Underground storage method for carbon dioxide

Abstract

The present invention provides an underground storage method for carbon dioxide that ensures the strength of a carbon dioxide–blocking layer and makes it possible to reliably seal carbon dioxide at an appropriate depth range in the layers of the ground and efficiently store a large amount of carbon dioxide. The present invention provides an underground storage method for carbon dioxide that involves forming a carbon dioxide storage layer (G) by pressing carbon dioxide below a carbon dioxide–sealing region S that is a layer of ground that exists to a prescribed depth from the surface F of the seabed and satisfies pressure and temperature conditions that make it possible to generate carbon dioxide hydrate, forming a carbon dioxide–blocking layer (C) in the carbon dioxide–sealing region (S) by allowing at least a portion of the carbon dioxide pressed into the carbon dioxide storage layer (G) to naturally rise to the carbon dioxide–sealing region (S) side by the buoyancy of the carbon dioxide and thereby generating carbon dioxide hydrate, and storing the carbon dioxide at a depth range in the layers of ground below the surface (F) of the seabed that satisfies pressure and temperature conditions that balance the density of the carbon dioxide and the density of pore water.

Description

Methods for underground storage of carbon dioxide

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

[0002] While generating electricity using fossil fuels, carbon dioxide (CO ２ Carbon dioxide capture and storage (hereinafter simply referred to as "CCS") has been attracting attention as an innovative technology that can curb carbon dioxide emissions. Furthermore, 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, namely, gas, liquid, solid, and supercritical, depending on the pressure and temperature conditions. Carbon dioxide becomes solid dry ice at atmospheric pressure and at a temperature of 194 K (-79.15°C) or less, but when mixed with water, it is known to become hydrate (solid) under different temperature and pressure conditions. Carbon dioxide hydrate (CO ２ Carbon dioxide hydrate is produced when liquid carbon dioxide mixes with water at a temperature of 10°C or less and a pressure of 4.5 MPa or more, and overlaps part of the liquid carbon dioxide region (liquid phase region).

[0004] Carbon dioxide hydrate as described above is a solid with a crystalline structure similar to that of methane hydrate, and has a structure in which gas molecules are trapped within a three-dimensional lattice formed by water molecules. Such carbon dioxide hydrate is also commonly called gas hydrate, and Non-Patent Document 1 is the first research report to confirm that carbon dioxide hydrate causes clogging in natural gas transfer lines. Carbon dioxide hydrate has also been confirmed as a blocking substance in plant piping for oil and gas treatment, and research is also progressing in the field of Earth and planetary science.

[0005] Furthermore, 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 as the pressure increases, but because it enters a supercritical state at temperatures above 31°C and pressures above 7.4 MPa, density changes due to further pressure increases are relatively slow. Since the density of carbon dioxide in a liquid or supercritical state is lower than that of seawater, for example, when attempting to store carbon dioxide in a geological layer below 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 conducting research into suitable locations for Deep Saline Aquifer Storage (DSA) as a method of underground storage of carbon dioxide since the 1990s, but commercial storage locations and storage capacities for this method have not yet been determined. This is thought to be due to the fact that Japan's geological strata contain little natural cap rock (a shielding layer such as mudstone) that is commonly found in the Middle East, Europe, the United States, and Asia, as suggested by Japan's limited production of oil and natural gas.

[0008] For this reason, as another 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 to hydrate and become a solid, and by using the hydrated carbon dioxide as a shielding layer, it has the sealing function to contain any carbon dioxide that rises to the surface, and active research is being conducted into this.

[0009] The hydrated carbon dioxide barrier layer described above has the advantage of not requiring a cap rock such as mudstone. Furthermore, the underground storage layer targeted for carbon dioxide hydrate storage is, for example, a Quaternary sedimentary layer (geologically dating from the present to approximately 2.58 million years ago) in which sediment transported from rivers on land has accumulated on an acoustic bedrock to a thickness of approximately 400 m, and this layer is widely distributed in the seas surrounding Japan.

[0010] Furthermore, the presence of methane hydrate in the seas surrounding Japan has revealed the existence of methane gas deep underground. However, this methane gas is not trapped in the cap rock 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 in, for example, 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 laboratory experiments have been reported as part of a nationally led project.

[0012] Here, Patent Document 1 proposes a method for achieving both carbon dioxide immobilization and methane gas production without relying on methane hydrate layers. Patent Document 1 describes a method including the steps of: forming a methane gas production layer 4 by adding a microbial community including at least methanogenic bacteria to gaps in a geological formation under temperature and pressure conditions at which the methanogenic bacteria produce methane gas; forming a carbon dioxide hydrate seal layer by injecting an emulsion of liquid carbon dioxide particles dispersed in water into gaps in the geological formation shallower than the methane gas production layer and under temperature and pressure conditions at which the carbon dioxide becomes hydrate; injecting liquid carbon dioxide as a methane gas production feedstock into gaps in the geological formation between the methane gas production layer and the seal layer; and recovering methane gas 5 produced in the methane gas production layer to the surface.

[0013] Furthermore, one concern in promoting carbon dioxide hydrate storage is the possibility that the hydration of carbon dioxide as shown in Non-Patent Document 1 may block and clog the pores of geological formations, making it difficult to inject carbon dioxide into the underlying geological formations. To solve this problem, Patent Document 2 proposes a method and 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 predetermined depth and satisfies the pressure and temperature conditions that allow carbon dioxide hydrate to be produced. The technology described in Patent Document 2 prevents clogging when carbon dioxide is stored underground, making it possible to efficiently store large amounts of carbon dioxide.

[0014] JP 2010-239962 A JP 2019-126787 A

[0015] Hammerschmidt, EG, “Formation of Gas Hydrates in Natural Gas Transmission Lines”, Industrial Engineering Chemistry, 26(8), (US), 1934, p851-855Sakai 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, p1093-1096Hitoshi 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, pp. 505-508, 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 Earth's Crust by Utilizing Deep Underground and Seabed Environments) Results Report," March 2004

[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 further 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 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 the geological layer, and can efficiently store large amounts of carbon dioxide.

[0018] In order to solve the above problems, the present invention employs the following configuration: [1] A method for storing liquefied carbon dioxide in a subseafloor stratum consisting of sediments on an acoustic platform, comprising: forming a carbon dioxide reservoir by injecting liquid carbon dioxide below a carbon dioxide sealed zone consisting of a geological layer that exists from the seabed surface to a predetermined depth and that satisfies pressure and temperature conditions that allow carbon dioxide hydrate to be produced; forming a carbon dioxide shielding layer in the carbon dioxide sealed zone by causing at least a portion of the carbon dioxide injected into the carbon dioxide reservoir to rise naturally toward the carbon dioxide sealed zone due to the buoyancy of the carbon dioxide to produce carbon dioxide hydrate; adjusting the injection amount of the liquid carbon dioxide using a threshold pressure of 0.02 MPa set 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 subseafloor stratum that satisfies pressure and temperature conditions that allow the density of the carbon dioxide and the density of pore water to equilibrate. [2] The method for underground storage of carbon dioxide according to the above [1], 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 method for underground storage of carbon dioxide according to the above [1], comprising calculating 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 seabed substratum, and controlling the carbon dioxide density ρ ＣＯ２ is the density of seawater ρ ３５ｓｗ[4] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above. [5] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above. [6] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above. [7] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above. [8] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above. [9] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above.

[10] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above.

[11] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above.

[12] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above.

[13] A method for searching for an ocean area suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match the above. ＣＯ２ is the density of seawater ρ ３５ｓｗ This program searches for suitable ocean areas for underground storage of carbon dioxide based on temperature and pressure conditions that match the above.

[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 formation, thereby making it possible to efficiently store large amounts of carbon dioxide.

