Method for dissolving metal components in rocks, method for storing carbon dioxide underground, and method for fixing carbon dioxide

Abstract

[Problem] To provide: a method for dissolving metal components in rock and stone, capable of promoting dissolution of, and increasing permeability of mafic rocks such as basalt or ultramafic rocks such as peridotite at a temperature lower than 200°C in the ground; a method for storing carbon dioxide in the ground using said method; and a carbon dioxide fixation method. [Solution] In the present invention, a solution comprising a liquid containing a chelating agent is injected into mafic rocks or ultramafic rocks at a temperature lower than 200°C in the ground to extract, into the solution, metal components contained in the mafic rocks or ultramafic rocks. Further, carbon dioxide or a storage liquid having carbon dioxide dissolved therein is injected into the mafic rocks or ultramafic rocks from which metal components have been extracted into the solution.

Description

[Technical Field]

[0001] The present invention relates to a method for dissolving metal components in rocks, a method for storing carbon dioxide underground, and a method for fixing carbon dioxide. [Background technology]

[0002] In recent years, technologies for reducing carbon dioxide (CO2) emitted into the atmosphere have attracted worldwide attention, including methods of storing carbon dioxide in underground basalt layers and further methods of chemically reacting the carbon dioxide with metal components such as Ca, Mg, and Fe in the basalt layers to form carbonate minerals and fix the carbon dioxide (mineral fixation). For example, in Iceland, the Carbfix (CCS) project has already stored water with dissolved carbon dioxide in basalt layers (see, for example, Non-Patent Document 1), and it has been reported that within two years, the carbon dioxide reacted with metal components such as Ca eluted from the basalt and was fixed in the minerals (see, for example, Non-Patent Document 2).

[0003] In this method, in order to efficiently store and fix more carbon dioxide in basalt layers, it is necessary to increase the amount and connectivity of pores in the basalt to increase permeability, and to increase the rate at which metal components such as calcium are dissolved from the basalt.

[0004] Conventionally, the present inventors have developed a method for increasing the permeability of rocks by injecting a weakly acidic to weakly alkaline aqueous solution of a chelating agent into volcanic rocks such as granite or basalt at 200°C that can be used for geothermal power generation (see, for example, Non-Patent Documents 3 and 4). It has been confirmed that this method promotes the dissolution of granite or volcanic rock, forming pores and increasing permeability. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Carbfix, [online], [Retrieved March 7, 2023], Internet〈URL: https: / / www.carbfix.com〉 [Non-patent document 2] Sandra O. Snaebjornsdottir et al., “Carbon dioxide storage through mineral carbonation”, Nature Reviews Earth & Environment, 2020, 1, p.90-102 [Non-patent document 3] Ryota Takahashi, Jiajie Wang, Noriaki Watanabe, “Process and optimum pH for permeability enhancement of fractured granite through selective mineral dissolution by chelating agent flooding”, Geothermics, 2023, 109, 102646 [Non-patent document 4] Luis Salala, Ryota Takahashi, Jonathan Argueta, Jiajie Wang, Noriaki Watanabe, Noriyoshi Tsuchiya, “Permeability enhancement and void formation by chelating agent in volcanic rocks (Ahuachapan and Berlin geothermal fields, El Salvador)”, Geothermics, 2023, 107, 102586 Summary of the Invention [Problem to be solved by the invention]

[0006] The methods described in Non-Patent Documents 3 and 4 can promote the dissolution of granite and volcanic rocks at 200°C and increase their permeability. However, the methods described in Non-Patent Documents 3 and 4 have not been applied to other temperatures, particularly to mafic rocks such as basalt or ultramafic rocks such as peridotite, which are suitable for carbon dioxide storage at temperatures below 200°C. Since it is impossible to predict the temperature dependence of the dissolution rate of rocks composed of multiple minerals, it is unclear what results would be obtained if the methods described in Non-Patent Documents 3 and 4 were applied to mafic rocks such as basalt or ultramafic rocks such as peridotite at temperatures below 200°C. Furthermore, there have been no experiments or reports to date on the use of aqueous chelating agents for mafic rocks such as basalt or ultramafic rocks such as peridotite at temperatures below 200°C, including the methods described in Non-Patent Documents 3 and 4.

