Co2 mineralization promoter

Abstract

[PROBLEM] To provide a mineralization promoter that contributes to the promotion of mineralization of CO2. [SOLUTION] This CO2 mineralization promoter contains silica sol that includes silica particles (a) having an average secondary particle size of 5-200 nm according to dynamic light scattering, and an aqueous medium (b).

Description

CO2 mineralization promoter

[0001] The present invention is ２ This relates to mineralization promoters.

[0002] CO ２ Carbon dioxide capture and storage (CCS) technology is a technology that captures and stores CO2 emitted from sources such as power plants and steelworks. ２ This technology separates and captures CO and stores it underground or underwater. ２ This prevents CO2 from being released into the atmosphere and is considered an important option for global warming countermeasures. ２ Deep underground aquifers (such as sandstone layers containing many voids) are considered to be promising storage locations for CO, and for example, a geological storage system using deep aquifers has been proposed, as shown in Patent Document 1. Patent Document 2 also proposes the storage of CO in an unstructured reservoir that does not have a sealed structure as an underground storage target layer. ２ CO storage achieved ２ Proposals have been made for underground storage facilities.

[0003] CO injected into a saline aquifer (reservoir) deep underground ２ CO displaces the formation water in the reservoir and spreads, and is trapped in the reservoir by four mechanisms: structural trapping, residual gas trapping, dissolution trapping, and mineral trapping. ２ The formation water containing dissolved cations reacts chemically with rock minerals, and the cations (Ca ２＋ , Mg ２＋ etc.) and CO ２ The mineral traps where CO reacts (carbonation) and is fixed underground as secondary minerals such as calcium carbonate and magnesium carbonate are the most stable form of CO. ２ However, as described in Non-Patent Document 1, the injected CO ２However, the drawback is that it is predicted to take 100-1000 years for the rocks to reach mineral traps. Non-Patent Document 2 shows that if the geological layer is rich in rocks containing basic minerals such as basalt, the time required to reach mineral traps can be significantly reduced. On the other hand, Non-Patent Document 3 shows that the supercritical CO ２ It has been shown that the dissolution of basalt is promoted in the presence of chlorine, but not at pH 5-6. Furthermore, Non-Patent Document 4 shows that the dissolution reaction of minerals is affected by the diffusion of metal ions.

[0004] JP 2008-307483 A JP 2011-147869 A

[0005] Nature Reviews Earth & Environment volume 1 (2020) 90-102Science 352 (6291) 1312-1314Journal of MMIJ 128 (2012) 94-102International Journal of Coal Science&Technology volume 12, article number 15 (2025)

[0006] As shown in Non-Patent Document 2, if the rocks containing basic minerals are abundant, CO ２ It is expected that the number of years required to reach mineral trapping due to mineralization of CO2 can be significantly reduced. ２ As mineralization progresses, the cations (metal ions) dissolved in the formation water in the reservoir are consumed, and it is expected that the progress of mineralization will slow down. ２ However, this reaction is slow above a neutral pH range, and if the reaction proceeds beyond a certain level and the pH in the system rises, the reaction slows down, preventing the supply of cations (metal ions), which slows mineralization. Furthermore, if the diffusion of cations (metal ions) in the formation water is slow, the dissolution reaction of minerals will be delayed, as shown in Non-Patent Document 4. Therefore, (although not proven) CO ２ It is also thought that this could be a cause of delays in mineralization itself.

[0007] In view of the above circumstances, the present inventors have conducted extensive research and have focused on the interaction between silanol groups on the surface of silica particles and positive charges (cations). ２ It was found that this contributes to the promotion of mineralization.