[0020] FIG. 1A is a diagram illustrating a method for underground storage of carbon dioxide according to an embodiment of the present invention, and is a schematic cross-sectional view showing a general structure in which carbon dioxide is stored in a geological layer under the seabed, with an image illustrating each layer over a wide area. FIG. 1B is a diagram illustrating a method for underground storage of carbon dioxide according to an embodiment of the present invention, and is a schematic cross-sectional view showing a general structure in which carbon dioxide is stored in a geological layer under the seabed, with an enlarged view of a key portion of FIG. 1A. FIG. 2 is a diagram illustrating a method for underground storage of carbon dioxide according to an embodiment of the present invention, and is a graph showing isopleths of carbon dioxide density in the temperature-pressure relationship and the stability region of carbon dioxide hydrate. FIG. 3 is a diagram illustrating a method for underground storage of carbon dioxide according to an embodiment of the present invention, and is a graph showing a temperature-pressure phase diagram of carbon dioxide hydrate. FIG. 4 is a diagram illustrating a method for underground storage of carbon dioxide according to an 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 geological layer under the seabed. FIG. 5 is a diagram illustrating a method for underground storage of carbon dioxide according to one embodiment of the present invention, and is a schematic diagram showing the general structure of a pressure cell used in a laboratory experiment to demonstrate the present invention. FIGS. 6( a) to 6(c) are diagrams illustrating a method for underground storage of carbon dioxide according to 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. 5. FIG. 7 is a diagram illustrating a method for underground storage of carbon dioxide according to one embodiment of the present invention, and is a graph showing the temperature increase due to the heat of formation of carbon dioxide hydrate when carbon dioxide permeates into a geological layer, obtained in an experiment using the pressure cell shown in FIG. 5. FIG. 8 is a diagram illustrating a method for underground storage of carbon dioxide according to one embodiment of the present invention, and is a graph showing the relationship between retention time and differential pressure when the sample is held for 17 days under conditions where the differential pressure is approximately 2 m or less in terms of head, obtained in an experiment using the pressure cell shown in FIG.FIG. 9 is a diagram illustrating a method for underground storage of carbon dioxide according to 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 differential pressure for 17 days under conditions where the pressure head was approximately 2 m or less, as obtained in an experiment using the pressure cell shown in FIG. 5 . FIG. 10 is a diagram illustrating a method for underground storage of carbon dioxide according to 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 was increased stepwise, as obtained in an experiment using the pressure cell shown in FIG. 5 . FIG. 11 is a diagram illustrating a method for underground storage of carbon dioxide according to 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 in the reverse direction is applied, as obtained in an experiment using the pressure cell shown in FIG. 5 . FIG. 12 is a diagram illustrating a method for underground storage of carbon dioxide according to one embodiment of the present invention, and is a graph showing contour lines of the optimal volume ratio of {carbon dioxide / water} over the temperature-pressure relationship. FIG. 13 is a diagram illustrating a method for underground storage of carbon dioxide according to an embodiment of the present invention, and is a diagram that considers the shielding mechanism by a carbon dioxide hydrate membrane based on the results of an experiment using the pressure cell shown in FIG. 5. FIG. 14 is a diagram illustrating a method for underground storage of carbon dioxide according to an embodiment of the present invention, and is a graph showing the relationship between temperature and pressure conditions at a depth in a geological layer below the seabed where the density of carbon dioxide and the density of seawater are in equilibrium. FIG. 15 is a diagram illustrating a method for underground storage of carbon dioxide according to an embodiment of the present invention, and is a diagram that schematically illustrates a storage method assuming islands, seamounts, and ocean plateaus, and is a graph showing the relationship between horizontal distance and water depth. FIG. 16 shows CO in the temperature-pressure relationship. ２ 17 is a graph showing the stable region of hydrate. ２ 18 is a graph showing the region where hydrates exist stably. ２19 is a graph showing the region where hydrate exists stably. ２ This is a graph showing the region where hydrates exist stably. Figure 20 is Fig. 2 from Reference 10 (House et al., 2006). Figure 21 is a graph showing the density-pressure relationship (2) with the same x- and y-axis scales as in Figure 20. Figure 22 is a graph showing the superposition of Figure 20 and Figure 21 (2). Figure 23 is a graph showing the CO hydrate in the temperature-pressure relationship. ２ Figure 24 shows the contours of seawater density based on TEOS-10 overlaid on Figure 23. Figure 25 shows the geothermal gradient in the sea area around Japan. Figure 26 shows the difference between injected CO ２ Figure 28A shows the long-term evolution of CO2 in the case of HFZ > NBZ. This is Fig. 4 from Reference 10 (House et al., 2006). Figure 27 shows the time evolution of physical properties in relation to the radial distance (m) from the injection point and the depth (mbsf) from the seafloor. This is Fig. 2 from Reference 2 (Teng and Zhang, 2018). Figure 28B shows the time evolution of CO2 in the case of HFZ > NBZ. ２ Storage model (ρ ＣＯ２ = ρ ｓｗ ) is a diagram showing the CO ２ Storage model (ρ ＣＯ２ = ρ ｓｗ ) is a diagram showing the CO ２ This is a graph showing the time course of geological storage. This is a graph from p. 208 of Reference 15 (IPCC, 2005). Figure 30 is a diagram showing the water depth and potential sea area in the sea area around the U.S.A. This is Fig. 5 from Reference 10 (House, 2006). Figure 31 is a diagram showing the potential sea area for storage in the sea area around Japan. Figure 32 shows the temperature-pressure relationship for CO2 storage when the seabed is 3,000 m deep and the geothermal gradient is 30°C / km. ２ 33 is a graph showing the area where hydrates are stable. ＣＯ２ = ρｓｗ ), and GG = 100°C / km. Figure 34 is a diagram showing an example of calculation for the Pacific Ocean taking into account a cross section of the ocean floor topography. Figure 35 is a diagram showing an example of calculation for the Sea of ​​Japan taking into account a cross section of the ocean floor topography.

[0021] A method for underground storage of carbon dioxide, which is one embodiment of the present invention, will be described in detail below with reference to the drawings. Note that the drawings used in the following description may show characteristic parts enlarged for convenience in order to make the characteristics easier to understand, and the dimensional proportions of each component may not necessarily be the same as those 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 views 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 an enlarged diagram showing a key part of Figure 1A.

[0023] The storage method of this embodiment is to store carbon dioxide (CO ２ ) alone or in a liquefied state as a mixed gas containing carbon dioxide as a main component, is stored in a stratum U below the seabed surface F consisting of sediments on an acoustic bedrock 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 exists to a predetermined depth from the seabed (seabed surface) F or the ground in a land area (not shown) and that satisfies pressure and temperature conditions that allow carbon dioxide hydrate to be produced. In addition, in the storage method of this embodiment, at least a portion of the carbon dioxide injected into the carbon dioxide reservoir G naturally rises toward the carbon dioxide sealed area S due to the buoyancy of the carbon dioxide, thereby producing carbon dioxide hydrate, and a carbon dioxide shielding layer C is formed 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 F or the stratum below the ground on land, but in this embodiment, an example will be described in which carbon dioxide is stored in the stratum U below the seabed F. On the other hand, in this embodiment, it is also possible to use a stratum below the ground on land as the storage location if predetermined pressure and temperature conditions are met.

[0025] In addition, the carbon dioxide described in this embodiment is pure carbon dioxide (CO ２ ) alone, but also includes a liquefied state of a mixed gas containing carbon dioxide as the main component, as described above. That is, the carbon dioxide to be stored in this embodiment may contain water by using existing countermeasure technology in combination. Furthermore, the carbon dioxide to be stored in this embodiment may contain, in addition to carbon dioxide itself, other components other than carbon dioxide, such as carbon monoxide (CO), hydrogen (H ２ ), methane (CH ４ ), water (H ２ O), hydrogen sulfide (H ２ S) and the like.

[0026] <Geological storage device used for underground storage of carbon dioxide> First, a description will be given of 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. Fig. 1A shows an underground carbon dioxide storage device (hereinafter sometimes simply referred to as "storage device") 1 of this embodiment (see also the enlarged view of the main part of Fig. 1B). The storage device 1 shown in the figure injects and stores carbon dioxide alone or a liquefied mixed gas containing carbon dioxide as a main component into a geological stratum U below the seabed 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, which exists from the seabed surface F to a predetermined depth and is made of a geological layer that satisfies pressure and temperature conditions capable of producing carbon dioxide hydrate, and extends to a carbon dioxide reservoir G below the carbon dioxide sealed area S; a carbon dioxide supply source that is loaded on a ship or the like (not shown) on the sea surface 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 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 the pumping facility installed on the ship to the injection well 2, which is provided with its origin at the seabed surface F. Furthermore, the storage device 1 of the example shown in FIGS. 1A and 1B is provided with an artificial water seal facility 6 that increases 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 geological layer U below the seabed surface F.

[0027] In the schematic cross-sectional view of Figure 1A, the pressure in a geological layer U below the seabed surface F located below seawater W typically varies depending on the depth from the sea surface M to the seabed surface F and on any depth from the seabed surface F to the geological layer U. The storage device 1 in 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 seabed surface F to a predetermined depth and that satisfies the pressure and temperature conditions that allow carbon dioxide hydrate to be produced.

[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 include devices that capture carbon dioxide derived from coal, oil, etc. emitted from power generation facilities using fossil fuels in various facilities such as offshore plants that extract oil and gas, and devices that temporarily receive and store carbon dioxide captured by these devices from ships or pipelines that transport them.

[0029] Furthermore, in the storage device 1, the carbon dioxide supply source (not shown) is preferably a supply source exposed above the sea level M, although this depends on the characteristics of the device, from the viewpoints of the supplyability of materials such as fuel and carbon dioxide, ease of maintenance, device life, etc. As such, an example of a configuration in which the carbon dioxide supply source is exposed above the sea level M is to construct an offshore platform similar to those installed to extract resources such as oil and gas from the seabed, and install the carbon dioxide supply source on top of 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 directly fixing a structure assembled from high-strength steel or the like to the seabed F. An example of a floating offshore platform is one consisting of a semi-submersible vessel or the like.

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

[0033] As the pressure-transfer equipment, pumps for pressure-transferring liquids that have been conventionally used in this field can be used without any restrictions. Furthermore, as the supply line (not shown) that connects the pressure-transfer equipment to 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. Note that the pressure-transfer equipment is not limited to those installed on offshore platforms such as ships, as described above, and it is also possible to install pressure-transfer 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, which 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 is configured to inject carbon dioxide into the carbon dioxide reservoir G by controlling the density (ρ ＣＯ２ ) and pore water density (ρ ｓｗ The carbon dioxide is injected and stored in a deep region in the stratum U that satisfies the pressure and temperature conditions for equilibrium between the carbon dioxide and the stratum U. The injection well 2 is a carbon dioxide injection / injection hole that is formed, for example, by a borehole excavated by boring or the like.

[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, the storage device 1 in the illustrated example is provided with an artificial water seal facility 6 for increasing the pore pressure in the carbon dioxide seal area S, and in the illustrated example, the artificial water seal facility 6 is arranged so that a portion of it is exposed on the upper surface of the carbon dioxide seal area S. Furthermore, as shown in the enlarged view of FIG. 1B , the artificial water seal facility 6 is composed of multiple pipes arranged in an annular shape so as to surround the periphery of the injection well 2. Furthermore, in the illustrated example, the artificial water seal facility 6 is composed of multiple pipes whose cross section has a stepped shape extending 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 equipment 6 injects at least one of seawater and water as pore water into the carbon dioxide sealing area S. This increases the pore pressure in the carbon dioxide sealing area S, thereby improving the strength of the entire carbon dioxide sealing 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 also 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 produced.

[0039] <Method 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] [Depth Region in the 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 for storing carbon dioxide (CO ２) alone or in a liquefied state as 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 exists to a predetermined depth from the seabed (seabed surface) F or the ground in a land area (not shown) and that satisfies pressure and temperature conditions that allow carbon dioxide hydrate to be produced. Also, at least a portion of the carbon dioxide injected into the carbon dioxide reservoir G is allowed to rise naturally toward the carbon dioxide sealed area S due to the buoyancy of the carbon dioxide, thereby producing carbon dioxide hydrate, and a carbon dioxide shielding layer C is formed in the carbon dioxide sealed area S. In the storage method of this embodiment, the density (ρ ＣＯ２ ) and pore water density (ρ ｓｗ The method involves storing carbon dioxide in a deep region of the geological formation that satisfies the pressure and temperature conditions for equilibrium between the water and the atmosphere.

[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 basement B using the storage device 1 shown in Figures 1A and 1B.

[0042] That is, in this embodiment, by conducting experiments and field surveys in advance, for example, the density of carbon dioxide (ρ ＣＯ２ ) and pore water density (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 ｓｗ It 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 seabed surface F or into the stratum on land based on the calculated value.

[0044] Considering the geothermal gradient in the 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 ) matches (ρ ＣＯ２ = ρ ｓｗ In other words, it is thought that carbon dioxide can be stored by utilizing the fact that carbon dioxide injected as a liquid in the geological formations at these depths remains without sinking or rising.