[0007] The present invention has been made with a focus on these problems, and aims to provide a method for dissolving metal components in rocks that can promote the dissolution of mafic rocks such as basalt and ultramafic rocks such as peridotite underground at temperatures below 200°C and increase permeability, as well as a method for storing carbon dioxide underground and a method for carbon dioxide fixation that utilize this method. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the method of the present invention for dissolving metal components in rocks is characterized in that a dissolving solution containing a liquid containing a chelating agent is injected into mafic or ultramafic rocks underground at a temperature of less than 200°C, thereby extracting metal components contained in the mafic or ultramafic rocks into the dissolving solution.

[0009] In the method for dissolving metal components in rocks according to the present invention, a dissolving solution containing a liquid containing a chelating agent dissolves metal components such as Ca, Mg, and Fe contained in underground mafic or ultramafic rocks, allowing these metal components to be extracted into the dissolving solution. This promotes the dissolution of underground mafic and ultramafic rocks. Furthermore, dissolving underground mafic and ultramafic rocks can create pores in the rocks, increasing the amount and connectivity of pores and improving the permeability of the rocks.

[0010] The method for dissolving metal components in rock according to the present invention allows carbon dioxide to be stored in the pores by injecting carbon dioxide or a liquid containing dissolved carbon dioxide into the formed pores or connected pores. Furthermore, by chemically reacting the metal components, such as Ca, Mg, and Fe, extracted into the solution with carbon dioxide, carbonate minerals can be produced, allowing carbon dioxide to be fixed in the minerals.

[0011] In the method for dissolving metal components in rock according to the present invention, when injecting the dissolving solution, it is preferable to drill a well from the surface of the earth to the underground mafic or ultramafic rock layer and use the well to inject the dissolving solution into the underground mafic or ultramafic rock. The dissolving solution from which the metal components have been extracted may be left underground, for example, to react the metal components with carbon dioxide underground. Alternatively, the dissolving solution may be recovered to the surface of the earth, for example, to react the metal components with carbon dioxide at the surface or to empty pores.

[0012] The method for dissolving metal components in rock according to the present invention can utilize mafic or ultramafic rocks that are highly porous, such as basalt or peridotite that exist at relatively shallow depths underground or under the seafloor, and can increase the amount and connectivity of pores in the rocks to enhance permeability. It is particularly preferable that the temperature of the underground mafic or ultramafic rocks be 100°C or lower.

[0013] In the method for dissolving metal components in rock according to the present invention, the chelating agent is preferably a biodegradable or thermally decomposable chelating agent. In this case, the chelating agent contained in the dissolution solution extracts metal components such as Ca, Mg, and Fe from mafic or ultramafic rock underground and then biodegrades or thermally decomposes to release the metal components. Therefore, for example, by injecting carbon dioxide or a liquid containing dissolved carbon dioxide into the formed pores or connected pores, the released metal components such as Ca, Mg, and Fe can be chemically reacted with carbon dioxide, thereby promoting mineral fixation of carbon dioxide. Furthermore, the chelating agent is environmentally friendly because it can be biodegraded or thermally decomposed. The biodegradable chelating agent may be any agent, such as L-glutamic acid diacetate (GLDA). Furthermore, the thermally decomposable chelating agent is preferably one that is decomposed by geothermal heat.

[0014] In the method for dissolving metal components in rock according to the present invention, the dissolving solution may have a pH value in the range of 1 to 14. For example, when the dissolving solution is acidic, it is particularly effective in promoting the dissolution of mafic and ultramafic rocks underground, and can particularly effectively increase the permeability of these rocks.

[0015] In the method for dissolving metal components in rock according to the present invention, the dissolution solution may be a liquid containing the chelating agent and carbon dioxide. In this case, the pH of the dissolution solution can be adjusted to a desired level by, for example, adding carbon dioxide to an alkaline chelating agent. Furthermore, simply by injecting the dissolution solution, carbon dioxide can be stored in the pores of underground mafic or ultramafic rocks whose dissolution has been promoted by the chelating agent, or carbon dioxide can be immobilized in minerals by chemically reacting with metal components such as Ca, Mg, and Fe extracted from the mafic or ultramafic rocks.

