of 3000 m or more, the following formula {ρ CO2 =ρ sw Considering the distribution of isopycnals satisfying the relationship expressed by}, in order to store carbon dioxide near the depth of this isopycnal, it is necessary to drill a well below the seafloor surface F to a depth of more than 3000 m. In this way, in order to drill a well below the seafloor surface F to a depth of more than 3000 m, it is necessary to use the latest drilling technology, and a significant increase in construction costs is expected. Therefore, as shown in Figure 15, the isopycnals (ρ CO2 =ρ sw ), there may be locations where wells can be drilled from the seafloor surface F at depths of less than 3,000 m. By establishing injection wells in such locations using more general-purpose, low-cost drilling techniques, it would be possible to store carbon dioxide in an area extending beyond depths of 3,000 m, thereby achieving a larger storage capacity.

[0061] In particular, islands, seamounts, and ocean plateaus made of volcanic rock are the remains of the shapes of the ducts for volcanic gases released from lava that erupted on the seafloor (F surface) and were then rapidly cooled by seawater. Volcanic rocks such as those on Mount Fuji contain cavities (pores) that are connected to the lava path (vent) and minute continuous pores (airtight for volcanic gases), and carbon dioxide is stored in these cavities.

[0062] The above-mentioned "island" refers to land surrounded on all sides by bodies of water such as ocean or lakes. In addition, the above-mentioned "sea mount" is a type of underwater landform, a relatively isolated high area with a relative elevation of 1000m or more from the seabed, and whose summit diameter is not large. The above-mentioned "ocean plateau" refers to a relatively flat-topped plateau-like landform on the ocean floor, with an area of ​​100 km2 or more. 2 This refers to an area that rises more than 200m above the surrounding seabed. Additionally, "volcanic gas" is a mixture of gases such as water vapor, CO2, hydrogen sulfide, and carbon monoxide.

[0063] [Relationship between time and strength after carbon dioxide hydrate formation] In the method for underground storage of carbon dioxide according to the present embodiment, the density of carbon dioxide (ρ CO2 ) and the density of pore water (ρ sw ) in a deep region of the stratum that satisfies the conditions for equilibrium between the carbon dioxide hydrate and the carbon dioxide barrier layer C. It is also possible to adopt a method in which the strength of the carbon dioxide hydrate at the initial stage of its formation corresponds to the pressure acting on the carbon dioxide shielding layer C when the carbon dioxide liquid layer G1 formed in the upper part of the carbon dioxide storage layer G has a thickness of 10 m to 40 m.

[0064] The strength of the carbon dioxide shielding layer C made of carbon dioxide hydrate described in this embodiment is what is called a threshold pressure, which is the pressure at which carbon dioxide starts to penetrate. In other words, the strength of the carbon dioxide shielding layer C must be equal to or higher than the threshold pressure below which carbon dioxide does not penetrate (is shielded). The threshold pressure described in this embodiment is measured by mercury porosimetry or the like as the pressure at which carbon dioxide (CO2) starts to penetrate the carbon dioxide shielding layer C, according to the method described in "Application for permission to dispose of specific carbon dioxide gas under the seabed" (http: / / www.env.go.jp / press / 102082.html) in the demonstration test of the DSA based on the "Law Concerning the Prevention of Marine Pollution, etc. and Maritime Disasters" (Law No. 136 of 1970). Note that the carbon dioxide shielding layer C is considered to be formed mainly by filling the pores of the strata with carbon dioxide hydrate, and the shielding mechanism is considered to be different from the threshold pressure in the DSA caused by capillary pressure.

[0065] As mentioned above, carbon dioxide (CO2) exists in one of four phases: gas, liquid, solid, or supercritical, within a certain temperature and pressure range. In addition, carbon dioxide (CO2) becomes a solid (dry ice) at temperatures below 194K (-79.15°C) under normal pressure, but when mixed with water, it becomes a hydrate (solid) under different temperature and pressure conditions.

[0066] Carbon dioxide (CO2) also becomes supercritical when the temperature is 31°C or higher and the pressure is 7.4MPa or higher, and the density change is relatively slow. Carbon dioxide (CO2) also has a lower density than seawater when the temperature is 35°C or lower and the pressure is 20MPa or lower, and is lower than the density of water except when the temperature is around 0°C and the pressure is 14MPa or higher. That is, since carbon dioxide (CO2) has a lower density than seawater when in a liquid or supercritical state, in order to store carbon dioxide (CO2) in the stratum U (see Figure 1) below the seafloor F, a sealing function is required to contain the carbon dioxide that rises to the surface due to the density difference.

[0067] Natural gas generally exists in geological formations in the form of conventional natural gas, shale gas, methane hydrate, etc. Conventional natural gas is stored in geological formations due to the functioning of a cap rock, such as highly airtight mudstone, which acts as a sealing layer. DNA analysis has revealed that shale gas is produced simultaneously with shale oil from shale, which is considered to be the source rock of petroleum. It has been confirmed that methane hydrate exists in a solid form in the subsea layers of the continental margins and in permafrost. Although there is no cap rock as described above, a seal layer that performs a so-called sealing function is formed depending on the temperature and pressure conditions for hydration.

[0068] The existence of natural gas in a natural state as described above can be considered a natural analogue of carbon dioxide storage. That is, in conventional natural gas and general underground storage of carbon dioxide (also called aquifer storage), the cap rock functions as a sealing layer. Also, methane hydrate and carbon dioxide hydrate can be used as a sealing layer by setting the temperature and pressure conditions for hydration. Shale gas exists when methane is adsorbed into shale or coal layers, and is known to also adsorb carbon dioxide. As mentioned above, naturally occurring natural gas is trapped in the earth's layers by different sealing mechanisms. By utilizing these sealing mechanisms, i.e., natural conditions, it is also possible to store carbon dioxide underground.

[0069] As a result of extensive investigations, the present inventors have found that, when generating the above-mentioned carbon dioxide hydrate to form the carbon dioxide shielding layer C, the strength of the carbon dioxide hydrate in the early stages of generation can be optimized to correspond to the pressure acting on the carbon dioxide shielding layer C when the carbon dioxide liquid layer G1 formed in the upper part of the carbon dioxide reservoir G has a predetermined layer thickness. In other words, it has been found that by optimizing the strength of the carbon dioxide hydrate in the carbon dioxide shielding layer C in the early stages of generation based on the pressure acting on the carbon dioxide shielding layer C when the carbon dioxide liquid layer G1 formed in the upper part of the carbon dioxide reservoir G has a layer thickness of 10 m to 40 m, the carbon dioxide shielding layer C becomes a sealing layer having sufficient strength.

[0070] Furthermore, the present inventors have discovered that by optimizing the relationship between the time and strength after the formation of carbon dioxide hydrate in the carbon dioxide shielding layer C, the carbon dioxide shielding layer C becomes a sealing layer with sufficient strength, making it possible to store large amounts of carbon dioxide more efficiently.

[0071] The inventor has discovered that by optimizing the relationship between the time from the generation of carbon dioxide hydrate in the carbon dioxide shielding layer C and its strength, as described above, it is possible to form a carbon dioxide shielding layer C with sufficient strength, thereby preventing the carbon dioxide injected into the carbon dioxide storage layer G from leaking, and making it possible to efficiently store large amounts of carbon dioxide without the need for a cap rock.

[0072] The strength at the initial stage of carbon dioxide hydrate formation in the carbon dioxide shielding layer as specified in the present invention is, for example, the strength at a time point 0 to 40 hours after the start of carbon dioxide hydrate formation.

[0073] Below, we will explain in detail the behavior of carbon dioxide hydrate in the pores of geological formations, as well as an image of carbon dioxide hydrate stored in geological formations. In the schematic cross-sectional view of Figure 1A, the carbon dioxide sealed area S below the seafloor surface F is a geological layer that satisfies the pressure and temperature conditions capable of producing carbon dioxide hydrate, i.e., an area of ​​pressure and temperature at which carbon dioxide hydrate exists stably. In addition, the carbon dioxide reservoir G located below the carbon dioxide sealed region S is a region of pressure and temperature conditions that allow carbon dioxide to be stored in liquid form (see also the temperature-pressure phase diagram of carbon dioxide hydrate in Figure 3). In addition, the acoustic bedrock B below the carbon dioxide reservoir G is a bedrock defined by the reflection method used to interpret ocean-floor geological maps, and is a bedrock that may contain, for example, basalt with a composition similar to that of magma. Furthermore, a sedimentary layer (Quaternary sedimentary layer) of several hundred meters exists above the acoustic bedrock B, and based on the stable region of carbon dioxide hydrate shown in the graph in Figure 3, this is divided into the above-mentioned carbon dioxide sealed region S and carbon dioxide storage layer G.

[0074] In the carbon dioxide reservoir G shown in FIG. 1A, when carbon dioxide is injected at the tip 21 of the injection well 2, the carbon dioxide rises due to buoyancy and permeates to the lower position of the carbon dioxide sealed area S. Then, when the temperature of the carbon dioxide that has permeated into the carbon dioxide sealed area S drops, carbon dioxide hydrate is produced and a carbon dioxide shielding layer C is formed at the lower position of the carbon dioxide sealed area S. After this, the carbon dioxide injected into the carbon dioxide reservoir G is prevented from floating upward from that position by the carbon dioxide shielding layer C, so that the storage range spreads horizontally and a carbon dioxide liquid layer G1 is thought to be formed in the upper part of the carbon dioxide reservoir G.

[0075] The following provides a detailed description of the calculation of the pressure acting on the carbon dioxide shielding layer C due to the rising pressure of the carbon dioxide liquid layer G1. In parallel with laboratory experiments described later in detail, the inventors calculated the pressure acting on the carbon dioxide barrier layer C made of carbon dioxide hydrate shown in Fig. 1A. This pressure is based on the buoyancy of liquid carbon dioxide in water (seawater), and is calculated by integrating the difference between the density of water or seawater and the density of carbon dioxide, as shown in the graph of Fig. 4.

[0076] Here, the carbon dioxide density contour lines used to estimate the pressure acting on the carbon dioxide barrier layer C are shown in the graph of the temperature-pressure phase diagram of carbon dioxide hydrate in Figure 2. The region indicated by R1 in the graph in Figure 2 is the region showing the pressure and temperature conditions under which carbon dioxide hydrate exists stably.