[0045] The graph in FIG. 14 shows the density of carbon dioxide (ρ ＣＯ２ ) and the density of seawater (ρ ｓｗ 14 shows the results of calculations of the relationship between temperature and pressure conditions at a depth where equilibrium is achieved between the two. The calculations for the results shown in the graph in Figure 14 were performed under the conditions of a seawater salinity of 35‰ psu and a geothermal gradient of 30°C / km, and the seawater temperature was calculated using an example of Argo data for 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 ) and the following equation {ρ ＣＯ２ <ρ ｓｗ}, and the temperature range deeper than this position is the range where carbon dioxide liquefies. Also, (2) in the graph of FIG. 14 is the temperature range where the density of carbon dioxide (ρ ＣＯ２ ) and the density of seawater (ρ ｓｗ ) and the following equation {ρ ＣＯ２ = ρ ｓｗ}, and the density of carbon dioxide (ρ ＣＯ２ ) and the density of seawater (ρ ｓｗ ) is the water depth / area that satisfies the pressure and temperature conditions for equilibrium. ＣＯ２ ) and the density of seawater (ρ ｓｗ ) and the following equation {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 As a result of calculating the water depth that satisfies the relationship expressed by {, the water depth = 3964.645 m, the water depth at seabed surface F = 3600.000 m, and the depth from seabed surface F = 364.645 mbsf were calculated.

[0048] The conditions for applying 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 ) and the following equation {ρ ＣＯ２ >ρ ｓｗ} and 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 on the base points of the X and Y axes represent the density of carbon dioxide (ρ ＣＯ２ ) is the density of seawater (ρ ｓｗ ) is larger than the temperature and pressure conditions. ＣＯ２ >ρ ｓｗ A method of storing carbon dioxide in a concave topography on the seafloor by utilizing an environment where the relationship expressed as {\displaystyle \mathbb {\text ...

[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 the stratum U below the seafloor surface F, and the density of carbon dioxide (ρ ＣＯ２ ) and the density of seawater (ρ ｓｗ ) and the following equation {ρ ＣＯ２ = ρ ｓｗ} and found that a density-matched depth appears where the relationship is satisfied and the depths match (equilibrium) with each other. The solid line in the graph of Figure 14 indicates the seawater temperature from 0 to 3500 m, and at depths of 3500 m or deeper, the temperature rises due to the influence of the ground temperature of the stratum U below the seafloor surface F. The dashed line in the graph of Figure 14 also indicates the seawater density (ρ ｓｗ ) in the graph of FIG. 14. The dashed line in the graph of FIG. 14 shows the density of carbon dioxide (ρ ＣＯ２ ) is shown.

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

[0053] In Figure 14, the transitional zone (2500-3000 m depth) in seawater is shown in the geological layer U below the seafloor surface F, as described in the 2005 IPCC report (15, p. 280, Fig. 6, 9 / p. 286). ＣＯ２ = ρ ｓｗ Similarly, in the layer U below the seafloor F, the geothermal gradient varies from point to point, so the following equation {ρ ＣＯ２ = ρ ｓｗ The depth from the seabed F changes, and the relationship expressed as

[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 below 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 that underground storage of carbon dioxide is possible at the above location in stratum U below 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. Conventionally, it was thought 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, if carbon dioxide is stored at the depth described above, carbon dioxide will not sink or rise.

[0056] In addition, the density of the pore water with dissolved carbon dioxide (carbon dioxide dissolved water) becomes greater than the density of the pore water, so it sinks and when it comes into contact with a rock mass rich in alkali metals such as calcium (Ca), such as the basalt present in the acoustic bedrock B below, it becomes calcium carbonate (CaCO ３ ) and eventually carbon dioxide (CO ２ ) will be placed in isolation.

[0057] The density of the layer is in equilibrium, that is, the following equation {ρ ＣＯ２ = ρ ｓｗ}, even if a highly permeable fault or the like occurs due to an earthquake or other event, the risk of carbon dioxide leakage due to buoyancy is more likely to be reduced, and the above-mentioned cap rock is no longer necessary.

[0058] Here, when the strata are deformed by an earthquake or the like, 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 a 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, so a double defense against the above-mentioned leakage risk functions. This double defense is defined by the following equation {ρ ＣＯ２ = ρ ｓｗ} and the formation of a carbon dioxide barrier layer C made 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, considering the geothermal gradient underground, it is possible to draw an isotherm parallel to the seafloor surface F. When this isotherm is considered around islands, seamounts, and oceanic plateaus, as shown in Figure 15, it is possible to obtain the following equation: ＣＯ２ = ρ ｓｗ It is considered that the isopycnals satisfying the relationship expressed by {ρ} 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 the relationship between horizontal distance and water depth is shown with an aspect ratio of 1:10. In areas shallower than the isopycnals, as shown in Figure 14, the following equation {ρ ＣＯ２ >ρ ｓｗ}, and conversely, the deep part is expressed by the following equation {ρ ＣＯ２ <ρ ｓｗ}, carbon dioxide is in the region where the relationship is expressed by the following formula {ρ ＣＯ２ = ρ ｓｗ} and stays along the isopycnal.

[0060] As described above, in the stratum U below the seafloor F at a depth of 3000 m or more, the following equation {ρ ＣＯ２ = ρ ｓｗ}, in order to store carbon dioxide near the depth of this isopycnal, it is necessary to drill a well below the seabed surface F to a depth of more than 3000 m. In this way, in order to drill a well below the seabed surface F to a depth of 3000 m or more, it is necessary to use the latest drilling technology, which is expected to result in a significant increase in construction costs. Therefore, as shown in Figure 15, ＣＯ２ = ρ ｓｗ ), there may be locations where wells can be drilled from the seafloor surface F at a depth of 3,000 m or less. By establishing injection wells in such locations using more general-purpose, low-cost drilling technology, it will be possible to store carbon dioxide in an area extending to a depth of 3,000 m or more, thereby realizing a larger storage capacity.

[0061] In particular, islands, seamounts, and oceanic plateaus made of volcanic rock are the remains of the volcanic gas channels released by lava erupting onto the seafloor (F) and then rapidly cooled by seawater. Volcanic rocks such as those on Mount Fuji contain cavities (pores) that are continuous with the lava channels (vents) and minute continuous pores (airtight for volcanic gas), and carbon dioxide is stored in these cavities.

[0062] The above-mentioned "islands" refer to land areas surrounded on all sides by bodies of water such as oceans or lakes. The above-mentioned "sea mount" is a type of underwater landform, and refers to a relatively isolated high place with a relative height of 1000m or more from the seabed, and the diameter of the summit is not large. The above-mentioned "ocean plateau" is a plateau-like landform with a relatively flat top located on the ocean floor, and is 100km wide. ２ Volcanic gases are gases that are generated by volcanoes, such as water vapor and CO. ２ , 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 pore water density (ρ ｓｗ ) and the carbon dioxide hydrate. In addition to storing carbon dioxide in a deep region in the stratum that satisfies the conditions for equilibrium between the carbon dioxide hydrate and the carbon dioxide shielding layer C, it is also possible to adopt a method in which the strength of the carbon dioxide hydrate in the early stages of its generation 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 reservoir G has a thickness of 10 m to 40 m.

[0064] The strength of the carbon dioxide barrier layer C made of carbon dioxide hydrate explained in this embodiment is what is called a threshold pressure, which is the pressure at which carbon dioxide begins to permeate. In other words, the strength of the carbon dioxide barrier layer C needs to be equal to or greater than the threshold pressure below which carbon dioxide does not permeate (is blocked). The threshold pressure explained in this embodiment is determined by applying carbon dioxide CO hydrate to the carbon dioxide barrier layer C by the method described in "Application for Permit for Subsea Disposal of Specified Carbon Dioxide Gas" (http: / / www.env.go.jp / press / 102082.html) in the demonstration test of the DSA, based on the "Act on Prevention of Marine Pollution, etc. and Maritime Disasters" (Act No. 136 of 1970). ２ This pressure is measured by mercury porosimetry, etc., as the pressure at which carbon dioxide begins to seep in. The carbon dioxide barrier layer C is thought to be formed mainly by carbon dioxide hydrate filling the pores of the formation, and its shielding mechanism is thought to be different from the threshold pressure in the DSA described above, which is caused by capillary pressure.

[0065] As mentioned above, carbon dioxide (CO ２ ) exists in one of four phases: gas, liquid, solid, or supercritical within a given temperature and pressure range. ２ ) becomes a solid (dry ice) at temperatures below 194 K (-79.15°C) under normal pressure, but when mixed with water, it becomes a hydrate (solid) under different temperature and pressure conditions.

[0066] In addition, carbon dioxide (CO ２ ) enters a supercritical state when the temperature is 31°C or higher and the pressure is 7.4 MPa or higher, and the density change is relatively slow. ２ ) has a density lower than that of seawater when the temperature is 35°C or lower and the pressure is 20 MPa or lower, and is lower than that of water except when the temperature is around 0°C and the pressure is 14 MPa or higher. ２ In a liquid or supercritical state, carbon dioxide (CO) is present in the layer U below the seafloor F (see Figure 1). ２ In order to store carbon dioxide, 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, or the like. Conventional natural gas is stored in geological formations due to the sealing effect of a cap rock, such as highly airtight mudstone. DNA analysis has revealed that shale gas is produced simultaneously with petroleum (shale oil) from shale, which is considered to be the petroleum source rock. Methane hydrate has been confirmed to exist as a solid in the subsea layer of the continental margin and in permafrost. Although the above-mentioned cap rock does not exist, a sealing layer that performs a so-called sealing function is formed depending on the temperature and pressure conditions for hydration.

[0068] The natural existence of natural gas, as described above, can be considered a natural analog of carbon dioxide storage. In other words, the cap rock described above functions as a sealing layer for conventional natural gas and general underground storage of carbon dioxide (also known as aquifer storage). Methane hydrate and carbon dioxide hydrate can also be used as sealing layers by setting the appropriate temperature and pressure conditions for hydration. Shale gas exists when methane is adsorbed into shale or coal layers, and it is known that it also adsorbs carbon dioxide. As described above, naturally occurring natural gas is trapped within geological formations through different sealing mechanisms. By utilizing these sealing mechanisms, i.e., natural conditions, carbon dioxide can also be stored underground.