[0016] The method for storing carbon dioxide underground according to the present invention is characterized in that carbon dioxide or a storage liquid in which carbon dioxide has been dissolved is injected into the mafic rock or ultramafic rock after the metal components have been extracted into the solution by the method for dissolving metal components in rock according to the present invention.

[0017] The method for storing carbon dioxide underground according to the present invention involves forming pores using the method for dissolving metal components in rock according to the present invention, increasing the volume and connectivity of pores, and increasing permeability of the mafic or ultramafic rock. Then, by injecting a storage solution into the rock, carbon dioxide can be stored in the pores of the rock. Furthermore, the metal components such as Ca, Mg, and Fe extracted into the solution react chemically with the stored carbon dioxide to produce carbonate minerals, allowing the carbon dioxide to be fixed within the pores.

[0018] In the method for storing carbon dioxide underground according to the present invention, when carbon dioxide is injected, a larger amount of carbon dioxide can be stored compared to when a storage liquid is injected. On the other hand, when a storage liquid is injected, a chemical reaction occurs between the metal components and carbon dioxide in the storage liquid, which is advantageous in that mineral fixation of carbon dioxide can be promoted compared to when carbon dioxide is injected.

[0019] When a storage liquid is injected in the method for storing carbon dioxide underground according to the present invention, the storage liquid may have a pH value in the range of 1 to 14. For example, when the storage liquid is alkaline, it has a high effect of promoting the chemical reaction between metal components and carbon dioxide, and can particularly promote the mineral fixation of carbon dioxide.

[0020] In the method for storing carbon dioxide underground according to the present invention, the storage liquid preferably contains a chelating agent. In this case, the effect of promoting the dissolution of mafic and ultramafic rocks underground and / or the effect of promoting carbon dioxide storage and mineral fixation are high. Furthermore, the chelating agent contained in the storage liquid is preferably a biodegradable chelating agent or a thermally decomposable chelating agent. In this case, the chelating agent is environmentally friendly because it is biodegradable or thermally decomposed. The biodegradable chelating agent may be any agent, such as L-glutamic acid diacetate (GLDA). Furthermore, the thermally decomposable chelating agent is preferably one that is decomposed by geothermal heat.

[0021] The carbon dioxide fixation method according to the present invention is characterized in that the solution in which the metal components have been extracted by the method for dissolving metal components in rock according to the present invention is recovered, and the metal components contained in the recovered solution are chemically reacted with carbon dioxide to produce carbonate minerals.

[0022] The carbon dioxide fixation method according to the present invention allows carbon dioxide to be fixed by minerals at the site of recovery, such as the earth's surface, by chemically reacting carbon dioxide with metal components such as Ca, Mg, and Fe contained in the recovered solution. Since the mafic and ultramafic rocks underground after the solution has been recovered have increased pore volume and pore connectivity, making them more permeable, injection of carbon dioxide or a liquid containing dissolved carbon dioxide into the pores allows for the storage of more carbon dioxide than before the solution was injected. [Effects of the Invention]