[0077] In addition, in the graph of the temperature-pressure phase diagram of carbon dioxide hydrate in Figure 3, the stable region of carbon dioxide hydrate is shown by enlarging the temperature axis. In Figure 3, the above-mentioned carbon dioxide density isoline, seawater density isoline, and density difference isoline are also superimposed to make it possible to read the buoyancy of carbon dioxide. Note that in Figure 3, curves are plotted showing the relationship between seawater temperature and pressure (water depth) in the Pacific Ocean and the Sea of ​​Japan. In addition, in Figure 3, the carbon dioxide isodensity lines calculated using the Span & Wagner equation of state (Reference 3) are also shown. In addition, in Figure 3, the equivalent density line of 35‰ seawater calculated using EOS80 (Reference 4; Reference 5) is also shown. In addition, in Figure 3, the isodensity lines of density difference Δ (carbon dioxide density - seawater density) are also shown. In addition, Figure 3 also shows the temperature and pressure of the formation shown in the schematic cross-sectional diagram of Figure 1A {carbon dioxide sealed area S (1) - carbon dioxide reservoir G (2) - acoustic bedrock B (3)}.

[0078] Here, the region indicated by R2 in the graph of FIG. 3 is a region in which the pressure acting on the carbon dioxide shielding layer C can be read when the thickness of the carbon dioxide liquid layer G1 formed in the upper part of the carbon dioxide reservoir G is 10 m to 40 m. The region R2 shown in FIG. 3 is a region showing the pressure and temperature of the carbon dioxide stored in the carbon dioxide reservoir G. Here, reading the value of the carbon dioxide equivalent density line L1 shown in FIG. 3, the density of carbon dioxide is 850 to 970 kg / m 3 If we read the value of the seawater density line L2, the density of seawater is 1029 to 1035 kg / m 3 These densities are necessary for calculating the buoyancy of carbon dioxide, and when the thickness of the carbon dioxide liquid layer G1 formed in the upper part of the carbon dioxide storage layer G is 10 m to 40 m, the pressure acting on the carbon dioxide shielding layer C can be calculated based on the above density values.

[0079] The region indicated by R3 in the graph of Figure 3 shows the range of the density of carbon dioxide stored by carbon dioxide hydrate storage (when the thickness of the carbon dioxide liquid layer G1 is 50 m). Looking at this region R3, the density of the stored carbon dioxide is 900 to 950 kg / m 3 This is about the same as the density of seawater, which is 90 to 95% of the density of seawater. This shows that when the density of the carbon dioxide stored in the carbon dioxide storage layer G (liquid carbon dioxide layer G1) approaches the density of seawater, the pressure generated by the buoyancy acting on the carbon dioxide barrier layer C becomes smaller.

[0080] Lines (a)-(b)-(c) shown in the graph in Figure 3 respectively connect the plot positions showing the temperature and pressure at the seafloor surface F (a), the temperature and pressure at the bottom end of the carbon dioxide shielding layer C (b), and the temperature and pressure at the carbon dioxide injection point (c), and show the cases where the temperature at the seafloor surface F is 5°C·1000m (approximately 1,000 decibar), the geothermal gradient in the subseafloor layer U is 30°C / km, and the temperature at the carbon dioxide injection point is 15°C. These temperature and pressure change depending on the water depth and geothermal gradient at the storage site, so lines (a)-(b)-(c) differ for each site.

[0081] On the other hand, the relationship between temperature and pressure (depth) of the ocean bottom F in the Pacific Ocean and the Sea of ​​Japan has been confirmed and reported (Reference 6). The relationship between temperature and pressure (depth) in the Pacific Ocean and the Sea of ​​Japan as reported in Reference 6 is plotted as two nearly vertical curves on the left side of the X-axis in the graph in Figure 3, with the curve on the lower temperature side showing the relationship in the Sea of ​​Japan and the other showing the relationship in the Pacific Ocean. Both of these two curves are plotted as curves at depths where the standard deviation is 0.5°C or less, with the Sea of ​​Japan being plotted in the depth range of 500 to 2000 m and the Pacific Ocean being plotted in the depth range of 1000 to 4000 m. Here, since the Pacific Ocean has a large temperature change that is thought to be due to the Kuroshio Current, the envelope of maximum and minimum values ​​is shown as a dashed line in the graph in Figure 3. The above two curves shown in Figure 3 are the results for the depth range confirmed in the database targeted in Reference 6. Regarding the geothermal gradient (GG), the average geothermal gradient on land is assumed to be 30°C / km, and is assumed to be similar in oceanic areas, but the geothermal gradient varies from point to point within a range of approximately 0 to 200°C / km.

[0082] Based on the above relationship between seawater temperature and water depth and the formula for the boundary (three-phase line) of the stable region of CO2 hydrate shown in Reference 6, the temperature and pressure (T, P) at the bottom of the carbon dioxide barrier layer C are T sf , P sf , it can be calculated using the following general formula (1) with GG as a known input.

[0083]

number

[0084] On the other hand, there are some points to note about the upper end of the carbon dioxide barrier layer C, such as the phenomenon of supercooling of CO2 hydrate, the condition of the molar ratio of CO2 to water (1:5.5-6) ​​that allows the formation of CO2 hydrate, and the freezing point depression that occurs when the pore water in the stratum U under the seafloor contains salt like seawater. Therefore, there may be insufficient data to calculate with precision. Here, the above-mentioned supercooling (SUPER-COOL) is a phenomenon in which a liquid remains liquid even at temperatures and pressures that cause a phase change to a solid, and it is known that when a minute part starts to change to a solid due to a trigger such as vibration, the whole changes to a solid all at once. In addition, it is known that freezing point depression is when ice is floated in salt water with a concentration of about 3%, the melting point of this ice drops to about -5℃, and it is also known that the melting point drops further as the salt concentration increases.

[0085] As described above, the pore water contained in the stratum U beneath the seafloor is considered to contain salt like seawater, so the freezing point depression in the formation of CO2 hydrate will be explained below. When CO2 hydrate is formed, it is formed in such a way that it expels salt, just like ice is formed. This causes the phenomenon that the salinity of the remaining water becomes high. In addition, the freezing point depression is not uniform because it varies with local salinity, and there is a variation of about ±2°C. Furthermore, when the change in heat quantity is measured near the freezing point of highly pure water, a clear exothermic peak appears at 0°C, making it possible to determine the freezing point. However, when the change in heat quantity is measured near the freezing point of water that contains salt, no clear peak appears, and there is a 1-2°C range around the freezing point, making it difficult to clearly determine the value.

[0086] As described above, there is uncertainty in determining the temperature and pressure at the top of the carbon dioxide shielding layer C in terms of supercooling, freezing point depression, and the molar ratio of CO2 to water, but the range is within a certain range. For example, the freezing point depression in the case of average seawater of 35‰ is 2°C on average, with a further range of about ±2°C. In the graph of FIG. 3, this freezing point depression is shown by multiple line segments L3 to L8 parallel to the temperature axis (X-axis). In the graph of FIG. 3, the above line segments L3 to L8 are located on the low temperature side with respect to the boundary (three-phase line) of the stable region of CO2 hydrate shown by line segment L9, and thus the freezing point depression appears. This data shows the results of an indoor experiment conducted by the present applicant, and the line segments L3 to L8 are drawn to connect the maximum and minimum values.

[0087] From the above findings, the temperature at the upper end of the carbon dioxide shielding layer C is 2°C lower on average than the temperature at the lower end, with a range of about ±2°C. Furthermore, when the geothermal gradient is 30°C / km, a temperature change of 2°C corresponds to a layer thickness of 66.7m, with a range of ±66.7m, but it is highly likely that this layer thickness and ± range are close to the average value. Therefore, it is considered that the carbon dioxide shielding layer C occurs on the lower end side of the carbon dioxide sealing area S, that is, between 0 and 133 m from the boundary with the carbon dioxide storage layer G. Note that this finding is based on a geothermal gradient of 30°C / km and a salinity of 35‰, so if these change, the above layer thickness will also change.

[0088] In the graph of Figure 4, the right end of the line for Cases 1-3 shows the water depth (m) when the reservoir thickness is 0m (top of the tank), and the pressure value when the reservoir thickness is 200m is shown as a downward positive value. In Figure 4, the pressure when the reservoir thickness is 20m is about 1 / 10 of when the reservoir thickness is 200m. In Figure 4, Cases 1 and 2 are carbon dioxide hydrate storage, and Case 3 is aquifer storage, and it can be seen that in aquifer storage, carbon dioxide becomes supercritical, the carbon dioxide density is low, and buoyancy is large, resulting in high pressure.

[0089] 5 shows a schematic diagram of the pressure cell 100 used in the laboratory experiment to demonstrate the present invention. The maximum pressure in the pressure cell 100 is 70 MPa, which corresponds to the pressure at a depth of 7000 m in the ocean. In this demonstration experiment, No. 7 silica sand (fine sand) or No. 9 silica sand (silt) was placed in the pressure cell 100 shown in Figure 5, and conditions simulating the pressure and temperature in the strata under the seabed were reproduced, after which an experiment was performed in which liquid carbon dioxide was allowed to permeate from the bottom of the pressure cell 100. The items measured at this time were the temperature measured by thermometers installed at 100 mm intervals from the bottom side, and the pressure at the top and bottom ends of the pressure cell 100.

[0090] Each of the graphs in FIGS. 6(a) to (c) shows the strength of a membrane made of carbon dioxide hydrate obtained in an experiment using the pressure cell 100 shown in FIG. In each of the graphs in Figures 6(a) to (c), the flow rate of carbon dioxide shown by the solid line in each lower graph is set to be slower in the order of Figure 6(a): 0.006 mL / min, Figure 6(b): 0.003 mL / min, and Figure 6(c): 0.0015 mL / min, so that the time interval at which the carbon dioxide hydrate membrane breaks becomes longer. If this time interval is short, it can be considered that the carbon dioxide hydrate membrane breaks before it has sufficient strength. In Figures 6(a) to (c), as the time interval is longer, a pressure difference (=strength) of about 0.2 MPa is measured after 115 hours has passed, and there is a tendency for the strength of the carbon dioxide hydrate membrane to increase.