[0008] That is, the present invention provides, as a first aspect, ２ The present invention relates to a mineralization accelerator, wherein the silica sol contained in the mineralization accelerator contains silica particles (a) having an average secondary particle diameter of 5 to 200 nm as measured by a dynamic light scattering method, and an aqueous medium (b). ２ The present invention relates to the mineralization accelerator according to the first aspect, wherein in Mineral Adsorption Test-1, a mixture obtained by adding the mineralization accelerator to a saltwater solution of pH 3.0 containing 8% by mass of calcium carbonate and 2% by mass of NaCl at a concentration such that the silica particle concentration becomes 1% by mass is stored at 60°C for 1 hour, and the mineral adsorption rate is evaluated, the mineral adsorption rate (%) calculated by the following formula 1 from the Ca concentration in the mixture before storage and the Ca concentration in the filtrate obtained by ultrafiltration of the mixture after storage is 0.1 to 30%. [Mathematical Formula 1] Mineral adsorption rate (%) = {[(Ca concentration in the mixture before storage) - (Ca concentration in the filtrate of the mixture after storage)] / (Ca concentration in the mixture before storage)} × 100 As a third aspect, ２ The present invention relates to the mineralization accelerator according to the first aspect, wherein in a mineral adsorption test-2 in which the mineralization accelerator is added to a saltwater solution of pH 3.0 containing 8% by mass of PEG and 2% by mass of NaCl at a concentration such that the silica particle concentration becomes 1% by mass, the mixture is stored at 25°C for 1 hour, and the silica particle size in the mixture after storage is evaluated, the ratio (DLS average particle size / BET particle size) of the average secondary particle size (DLS average particle size) of the silica particles contained in the mixture after storage measured by dynamic light scattering to the specific surface area diameter (BET particle size) of the silica particles contained in the mineralization accelerator measured by nitrogen adsorption is 2.0 or more and 5.0 or less. As a fourth aspect, the present invention relates to the mineralization accelerator according to the first aspect, wherein the silica particles are added to a saltwater solution of pH 3.0 containing 8% by mass of PEG and 2% by mass of NaCl at a concentration such that the silica particle concentration becomes 1% by mass, the mixture is stored at 25°C for 1 hour, and the silica particle size in the mixture after storage is evaluated Ｎ２ ) is 25 to 550 m ２As a fifth aspect, the present invention relates to the mineralization accelerator according to the first aspect, wherein a silica particle powder obtained by removing components unbonded to the silica particles by ultrafiltration and then heating and drying the resulting mineralization accelerator to remove the aqueous medium has a silanol group amount of 3.5 to 20 mmol / g as calculated by the following formula 2 from the mass loss when heated from 25°C to 600°C in thermogravimetric analysis and the molecular weight of water molecules: [Formula 2] Amount of silanol groups (mmol / g)=2×(M2−M1)÷M Ｈ２Ｏ ÷ (M0) × 1000 (where M0 is the mass of the silica particles subjected to thermogravimetric analysis at 25°C or less before heating, M1 is the mass loss of the silica particles when the temperature reaches 200°C, M2 is the mass loss of the silica particles when the temperature reaches 600°C, and M Ｈ２Ｏ represents the molecular weight of a water molecule.) As a sixth viewpoint, ２９ The present invention relates to the mineralization accelerator according to the first aspect, wherein when the peak area from −136 ppm to −82 ppm in a Si-NMR spectrum is PA, the peak area from −116 ppm to −106 ppm is PA4, the peak area from −105.5 ppm to −96 ppm is PA3, and the peak area from −95 ppm to −85 ppm is PA2, the silica particles have a silanol group amount (%) calculated by the following formula 3 of 30 or less: [Equation 3] Amount of silanol groups (%) = ((PA2 / PA) x 100) x 2 + ((PA3 / PA) x 100) x 1 + ((PA4 / PA) x 100) x 0 As a seventh aspect, the present invention relates to the mineralization accelerator according to the first aspect, wherein at least a portion of the silica particles is coated with a silane compound (c) having a hydrophilic organic group, and the hydrophilic organic group is an epoxy group-containing organic group, an amino group-containing organic group, a hydroxy group-containing organic group, or a carboxy group-containing organic group. As an eighth aspect, the present invention relates to the mineralization accelerator according to the first aspect, wherein at least a portion of the silica particles is coated with a silane compound (c) having a hydrophilic organic group, and the hydrophilic organic group is an epoxy group-containing organic group, an amino group-containing organic group, a hydroxy group-containing organic group, or a carboxy group-containing organic group. ２In a salt tolerance test in which a mixture obtained by adding the mineralization accelerator to a saltwater solution of pH 3.0 containing 8% by mass of CaCl and 2% by mass so that the silica particle concentration becomes 1% by mass is stored at 60°C for 7 days, the ratio of the DLS average particle size of the mixture after storage to the DLS average particle size of the mixture before storage (DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage) is 0.9 to 3.0. ２ and 2% by mass of NaCl, and the mineralization accelerator was added to a concentration such that the silica particle concentration was 1% by mass. ２ The present invention relates to the mineralization accelerator according to the first aspect, wherein, in a high-pressure salt resistance test in which CO is injected into the mixture at 8 MPa and the mixture is stored at 60°C and 8 MPa for 7 days, the ratio of the DLS average particle size of the mixture after storage to the DLS average particle size of the mixture before storage (DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage) is 2.4 or less. ２ The mineralization promoting method of claim 1, wherein the mineralization promoting agent according to the first aspect or a diluted solution of the mineralization promoting agent adjusted to a solids concentration of 0.001 to 30 mass % in an aqueous medium is mixed with CO ２ and a step of injecting the mineral into a subterranean formation before or after the injection of the mineral into the subterranean formation.