[0023] The present invention provides a method for dissolving metal components in rocks, which can promote the dissolution of underground mafic rocks such as basalt and ultramafic rocks such as peridotite at temperatures below 200°C and increase permeability, as well as a method for storing carbon dioxide underground and a method for carbon dioxide fixation using the method. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a horizontal cross-sectional view of an underground basalt layer, illustrating a method for dissolving metal components in rock and a method for storing carbon dioxide underground according to an embodiment of the present invention. [Figure 2] 1 is a cross-sectional view showing an experimental device used in an experiment on dissolving basalt with a dissolving solution in a method for dissolving metal components in rock according to an embodiment of the present invention. [Figure 3]Graphs showing (a) the change in pressure difference (differential pressure; pressure at the upstream end face - pressure at the downstream end face) over time between both ends of the sample when the dissolving solution is adjusted to pH 8 with nitric acid, (b) the change in concentration of various elements contained in the wastewater over time, (c) the change in pressure difference between both ends of the sample when the dissolving solution is adjusted to pH 6 with nitric acid, (d) the change in concentration of various elements contained in the wastewater over time, (e) the change in pressure difference between both ends of the sample when the dissolving solution is adjusted to pH 4 with nitric acid, and (f) the change in concentration of various elements contained in the wastewater over time, in an experiment using the experimental apparatus shown in Figure 2 for a method for dissolving metal components in rock according to an embodiment of the present invention. [Figure 4] 3A and 3B are X-ray CT images showing the distribution of isolated pores by volume in a sample before and after an experiment using the experimental apparatus shown in FIG. 2, in accordance with an embodiment of the present invention, of a method for dissolving metal components in rock. [Figure 5] 1 is a graph showing the change over time in the concentration of various elements contained in wastewater in an experiment using the experimental apparatus shown in FIG. 2 for the method for dissolving metal components in rock according to an embodiment of the present invention, in which (a) the dissolution solution was adjusted to pH 8 with nitric acid, (b) the dissolution solution was adjusted to pH 8 with carbon dioxide, and (c) a comparative example was used in which a liquid adjusted to pH 4 by dissolving carbon dioxide in pure water was used. [Figure 6] 1 is a graph showing the concentration of various elements contained in the solution after reaction when the reaction temperatures are (a) 22°C, (b) 50°C, and (c) 100°C in an experiment to investigate the temperature dependence of the dissolution effect of the solution in the method for dissolving metal components in rock according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings and examples. 1 to 6 show a method for dissolving metal components in rocks, a method for storing carbon dioxide underground, and a method for fixing carbon dioxide according to an embodiment of the present invention.

[0026] As shown in Figure 1, in the method for dissolving metal components in rock and the method for storing carbon dioxide underground according to the embodiment of the present invention, first, a well 2 is dug from the surface of the earth to an underground basalt layer 1, and then, according to the method for dissolving metal components in rock according to the embodiment of the present invention, a dissolving solution is injected into the underground basalt layer 1 using the well 2. The dissolving solution is composed of a liquid containing a chelating agent.

[0027] When the dissolution liquid is injected, the dissolution liquid dissolves metal components such as Ca, Mg, and Fe contained in the underground basalt layer 1, allowing these metal components to be extracted into the dissolution liquid. This promotes the dissolution of the underground basalt layer 1. Furthermore, dissolving the basalt layer 1 creates pores in the basalt layer 1, increasing the amount and connectivity of the pores and improving the permeability of the basalt layer 1.

[0028] In Figure 1, the rock into which the dissolving solution is injected is basalt. However, other mafic or ultramafic rocks may also be used. For example, rocks with relatively high porosity, such as basalt or peridotite at temperatures below 200°C, which exist relatively shallow underground or under the seafloor, are preferred. The chelating agent in the dissolving solution is preferably a biodegradable chelating agent, adjusted to a strong to weak acidity of pH 1 to 6 with a pH adjuster such as nitric acid. This enhances the dissolution-promoting effect of the basalt and effectively increases the permeability of the basalt. Furthermore, since the agent is biodegradable underground, it is environmentally friendly.

[0029] Next, the metal components contained in the underground basalt layer 1 are extracted into a solution, and then carbon dioxide or a storage solution containing dissolved carbon dioxide is injected into the basalt layer 1 using an existing well 2 according to a method for storing carbon dioxide underground according to an embodiment of the present invention. At this time, the solution increases the volume and connectivity of the pores in the basalt layer 1, improving permeability, allowing carbon dioxide to be stored in the pores. Furthermore, the metal components, such as Ca, Mg, and Fe, extracted into the solution react chemically with the stored carbon dioxide to produce carbonate minerals, allowing carbon dioxide to be fixed within the pores.