[0091] The above pressure difference is the pressure acting on the carbon dioxide hydrate membrane. When this pressure difference becomes higher than the strength of the carbon dioxide hydrate membrane, part of the membrane breaks, carbon dioxide permeates, the pressure difference drops to "0", and then the carbon dioxide permeation stops, and the carbon dioxide hydrate membrane is repeatedly generated.

[0092] From the results shown in the graphs of Figures 6(a) to (c), the initial strength of the carbon dioxide hydrate membrane is assumed to be 0.02 MPa, and in the following experiments, a pump (not shown) is controlled so that the differential pressure acting on the initial strength of the carbon dioxide hydrate membrane is 0.02 MPa.

[0093] The graph in Fig. 7 shows the temperature rise due to the heat of formation of carbon dioxide hydrate when carbon dioxide permeates into a layer, obtained in an experiment using the pressure cell 100 shown in Fig. 5. In this experiment, the position of the penetration front of liquid carbon dioxide into a sample (No. 7 silica sand) saturated with seawater was confirmed by temperature change.

[0094] The graph in Figure 8 shows the relationship between retention time and differential pressure when the pressure was maintained for 17 days under conditions where the differential pressure was approximately 2m or less in terms of head, as obtained in an experiment using the pressure cell 100 shown in Figure 5. In this experiment, a sample (No. 9 silica sand) saturated with 40‰ seawater was used. The initial conditions in the experiment whose results are shown in the graph in Figure 8 were such that carbon dioxide was allowed to permeate into the pores of sand saturated with seawater up to the position of a thermometer installed 400 to 500 mm from the bottom. The relationship between time and pressure difference shown by the solid line in the third graph from the top (third from the bottom) in Figure 8 is for an initial carbon dioxide hydrate membrane that is assumed to have formed at the tip where carbon dioxide has penetrated, and the pressure difference was maintained at 0.02 MPa for 400 hours (approximately 17 days).

[0095] The graph in Figure 9 shows the strength of the carbon dioxide barrier layer C made of a carbon dioxide hydrate membrane when the differential pressure was maintained for 17 days under conditions of a head pressure of approximately 2 m or less, and then the differential pressure was maintained at 1 MPa for 24 hours, as obtained from an experiment using the pressure cell 100 shown in Figure 5.

[0096] In this experiment, a differential pressure of 0.08 MPa was forcibly applied to the carbon dioxide barrier layer C, which was made of carbon dioxide hydrate and whose strength was thought to have increased after 410 hours in the above experiment, and this condition was maintained for 24 hours. After 24 hours had passed in this maintained condition, the differential pressure was increased to 1 MPa and maintained for 4 hours. As a result, it was confirmed that the strength of the carbon dioxide barrier layer C had increased from 0.02 MPa, which was the initial strength of the carbon dioxide hydrate membrane, to 1 MPa.

[0097] Here, the above-mentioned pressure difference of 0.02 MPa is the pressure difference that occurs when carbon dioxide is stored in carbon dioxide hydrate storage so that the carbon dioxide storage layer G has a layer thickness of about 20 m. In actual carbon dioxide storage, it is expected that 5 million tons of carbon dioxide will be injected continuously per year for 20 to 30 years, so the layer thickness of the carbon dioxide storage layer G is thought to gradually increase. For example, if it takes more than two weeks for the layer thickness of the carbon dioxide storage layer G to reach 0 to 20 m, it is expected that the amount of storage will be increased by using the effective shielding function of the carbon dioxide shielding layer C, which has a strength of 1 MPa or more.

[0098] In this experiment, first, as explained with reference to the graph in Fig. 8, an experiment was conducted in which the conditions of temperature 2°C, pressure 7 MPa, and differential pressure of 2 m or less in head were maintained for 17 days, and then an experiment was conducted in which the carbon dioxide pressure was increased to a differential pressure of 1 MPa and maintained for about 24 hours. As a result, it was confirmed that the strength of the carbon dioxide barrier layer C (the pressure at which carbon dioxide begins to permeate) was 1 MPa or more, as described above.

[0099] The graph in Fig. 10 shows the strength of the carbon dioxide barrier layer made of a carbon dioxide hydrate membrane when the pressure difference is increased stepwise, obtained in an experiment using the pressure cell 100 shown in Fig. 5. Here, the strength shown in the graph in Fig. 10 is the strength of the carbon dioxide hydrate membrane (the pressure at which carbon dioxide starts to permeate).

[0100] The data shown in FIG. 10 is the result after data on the carbon dioxide shielding layer C having a strength of 1 MPa or more was obtained three times in succession over a period of about two weeks. The reason for conducting this experiment was that, based on previous knowledge, it was predicted that the strength of the carbon dioxide barrier layer C would gradually increase over a period of approximately two weeks, and it was necessary to confirm this. In this experiment, as an initial condition, sand saturated with seawater was maintained at a temperature of about 4°C at a water pressure (8 MPa) equivalent to a depth of 800 m. Then, various treatments were carried out at the respective times shown in (1) to (6) below, and the results were observed over time.

[0101] (1)0~7 hours Carbon dioxide was allowed to permeate while adjusting the flow rate so that the above-mentioned pressure difference became approximately 0.02 MPa (see the second graph from the bottom in FIG. 10). During this time, carbon dioxide permeated at a maximum rate of 0.04 mL / min, and the seawater with the same amount of carbon dioxide permeated was discharged from the upper side of the pressure cell 100 (see the second graph from the bottom in the graph of FIG. 10).

[0102] As the pressure difference at this time, a graph with a pressure scale of 0 to 3 MPa is shown in the third graph from the top in Fig. 10, and a graph with a pressure scale of 0 to 0.25 MPa is shown in the fourth graph from the top in Fig. 10. That is, in Fig. 10, in the case where the pressure difference is small, 0.2 MPa or less, the pressure can be read from the fourth graph from the top.

[0103] (2)7~18 hours After 7 hours had passed, the amount of carbon dioxide permeating had decreased, so the pressure difference was increased to 0.025 MPa. At this time, seawater containing the amount of carbon dioxide permeating was discharged from the upper side of the pressure cell 100, and it was confirmed that the cell was not blocked.

[0104] (3)18~42 hours When the pressure difference was forcibly increased to 0.05 MPa, it appeared that part of the carbon dioxide hydrate membrane broke, and the amount of carbon dioxide permeation increased sharply to 0.05 mL / min or more. However, after another four hours, 22 hours later, the amount of carbon dioxide permeation decreased, and the seawater discharged from the upper side of the pressure cell 100 (discharge is a negative flow rate) gradually changed from discharge to injection (injection is a positive flow rate), indicating that the carbon dioxide barrier layer C is gradually being formed. The reason for this is thought to be that the formation of carbon dioxide hydrate causes a volumetric shrinkage of about 10%, and both carbon dioxide and seawater flow in during this formation process.

[0105] (4)42~50 hours When the pressure difference was gradually increased to 0.2 MPa, no penetration through the carbon dioxide barrier layer C was observed, confirming that the strength of the carbon dioxide barrier layer C was 0.2 MPa or more.

[0106] (5)50~67 hours When the above differential pressure was maintained at 0.5 MPa for 17 hours, it was confirmed that the strength of the carbon dioxide barrier layer C was 0.5 MPa or more.

[0107] (6)67~72 hours The pressure difference was gradually increased, and the experiment was terminated when it was confirmed that the strength of the carbon dioxide barrier layer C was 3 MPa or more.

[0108] From the above results, in this experiment, although a sudden increase is seen in the flow rate of carbon dioxide at 50 hours and 65 to 72 hours (the third graph from the bottom in FIG. 10), this is an apparent flow rate caused by the compression of carbon dioxide. In other words, during the same time periods, seawater was not discharged from the top of the pressure cell 100, and both were in an injected state, so it was confirmed that there was no penetration of carbon dioxide through the carbon dioxide shielding layer C.

[0109] The above results are believed to show the time-strength relationship of the carbon dioxide barrier layer C, and show that its strength increases over time. In addition, it is believed that the carbon dioxide hydrate membrane broke in the time period from 18 to 22 hours, but for breaks caused by this level of pressure difference, the repair function of the carbon dioxide barrier layer C itself is thought to have acted, shortening the time it took to reach a strength of 1 MPa or more to approximately 67 hours (just under 3 days).

[0110] FIG. 11 is a graph showing the strength of the carbon dioxide barrier layer C made of a carbon dioxide hydrate film when a reverse pressure difference is applied, obtained in an experiment using the pressure cell 100 shown in FIG. In this experiment, the "reverse pressure difference" refers to the density of carbon dioxide (ρ CO2 ) and the density of pore water (ρ sw ) is expressed by the following equation {ρ CO2 <ρ sw} This refers to the case where the relationship is expressed as

[0111] In this experiment, as an initial condition, carbon dioxide was first permeated into sand saturated with seawater so that the tip of the carbon dioxide permeation was located within a range of 400 to 500 mm from the bottom of the pressure cell 100, and then the differential pressure was maintained in the range of 0 to 0.02 MPa for approximately 15 hours. At this initial stage, a temperature rise due to the heat of carbon dioxide hydrate formation is observed at the front of the carbon dioxide permeation. This temperature rise is observed at equal time intervals at positions 100 mm, 200 mm, and 300 mm from the bottom of the pressure cell 100, and the front of the carbon dioxide permeation can be confirmed by this.

[0112] The graph in FIG. 11 shows the results when the carbon dioxide pressure was suddenly reduced after 15 hours. Due to this sudden drop in carbon dioxide pressure, a significant temperature increase due to the heat of carbon dioxide hydrate formation was observed at a point 400 mm from the bottom of the pressure cell 100, and it is believed that a carbon dioxide shielding layer C was formed at this position. This timing also coincides with the time when the carbon dioxide pressure drops from 20 MPa to around 17.5 MPa and then rises back to 20 MPa.