[0069] As a result of extensive research, the present inventors have found that, when generating the above-described 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, the inventors have 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 with sufficient strength.

[0070] Furthermore, the present inventors have discovered that by optimizing the relationship between the time after the formation of carbon dioxide hydrate in the carbon dioxide shielding layer C and its strength, 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 inventors have 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 defined in the present invention is, for example, the strength at a time point between 0 and 40 hours after the start of carbon dioxide hydrate formation.

[0073] The behavior of carbon dioxide hydrate in the pore spaces of geological formations and an image of carbon dioxide hydrate stored in the formations are described in detail below. In the schematic cross-sectional view of FIG. 1A , the carbon dioxide sealed region S below the seafloor surface F is a geological formation that satisfies the pressure and temperature conditions that allow carbon dioxide hydrate to be generated, i.e., a region of pressure and temperature at which carbon dioxide hydrate stably exists. Furthermore, 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 FIG. 3 ). Furthermore, the acoustic basement B below the carbon dioxide reservoir G is a basement defined by a reflection method used to interpret ocean-floor geological maps, etc., and is a basement that is thought to contain, for example, basalt with a composition similar to that of magma. Furthermore, a sedimentary layer (Quaternary sedimentary layer) about several hundred meters deep exists above the acoustic basement B, and is divided into the carbon dioxide sealed region S and the carbon dioxide reservoir G based on the stable region of carbon dioxide hydrate shown in the graph of FIG. 3 .

[0074] In the carbon dioxide reservoir G shown in Figure 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 a position below 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 a position below 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, and it is thought that the storage range spreads horizontally and a carbon dioxide liquid layer G1 is formed at the top of the carbon dioxide reservoir G.

[0075] Below, a detailed description will be given of a trial 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 a laboratory experiment, the details of which will be described later, the present inventors have calculated the pressure acting on the carbon dioxide shielding 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, resulting in the results shown in the graph of Fig. 4.

[0076] Here, the carbon dioxide density contour lines used in the trial calculation of the pressure acting on the carbon dioxide shielding 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 of Figure 2 is the region showing the pressure and temperature conditions under which carbon dioxide hydrate exists stably.

[0077] In addition, the stable region of carbon dioxide hydrate is shown in the graph of the temperature-pressure phase diagram of carbon dioxide hydrate in Figure 3, with the temperature axis enlarged. In Figure 3, the carbon dioxide density isolines, seawater density isolines, and density difference isolines are also superimposed, making it possible to read the buoyancy of carbon dioxide. Furthermore, Figure 3 also shows curves plotting the relationship between seawater temperature and pressure (depth) for the Pacific Ocean and the Sea of ​​Japan. Also shown in Figure 3 are carbon dioxide isopycnals calculated using the Span & Wagner equation of state (Reference 3). Also shown in Figure 3 are isopycnals for 35‰ psu seawater calculated using EOS80 (Reference 4; Reference 5). Also shown in Figure 3 are isopycnals for the density difference Δ (carbon dioxide density - seawater density). Furthermore, Figure 3 also shows the temperature and pressure of the formation shown in the schematic cross-sectional view of Figure 1A (carbon dioxide sealed area S (1) - carbon dioxide reservoir G (2) - acoustic base B (3)).

[0078] Here, the region indicated by R2 in the graph of Figure 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 Figure 3 is a region showing the pressure and temperature of the carbon dioxide stored in the carbon dioxide reservoir G. Here, when the value of the carbon dioxide equivalent density line L1 shown in Figure 3 is read, the density of carbon dioxide is 850 to 970 kg / m ３ When the value of the density line L2 of seawater is read, 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-mentioned 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 ３This density is about 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 shielding layer C decreases.

[0080] The lines (a)-(b)-(c) shown in the graph of Figure 1 respectively connect the plot positions showing the temperature and pressure (a) at the seafloor surface F, the temperature and pressure (b) at the lower end of the carbon dioxide shielding layer C, and the temperature and pressure (c) at the carbon dioxide injection point, and show the case where the temperature at the seafloor surface F is 5°C / 1000m (approximately 1000 decibars), the geothermal gradient of the subseabed stratum U is 30°C / km, and the temperature at the carbon dioxide injection point is 15°C. Because these temperature and pressure change depending on the water depth and geothermal gradient at the storage point, the lines (a)-(b)-(c) differ for each point.

[0081] Meanwhile, the temperature and pressure of the ocean floor F have been confirmed and reported in the Pacific Ocean and the Sea of ​​Japan (Reference 6). The temperature-pressure (depth) relationships for the Pacific Ocean and the Sea of ​​Japan reported in Reference 6 are plotted in Figure 3 as two nearly vertical curves on the left side of the X-axis. The curve at the lower temperature side represents the relationship for the Sea of ​​Japan, while the curve at the other side represents the relationship for the Pacific Ocean. Both of these curves are plotted at depths where the standard deviation is 0.5°C or less. The Sea of ​​Japan is plotted at a depth range of 500 to 2000 m, while the Pacific Ocean is plotted at a depth range of 1000 to 4000 m. Because the Pacific Ocean experiences large temperature fluctuations, likely due to the influence of the Kuroshio Current, the envelopes of maximum and minimum values ​​are shown as dashed lines in the graph in Figure 3. The two curves shown in Figure 3 represent the results for the depth range confirmed in the database covered 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 marine areas, but the geothermal gradient varies from point to point within a range of approximately 0 to 200°C / km.

[0082] The relationship between seawater temperature and depth as described above and the CO ２Based on the formula for the boundary (three-phase line) of the stable region of hydrate, the temperature and pressure (T, P) at the bottom of the carbon dioxide barrier layer C are T ｓｆ , P ｓｆ , GG are known inputs, the calculation can be performed by the following general formula (1).

[0083]

[0084] On the other hand, the upper end of the carbon dioxide barrier layer C is ２ There is a phenomenon of supercooling of hydrate, CO ２ CO that can produce hydrate ２ There are some points to note, such as the condition of a molar ratio of mol / mol of water (1:5.5-6), and the fact that freezing point depression occurs when the pore water in the stratum U below the seafloor contains salts similar to seawater, so there may not be enough data to accurately calculate it. Here, the above-mentioned supercooling (SUPER-COOL) is a phenomenon in which a substance remains liquid even at temperatures and pressures that would cause it to undergo a phase change to a solid. It is known that when a small portion of a substance begins to change to a solid due to a trigger such as vibration, the entire substance suddenly changes to a solid. Freezing point depression is also known to occur when ice is floated in a salt solution with a concentration of about 3%. This phenomenon causes the melting point of the ice to drop to about -5°C, and it is also known that the melting point drops further as the salt concentration increases.

[0085] As mentioned above, the pore water contained in the stratum U under the seafloor is thought to contain salt like seawater. ２ The freezing point depression in the formation of hydrate is explained below. ２ When hydrate forms, it expels salt, just like ice does. This results in the phenomenon of the remaining water becoming more saline. Furthermore, the freezing point depression is not uniform because it varies depending on the local salinity, resulting in 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 containing salt, no clear peak appears, and there is a 1-2°C difference in the freezing point, making it difficult to determine the exact value.

[0086] As described above, in order to determine the temperature and pressure at the top end of the carbon dioxide shielding layer C, the supercooling, freezing point depression, and CO ２ Although there is uncertainty in terms of the molar ratio of CO to water, the range is within a certain range. For example, the freezing point depression for an average seawater of 35‰ psu is 2°C on average, with a further range of about ±2°C. In the graph of Figure 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 Figure 3, the CO ２ The freezing point depression appears because the above line segments L3 to L8 are located on the low temperature side of the boundary of the stable region of hydrate (three-phase line). This data shows the results of laboratory experiments conducted by the applicant, and line segments L3 to L8 are drawn to connect the maximum and minimum values.

[0087] From the above findings, it can be seen that the temperature at the upper end of the carbon dioxide shielding layer C is, on average, 2°C lower than the temperature at the lower end, with a range of approximately ±2°C. Furthermore, when the geothermal gradient is 30°C / km, a temperature change of 2°C corresponds to a layer thickness of 66.7 m, with a range of ±66.7 m, but it is highly likely that this layer thickness and ± range are close to the average value. Therefore, it is believed 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‰ psu, and therefore 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 0 m (top end of the storage tank), and the pressure value when the reservoir thickness is 200 m is shown as a downward positive value. Also in Figure 4, the pressure when the reservoir thickness is 20 m is about 1 / 10 of the pressure when the reservoir thickness is 200 m. Also in Figure 4, Cases 1 and 2 are carbon dioxide hydrate storage, and Case 3 is aquifer storage. 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] The schematic diagram in Figure 5 shows the general structure of a pressure cell 100 used in a laboratory experiment to demonstrate the present invention. The maximum pressure within this pressure cell 100 was 70 MPa, which corresponds to the pressure at a depth of 7,000 m underwater. 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 to reproduce conditions simulating the pressure and temperature of the strata below the seabed. Liquid carbon dioxide was then allowed to permeate from the bottom of the pressure cell 100. The measurements taken during this experiment included the temperature measured by thermometers installed at 100 mm intervals from the bottom, and the pressure at the top and bottom of the pressure cell 100.

[0090] Each of the graphs in Figures 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 Figure 5. In each of the graphs in Figures 6(a) to (c), the carbon dioxide flow rate shown by the solid line in the 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 until 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 sufficient strength is achieved. In Figures 6(a) to (c), as the time interval is increased, a differential pressure (= strength) of about 0.2 MPa is measured after 115 hours, indicating a tendency for the strength of the carbon dioxide hydrate membrane to increase.

[0091] The above-mentioned 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, a part of the membrane breaks, carbon dioxide permeates, the pressure difference drops to "0", and then the permeation of carbon dioxide stops, and the carbon dioxide hydrate membrane is repeatedly produced.

[0092] From the results shown in the graphs of FIGS. 6(a) to 6(c), it is assumed that the initial strength of the carbon dioxide hydrate membrane is 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 becomes 0.02 MPa.