[0030] Furthermore, by injecting carbon dioxide into the basalt layer 1, more carbon dioxide can be stored than when a storage liquid is injected. Furthermore, by injecting a storage liquid into the basalt layer 1, the chemical reaction between the metal components in the solution and the carbon dioxide can be promoted, compared to when only carbon dioxide is injected, and the mineral fixation of carbon dioxide can be promoted. Note that when a storage liquid is used, it is preferable that the storage liquid be a liquid containing carbon dioxide and a biodegradable chelating agent adjusted to a weak to strong alkaline pH of 8 to 14. This further promotes the chemical reaction between the metal components and the carbon dioxide, and further promotes the mineral fixation of carbon dioxide. Furthermore, since it is biodegradable underground, it is environmentally friendly.

[0031] Furthermore, if the chelating agent in the solution is a biodegradable chelating agent, the chelating agent will extract metal components such as Ca, Mg, and Fe from the basalt, and then biodegrade to release the metal components.Therefore, by subsequently injecting carbon dioxide and the storage liquid into the pores of the basalt, the chemical reaction between the released metal components such as Ca, Mg, and Fe and carbon dioxide can be further promoted, and the mineral fixation of carbon dioxide can be further promoted.

[0032] In the example shown in Figure 1, by injecting a solution from well 2, the solution permeates up to position A, increasing the amount and connectivity of pores in basalt layer 1 and enhancing permeability, and by further injecting carbon dioxide or storage liquid from well 2, the carbon dioxide or storage liquid permeates up to position B, enabling carbon dioxide to be stored in the pores of basalt layer 1. If the permeability of basalt layer 1 is dramatically increased by the solution, position B may approach position A and may even overlap.

[0033] The chelating agents in the dissolving solution and the storage solution may be thermally decomposable. In this case, as with biodegradable chelating agents, they can also promote mineral fixation of carbon dioxide and are environmentally friendly.

[0034] The chelating agent in the dissolution solution may be adjusted to a weak acidity or alkalinity (pH of approximately 4 to 10) using carbon dioxide. In this case, simply by injecting the dissolution solution, carbon dioxide can be stored in the pores of the basalt layer 1, whose dissolution has been promoted by the chelating agent, or the carbon dioxide can be chemically reacted with metal components, such as Ca, Mg, and Fe, extracted from the basalt layer 1, to thereby immobilize the carbon dioxide in the mineral. When the solution is adjusted to a weak acidity (pH of approximately 4 to 6) using carbon dioxide, the dissolution promotion effect of the basalt layer 1 is enhanced, particularly increasing permeability, and carbon dioxide can be immobilized in the pores of the basalt layer 1. When the solution is adjusted to a weak alkalinity (pH of approximately 8 to 10) using carbon dioxide, the permeability of the basalt layer 1 is increased, although not as much as when it is weakly acidic, and the mineral immobilization of carbon dioxide injected into the pores of the basalt layer 1 can be further promoted. In either case, carbon dioxide can be stored and fixed in minerals simply by injecting the solution, so after injecting the solution, carbon dioxide or storage solution may be further injected into the basalt layer 1, but it is not necessary to do so.

[0035] Alternatively, metal components contained in a basalt layer 1 underground may be extracted into a solution using a method for dissolving metal components in rock according to an embodiment of the present invention, and then the solution may be recovered from a well other than the existing well 2 using a method for carbon dioxide fixation according to an embodiment of the present invention. The metal components, such as Ca, Mg, and Fe, contained in the recovered solution may then be chemically reacted with carbon dioxide to produce carbonate minerals, thereby sequestrating the carbon dioxide. In this case, the carbon dioxide can be sequestrated at the site where the solution was recovered, such as the surface of the earth. After the solution was recovered, the basalt layer 1 has increased pore volume and pore connectivity, resulting in enhanced permeability. Therefore, by injecting carbon dioxide or a liquid containing dissolved carbon dioxide into the pores, it is possible to store more carbon dioxide than before the solution was injected. In the example shown in Figure 1, the well from which the solution is recovered may be located inside position A, where the solution permeates. [Example]

[0036] As a method for dissolving metal components in rock according to an embodiment of the present invention, an experiment on dissolving basalt using a dissolving solution was conducted. The experiment was conducted using the apparatus shown in Figure 2. In the experiment, a 20 wt% GLDA-Na4 aqueous solution with an adjusted pH was used as the dissolving solution. This dissolving solution is an aqueous solution containing L-glutamic acid diacetate (GLDA), a biodegradable chelating agent, and the GLDA-Na4 aqueous solution is strongly alkaline with a pH of 11 or higher. Porous olivine basalt collected from Daikon Island in Shimane Prefecture was used as the basalt sample 11. The contents of minerals and other components contained in this sample 11 are shown in Table 1. As shown in Figure 2, the sample (Viton-sleeved sample) 11 was formed into a cylindrical shape with a diameter of 25 mm and a height of 25 mm.