[0113] In addition, while the carbon dioxide pressure was decreasing from 20 MPa to 17.5 MPa, the carbon dioxide shielding layer C maintained sufficient strength and did not break, so a pressure difference was generated. However, when the carbon dioxide pressure decreased to 17.5 MPa, the carbon dioxide shielding layer C broke once. At the timing when the carbon dioxide barrier layer C is broken once, water starts to permeate toward the vicinity of the carbon dioxide barrier layer C, and this permeation causes the pressure on the carbon dioxide side to rise to 20 MPa. In addition, since the temperature rise caused by the heat generated by the formation of carbon dioxide hydrate is remarkable at a position 400 mm from the bottom of the pressure cell 100, it is considered that the carbon dioxide barrier layer C made of a stronger carbon dioxide hydrate membrane is regenerated at this position.

[0114] Furthermore, the flow rate of carbon dioxide was 0.04 mL / m at the 15-hour mark, and stopped at 0 mL / m 15 hours later. As mentioned above, after carbon dioxide barrier layer C is ruptured and then regenerated, the pressure on the carbon dioxide side gradually decreases, and the differential pressure acting on carbon dioxide barrier layer C becomes negative (negative). This differential pressure indicates that carbon dioxide barrier layer C has a strength equivalent to 10 MPa (1000 m of water head).

[0115] Here, in the pressure cell 100 shown in Fig. 5, when artificial water sealing, the details of which will be described later, is performed in a direction in which water is allowed to permeate into the pores in the sand into which carbon dioxide has permeated (see artificial water sealing equipment 6 in Fig. 1), it is expected to have the effect of reducing the differential pressure acting on the carbon dioxide shielding layer C. At the same time, when the pressure rise due to the artificial water sealing becomes excessive, it is expected that the carbon dioxide shielding layer C will have an increased strength in a short period of time (1 to 2 hours in this experiment).

[0116] The graph in Figure 12 shows contour lines of the optimal volume ratio of {carbon dioxide / water} as a function of temperature and pressure. As shown in Figure 12, the volume ratio was calculated taking into account the molecular formula of carbon dioxide hydrate crystals (5.75H2O·CO2), and the volume of water required was 2.3 times the volume of carbon dioxide. Therefore, it is highly likely that there is a lack of water / seawater in order for the pores into which carbon dioxide has permeated to become blocked. In other words, it is believed that the water has been used up (dried up), leaving carbon dioxide in an excess.

[0117] On the other hand, Fig. 13 shows a schematic diagram of the mechanism of blocking carbon dioxide hydrate by a membrane, based on the experimental results using the pressure cell 100 shown in Fig. 5. That is, Fig. 13 shows a partial cross section of the cylindrical pressure cell 100 shown in Fig. 5. As shown in the schematic diagram in Figure 13, near the interface between liquid carbon dioxide and water / seawater in the pores of the sand, the water saturation gradually decreases from 100%, and it can be seen that the interface is by no means flat. Moreover, as shown in FIG. 13, near the front of the carbon dioxide permeation, there is a region close to the volume ratio according to the molecular formula described above.

[0118] It is believed that a water film exists on the surface of the sand due to the wettability of water, and that the carbon dioxide hydrate film has formed at the interface that has infiltrated into the complex pores of the sand. Therefore, it is highly likely that the carbon dioxide barrier layer C is not a horizontal, one-sided film, but rather an area with a certain thickness.

[0119] Furthermore, assuming that carbon dioxide hydrate with a thickness of about 20 μm (see Reference 1 above) is formed in the pores of the sand in the shape of a straw, and assuming further that the average pore size is 60 μm, liquid carbon dioxide is thought to exist in the 20 μm region remaining in the center of the straw shape. It takes about two weeks for this remaining part of liquid carbon dioxide to turn into carbon dioxide hydrate, and it is thought that this part will have a strength of more than 1 MPa. It is believed that carbon dioxide molecules and water molecules are supplied to the remaining liquid carbon dioxide portion by diffusion based on a concentration gradient, and this supply by diffusion is believed to be the reason why it takes time for the carbon dioxide barrier layer C to form.

[0120] According to the above-mentioned demonstration experiment of the present invention, the strength of the carbon dioxide hydrate in the pores of the carbon dioxide shielding layer C at the initial stage of its formation is preferably 0.01 to 0.04 MPa, and more preferably about 0.02 MPa on average. By ensuring that the strength of the carbon dioxide hydrate constituting the carbon dioxide shielding layer C at the initial stage of its formation falls within the above range, the carbon dioxide injected below the carbon dioxide shielding layer C can be more reliably contained, and large amounts of carbon dioxide can be more efficiently stored. The strength of 0.02 MPa at the initial stage of carbon dioxide hydrate formation is an average value for the grain sizes of the above-mentioned silica sand No. 9 to No. 7.

[0121] In the storage method of the present embodiment, the initial strength of carbon dioxide hydrate generation in the carbon dioxide shielding layer C and the pressure acting on the carbon dioxide shielding layer C when the thickness of the carbon dioxide liquid layer G1 formed in the upper part of the carbon dioxide storage layer G is 10 m to 40 m are calculated, and at least one of the flow rate of carbon dioxide injected into the stratum U below the seabed surface F, the number of injection wells 2 used, and the injection depth can be controlled based on each calculated value. Each calculation as described above can be performed using a calculation program on a computer, tablet terminal, or the like, and the injection flow rate of carbon dioxide and the number and depth of the injection wells 2 used can be controlled based on the calculation results. In this case, for example, by installing multiple injection wells 2 with different inner diameters and injection depths, it becomes easy to appropriately change the flow rate and the number of injection wells used.

[0122] According to the storage method of this embodiment, by employing the control method as described above, it is possible to adjust the carbon dioxide injection flow rate, injection position, and the like while constantly monitoring the carbon dioxide storage state in the carbon dioxide storage layer G and the strength of the carbon dioxide shielding layer C, thereby enabling efficient operation of the carbon dioxide storage facility. Also, as described above, it is possible to reliably and effectively contain the carbon dioxide injected below the carbon dioxide shielding layer C, making it possible to store a large amount of carbon dioxide with high efficiency.

[0123] Furthermore, the carbon dioxide shielding layer C preferably has a strength of 1 MPa or more 10 to 20 days after the start of carbon dioxide hydrate generation. When the strength of the carbon dioxide shielding layer C 10 to 20 days after the start of carbon dioxide hydrate generation is within the above range, the strength of the carbon dioxide shielding layer C can be more reliably ensured. This makes it possible to more reliably contain the carbon dioxide injected below the carbon dioxide shielding layer C, and therefore to more efficiently store large amounts of carbon dioxide.

[0124] Furthermore, in the storage method of this embodiment, a method can be adopted in which the strength after 10 to 20 days has elapsed since the start of carbon dioxide hydrate generation in the carbon dioxide shielding layer C is calculated, and at least any one of the flow rate of carbon dioxide injected into the stratum U below the seabed surface F, the number of injection wells 2 in use, and the injection depth is controlled based on the calculated value obtained by the calculation. The above calculated values ​​can be obtained using a calculation program on a computer, tablet terminal, or the like, and similarly to the above, the flow rate of carbon dioxide injected and the number and depth of injection wells 2 in use can be controlled based on the calculation results. This allows for more efficient operation of the carbon dioxide storage facility, as it is possible to adjust the carbon dioxide injection flow rate and injection position while constantly monitoring the carbon dioxide storage state in the carbon dioxide storage layer G and the strength of the carbon dioxide shielding layer C, as described above. Also, as described above, it is possible to reliably and effectively contain the carbon dioxide injected below the carbon dioxide shielding layer C, making it possible to store large amounts of carbon dioxide with even higher efficiency.

[0125] In this embodiment, the thickness of the stratum in the entire carbon dioxide sealing area S is not particularly limited. However, in order to improve the sealing property of the carbon dioxide sealing area S, that is, to reliably prevent the carbon dioxide (CO2) stored in the carbon dioxide reservoir G from leaking into the seawater W, it is preferable to ensure that the stratum thickness in the carbon dioxide sealing area S is at least 100 m or more. In this way, as a method for ensuring the desired thickness of the stratum in the carbon dioxide sealing area S, for example, it is preferable to fully grasp the temperature and pressure conditions of the stratum U below the seabed surface F through a preliminary survey, and then set the depth of the injection well 2 so that the stratum thickness in the carbon dioxide sealing area S is a predetermined dimension, thereby optimizing the injection depth of carbon dioxide (injection depth).

[0126] [Artificial water seal in the carbon dioxide sealing region] In the method for underground storage of carbon dioxide according to the present embodiment, the density of carbon dioxide (ρ CO2 ) and the density of pore water (ρ sw In addition to storing carbon dioxide in a deep region of the stratum that satisfies the conditions for equilibrium between the carbon dioxide hydrate and the carbon dioxide barrier layer C and optimizing the relationship between time and strength during the formation of the carbon dioxide hydrate that constitutes the carbon dioxide barrier layer C, it is also possible to adopt a method of increasing the pore pressure in the carbon dioxide sealed region S by artificially sealing with water, in which at least one of seawater and water is injected into the carbon dioxide sealed region S as pore water.

[0127] An example of equipment for performing the above-mentioned artificial water sealing is an artificial water sealing equipment 6 provided in the storage device 1 of the example shown in Figures 1A and 1B. The artificial water sealing equipment 6 of the illustrated example is disposed on the upper surface side of the carbon dioxide sealed area S, and as described above, is capable of increasing the pore pressure of the carbon dioxide sealed area S by injecting at least one of seawater and water as pore water into the carbon dioxide sealed area S. By providing such an artificial water sealing device 6, the pore pressure in the carbon dioxide sealed area S increases, and the strength of the entire carbon dioxide sealed area S including the carbon dioxide shielding layer C is increased. The detailed configuration of the artificial water seal facility 6 has already been described.

[0128] The details of performing artificial water sealing in the storage method of this embodiment will be described below. In the schematic cross-sectional view of FIG. 1A explained above, the maximum pressure due to the buoyancy of carbon dioxide acts near point (b) located vertically above the carbon dioxide injection point. At this time, if the pressure of the stored carbon dioxide exceeds the time-strength relationship of the carbon dioxide barrier layer C, there is a risk of carbon dioxide leakage. It is highly likely that the carbon dioxide pressure will exceed the strength of the carbon dioxide barrier layer C in the early stages of formation of the carbon dioxide hydrate membrane that will become the carbon dioxide barrier layer C (for example, within two weeks).