[0009] The mineralization accelerator of the present invention adsorbs minerals (cations) such as Ca in the system by the cation adsorption action of the silanol groups (anionic) on the surface of the silica particles, thereby promoting the formation of carbonates on the surface of the silica particles, i.e., CO ２ Furthermore, the mineralization accelerator of the present invention uses silica particles with an average secondary particle size of 5 to 200 nm as measured by dynamic light scattering, i.e., does not contain coarse particles, and therefore can penetrate without blocking pores even when injected into underground bedrock, and adsorbs minerals (cations) such as Ca dissolved and released from basic minerals contained in the rocks that make up the pores, thereby absorbing CO ２ It is expected that this will contribute to the mineralization of the

[0010] The present inventors have developed a three-stage CO trapping system to address the issue of promoting mineral trapping (mineralization) in CCS technology. ２ The mineralization mechanism of CO ２ 2) Minerals (Ca) formed by dissolving rocks and minerals in the acid produced by the carbonate ionization. ２＋ , Mg ２＋ , Fe ２＋ The present inventors focused on a three-step mechanism: 2) dissolution and diffusion of minerals (cations, etc.), and 3) the formation of carbonates (minerals) through the binding of dissolved and diffused minerals (cations) with carbonate ions. They then noted that 2) in the mineral diffusion stage, mineral diffusion is rate-limiting for carbonate formation, especially in geological formations with low permeability and small pore sizes, and 3) in the carbonate (mineral) formation stage, the formation of nuclei carbonate particles is rate-limiting for carbonate growth (mineralization). On the other hand, the formation (precipitation) of carbonates and their dissociation (dissolution) are reversible reactions, and the readily soluble nature of the formed carbonates hinders carbonate formation and mineralization. The present inventors then considered the use of particles that could serve as pseudo-nuclei, and concluded that if the pseudo-nuclei particles could adsorb minerals (cations) and saturate the minerals (cations), they could be expected to promote mineral (cation) diffusion, promote carbonate formation, and further promote mineralization. They then proceeded to use silica particles with mineral adsorption capacity, which led to the completion of the present invention.

[0011] That is, the present invention is ２ The present invention is directed to a mineralization accelerator comprising (a) silica particles having an average secondary particle diameter of 5 to 200 nm as measured by dynamic light scattering, and (b) a silica sol containing an aqueous medium. As described above, the silica particles of the present invention are particles that adsorb minerals (cations). At least a portion of the (a) silica particles may be coated with (c) a silane compound having a hydrophilic organic group, as described below.

[0012] The mineral adsorption performance of the mineralization accelerator of the present invention can be evaluated by the following <Mineral Adsorption Evaluation Test 1> and <Mineral Adsorption Evaluation Test 2>. As mentioned above, the adsorbed minerals form carbonates, which in turn contribute to the mineralization of carbonates. Therefore, those with high mineral adsorption performance are considered to have high CO ２ The dispersion stability of the silica particles constituting the mineralization accelerator of the present invention can be evaluated by a salt tolerance test (stability test 1 in a geological formation environment) and a high-pressure salt tolerance test (stability test 2 in a geological formation environment). ２ This simulates the formation water (pH 3 to 4) after dissolution of chlorine.