[0037] [Table 1]

[0038] As shown in FIG. 2, in the experiment, sample 11 was placed inside pressure vessel 12 so that its central axis was horizontal, and the gap inside pressure vessel 12 was filled with silicone oil 13. Pressure sensors 14 were attached to both end faces 11a and 11b of sample 11 so that the pressure at these two end faces could be measured. A supply pipe 15a and a supply pump 15b were provided to supply liquid to one end face (upstream end face) 11a of sample 11, and a discharge pipe 16a and an adjustment valve 16b were provided to discharge liquid from the other end face (downstream end face) 11b of sample 11. Thermocouples 17 were provided to measure the temperature at one end face 11a of sample 11 and inside pressure vessel 12, respectively. A silicone oil pump 18 was provided to adjust the pressure inside pressure vessel 12. A heating and heat-retaining device (not shown) was provided to heat pressure vessel 12 from the outside.

[0039] In the experiment, the temperature inside the pressure vessel 12 was maintained at 100°C using a heating and insulation device, and the pressure inside the pressure vessel 12 was maintained at 6 MPa using a silicone oil pump 18. Under these conditions, pure water was first supplied to the sample 11 at a flow rate of 0.1 mL / min through the supply pipe 15a using the supply pump 15b, and then drained through the drain pipe 16a, allowing the water to circulate inside the sample 11. The drain pressure (back pressure) at the other end face 11b of the sample 11 was adjusted to 1 MPa using the control valve 16b. After the flow rate and back pressure stabilized, the pure water was replaced with the solvent, and the solution was allowed to flow at the same flow rate and back pressure for 6 hours. During the experiment, the pressure at both end faces 11a and 11b of the sample 11 was measured using a pressure sensor 14. When the pure water was replaced with the solvent and every 30 minutes thereafter, the wastewater was sampled, and the concentrations of various elements contained in the wastewater were measured.

[0040] Figure 3 shows the experimental results when the pH of the dissolving solution was adjusted to 8, 6, and 4 using nitric acid. Figures 3(a), (c), and (e) show the time change in the differential pressure between the upstream end face 11a and the downstream end face 11b of the sample 11 at each pH, while Figures 3(b), (d), and (f) show the time change in the concentration of various elements and the pH of the wastewater. Note that the pressure difference temporarily becomes zero around 0 minutes in Figures 3(a), (c), and (e), and then returns to nearly the original pressure difference when the solution was switched to.

[0041] As shown in Figure 3(a), at pH 8, the pressure difference gradually increased after switching to the solvent because the viscosity of the solvent was higher than that of pure water. However, after approximately 100 minutes, the pressure difference became almost constant. Since the pressure difference remained constant, it is believed that the permeability of Sample 11 remained almost constant. As shown in Figure 3(c), at pH 6, the pressure difference gradually increased after switching to the solvent, as in Figure 3(a). However, after approximately 100 minutes, the pressure difference gradually decreased. Since the pressure difference decreased to approximately one-third, it is believed that the permeability of Sample 11 was approximately three times higher than that at pH 8 in Figure 3(a). As shown in Figure 3(e), at pH 4, the increase in pressure difference as in Figures 3(a) and (c) was not observed, and it was confirmed that the pressure difference decreased significantly after approximately 100 minutes. Based on the decrease in pressure difference, it is believed that the permeability of Sample 11 was more than 11 times higher than that at pH 8 in Figure 3(a).