[0129] Meanwhile, hydrostatic pressure due to seawater W acts on the upper part of the carbon dioxide barrier layer C, while pressure generated by the buoyancy of carbon dioxide acts on the lower part of the carbon dioxide barrier layer C. If this pressure difference between the top and bottom of the carbon dioxide barrier layer C is equal to or less than the strength of the carbon dioxide barrier layer C, carbon dioxide will not start to permeate into the carbon dioxide barrier layer C. Therefore, in the early stages of formation of the carbon dioxide hydrate membrane when the carbon dioxide barrier layer C is weak (for example, two weeks after the start of formation), it is possible to provide an artificial water sealing facility 6 that utilizes a gap structure of multiple pipes (casing annulus) or the like installed on the seabed surface F when cutting the injection well 2 so that the pressure difference between the top and bottom of the carbon dioxide barrier layer C does not exceed 2m in hydraulic head. By installing such an artificial water sealing facility 6, it becomes possible to increase the pore pressure (pore pressure) of the carbon dioxide sealed area S including the carbon dioxide barrier layer C by injecting seawater / water from above the carbon dioxide sealed area S.

[0130] Carbon dioxide hydrate storage targets the unconsolidated Quaternary sedimentary layers, which are the newest in geological time, as mentioned above. When considering faults caused by earthquakes, etc., Quaternary sedimentary layers are unconsolidated and therefore allow deformation, and it is thought that dilatancy (volume change caused by shear deformation) occurs with this deformation. It is thought that the porosity and permeability of Quaternary sedimentary layers increases due to the above-mentioned dilatancy, and it is necessary to consider the extent to which this affects the risk of carbon dioxide leakage. For this reason, the inventors conducted a demonstration experiment using the pressure cell 100 shown in Fig. 5 regarding the time-strength relationship of the carbon dioxide shielding layer C assuming artificial water sealing, and the results are shown in the graph of Fig. 11 described above. As a result, when artificial water sealing is performed in the pressure cell 100 in a direction in which water is allowed to permeate into the pores in the sand into which carbon dioxide has permeated, the effect of reducing the differential pressure acting on the carbon dioxide shielding layer C was observed, and when the pressure rise due to the artificial water sealing becomes excessive, the effect of increasing the strength of the carbon dioxide shielding layer C in a short period of time (1 to 2 hours in this experiment) was observed.

[0131] <Storage of carbon dioxide at depths below the seafloor where the density of carbon dioxide and pore water are equal> Figure 16 shows the stable region of CO2 hydrate in terms of temperature and pressure. CO2 >Seawater density ρ sw A method was proposed to store carbon dioxide in a concave topography on the seabed by utilizing the temperature and pressure environment where the temperature and pressure become equal to or greater than 1000 K. However, at the time of filing this application, this was not feasible due to the international law-based "Act on the Prevention of Marine Pollution, etc. and Maritime Disasters" (abbreviated as the Coastal Defense Act; Act No. 136 of 1970). On the other hand, the "Ministry Ordinance on Permission for Subsea Disposal of Specified Carbon Dioxide Gas" (Ministry of the Environment Ordinance No. 23 of 2007) specifies the application for permission based on Article 18-8, paragraph 2 of the Coastal Defense Act. Therefore, a study on storing carbon dioxide in a subseabed strata at a depth of 3,000 m or more has been reported (Reference 2), but the geothermal gradient was not taken into consideration. Also, a calculation that took into account the geothermal gradient of the subseabed strata in the temperature-pressure relationship of Figure 14 showed that the densities matched (ρ CO2 =ρ sw) density-matched depth appears.

[0132] FIG. 17 is a graph showing the temperature-pressure relationship region in which CO2 hydrate exists stably.

[0133] Figure 17 (1) shows the boundary of the region where CO2 hydrate stably exists in terms of temperature-pressure relationship with a thick solid line and a dotted line extending from about 100m depth to the right. The solid line portion up to a depth of 2000m is the range where there is laboratory experimental data, and since there is no data beyond 2000m, it is shown as a dotted line because it has been extrapolated. The thick solid line extending downward from about 1000m depth to 3000m and the thick solid line extending downward to the right from a depth of 3000m show the layer temperature when the pressure (≒ depth) is 3000m at the seafloor surface, when the area shallower than 3000m is the Pacific ocean water temperature (Reference 4), and when the geothermal gradient is 30℃ / km below 3000m.

[0134] In addition, in Figure 17(1), the thin solid and dashed lines indicate the CO2 density ρ CO2 Density ρ of seawater with a salinity of 35‰ 35sw The contour line of the density difference Δρ minus Δρ is shown. For example, the density difference value Δρ = 20 kg / m 3 The higher the pressure, the lower the temperature, and the larger Δρ becomes. The dark grey area indicates that Δρ is 0 kg / m 3 That's it, ρ CO2 ρ 35sw The region of temperature and pressure conditions where the temperature is greater is shown.

[0135] In addition, the contour line of Δρ in Figure 17(1) is a maximum of 200 kg / m 3 The white area where no contour lines are plotted is Δρ>200kg / m 3 The thick line parallel to the x-axis at a depth of 400 to 500 m indicates the temperature and pressure at the gas-liquid boundary of CO2. Below this boundary, CO2 is liquid, and above it, it is gas.

[0136] Therefore, the thick solid line extending downward from a depth of 1000 m in Figure 17(1) represents Δρ = 0 kg / m 3Liquid CO2 sinks in seawater under temperature and pressure conditions below the solid line shown in Fig. 1. Conversely, the thick solid line extending downward from a depth of 1000 m corresponds to Δρ = 0 kg / m 3 Liquid CO2 rises in seawater under temperature and pressure conditions in the region above the solid line indicating Δρ=0kg / m 3 At the intersection with the solid line showing CO2 =ρsw and density are equal.

[0137] The thick solid line in Figure 17(2) represents the CO2 density ρ CO2 and pressure. The thick black dashed line indicates the density of seawater ρ 35sw and pressure.

[0138] The thick black solid line in Figure 17(3) represents the density difference Δρ=ρ CO2 -ρ 35sw and pressure.

[0139] CO2 density ρ CO2 is calculated using the Span & Wegner equation of state (SWEOS) (Reference 8), which is used to calculate the values ​​shown in the NIST tables. Also, the seawater density ρ sw is calculated using the seawater equation of state TEOS-10 (Reference 9).

[0140] The densities shown in (2) and (3) of Figure 17 are the results for the temperature-pressure relationship shown by the black line extending downward from a depth of 1000 m in (1) of Figure 17. Therefore, the calculation results will change if the water depth or geothermal gradient at the seafloor surface changes.

[0141] Next, the diamond markers at depths of 3100 m shown in Figure 17 (1), (2), and (3) indicate the ρ CO2 =ρ 35sw The diamond marker at a depth of 2750 m indicates the lower limit of the temperature and pressure (NBZ: Negative Buoyancy Zone) where the density coincides with the temperature and pressure of the seawater. CO2 =ρ 35swThe square marker at a depth of about 3,450 m indicates the lowest temperature and pressure at which CO2 hydrate can stably exist in the subseafloor strata (HFZ: Hydrate Forming Zone). The circle marker at a depth of 3,000 m indicates the temperature and pressure at the seafloor surface, and the star at a depth of about 3,500 m indicates the temperature and pressure at the point where liquid CO2 is injected.

[0142] Figures 18 and 19 show the same content as Figure 17, but Figure 18 shows the case where the geothermal gradient GG is 60°C / km, and Figure 19 shows the case where the geothermal gradient GG is 100°C / km. Focusing on the NBZ and HFZ in Figures 17, 18, and 19, it can be seen that the larger the geothermal gradient GG, the closer it is to the seafloor and the shallower it becomes.

[0143] Previously, expectations have been expressed regarding shielding through the generation of CO2 hydrate, but no specific quantitative values ​​for shielding performance have been presented. Based on the results of laboratory experiments, this invention newly considers the threshold pressure (≒0.02MPa) at which liquid CO2 is blocked by a CO2 hydrate film formed in sand pores when the pressure difference is ≒0.02MPa. CO2 is injected into the subseafloor strata using this threshold pressure as the control standard. In addition, the blocking of CO2 movement due to some external force is considered by the threshold pressure. ρ CO2 =ρ sw In addition to the mechanism of CO2 geological storage (shielding) based on this, a double shielding mechanism is used to provide shielding through threshold pressure using a CO2 hydrate membrane in the HFZ, enabling safer CO2 geological storage. In addition, in unconsolidated deposits such as alternating layers of sand and mud, where there is no geological shielding structure such as a cap rock, ρ CO2 =ρ sw Furthermore, in islands, seamounts, and ocean plateaus made of volcanic rock, the air passages for volcanic gases from the erupted lava remain because they were rapidly cooled by seawater, and these cavities are filled with ρ CO2 =ρ swIt is possible to store CO2 underground using this method. Volcanic gas is a mixture of water vapor, CO2, hydrogen sulfide, carbon monoxide, etc., and volcanic rocks such as those on Mount Fuji have continuous cavities (pores) like lava paths (vents) and minute continuous pores (airtight for volcanic gas). Note that the air passages for volcanic gas are larger than the pores in sand, so a threshold pressure of ≒ 0.02 MPa using a CO2 hydrate membrane cannot be expected; it varies depending on the diameter of the air passage, but is, for example, a few percent of 0.02 MPa.

[0144] Conventionally, the relationship between density and water depth has been explained as follows, using the diagram (Fig. 2) in Reference 10 shown in Figure 20. "Assuming that the ocean floor depth is 3,500 m and the geothermal gradient is 0.03°C / m (30°C / km), the injected CO2 (liquid) will reach a temperature of (ρ CO2 =ρ sw ) ρ CO2 =ρ sw Above a depth of ρ, liquid CO2 becomes denser than the surrounding pore water. CO2 =ρ sw The range up to the sedimentation depth of the reservoir is called the Negative Buoyancy Zone (NBZ). The calculation at a depth of 3,500m in Figure 20 uses the CO2 density and seawater density calculated based on the table published by NIST. On the other hand, this high-precision calculation was performed only using Figure 20, while Figure 23, which will be described later, uses a constant seawater density for the calculation. Also, in Figure 31, the threshold value for the storage potential sea area is determined only from Figure 20.