[0093] The graph in Figure 7 shows the temperature rise due to the heat of formation of carbon dioxide hydrate when carbon dioxide permeates into a geological formation, obtained in an experiment using the pressure cell 100 shown in Figure 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 sample was maintained for 17 days under conditions where the differential pressure was approximately 2 m or less in terms of head, as obtained in an experiment using the pressure cell 100 shown in Figure 5. Furthermore, in this experiment, a sample (No. 9 silica sand) saturated with 40‰ seawater was used. The initial condition in the experiment whose results are shown in the graph in Figure 8 was that carbon dioxide was permeated into the pores of the sand saturated with seawater up to the position of a thermometer installed 400 to 500 mm from the bottom. The time-differential pressure relationship 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 assumed to have formed at the tip of the carbon dioxide permeation, maintained for 400 hours (approximately 17 days) so that the differential pressure was 0.02 MPa.

[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 at 1 MPa for 24 hours after being maintained for 17 days under conditions where the differential pressure was approximately 2 m or less in terms of head, as obtained in 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 made of carbon dioxide hydrate, which was thought to have increased in strength after 410 hours in the above experiment, and this state was maintained for 24 hours. After 24 hours had passed in this maintained state, 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 reservoir 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, and therefore the layer thickness of the carbon dioxide reservoir G is thought to gradually increase. For example, if it takes two weeks or more for the layer thickness of the carbon dioxide reservoir G to reach 0 to 20 m, a storage amount can be expected 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, an experiment was conducted in which the conditions, as explained with reference to the graph in Fig. 8, were maintained for 17 days at a temperature of 2°C, a pressure of 7 MPa, and a differential pressure of 2 m or less in terms of head, and then an experiment was conducted in which the carbon dioxide pressure was increased to a differential pressure of 1 MPa and maintained for approximately 24 hours. As a result, it was confirmed that the strength of the carbon dioxide shielding layer C (the pressure at which carbon dioxide penetration begins) 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 begins to permeate).

[0100] The data shown in Figure 10 are the results after data on the carbon dioxide barrier layer C having a strength of 1 MPa or more was obtained three times consecutively over a period of approximately 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, first, as an initial condition, sand saturated with seawater was maintained at a temperature of approximately 4°C at a water pressure equivalent to a depth of 800 m (8 MPa). 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 to 7 hours: Carbon dioxide was allowed to permeate while adjusting the flow rate so that the pressure difference was 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 seawater in the amount that the carbon dioxide permeated was discharged from the upper side of the pressure cell 100 (see the second graph from the bottom in FIG. 10).

[0102] Furthermore, as the differential pressure 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 cases where the differential pressure is small, at 0.2 MPa or less, the pressure can be read from the fourth graph from the top.

[0103] (2) 7 to 18 hours After 7 hours had passed, the amount of carbon dioxide permeation decreased, so the pressure difference was increased to 0.025 MPa. At this time, seawater equivalent to the amount of carbon dioxide permeated was discharged from the upper side of the pressure cell 100, confirming that the cell was not shielded.

[0104] (3) 18 to 42 hours: When the above differential pressure was forcibly increased to 0.05 MPa, it appeared that part of the carbon dioxide hydrate membrane was ruptured, and the amount of carbon dioxide permeation suddenly increased 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 that had been 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 shielding layer C was gradually being formed. The reason for this is thought to be that the generation of carbon dioxide hydrate causes a volumetric shrinkage of about 10%, and both carbon dioxide and seawater flow in during this generation process.

[0105] (4) 42 to 50 hours: When the differential pressure 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 to 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 shielding layer C was 0.5 MPa or more.

[0107] (6) 67 to 72 hours: The differential pressure was increased stepwise, 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 observed in the flow rate of carbon dioxide at 50 hours and from 65 to 72 hours (the third graph from the bottom in Figure 10), this is an apparent flow rate caused by the compression of carbon dioxide. In other words, during those 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 no carbon dioxide was penetrating through the carbon dioxide shielding layer C.

[0109] The above results are thought to show the time-strength relationship of the carbon dioxide barrier layer C, and it is clear that the strength increases with the passage of time. Furthermore, it is thought that the carbon dioxide hydrate membrane ruptured in the time period from 18 to 22 hours, but that the repair function of the carbon dioxide barrier layer C itself acted on ruptures caused by this level of pressure difference, shortening the time it took for the strength to reach 1 MPa or more to be reached to approximately 67 hours (slightly less than three days).

[0110] 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 differential acts, obtained in an experiment using the pressure cell 100 shown in FIG. 5. Note that the "reverse pressure differential" in this experiment refers to the difference between the density of carbon dioxide (ρ ＣＯ２ ) and pore water density (ρ ｓｗ ) is expressed by the following equation {ρ ＣＯ２ <ρ ｓｗ}.

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

[0112] The graph in Figure 11 shows the results when the carbon dioxide pressure was suddenly reduced after 15 hours. This sudden reduction in carbon dioxide pressure caused a significant temperature increase due to the heat of carbon dioxide hydrate formation at a point 400 mm from the bottom of the pressure cell 100, and it is believed that the carbon dioxide shielding layer C was formed at this position. This timing also coincides with the timing when the carbon dioxide pressure decreased from 20 MPa to about 17.5 MPa and then rose again to 20 MPa.

[0113] Furthermore, while the carbon dioxide pressure is decreasing from 20 MPa to 17.5 MPa, the carbon dioxide shielding layer C maintains sufficient strength and does not break, so a pressure difference is generated. However, when the carbon dioxide pressure decreases to 17.5 MPa, the carbon dioxide shielding layer C breaks once. At the same time that the carbon dioxide shielding layer C breaks once, water begins to permeate toward the vicinity of the carbon dioxide shielding layer C, and this permeation causes the pressure on the carbon dioxide side to rise to 20 MPa. Furthermore, since the temperature rise associated with the generation of heat from the formation of carbon dioxide hydrate is noticeable at a position 400 mm from the bottom of the pressure cell 100, it is believed that the carbon dioxide shielding layer C made of an even 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 after 15 hours. Furthermore, after the carbon dioxide shielding layer C was ruptured and then regenerated as described above, the pressure on the carbon dioxide side gradually decreased, and the differential pressure acting on the carbon dioxide shielding layer C became negative (negative). This differential pressure indicates that the carbon dioxide shielding layer C has a strength equivalent to 10 MPa (1000 m of hydraulic head).

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

[0116] The graph in FIG. 12 shows contour lines of the optimum volume ratio of {carbon dioxide / water} in the temperature-pressure relationship. As shown in FIG. 12, the molecular formula of the carbon dioxide hydrate crystal (5.75H ２ O.C.O. ２ As a result of calculating the volume ratio taking into account the above, the volume of water required was 2.3 times the volume of carbon dioxide. Therefore, it is thought that in order for the pores into which carbon dioxide has penetrated to become blocked, there is a high possibility that there is insufficient water / seawater. In other words, it is thought that the water has been used up (dry up), resulting in an excess of carbon dioxide.

[0117] On the other hand, Figure 13 shows a schematic diagram of the shielding mechanism of carbon dioxide hydrate by a membrane, based on the experimental results using the pressure cell 100 shown in Figure 5. That is, Figure 13 shows a partial cross section of the cylindrical pressure cell 100 shown in Figure 5. As shown in the schematic diagram of Figure 13, near the interface between liquid carbon dioxide and water / seawater in the pores of the sand, the degree of saturation of water gradually decreases from 100%, and it can be seen that the interface is by no means flat. Also, as shown in Figure 13, near the front of the carbon dioxide penetration, there is a region with a volume ratio close to that 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 where the sand has intricately penetrated into the pores. Therefore, it is highly likely that the carbon dioxide barrier layer C is not a horizontal, one-sided film, but rather a region with a certain thickness or more.

[0119] Furthermore, assuming that carbon dioxide hydrate with a thickness of about 20 μm (see Reference 1 above) is formed in the shape of a straw in the pores of the sand, and further assuming that the average pore diameter 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 liquid carbon dioxide to become carbon dioxide hydrate, and this portion is thought to have a strength of 1 MPa or more. Note that carbon dioxide molecules and water molecules are thought to be supplied to the portion where liquid carbon dioxide remains by diffusion based on a concentration gradient, and this supply by diffusion is thought to be the reason why it takes time to form the carbon dioxide shielding layer C.

[0120] According to the above-mentioned demonstration experiment of the present invention, the carbon dioxide shielding layer C preferably has a strength of 0.01 to 0.04 MPa, and more preferably an average value of approximately 0.02 MPa, at the initial stage of carbon dioxide hydrate generation in the pores of this carbon dioxide shielding layer C. Ensuring that the strength of the carbon dioxide hydrate constituting the carbon dioxide shielding layer C at the initial stage of generation is within the above range makes it possible to more reliably contain the carbon dioxide injected below the carbon dioxide shielding layer C, and to store large amounts of carbon dioxide with higher efficiency. The strength of 0.02 MPa at the initial stage of carbon dioxide hydrate generation is an average value for the particle sizes of the above-mentioned No. 9 to No. 7 silica sand.

[0121] In addition, the storage method of this embodiment can employ a method of calculating the strength of carbon dioxide hydrate in the carbon dioxide shielding layer C at the initial stage of formation, and 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, and controlling at least one of the flow rate of carbon dioxide injected into the stratum U below the seafloor surface F, the number of injection wells 2 in use, and the injection depth based on each calculated value. Each of the calculations described above can be performed using a calculation program on a computer, tablet terminal, or the like, and the carbon dioxide injection flow rate and the number and depth of injection wells 2 in use 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 number of injection wells 2 in use.

[0122] According to the storage method of this embodiment, by employing the control method described above, it is possible to adjust the carbon dioxide injection flow rate, injection position, etc. while constantly monitoring the carbon dioxide storage status 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. Furthermore, 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 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 production. If the strength of the carbon dioxide shielding layer C 10 to 20 days after the start of carbon dioxide hydrate production 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, thereby enabling a large amount of carbon dioxide to be stored with even higher efficiency.

[0124] Furthermore, the storage method of this embodiment can employ a method of calculating the strength of carbon dioxide hydrate in the carbon dioxide shielding layer C 10 to 20 days after the start of its generation, and controlling at least one of the flow rate of carbon dioxide injected into the stratum U below the seafloor surface F, the number of injection wells 2 in use, and the injection depth based on the calculated value. The calculated values ​​can be obtained using a calculation program on a computer, tablet terminal, or the like, and similarly to the above, the carbon dioxide injection flow rate and the number and depth of the injection wells 2 in use can be controlled based on the calculation results. This, similarly to the above, allows the carbon dioxide injection flow rate, injection position, and the like to be adjusted while constantly monitoring the carbon dioxide storage status in the carbon dioxide storage layer G and the strength of the carbon dioxide shielding layer C, thereby enabling even more efficient operation of the carbon dioxide storage facility. Furthermore, similarly to the above, it becomes possible to reliably and effectively contain carbon dioxide injected below the carbon dioxide shielding layer C, thereby enabling even more efficient storage of large amounts of carbon dioxide.