[0042] As shown in Figure 3(b), at pH 8, Fe, Ca, and K were primarily extracted into the wastewater. As shown in Figure 3(d), at pH 6, Mg and Si were also extracted in addition to Fe, Ca, and K. Furthermore, a greater amount of Fe was extracted compared to the pH of 8 in Figure 3(b). As shown in Figure 3(f), at pH 4, Fe, Mg, and Si were primarily extracted into the wastewater, with a smaller amount of Ca also extracted. Furthermore, a greater amount of Mg was extracted compared to the pH of 6 in Figure 3(d). Thus, it was confirmed that elements such as Ca, Mg, and Fe, which can react with carbon dioxide to produce carbonate minerals, were extracted using solutions with pHs between 4 and 8. It was also confirmed that the lower the pH, the shorter the time it took for each element to be extracted.

[0043] From the results shown in Figure 3, it can be said that the lower the pH, the faster the dissolution rate of minerals by the dissolving solution, and the greater the effect of promoting the dissolution of basalt. It can also be said that the lower the pH, the greater the effect of increasing the permeability of basalt. From this, it can be said that the lower the pH of the dissolving solution, the less injection pressure and shorter the injection time can be used when injecting carbon dioxide or storage liquid into basalt with increased permeability, thereby reducing the energy required for injection and improving the safety of the injection work.

[0044] Next, the three-dimensional spatial distribution of sample 11 before and after the experiment at pH 6 was determined using X-ray CT. From the determined three-dimensional spatial distribution, isolated pores in sample 11 were extracted and classified by volume, as shown in Figure 4. Comparing the before-experiment image shown in Figure 4(a) with the after-experiment image shown in Figure 4(b), it was confirmed that the volume of isolated pores had decreased after the experiment, as shown in the circled area in the figure.

[0045] The total porosity was calculated from the 3D spatial distribution using X-ray CT. It was 18.2% before the experiment and 18.4% after, confirming a 0.2% increase in pore volume. Furthermore, the proportion of isolated pores in the total pore volume was 17.6% before the experiment and 12.2% after, confirming a 5.4% decrease in isolated pore volume. From these results, the proportion of connected pores (i.e., voids) was calculated as 15.0% (= 18.2% × (100% - 17.6%)) before the experiment and 16.2% (= 18.4% × (100% - 12.2%)) after the experiment. Therefore, the amount of connected pores increased by 8% (= (16.2% / 15.0%) - 1) after 6 hours of chelating agent-containing solution flow.

[0046] The results of the X-ray CT analysis confirmed that the amount of pore space and the connectivity of the pores can be increased in a relatively short time (6 hours) by using a solution containing a chelating agent, which allows for more space for carbon dioxide storage when carbon dioxide or storage fluid is injected. [Example]

[0047] A basalt dissolution experiment was conducted in the same manner as in Example 1, using the experimental apparatus shown in Figure 2, with a 20 wt% GLDA-Na4 aqueous solution adjusted to pH 8 with carbon dioxide as the dissolution solution. As a comparative example, a similar experiment was conducted using pure water with carbon dioxide dissolved therein and adjusted to pH 4 instead of the dissolution solution. The liquid used in this comparative example corresponds to the storage liquid used in Non-Patent Document 1, a conventional storage method. The results of these experiments are shown in Figures 5(b) and (c), respectively. For comparison, Figure 5(a) shows the results of the experiment in Example 1, where the dissolution solution was adjusted to pH 8 with nitric acid. Figure 5(a) shows the scale of the concentration of each element on the vertical axis of Figure 3(b), adjusted to Figures 5(b) and (c).

[0048] As shown in Figure 5(b), even though the dissolving solution was weakly alkaline and contained carbon dioxide, it was confirmed that the minerals in the basalt were dissolved and each metal element was eluted. It was also confirmed that the extraction rate of Ca, Mg, and Fe in Figure 5(b) was slightly higher than in Figure 5(a), and the extracted amounts were also slightly larger. It was also confirmed that the extraction rate of each element in Figure 5(b) was higher than in the comparative example in Figure 5(c), and the extracted amounts were also larger.