[0145] In the present invention, a new program is used to create Figure 21(2) and plot it on the same axis scale as Figure 20. Furthermore, Figure 22 shows the overlap of Figure 20 and Figure 21(2). The CO2 density and seawater density in Figure 20 and Figure 21(2) are almost the same.

[0146] Figure 23 shows the figure (Fig. 1) from Reference 10 rotated so that the y-axis is pressure (depth). Figure 23 shows the difference between CO2 density and seawater density (CO2 density - seawater density) as isolines in the temperature-pressure relationship, and isoline values ​​of 0 or greater indicate temperatures and pressures where the CO2 density is greater than the seawater density. For example, contour lines represent the elevation / undulations of a topographical map in the longitude-latitude relationship, but the isolines shown above show lines connecting the same values ​​of density difference in the temperature-pressure relationship. Figure 23 is the same as the contour line of Δρ shown in Figure 17 (1). On the other hand, Figure 23 uses the CO2 density shown in the table of NIST. Also, the density of seawater is constant (1,027 kg / m 3 The calculation is performed assuming normal temperature and normal pressure. The difference between this table format and the equation of state is that, for example, when values ​​of 1000 decibar and 1500 decibar are given in the table format, the density at 1300 decibar is calculated by interpolation (linear), whereas the equation of state is formulated and verified based on values ​​actually measured in experiments, etc., so a value close to the actual measured value is calculated. In addition, in numerical calculations that require high speed, the table format interpolation calculation is generally used.

[0147] Figure 24 is a diagram in which the contour lines of Δρ in Figure 23 and Figure 17(1) are superimposed with the same scale of the xy axis. Note that the aspect ratio of Figure 23 has been changed to match the scale. In Figure 23, the density of seawater is kept constant (1,027 kg / m 3 The Δρ contour lines in Figure 17(1) are calculated based on temperature and pressure (normal temperature and normal pressure), which is thought to be the cause of the difference. On the other hand, for example, in Figure 20, seawater density is calculated based on temperature and pressure. Therefore, for Δρ in Figure 17(1), the CO2 density is calculated using the Span & Wagner equation of state (Reference 8) and the seawater density is calculated using the equation of state (TEOS-10) (Reference 9). For this reason, the Δρ contour lines in Figure 17(1) are more accurate.

[0148] Reference 10 shows an example where the ocean floor is 3,500 m deep (Figure 20), but does not include the temperature of the ocean floor. Also, the geothermal gradient is 0.3°C / m = 30°C / km. The seawater temperature-pressure relationship (formula) shown based on the results of measuring seawater temperature at approximately 300,000 points in the sea areas around Japan (Reference 6: Yojiro Ikegawa, Formulation of seawater temperature in the deep seas around Japan that govern CO2 hydrate storage, Journal of the Japan Society of Civil Engineers, Proceedings of the Japan Society of Civil Engineers G (Environment), Vol. 78, No. 1, pp. 30-41, January 14, 2022) is used. It is considered that the temperature of the seabed surface and the temperature of the seawater in contact with it are the same, and the seabed surface temperature is calculated using the formula (Reference 6). The ocean temperature-pressure relationship in the Pacific Ocean is shown in Figure 18 by a thick solid curve extending downward from a depth of about 1000m, and the ocean temperature-pressure relationship in the Sea of ​​Japan is shown by a thick solid curve extending downward from a depth of about 500m. Note that since no data could be found for the Pacific Ocean deeper than 4000m, the ocean temperature at depths of 4000m is assumed to be the same as the ocean temperature at depths of 4000m and is shown by a dotted line. Furthermore, since no data could be found for the Sea of ​​Japan deeper than 2000m, it is similarly shown by a dotted line. In Figure 16, the sea water temperature in the Pacific Ocean is about 2°C lower than that in the Sea of ​​Japan, and varies with depth.

[0149] In reference 10, the geothermal gradient is considered to be constant at 30°C / km. Considering that the geothermal gradient varies from point to point, the program allows for arbitrary ranges, for example 10 to 300°C / km. Figure 25 shows the geothermal gradient in the ocean around Japan (only the exclusive economic zone (EEZ) is shown).

[0150] Reference 10 does not provide a source for the stability region of CO2 hydrate, so the data and equations used are unclear. Here, we used the formula (Reference 6) for the boundary (three-phase line) of the region where CO2 hydrate exists stably, shown by the thick solid line and dashed line sloping downward to the right from about 100 m depth, for the temperature-pressure relationship in Figure 16. This formula is used when data is used for depths of 2,000 m or less. The black diamond data labeled Ikegawa2022w in the legend of Figure 16 is the result of new data obtained from indoor experiments at depths of 2,000 to 5,000 m using freshwater. Previously, data was only available at depths of 2,000 m or less. The thick black solid line sloping downward to the right from a depth of about 100 m is a boundary based on the data, while the dashed line indicates extrapolation (linear interpolation). However, as the water depth increases, the measured values ​​deviate from the extrapolation, with a difference of about 1.5°C at a depth of 5,000 m. Additionally, the data indicated by the grey diamonds, labeled Ikegawa2022sw35 in the legend, is data obtained through new indoor experiments at depths of 2,000-5,000m, but in the case of seawater. In the case of seawater, the boundary shifts to the lower temperature side due to freezing point depression. Freezing point depression is the lowering of the freezing point of a solution (seawater) compared to a pure solvent (freshwater), and is proportional to the number of moles of solute in a unit solvent (Reference 11). Furthermore, by using data obtained through new indoor experiments at depths of 2,000-5,000m, the reliability of calculation accuracy is improved, and safety against CO2 leakage is improved.

[0151] Previously, the long-term evolution of injected CO2 was shown as follows (a) to (c) in Figure 26 of Reference 10. (a) On the injection time scale (≈1 year), buoyancy forces drive the upper part of the CO2 plume into the HFZ, where a small amount of hydrate forms. The formed hydrate is expected to impede the upward migration of CO2 (liquid), forcing the CO2 (liquid) to flow laterally. (b) After 102 years, most of the CO2 will have reached the base of the HFZ and the self-forming hydrate cap is expected to expand laterally, storing large volumes of liquid CO2 beneath it. (c) Eventually, the liquid CO2 and CO2 hydrate dissolve, forming dissolved CO2 water (aqueous solution). This dissolved CO2 water permeates the stratum (in the porous matrix) and mixes with a sufficient amount of water, making it no longer buoyant. Once the dissolved CO2 water is no longer buoyant, any further dissolved CO2 water can only move by diffusion. Furthermore, the results of numerical calculations (Fig. 2 in Reference 2) for this conventional prediction (time course of a, b, and c in Figure 25) have been published as shown in Figure 27. These numerical calculations show that for (c) above, the density of the dissolved CO2 becomes greater than the density of the pore water (seawater), resulting in sinking. In this figure 27, the mole fraction of CO2 is X CO2 The change over time is shown in the third row of figures from the top. CO2 dissolved water has a higher density than seawater, so it sinks over the long term.

[0152] Here, two cases are shown: HFZ>NBZ as shown in Figure 28A, and NBZ>HFZ as shown in Figure 28B. Figure 28A shows the conventional case (Reference 2, Reference 10, Reference 12), and Figure 28B shows the new proposal of the present invention.

[0153] In the case of Figure 28A, the HFZ is deeper than the NBZ, and by storing liquid CO2 below the HFZ (first tank), a CO2 hydrate membrane creates a shield at the bottom of the HFZ with a threshold pressure of ≈0.02 MPa. In the unlikely event that the shielding at the bottom of the HFZ is damaged by deformation (shear) of the strata due to an earthquake or other cause, causing liquid CO2 to leak, if the pressure difference is ≒ 0.02 MPa or less, the CO2 hydrate membrane will regenerate and the liquid CO2 will be shielded. On the other hand, if the pressure difference is ≒ 0.02 or more, the regenerating CO2 hydrate membrane will be destroyed, so shielding cannot be expected, and the upward leakage of CO2 will stop in the upper NBZ (second storage tank) (ρ CO2 =ρ sw (shielding by means of a shield). Therefore, in the case of FIG. 28A, the liquid CO2 in the first storage tank has a double shielding function of being shielded by the CO2 hydrate membrane and by NBZ, so there is a high level of safety against leakage. In the case of Figure 28A, the second storage tank can store CO2 underground based on the dual shielding function of the NBZ and the threshold pressure of the CO2 hydrate membrane that is generated to cover the stored liquid CO2. If CO2 is injected into the lower part of the NBZ so that the pressure difference is 0.02 MPa or more (control standard), liquid CO2 can be injected even in the HFZ. By combining the first and second tanks, the storage capacity per unit area can be approximately doubled. In addition, the liquid CO2 stored in the first and second storage tanks dissolves in the pore water of the surrounding formations. This dissolution is not expected to occur by convection of the pore water in the formations, but is based on diffusion through the CO2 hydrate membrane, so the dissolution rate is essentially slow. Although the dissolution rate is very slow, the density of the dissolved CO2 water is greater than that of the pore water, so it sinks. Experimental values ​​(Reference 7) and calculation models (Reference 13; Reference 14) have shown that the density of this dissolved CO2 water is greater than the density of seawater. If a storage site is selected where basic rocks such as basalt, which are rich in alkali metal elements such as Ca and Na, are present in the deep part of the sinking area, the carbonate ions (CO3 2- ) and calcium ions (Ca + ) produces carbonate (CaCO3), and CO2 is fixed as a mineral (solid). Basic rocks are igneous rocks and their metamorphic rocks with a weight content of 45-52% silicon dioxide (SiO2). Ultrabasic rocks have a lower silicon dioxide content than basic rocks, below 45%, such as peridotite. The lack of silicon dioxide is the reason why they are rich in alkali metal elements. Basalt is a volcanic rock derived from the mantle, and the oceanic plates that form at mid-ocean ridges and sink into trenches are made of basalt. The Sea of ​​Japan is thought to have begun to open up due to plate tectonics about 200 million years ago, and is thought to be an environment in which the mantle can easily rise from deep within, like a mid-ocean ridge. Marine geological maps show the presence of basalt several hundred meters below the seafloor. The mineralization process leading to the above carbonate (CaCO3) is shown diagrammatically in the IPCC Special Report in Figure 29. However, Figure 28A shows a case where the water depth is more than 4,000 m, so the latest high-cost oil and natural gas development technology is required.