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

[0126] [Artificial water sealing in the carbon dioxide sealed region] In the carbon dioxide underground storage method of this embodiment, the density of carbon dioxide (ρ ＣＯ２ ) and pore water density (ρ ｓｗIn addition to storing carbon dioxide in a deep region in the stratum that satisfies the conditions for equilibrium between carbon dioxide hydrate and seawater, and optimizing the relationship between time and strength during the generation of carbon dioxide hydrate that forms the carbon dioxide shielding layer C, it is also possible to employ a method of increasing the pore pressure in the carbon dioxide sealed region S by artificially sealing with water by injecting at least one of seawater and water as pore water into the carbon dioxide sealed region S.

[0127] An example of equipment for performing the above-mentioned artificial water sealing is an artificial water sealing equipment 6 such as that 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 arranged on the upper surface side of the carbon dioxide sealing area S, and as described above, is capable of increasing the pore pressure of the carbon dioxide sealing area S by injecting at least one of seawater and water as pore water into the carbon dioxide sealing area S. By providing such artificial water sealing equipment 6, the pore pressure of the carbon dioxide sealing area S increases, thereby increasing the strength of the entire carbon dioxide sealing area S including the carbon dioxide shielding layer C. The detailed configuration of the artificial water sealing equipment 6 has been described above.

[0128] Hereinafter, details of the case where artificial water sealing is performed in the storage method of this embodiment will be described. In the schematic cross-sectional view of FIG. 1A described 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 shielding layer C, while pressure generated by the buoyancy of carbon dioxide acts on the lower part of the carbon dioxide shielding layer C. If the pressure difference between the top and bottom of the carbon dioxide shielding layer C is equal to or less than the strength of the carbon dioxide shielding layer C, carbon dioxide will not start to permeate into the carbon dioxide shielding layer C. Therefore, in the early stages of generation of the carbon dioxide hydrate membrane when the strength of the carbon dioxide shielding layer C is weak (for example, two weeks after the start of generation), it is possible to provide an artificial water seal facility 6 that uses a multi-pipe gap structure (casing annulus) or the like installed on the seabed surface F when the injection well 2 is cut, so that the pressure difference between the top and bottom of the carbon dioxide shielding layer C does not exceed 2 m in hydraulic head. By installing such an artificial water seal facility 6, it becomes possible to increase the pore pressure (pore pressure) of the carbon dioxide sealed area S, including the carbon dioxide shielding layer C, by injecting seawater / water from above the carbon dioxide sealed area S.

[0130] Carbon dioxide hydrate storage targets the unconsolidated Quaternary sedimentary layer, which is the newest in geological time, as described above. When considering faults caused by earthquakes, etc., Quaternary sedimentary layers are unconsolidated and therefore allow deformation, and it is thought that this deformation will cause dilatancy (volume change caused by shear deformation). It is thought that the porosity and permeability of Quaternary sedimentary layers increase due to the above-mentioned dilatancy, and it is necessary to consider the extent to which this will affect the risk of carbon dioxide leakage. For this reason, the inventors conducted a demonstration experiment using the pressure cell 100 shown in Figure 5 to examine the time-strength relationship of the carbon dioxide shielding layer C, which is assumed to be an artificial water seal, and the results are shown in the graph of Figure 11 described above. As a result, when artificial water sealing was performed in the pressure cell 100 in a direction that allowed water to infiltrate into the pores of the sand into which carbon dioxide had permeated, the effect of reducing the differential pressure acting on the carbon dioxide shielding layer C was observed, and when the pressure increase due to the artificial water sealing became 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] <Carbon dioxide storage at depths in the subseafloor layers where the carbon dioxide density and pore water density are the same> Figure 16 shows the CO ２The stable region of CO hydrate is shown. ２ density ρ ＣＯ２ >Seawater density ρ ｓｗ A method of storing carbon dioxide in a concave topography of the seabed has been proposed, taking advantage of the temperature and pressure environment where carbon dioxide is stored (Reference 7). However, at the time of filing this application, this method is 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). Meanwhile, 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, studies have been reported on storing carbon dioxide in subseabed strata at depths of 3,000 m or more (Reference 2), but this does not take into account the geothermal gradient. Furthermore, calculations that take into account the geothermal gradient of the subseabed strata in the temperature-pressure relationship shown in Figure 14 show that the density matches (ρ ＣＯ２ = ρ ｓｗ ) density-matched depth appears.

[0132] Figure 17 shows the relationship between temperature and pressure. ２ 1 is a graph showing a region in which hydrates exist stably.

[0133] (1) in Figure 17 shows the relationship between temperature and pressure. ２ The boundary of the region where hydrates exist stably is shown by a thick solid line and a dotted line extending downward to the right from a depth of approximately 100 m. The solid line portion up to a depth of 2000 m is the range for which laboratory experimental data exists, and since there is no data for depths of 2000 m or more, it is shown as a dotted line, which is an extrapolation. The thick solid line extending downward from a depth of approximately 1000 m to 3000 m and the thick solid line extending downward to the right from a depth of 3000 m show the layer temperature when the pressure (≒ depth) is 3,000 m at the seafloor surface, when the temperature of the Pacific Ocean is at depths shallower than 3,000 m (Reference 4), and when the geothermal gradient is 30°C / km at depths deeper than 3,000 m.

[0134] In addition, in FIG. 17(1), the thin solid line and the broken line indicate CO ２ density ρ ＣＯ２ From the density of seawater with a salinity of 35‰ psu, ３５ｓｗ The contour lines show the density difference Δρ minus the density difference Δρ. For example, the contour lines show the density difference value Δρ = 20 kg / m ３The higher the pressure and the lower the temperature, the larger Δρ becomes. The dark grey area indicates that Δρ is 0 kg / m ３ That's it, ρ ＣＯ２ is ρ ３５ｓｗ The temperature and pressure conditions are shown below.

[0135] 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 > 200 kg / m ３ The thick line parallel to the x-axis at a depth of 400-500m is the CO ２ Below this boundary, the temperature and pressure of CO ２ is liquid at the top and gas at the bottom.

[0136] Therefore, the thick solid line extending downward from a water depth of 1000 m in (1) of Figure 17 indicates Δρ = 0 kg / m ３ Under the temperature and pressure conditions in the region below the solid line shown in ２ Conversely, the thick solid line extending downward from a depth of 1000 m indicates Δρ = 0 kg / m ３ Under the temperature and pressure conditions in the region above the solid line indicating ２ The thick solid line extending downward from a depth of 1000 m and the line Δρ = 0 kg / m ３ At the intersection with the solid line showing ＣＯ２ =ρsw and density become equal.

[0137] The thick solid line in (2) of FIG. ２ density ρ ＣＯ２ The thick black dashed line represents the density of seawater ρ ３５ｓｗ The relationship between pressure and temperature is shown.

[0138] The thick black solid line in FIG. 17(3) represents the density difference Δρ=ρ in FIG. 17(2). ＣＯ２ -ρ ３５ｓｗ The relationship between pressure and temperature is shown.

[0139] More than CO ２ 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.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-shaped markers at a depth of 3100 m shown in (1), (2), and (3) of Figure 17 indicate the ρ ＣＯ２ = ρ ３５ｓｗ The diamond-shaped marker at a depth of 2750 m indicates the lower limit of the temperature and pressure where the density coincides with ρ ＣＯ２ = ρ ３５ｓｗ The square marker at a depth of 3,450 m indicates the temperature and pressure at which the density is consistent with that of the subseafloor layer. ２ The figure shows the lowest temperature and pressure limits (HFZ: Hydrate Forming Zone) where hydrates can stably exist. The round marker at a depth of 3000 m indicates the temperature and pressure at the seafloor, and the star at a depth of around 3500 m indicates the temperature and pressure at the liquid CO ２ The temperature and pressure at the injection point are shown.

[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 surface and the shallower it becomes.

[0143] Conventionally, CO ２ Although expectations regarding shielding due to the formation of hydrate are described, no specific quantitative values ​​of shielding performance have been shown. Based on the results of laboratory experiments, the present invention shows that when the pressure difference is approximately 0.02 MPa, CO generated in sand pores ２ Liquid CO is produced by a hydrate membrane ２ This threshold pressure is used as a control standard to determine the amount of CO ２ Inject CO ２Consider that the movement due to some external force is shielded by a threshold pressure. ＣＯ２ = ρ ｓｗ CO based on ２ In addition to the underground storage (shielding) mechanism, ２ Shielding by the threshold pressure of the hydrate membrane utilizes a double shielding mechanism, resulting in a safer CO ２ It also enables underground storage in unconsolidated deposits such as alternating layers of sand and mud where there is no conventional geological shielding structure such as a cap rock. ＣＯ２ = ρ ｓｗ Furthermore, islands, seamounts, and oceanic plateaus made of volcanic rocks have volcanic gas channels remaining from erupted lava that have been rapidly cooled by seawater, and these cavities are filled with ρ ＣＯ２ = ρ ｓｗ CO using ２ It can be stored underground. Volcanic gases contain water vapor and CO ２ For example, volcanic rocks such as those of Mount Fuji have continuous cavities (pores) like lava channels (vents) and minute continuous pores (airtight for volcanic gases). Note that the pores for volcanic gases are larger than those in sand, and CO ２ A threshold pressure of approximately 0.02 MPa due to a hydrate membrane cannot be expected, and although it differs depending on the diameter of the airway, it is, for example, a few percent of 0.02 MPa.

[0144] The relationship between density and water depth has been explained as follows using the diagram (Fig. 2) in Reference 10 shown in Figure 20: "If the seafloor depth is 3,500 m and the geothermal gradient is 0.03°C / m (30°C / km), the injected CO ２ (liquid) is 200m below the seafloor (ρ ＣＯ２ = ρ ｓｗ ) ρ ＣＯ２ = ρ ｓｗ At a deposition depth of 1000 m or more, liquid CO ２ The density of the seafloor is higher than the surrounding pore water. ＣＯ２ = ρ ｓｗThe area up to the deposition depth of CO is called the NBZ (Negative Buoyancy Zone). The calculation at a depth of 3,500 m in Figure 20 is based on the table published by NIST. ２ The density and seawater density are calculated. On the other hand, this highly accurate calculation is performed only in Figure 20, and in Figure 23 described later, the calculation is performed with the seawater density constant. Also, in Figure 31, the threshold value of the storage potential sea area is determined only from Figure 20.