[0049] These results suggest that by absorbing carbon dioxide into a solution containing a chelating agent and adjusting it to a weak alkaline solution, elements such as Ca, Mg, and Fe can be extracted, and the chemical reaction between these metal elements and the carbon dioxide contained in the solution can promote mineral fixation of carbon dioxide. Furthermore, by absorbing carbon dioxide into a solution containing an alkaline chelating agent, the carbon dioxide content per unit volume can be increased compared to the conventional method of dissolving carbon dioxide in pure water, as in the comparative example in Figure 5(c), and therefore the rate of mineral fixation of carbon dioxide can be increased. [Example]

[0050] An experiment was conducted to investigate the temperature dependence of the dissolution effect of the dissolution solution. In the experiment, 2.5 g of basalt powder and 50 mL of the dissolution solution were placed in a sealed reaction vessel. The inside of the reaction vessel was pressurized to 1 MPa with nitrogen gas, and the reaction was carried out at temperatures of 22°C, 50°C, and 100°C for one hour. The solution was then removed from the reaction vessel, and the concentrations of various elements contained therein were measured.

[0051] In the experiments, the dissolving solutions used were a 20 wt% GLDA-Na4 aqueous solution adjusted to pH 8 with carbon dioxide, and a 20 wt% GLDA-Na4 aqueous solution adjusted to pH 4 with nitric acid. As a comparative example, a similar experiment was conducted using pure water with carbon dioxide dissolved therein to adjust the pH to 4 instead of the dissolving solution. The experimental results at each reaction temperature are shown in Figures 6(a) to 6(c).

[0052] As shown in Figures 6(a) to (c), in the comparative example, even at an acidic pH of 4, the extracted concentration of elements was low regardless of the reaction temperature. However, when a dissolving solution containing a chelating agent was used, each metal element was extracted even at temperatures below 100°C or with a weakly alkaline dissolving solution, confirming a high mineral dissolving effect. In particular, as shown in Figure 6(a), elements such as Ca were extracted even at room temperature, confirming a high mineral dissolving effect compared to the comparative example. Furthermore, when a dissolving solution containing a chelating agent was used, the extracted concentration of each metal element tended to be higher at higher reaction temperatures, confirming a high mineral dissolving effect. [Explanation of symbols]

[0053] 1 basalt layer 2. Well 11 Sample 11a (one side, upstream) end face 11b (the other, downstream) end face 12 Pressure vessels 13 Silicone oil 14 Pressure Sensor 15a Supply pipe 15b Supply pump 16a Discharge pipe 16b Regulating valve 17 Thermocouple 18 Silicone oil pump

Claims

1. A method for dissolving metal components in rock, comprising the steps of injecting an acidic dissolving solution containing a liquid containing a chelating agent into underground mafic or ultramafic rock at a temperature below 200°C, thereby extracting metal components contained in the mafic or ultramafic rock into the dissolving solution.

2. 2. The method for dissolving metal components in rock according to claim 1, wherein the chelating agent is a biodegradable chelating agent or a thermally decomposable chelating agent.

3. 2. The method for dissolving metal components in rock according to claim 1, wherein the dissolving solution comprises a liquid containing the chelating agent and carbon dioxide.

4. 2. The method for dissolving metal components in rocks according to claim 1, wherein the dissolving solution is injected into basalt or peridotite as the mafic rock or ultramafic rock underground.

5. A method for storing carbon dioxide underground, comprising injecting carbon dioxide or a storage liquid containing dissolved carbon dioxide into the mafic rock or ultramafic rock after extracting the metal components into the solution by the method for dissolving metal components in rock according to any one of claims 1 to 4.

6. 6. The method for storing carbon dioxide underground according to claim 5, wherein the storage liquid is alkaline.

7. 6. The method for storing carbon dioxide underground according to claim 5, wherein the storage liquid contains a chelating agent.

8. 8. The method for storing carbon dioxide underground according to claim 7, wherein the chelating agent contained in the storage liquid is a biodegradable chelating agent or a chelating agent that can be decomposed by heat.

9. 5. A method for fixing carbon dioxide, comprising recovering a solution from which metal components have been extracted by the method for dissolving metal components in rock according to claim 1, and chemically reacting the metal components contained in the recovered solution with carbon dioxide to produce carbonate minerals.

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