[0154] In the case of Figure 28B, the NBZ is deeper than the HFZ, which is a new proposal that has not been previously considered, and the target water depth is less than 4,000 m. The lower part of the NBZ in the case of Fig. 28B (ρ CO2 =ρ sw ) liquid CO2 is injected. In this case, since it is not an HFZ, CO2 seeps upwards, and if it seeps horizontally, it will not be blocked by the formation of CO2 hydrate. In addition, CO2 can be safely injected by injecting it so that the pressure rise near the injection well blocks the threshold pressure (≒0.02MPa) based on the CO2 hydrate membrane in the HFZ in Figure 28B (control standard). In this case too, CO2 =ρ sw This provides double protection by providing a threshold pressure for the CO2 hydrate membrane, making underground storage of CO2 safer. In addition, the stored liquid CO2 dissolves in the pore water of the surrounding strata, as shown in Figure 28A, and as it settles, it is eventually fixed as carbonate (mineral) from Ca ions and carbonate ions near basic rocks such as basalt. In the case of Figure 28A, by looking at a cross section of the ocean floor geological map, it may be possible to consider suitable locations where basic rocks such as basalt exist several hundred meters below the seafloor surface. Furthermore, for depths of less than 3,000m, there are existing technologies that can be used at depths targeted for commercial oil and natural gas development, and there are also several vessels available in Japan that can be used, making this highly feasible.

[0155] Previously, potential storage areas in the ocean surrounding the United States were shown as in Figure 30. Areas with depths of 3,000m or more were considered to be areas where CO2 can be stored (potential storage areas).

[0156] In addition, previous proposals for suitable storage sites in the sea areas around Japan (Reference 12) based on Reference 10 have proposed areas with a depth of 3,000 m and within 100 km of land (within the EEZ). Here, the potential storage areas in the waters around Japan are shown in Fig. 31 (only within the EEZ). The Pacific Ocean is plotted using the seawater temperature - pressure relationship in the Pacific Ocean (Reference 6), and the Sea of Japan is plotted considering the seawater temperature - pressure relationship in the Sea of Japan (Reference 6). In addition, the 3,000 m isobath is shown as a dotted line in Fig. 31. The areas shown in gray shading on the Pacific Ocean side of Fig. 31 are the cases where the water depth of the NBZ > the water depth of the HFZ (HFZ - NBZ > 0 in the case of Fig. 28A), and the areas shown in gray shading on the Sea of Japan side are the cases where the water depth of the NBZ < the water depth of the HFZ as shown in Fig. 27 (HFZ - NBZ < 0 in the case of Fig. 28B). In addition, the shading of the gray area on the Sea of Japan side indicates the isopleths (50 m, 100 m, 150 m, 200 m) every 50 m based on the difference value of HFZ - NBZ. The shading of the gray area on the Pacific Ocean side also indicates the isopleths (-50 m, -100 m, -150 m, -200 m, ···) every 50 m based on HFZ - NBZ.

[0157] Fig. 32 is plotted considering the seawater temperature - pressure relationship in the Sea of Japan when the water depth of the seabed is 3,000 m and the geothermal gradient is 30 °C / km. Fig. 17 shows the case considering the seawater temperature - pressure relationship in the Pacific Ocean for the same seabed depth and geothermal gradient. As described above, since the seawater temperature in the Sea of Japan is about 2 °C lower than that in the Pacific Ocean, when comparing Fig. 32 of the Sea of Japan with Fig. 17 of the Pacific Ocean, the NBZ (the water depth where the density is the same (ρ CO2 = ρ sw ) in the formation) and the HFZ are deeper in the Sea of Japan. Due to this difference in seawater temperature, both the NBZ and the HFZ exist at shallower water depths in the Sea of Japan than in the Pacific Ocean. Compared with the 3,000 m isobath (blue dotted line) in Fig. 32, the potential area in the Pacific Ocean almost coincides with the 3,000 m isobath. On the other hand, the potential area in the Sea of Japan extends to shallower waters than 3,000 m. Also, by looking closely at the Pacific Ocean, it can be seen that the potential area also penetrates into the waters shallower than the 3,000 m isobath. This is due to improving the accuracy by obtaining the density based on the equation of state for seawater density and CO2 density. Furthermore, as a result of sorting out the geothermal gradients mentioned above, a geothermal gradient of 30°C / km is the average geothermal gradient on land in Japan, while the average geothermal gradient in the surrounding ocean areas is approximately 100°C / km. Figure 33 shows potential ocean areas when the geothermal gradient is 100°C / km. As the geothermal gradient becomes higher, both the HFZ and NBZ become shallower, and the difference between the HFZ and NBZ becomes smaller.

[0158] Figure 34 shows an example calculation for the Pacific Ocean that takes into account a cross section of the ocean floor topography. The thick solid line extending from a depth of around -7000m indicates the lower limit of the NBZ, and the thin solid line extending from a depth of around -6200m indicates the lower limit of the HFZ, and shows the results of calculations when the geothermal gradient is 30°C / km. If the depth of the ocean floor changes from 2000 to 5700m, the depths of the NBZ and HFZ will also change. Note that the cross section example in Figure 34 takes as an example the cross section of a submarine volcano with undulating topography in order to take into account the effects of changes in water depth, but active volcanoes are not eligible for CO2 geological storage. In addition, because the geothermal gradient varies from point to point, calculations must take into account the geothermal gradient at each point. The actual geothermal gradient may be, for example, 100°C / km, or even higher. When the geothermal gradient is high, the NBZ and HFZ are located at shallower positions closer to the seafloor, as shown in Figures 18 and 19. In the case of a geothermal gradient of 30°C / km in Figure 34, the NBZ and HFZ coincide at a depth of about 4,000 m, the NBZ is in a deeper position at depths below 4,000 m, and the NBZ is in a deeper position at depths below 4,000 m, with the positions reversed. The x-axis in Figure 34 indicates longitude, and 1° ≒ 111 km, so the aspect ratio of the x-axis in Figure 34 is 8 km:222 km ≒ 0.036. Therefore, although the ocean floor topography is depicted as steep in Figure 34, the actual ocean floor slope is gentle.

[0159] Figure 35 shows an example of a calculation for the Sea of ​​Japan taking into account the cross section of the ocean floor topography. When the water depth of the HFZ is greater than that of the NBZ, as shown in Figure 26, the HFZ is deeper than the NBZ. Reference 10 expects liquid CO2 to be stored at the bottom of the HFZ, and if it rises from the bottom of the HFZ, it will be trapped in the NBZ. In this proposal, rather than expecting liquid CO2 to be stored at the bottom of the HFZ, the possibility of leakage is reduced by injecting CO2 with a threshold pressure of -0.02MPa, which was confirmed in laboratory experiments as the control standard value. When the water depth of the NBZ is greater than that of the HFZ, this does not appear because Reference 10 defines storage potential areas as areas deeper than 3,000 m.

[0160] <Method and program for searching for marine areas suitable for underground storage of carbon dioxide> The method of the present invention for searching for an ocean area suitable for underground storage of carbon dioxide is to determine the time-strength relationship of a carbon dioxide shielding layer made of carbon dioxide hydrate under temperature and pressure conditions under which carbon dioxide hydrate stably exists in the seabed substrata, and the carbon dioxide density ρ CO2 is the density of seawater, ρ 35sw We will explore offshore areas suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match those of the abovementioned ocean.

[0161] The above-mentioned method for searching for an ocean area suitable for underground storage of carbon dioxide (hereinafter also referred to as "search method") will be described in more detail. One embodiment of the search method includes (Step 1) to (Step 7) which will be described later.

[0162] (Step 1) Select a point (grid) P in the sea area surrounding Japan.

[0163] (Step 2) If the point P is a point in the Pacific Ocean, the temperature and density at the seabed of the point P (hereinafter also referred to as "point (a)") are calculated using equation (6) in Reference 3. If the point P is a point in the Sea of ​​Japan, equation (7) in Reference 3 is used.

[0164] (Step 3) The temperature and pressure at the HFZ (Hydrate Forming Zone, hereinafter also referred to as "point (b)") of point P are calculated based on the geothermal gradient (input value: GG) and the stable region of CO2 hydrate (Equations (2) and (3) in Reference 3). In addition, the temperature T b and pressure P b Based on this, calculate the CO2 density and seawater density at point (b).

[0165] (Step 4) Calculate the CO2 density and seawater density at point (c) based on the CO2 injection point at point P (hereinafter also referred to as "point (c)") (input value: CO2 injection depth).

[0166] (Step 5) Based on the geothermal gradient (input value: GG) of the NBZ (Negative Buoyancy Zone, hereafter also referred to as point (d)) at point P and the relationship between CO2 density (Non-Patent Document 2) and seawater density in the temperature (≒pressure) relationship (References 4 and 5), calculate the water depth and temperature at which the CO2 density and seawater density match in the subseafloor layer. Furthermore, calculate the CO2 density and seawater density at point (d).

[0167] (Step 6) Calculate the temperature and pressure of the point (hereinafter also referred to as "point (e)") where the CO2 density and seawater density are equal in the seawater of the NBZ at point P. Furthermore, calculate the CO2 density and seawater density at point (e).

[0168] (Step 7) The above steps (Step 1) to (Step 6) are carried out for all points (grids) P selected in the waters around Japan, and a potential map (e.g., Figures 31 and 33) is calculated and plotted based on the temperature, pressure (≒water depth), seawater density, and CO2 density at points (a), (b), and (c) at each point P.