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

[0146] Figure 23 shows the diagram (Fig. 1) from Reference 10 rotated so that the y-axis represents pressure (depth). ２ The difference between the density of water and the density of seawater (CO ２ The density of seawater is shown as an isoline. If the isoline value is 0 or greater, the CO ２ This indicates that the temperature and pressure are such that the density becomes greater than the density of seawater. For example, contour lines represent the elevations / undulations of a topographical map in the longitude-latitude relationship, but the above contour lines represent lines connecting the same values ​​of density difference in the temperature-pressure relationship. Figure 23 is the same as the contour lines of Δρ shown in 1) of Figure 17. On the other hand, Figure 23 shows the CO ２ The density of seawater is constant (1,027 kg / m ３ 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 measured value is calculated. Note that interpolation calculations in the table format are generally used for numerical calculations that require high speed.

[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 on the x and y axes. 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 Since the density of seawater is calculated based on the temperature and pressure, it is thought that there is a difference with the contour line of Δρ in Figure 17 (1). On the other hand, for example, in Figure 20, the density of seawater is calculated based on the temperature and pressure. Therefore, Δρ in Figure 17 (1) is calculated based on the CO ２ The density is calculated using the Span & Wagnar equation of state (Reference 8) and the seawater density 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 depth is 3,500 m (Figure 20), but does not mention the temperature of the ocean floor surface. It also assumes a geothermal gradient of 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 waters around Japan (Reference 6: Yojiro Ikegawa, CO ２ The formula for seawater temperature in the deep ocean around Japan that governs hydrate storage, Journal of the Japan Society of Civil Engineers, Proceedings of the Environment, Vol. 78, No. 1, pp. 30-41, January 14, 2022, is used. The seawater temperature is calculated using a formula (Reference 6) based on the assumption that the seafloor temperature and the seawater temperature at the contact point are the same. The seawater temperature-pressure relationship for the Pacific Ocean is shown in Figure 18 as a thick solid curve extending downward from a depth of approximately 1,000 m, while the seawater temperature-pressure relationship for the Sea of ​​Japan is shown as a thick solid curve extending downward from a depth of approximately 500 m. Since no data could be found for the Pacific Ocean at depths of more than 4,000 m, the seawater temperature at depths of more than 4,000 m is assumed to be the same as the seawater temperature at depths of 4,000 m and is shown as a dotted line. Furthermore, since no data could be found for the Sea of ​​Japan at depths of more than 2,000 m, the same is shown as a dotted line. In Figure 16, the seawater temperature in the Pacific Ocean is about 2°C lower than that in the Sea of ​​Japan, and varies depending on the water depth.

[0149] In Reference 10, the geothermal gradient is considered to be a constant 30°C / km. However, taking into account that the geothermal gradient varies depending on the location, the program allows for any range, for example, from 10 to 300°C / km, to be considered. Figure 25 shows the geothermal gradient in the oceans around Japan (only the exclusive economic zone (EEZ) is shown).

[0150] In reference 10, CO ２ Since there is no source for the stable region of hydrate, the data and formula used are unknown. Here, the temperature-pressure relationship in Figure 16 is shown with a thick solid line and a dashed line sloping downward to the right from a depth of about 100 m. ２ The formula for the boundary (three-phase line) of the region where hydrates exist stably (Reference 6) was used. This formula is for use when data from depths of 2,000 m or less is used. The black diamond data labeled "Ikegawa2022w" in the legend of Figure 16 is the result of newly obtained data from laboratory experiments at depths of 2,000 to 5,000 m using freshwater. Previously, data was only available from depths of 2,000 m or less. The thick black solid line sloping downward to the right from about 100 m represents the boundary based on the data, while the dashed line represents extrapolation (linear interpolation). However, as the water depth increases, the measured values ​​deviate from the extrapolation, resulting in a difference of approximately 1.5°C at a depth of 5,000 m. Additionally, the gray diamond data labeled "Ikegawa2022sw35" in the legend is newly obtained data from laboratory experiments at depths of 2,000 to 5,000 m using seawater. In the case of seawater, the boundary shifts to the lower temperature side due to freezing point depression. Freezing point depression is the decrease in 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 newly obtained from laboratory experiments at depths of 2,000 to 5,000 m, the reliability of calculation accuracy has been improved, and CO ２ This improves security against data leaks.

[0151] Conventionally, as shown in FIG. 26 in Reference 10, injected CO ２ The long-term evolution of CO is shown in (a) to (c) below. (a) On the injection time scale (≒1 year), CO is buoyant. ２The top of the plume enters the HFZ and a small amount of hydrate forms. ２ (liquid) upward movement, ２ (b) After 102 years, most of the CO ２ Most of the CO reaches the bottom of the HFZ, and the self-forming hydrate cap expands laterally, creating a large volume of liquid CO below it. ２ (c) Finally, liquid CO ２ and CO ２ The hydrate dissolves and CO ２ Dissolved CO2 (aqueous solution) is formed. This CO ２ The dissolved water percolate through the formation (in the porous matrix) and mixes with enough water to become non-buoyant. ２ Once the dissolved water is no longer buoyant, any further CO ２ Dissolved water moves only by diffusion. The results of numerical calculations (Fig. 2 in Reference 2) for this conventional prediction (time course of a, b, and c in Fig. 25) have been published as shown in Fig. 27. In this calculation, the dissolved CO ２ The density of CO becomes larger than the density of pore water (seawater), causing it to sink. ２ Molar fraction X ＣＯ２ The change over time is shown in the third row from the top. ２ The dissolved water has a density greater than that of seawater and therefore sinks over the long term.

[0152] Here, two cases are shown: the case of HFZ>NBZ shown in Figure 28A, and the case of NBZ>HFZ 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 FIG. 28A, the HFZ is deeper than the NBZ, and liquid CO is stored in the lower part of the HFZ (first storage tank). ２ By storing CO in the lower part of the HFZ ２ The hydrate membrane creates a shielding threshold pressure of approximately 0.02 MPa. In the unlikely event that deformation (shear) of the strata due to an earthquake or other cause damages the shielding at the bottom of the HFZ, liquid CO２ If there is a leak, and the pressure difference is ≒ 0.02 MPa or less, ２ The hydrate membrane is regenerated and liquid CO ２ On the other hand, when the pressure difference is ≈0.02 or more, the regenerated CO ２ The hydrate membrane is destroyed, so shielding is not possible, and CO ２ The upward leakage of ＣＯ２ = ρ ｓｗ Therefore, in the case of FIG. 28A, the liquid CO 2 in the first storage tank ２ is CO ２ The double shielding function of the hydrate membrane and the NBZ provides high safety against leakage. In the case of Figure 28A, the second storage tank is shielded by the NBZ and the stored liquid CO ２ CO is generated to cover ２ CO2 hydride membrane based on the dual barrier function of threshold pressure ２ This CO2 stored underground in the lower part of the NBZ is ２ Injection should be carried out so that the pressure difference is 0.02 MPa or more (control standard). ２ If liquid CO is injected, it can be used in the HFZ. ２ By combining the first and second storage tanks, the storage volume per unit area can be increased by about two times. Furthermore, the liquid CO stored in the first and second storage tanks can be injected into the ２ dissolves in the pore water of the surrounding formation. This dissolution is not expected to occur due to the convection of the pore water in the formation. ２ The dissolution rate is essentially slow because it is based on diffusion through the hydrate membrane. Although the dissolution rate is very slow, CO ２ The density of dissolved water is greater than that of pore water, so it settles. ２ Experimental values ​​(Reference 7) and calculation models (Reference 13; Reference 14) have reported that the density of dissolved water will be greater than that 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, exist in the depths of the sinking water, CO ２ Dissolved water carbonate ions (CO ３ ２－ ) and calcium ions (Ca＋ ) to carbonate (CaCO ３ ) is produced and CO ２ is fixed. Basic rocks are made up of silicon dioxide (SiO ２ ) by weight of 45-52% and metamorphic rocks thereof. Ultrabasic rocks have a lower silicon dioxide content than basic rocks, at 45% or less, and include peridotite. The low silicon dioxide content 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 basalt. The Sea of ​​Japan is believed to have begun to open up due to plate tectonics about 200 million years ago, and is thought to be an environment where the mantle can easily rise from deep within, like at mid-ocean ridges, and marine geological maps show the presence of basalt several hundred meters below the seafloor surface. The above carbonates (CaCO ３ The process of mineralization that results from this 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, and this is a new proposal that has not been considered before, and the target water depth is less than 4,000 m. ＣＯ２ = ρ ｓｗ ) to liquid CO ２ In this case, since it is not an HFZ, CO ２ In the case of upward penetration, CO ２ No shielding due to hydrate formation occurs. ２ The pressure rise near the injection well is the CO in the HFZ in Figure 28B. ２ Safe injection is possible by injecting the hydrate so that the threshold pressure (≒0.02 MPa) based on the hydrate membrane is shielded (control standard). ＣＯ２ = ρ ｓｗ and CO ２ Double shielding by the hydrate membrane threshold pressure ensures safer CO ２ It can be stored underground. ２As in Figure 28A, the calcium carbonate dissolves in the pore water of the surrounding strata, settles, and is eventually fixed as carbonate (mineral) from calcium ions and carbonate ions near basic rocks such as basalt. In the case of Figure 28A, by examining a cross section of the ocean floor geological map, it is possible to consider suitable locations where basic rocks such as basalt exist several hundred meters below the seafloor surface. Furthermore, for water depths of 3,000 meters or less, there are existing technologies that can be used at depths targeted for commercial oil and natural gas development. Furthermore, there are several vessels available in Japan that can be used, making this highly feasible.

[0155] Previously, potential storage areas in the oceans surrounding the United States were shown as in Figure 30. Depths of 3,000 m or more were designated as CO ２ These areas are designated as areas with storage potential (storage potential areas).