[0169] The program for searching for an ocean area suitable for underground storage of carbon dioxide of the present invention is a program for searching for an ocean area suitable for underground storage of carbon dioxide by inputting into a computer a time-strength relationship of a carbon dioxide shielding layer made of carbon dioxide hydrate under temperature and pressure conditions under which carbon dioxide hydrate exists stably in the subsea layer, and a carbon dioxide density ρ CO2is the density of seawater, ρ 35sw This is a program that searches for ocean areas suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match those of the ocean.

[0170] The above-mentioned program for searching for marine areas suitable for underground storage of carbon dioxide (hereinafter also referred to as the "search program") will be described in more detail. In one embodiment of the search program, the program causes a computer to execute steps 1 to 7 described below. The computer includes at least an input device, a calculation device, a storage device, and a display device.

[0171] (Step 1) A point (grid) P selected from the sea area around Japan is input from an input device and stored in a storage device.

[0172] (Step 2) If the point P is a point in the Pacific Ocean, the temperature and density at the seabed of the point P (hereinafter also referred to as "point (a)") are calculated by a calculation device using equation (6) in Reference 3, and if the point P is a point in the Sea of ​​Japan, equation (7) in Reference 3 is used.

[0173] (Step 3) The temperature and pressure at the HFZ (hereinafter also referred to as "point (b)") of point P are calculated by a calculation device based on the geothermal gradient (input value: GG) and the stable region of CO2 hydrate (Equations (2) and (3) in Reference 3), and the calculation results are stored in a storage device. In addition, the temperature T b and pressure P b Based on this, the CO2 density and seawater density at point (b) are calculated by a computing device, and the calculation results are stored in a storage device.

[0174] (Step 4) Based on the CO2 injection point (hereinafter referred to as "point (c)") at said point P (input value: CO2 injection depth), the CO2 density and seawater density at point (c) are calculated by a calculation device, and the calculation results are stored in a storage device.

[0175] (Step 5) Based on the geothermal gradient (input value: GG) at the NBZ (Negative Buoyancy Zone) of point P (hereinafter also referred to as point (d)), and the relationship between CO2 density (Non-Patent Document 2) and seawater density in the temperature (≒pressure) relationship (References 4 and 5), the water depth and temperature at which the CO2 density and seawater density match in the subsea layer are calculated by a computing device, and the calculation results are stored in a storage device. Furthermore, the CO2 density and seawater density at point (d) are calculated by a computing device, and the calculation results are stored in a storage device.

[0176] (Step 6) The temperature and pressure of the point where the CO2 density and seawater density match in the seawater of the NBZ at point P (hereinafter also referred to as "point (e)") are calculated by the arithmetic device, and the calculation results are stored in the storage device. Furthermore, the CO2 density and seawater density at point (e) are calculated by the arithmetic device, and the calculation results are stored in the storage device.

[0177] (Step 7) The computer executes the above steps (Step 1) to (Step 6) for all input points P, and the calculation device calculates a potential map (e.g., Figures 31 and 33) based on the temperature, pressure (≒water depth), seawater density, and CO2 density at points (a), (b), and (c) at each point P, and plots it on the output device.

[0178] <Action and effect> As described above, according to the method for underground storage of carbon dioxide of this embodiment, by employing the above-mentioned configuration, it is possible to reliably ensure the strength of the carbon dioxide shielding layer C formed by hydrated carbon dioxide, and to reliably seal carbon dioxide in an optimal depth region in the stratum U. This makes it possible to efficiently store a large amount of carbon dioxide.

[0179] <Other forms> The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, an example of storing carbon dioxide in a geological layer under the seabed has been described, but the method for storing carbon dioxide underground according to the present invention is not limited to this, and as described above, can also be applied to storing carbon dioxide in a geological layer on land.

[0180] <References> Reference 1: Takehiko Yanai, Yutaka Abe, Akiko Kaneko, Kenji Yamane. Effect of flow field on CO2 hydrate film thickness, Transactions of the Japan Society of Mechanical Engineers, Series B, Vol. 78, No. 787, 2012. ·Reference 2: Yihua Teng and Dongxiao Zhang. Long-term viability of carbon sequestration in deep-sea sediments, Science Advances, Vol.4, No.7, 04 Jul 2018. ·Reference 3: Span, R. and W. Wagner. A New Equation of State for Carbon Dioxide Covering the Fluid Region from the Triple-Point Temperature to 1100 K at Pressures up to 800 MPa, J. of Physical and Chemical Reference Data 25, (6), pp.1509-1596,1996. ·Reference 4: Millero, FJ, Poisson, A. International one- atmosphere equation of state of seawater, Deep Sea Research Part A. Oceanographic Research Papers, Vol.28, Issue 6, pp.625-629, 1981. ·Reference 5: Millero, FJ, Poisson, A.. Density of seawater and the new International Equation of State of Seawater 1980, IMS Newsletter, Number 30, Wright (Ed), Special Issue 1981-1982, UNESCO, Paris, France, p.3, 1981. Reference 6: Yojiro Ikekawa. Formulation of deep seawater temperature around Japan that governs CO2 hydrate storage, Journal of the Japan Society of Civil Engineers, Proceedings of the Society of Civil Engineers, Vol. 78, No. 1, pp. 30-41, 2022.01.14. Reference 7: Takashi Osumi, Norikazu Nakashiki, Kiminori Shimojima, Kenichi Hirama. Carbon dioxide deep sea disposal technology (part 1) - Presentation of concept and examination of environmental impact assessment method -, Research Report of the Central Research Institute of the Electric Power Industry: U92062, March 1993. ·Reference 8: Tsutomu Omori, EOS-SCx Ver. 0.2w, http: / / hp.vector.co.jp / authors / VA030090 / , accessed March 27, 2023. Reference 9: Thermodynamic Equation Of Seawater - 2010 (TEOS-10), https: / / www.teos-10.org / , accessed March 27, 2023 ·Reference 10: KZ House, DP Schrag, CFHarvey, KSLackner. Permanent carbon dioxide storage in deep-sea sediments, Proceedings of the National Academy of Sciences, Vol.103, No.33, pp.12291-12295, 2006. Reference 11: McGraw-Hill Scientific and Technical Dictionary Editorial Committee, ed. McGraw-Hill Scientific and Technical Dictionary, revised 3rd edition, October 2001. Reference 12: Japan Science and Technology Agency (JST). Proposal for the Low Carbon Society Strategy Center, Storage of Liquid Carbon Dioxide in Deep Sea Sediments - Search for Suitable Sites in Japan, March 2022 Reference 13: Akira Ijiri, Wataru Tanigawa, Fumio Inagaki, Yojiro Ikegawa. Joint research on engineering analysis and evaluation technology of pore water salinity in marine sediments (2019-2020), Final project report, March 2021. ·Reference 14: Nicolas Spycher, Karsten Pruess and Jonathan Ennis-King. CO2-H2O mixtures in the geological sequestration of CO2. I. Assessment and calculation of mutual solubilities from 12 to 100℃ and up to 600 bar, Geochimica et Cosmochimica Acta, Vol. 67, No. 16, pp. 3015-3031, 2003. ·Reference 15: IPCC. IPCC Special Report on Carbon Dioxide Capture and Storage . Prepared by Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp.442, 2005. [Industrial Applicability]

[0181] The method for underground storage of carbon dioxide of the present invention is a method capable of efficiently storing a large amount of carbon dioxide by ensuring the strength of a carbon dioxide shielding layer formed by hydrated carbon dioxide and reliably sealing carbon dioxide in an optimal depth region in a geological layer. Therefore, the present invention is very suitable for applications such as capturing carbon dioxide while generating power using fossil fuels in various plants and storing the carbon dioxide underground to reduce emissions. [Explanation of symbols]

[0182] 1... Carbon dioxide underground storage device (storage device) 2…Injection well 21…Tip 6…Artificial water seal equipment 100…Pressure cell M…Sea surface W…Seawater F…Seafloor surface (seafloor) U...Geological strata S… Carbon dioxide sealing area C… Carbon dioxide shielding layer G…Carbon dioxide reservoir G1: Carbon dioxide liquid layer B…Acoustic base

Claims

1. 1. A method for storing carbon dioxide in a liquefied state in a subseafloor formation of sediments on an acoustic base, comprising the steps of: forming a carbon dioxide reservoir by injecting liquid carbon dioxide below a carbon dioxide seal area consisting of a geological layer that exists from the seabed surface to a predetermined depth and satisfies pressure and temperature conditions capable of generating carbon dioxide hydrate; At least a portion of the carbon dioxide injected into the carbon dioxide reservoir is naturally raised toward the carbon dioxide sealed region by the buoyancy of the carbon dioxide to generate carbon dioxide hydrate, thereby forming a carbon dioxide shielding layer in the carbon dioxide sealed region; The amount of liquid carbon dioxide injected is adjusted based on a threshold pressure of 0.02 MPa due to the carbon dioxide shielding layer made of carbon dioxide hydrate as a control standard; A method for underground storage of carbon dioxide, comprising storing the carbon dioxide in a depth region of the subsea stratum that satisfies pressure and temperature conditions under which a density of the carbon dioxide and a density of pore water are in equilibrium.

2. 2. The method for underground storage of carbon dioxide according to claim 1, further comprising: calculating a depth of the stratum in the seabed substratum that satisfies pressure and temperature conditions under which the density of the carbon dioxide and the density of pore water are in equilibrium; and controlling an injection position of the carbon dioxide into the seabed substratum based on the calculated value.

3. The time-strength relationship of the carbon dioxide shielding layer consisting of carbon dioxide hydrate under the temperature and pressure conditions under which carbon dioxide hydrate can stably exist in the seafloor substrata, and the carbon dioxide density ρ in the seafloor substrata ＣＯ２ is the density of seawater ρ ３５ｓｗ A method for exploring marine areas suitable for underground storage of carbon dioxide based on matching temperature and pressure conditions.

4. The time-strength relationship of the carbon dioxide shielding layer made of carbon dioxide hydrate under the temperature and pressure conditions under which carbon dioxide hydrate is stable in the seafloor substrata and the carbon dioxide density ρ in the seafloor substrata are calculated by the computer. ＣＯ２ is the density of seawater ρ ３５ｓｗ A program to search for ocean areas suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above.

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