[0156] Furthermore, a previous proposal for suitable storage sites in the waters surrounding Japan (Reference 12) based on Reference 10 proposed a 3,000-m-deep ocean area within 100 km of land (within the EEZ). Figure 31 shows potential storage areas in the waters surrounding Japan (within the EEZ only). The Pacific Ocean is plotted using the temperature-pressure relationship of the Pacific Ocean (Reference 6), while the Sea of ​​Japan is plotted based on the temperature-pressure relationship of the Sea of ​​Japan (Reference 6). Figure 31 also shows the 3,000-m-deep isobath with a dotted line. The areas shown in gray on the Pacific Ocean side of Figure 31 represent areas where the NBZ depth is greater than the HFZ depth (HFZ - NBZ > 0, as in Figure 28A). The areas shown in gray on the Sea of ​​Japan side represent areas where the NBZ depth is less than the HFZ depth (HFZ - NBZ < 0, as in Figure 28B), as in Figure 27. The shading of the gray area on the Sea of ​​Japan side is based on the difference between the HFZ and NBZ, and indicates isolines every 50 m (50 m, 100 m, 150 m, 200 m). The shading of the gray area on the Pacific side is also based on the HFZ and NBZ, and indicates isolines every 50 m (-50 m, -100 m, -150 m, -200 m, ...).

[0157] Figure 32 plots the seawater temperature-pressure relationship for the Sea of ​​Japan when the seafloor depth is 3,000 m and the geothermal gradient is 30°C / km. Figure 17 plots the seawater temperature-pressure relationship for the Pacific Ocean when the seafloor depth and geothermal gradient are the same. As mentioned above, the seawater temperature in the Sea of ​​Japan is about 2°C lower than that in the Pacific Ocean. Therefore, comparing Figure 32 for the Sea of ​​Japan with Figure 17 for the Pacific Ocean, it is clear that the NBZ (density consistency within the strata (ρ ＣＯ２ = ρ ｓｗ ) and the HFZ are deeper in the Sea of ​​Japan. Due to this difference in seawater temperature, both the NBZ and HFZ exist in the Sea of ​​Japan at shallower depths than in the Pacific Ocean. Comparing it to the 3,000m depth isobath (blue dotted line) in Figure 32, the potential sea area in the Pacific Ocean almost coincides with the 3,000m depth isobath. On the other hand, the potential sea area in the Sea of ​​Japan extends to areas shallower than the 3,000m depth isobath. Also, if you look closely at the Pacific Ocean, you can see that the potential sea area extends into areas shallower than the 3,000m depth isobath. This is due to the seawater density and ＣＯ２ This is because accuracy has been improved by calculating density based on the equation of state. Furthermore, as a result of sorting out the geothermal gradients mentioned above, a geothermal gradient of 30°C / km is the average geothermal gradient for Japan's land areas, while the average geothermal gradient for the surrounding ocean areas is approximately 100°C / km. Therefore, Figure 33 shows the potential ocean area for a geothermal gradient of 100°C / km. As the geothermal gradient increases, both the HFZ and NBZ become shallower, and the difference between the HFZ and NBZ becomes smaller.

[0158] Figure 34 shows an example of a calculation for the Pacific Ocean that takes into account the cross section of the ocean floor topography. The thick solid line extending from around -7000m depth indicates the lower limit of the NBZ, and the thin solid line extending from around -6200m depth indicates the lower limit of the HFZ. The calculation results are shown for a geothermal gradient of 30°C / km. If the water depth at the seafloor surface changes from 2000 to 5700m, the water 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 effect of changes in water depth, but in the case of an active volcano, CO ２It is not a candidate for geological storage. Furthermore, because the geothermal gradient varies from site to site, calculations must take into account the geothermal gradient at each site. 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 approximately 4,000 m. At depths greater than 4,000 m, the NBZ is located at deeper positions, and at depths less than 4,000 m, the NBZ is located at deeper positions, with the positions reversed. The x-axis in Figure 34 indicates longitude, with 1° ≒ 111 km. Therefore, the aspect ratio of the x-axis in Figure 34 is 8 km:222 km ≒ 0.036. Therefore, although the seabed topography appears steep in Figure 34, the actual seabed slope is gentle.

[0159] Figure 35 shows an example of calculation for the Sea of ​​Japan, taking into account the cross section of the seabed 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. In Reference 10, liquid CO ２ It is expected that liquid CO will be stored in the NBZ if it rises from the bottom of the HFZ. ２ Rather than hoping that CO will be stored, we set the threshold pressure of -0.02 MPa, which was confirmed in an indoor experiment, as the control standard value. ２ Injecting the NBZ reduces the possibility of leakage. If the water depth of the NBZ is greater than the water depth of the HFZ, this will not occur because Reference 10 defines the potential storage area as an area with a water depth of 3,000 m or more.

[0160] <Method and program for searching for a sea area suitable for underground storage of carbon dioxide> The method for searching for a sea area suitable for underground storage of carbon dioxide of the present invention is a method for searching for a sea area suitable for underground storage of carbon dioxide by calculating 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 exists stably in the subseafloor strata and the carbon dioxide density ρ ＣＯ２ is the density of seawater ρ ３５ｓｗ We will explore marine areas suitable for underground storage of carbon dioxide based on temperature and pressure conditions that match these.

[0161] The method for searching for a sea area suitable for underground storage of carbon dioxide (hereinafter also referred to as the "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 around Japan.

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

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

[0165] (Step 4) CO at the point P ２ Injection point (hereinafter also referred to as "point (c)") (input value: CO ２ (c) based on the injection depth of ２ Calculate density and seawater density.

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

[0167] (Step 6) CO in the seawater of the NBZ at the point P ２The temperature and pressure at the point where the density of CO equals the density of seawater (hereinafter referred to as "point (e)"). ２ Calculate density and seawater density.

[0168] (Step 7) The above steps (Step 1) to (Step 6) are carried out at each point (grid) P selected in the sea area around Japan, and the temperature, pressure (≒ water depth), seawater density, CO ２ Based on the density, potential maps (e.g., Figures 31 and 33) are calculated and plotted.

[0169] The program for searching for a sea area suitable for underground storage of carbon dioxide of the present invention is a program for searching for a sea area suitable for underground storage of carbon dioxide by inputting into a computer the time-strength relationship of a carbon dioxide shielding layer made of carbon dioxide hydrate under the temperature and pressure conditions under which carbon dioxide hydrate exists stably in the subsea bed strata and the carbon dioxide density ρ ＣＯ２ is the density of seawater ρ ３５ｓｗ This program searches for suitable ocean areas for underground storage of carbon dioxide based on temperature and pressure conditions that match the above.

[0170] The above-mentioned program for searching for marine areas suitable for underground storage of carbon dioxide (hereinafter also referred to as "search program") will be described in more detail. In one embodiment of the search program, a computer is caused to execute (Step 1) to (Step 7) described below. Here, 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 point P is a point in the Pacific Ocean, equation (6) of Reference 3 is used, and if point P is a point in the Sea of ​​Japan, equation (7) of Reference 3 is used to calculate the temperature and density at the seabed of point P (hereinafter also referred to as "point (a)") using a computing device.

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

[0174] (Step 4) CO at point P ２ Injection point (hereinafter also referred to as "point (c)") (input value: CO ２ (c) based on the injection depth of ２ The density and seawater density are calculated by a computing device, and the calculation results are stored in a storage device.

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

[0176] (Step 6) CO ２ The temperature and pressure at the point where the density of CO 2 at point (e) is equal to the density of seawater are calculated by a computing device, and the calculation results are stored in a storage device. ２ The density and seawater density are calculated by a computing device, and the calculation results are stored in a storage device.

[0177] (Step 7) The computer executes the above (Step 1) to (Step 6) at all input points P, and calculates the temperature, pressure (≒ water depth), seawater density, and CO at points (a), (b), and (c) at each point P. ２A potential map (e.g., Fig. 31, Fig. 33) is calculated based on the density by a computing device and plotted on an output device.

[0178] <Effects> As described above, according to the method for underground storage of carbon dioxide of this embodiment, by employing the above-described 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 large amounts of carbon dioxide.

[0179] <Other Embodiments> The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above-described embodiments, an example of storing carbon dioxide in a geological layer under the seabed has been described, but the method for underground storage of carbon dioxide 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. CO ２Effect of Flow Field on 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 Ikegawa. CO ２Formulation of Seawater Temperature in the Deep Ocean around Japan that Controls Hydrate Storage, Journal of the Japan Society of Civil Engineers, Proceedings of the Society of Civil Engineers, G (Environment), Vol. 78, No. 1, pp. 30-41, 2022.01.14. ・Reference 7: Osumi, Takashi, Nakashiki, Norikazu, Shimojima, Kiminori, Hirama, Kenichi. Deep Sea Disposal Technology for Carbon Dioxide (Part 1) - Presentation of Concept and Consideration of Environmental Impact Assessment Methods -, Central Research Institute of the Electric Power Industry Research Report: U92062, 1993.03. ・Reference 8: Omori, Tsutomu, 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: Edited by the McGraw-Hill Dictionary of Science and Technology Terminology Editorial Committee. McGraw-Hill Dictionary of Science and Technology Terminology, revised 3rd edition, October 2001. ・Reference 12: Japan Science and Technology Agency (JST). Proposal from 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 Techniques for 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.

[0181] The method for underground storage of carbon dioxide of the present invention is a method that can efficiently store large amounts of carbon dioxide by ensuring the strength of a carbon dioxide shielding layer formed by hydrated carbon dioxide and reliably sealing off carbon dioxide at an optimal depth in the geological formation. 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.

[0182] DESCRIPTION OF SYMBOLS 1... Carbon dioxide underground storage device (storage device) 2... Injection well 21... Tip 6... Artificial water seal facility 100... Pressure cell M... Sea surface W... Seawater F... Seabed surface (seabed) U... Stratum S... Carbon dioxide sealing area C... Carbon dioxide shielding layer G... Carbon dioxide storage layer G1... Carbon dioxide liquid layer B... Acoustic base

Claims

1. A method for storing carbon dioxide in a liquefied state in a seafloor substrata consisting of sediments on an acoustic platform, comprising: 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; forming a carbon dioxide shielding layer in the carbon dioxide sealed area by 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 by the buoyancy of the carbon dioxide to produce carbon dioxide hydrate; 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 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 as described in claim 1, comprising calculating a depth of the stratum in the seabed that satisfies pressure and temperature conditions under which the density of the carbon dioxide and the density of pore water are in equilibrium in the seabed, and controlling the position at which the carbon dioxide is injected into the seabed 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 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 can stably exist in the subsea layer and the carbon dioxide density ρ in the subsea layer are calculated by a 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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