[0013] <Mineral Adsorption Evaluation Test 1> In the mineral adsorption evaluation test 1, the target mineralization accelerator is stored in a saltwater environment containing minerals, and the amount of minerals adsorbed onto the silica particles constituting the mineralization accelerator is evaluated. ２ The mineralization accelerator is added to a saltwater solution containing 8% by mass of calcium carbonate and 2% by mass of NaCl and having a pH of 3.0 at a concentration such that the silica particle concentration becomes 1% by mass, and the mixture is stored at 60°C for 1 hour. In order to adjust the saltwater to pH 3.0, an inorganic acid that does not easily form precipitates with the mineral components contained in the saltwater, such as hydrochloric acid, can be used. ２＋In the case where the silica particles have adsorbed calcium carbonate, the amount of minerals adsorbed to the silica particles can be evaluated by filtering the silica particles (with minerals adsorbed) after storage with the amount of minerals in the mixture before storage and comparing the amount of minerals in the filtrate. The method for filtering the silica particles is not particularly limited as long as it can filter out the silica particles, but for example, an ultrafiltration filter with a molecular weight cutoff of 100,000 can be used. That is, the mineral adsorption ability, and therefore the mineralization promotion ability, of the mineralization accelerator according to the present invention can be evaluated based on the mineral adsorption rate (%) calculated using the following formula from the Ca concentration in the mixture before storage and the Ca concentration in the filtrate after ultrafiltration of the mixture after storage. [Mathematical Expression 1] Mineral adsorption rate (%) = {[(Ca concentration in the mixture before storage) - (Ca concentration in the filtrate of the mixture after storage)] / (Ca concentration in the mixture before storage)} x 100 In the mineralization accelerator of the present invention, the mineral adsorption rate (%) is preferably 0.1 to 30%, 0.1 to 20%, 0.1 to 15%, 0.1 to 10%, or 1 to 10%. Note that although a higher mineral adsorption rate can be expected to have a greater mineralization-accelerating ability, if it is too high, i.e., if the amount of adsorption becomes too great, it may induce aggregation of the silica particles and impair dispersibility, and therefore it is desirable to keep it at around 30% or less.

[0014] <Mineral Adsorption Test Evaluation 2> The mineralization accelerator according to the present invention can be evaluated as a change in particle size to determine whether or not the silica particles adsorb minerals in a saltwater environment containing minerals, based on the ratio between the primary particle size of the silica particles in the target mineralization accelerator and the secondary particle size of the silica particles constituting the mineralization accelerator in a saltwater environment containing minerals similar to that in the <Mineral Adsorption Evaluation Test 1>. ２The mineralization accelerator is added to a pH 3.0 saltwater solution containing 8% by mass of silica and 2% by mass of NaCl at a concentration of 1% by mass of silica particles, and the mixture is stored at 25°C for 1 hour. After storage, the mineral adsorption capacity of the mineralization accelerator according to the present invention, and therefore its mineralization promotion ability, can be evaluated by the ratio (DLS average particle size / BET particle size) of the average secondary particle size (DLS average particle size) of the silica particles contained in the mixture measured by dynamic light scattering to the specific surface area diameter (BET particle size) of the silica particles contained in the mineralization accelerator measured by nitrogen adsorption. For the mineralization accelerator of the present invention, this ratio is preferably 2.0 to 5.0, or 2.0 to 3.0. A ratio exceeding 5.0 is undesirable, since it reflects silica particle aggregation rather than mineral adsorption.

[0015] <Salt Tolerance Test (Stability Evaluation Test 1 in a Geological Formation Environment)> Furthermore, the mineralization accelerator according to the present invention can be evaluated for its maintenance of particle size by storing the target mineralization accelerator in a saltwater environment containing minerals at 60°C for 7 days and evaluating the particle size of the mineralization accelerator (silica particles) before and after storage. ２The mineralization accelerator is added to a saltwater solution of pH 3.0 containing 8% by mass of HCl and 2% by mass of NaCl at a concentration such that the silica particle concentration becomes 1% by mass, and the mixture is stored at 60°C for 7 days. The average secondary particle size of the silica particles in the mixture is measured by dynamic light scattering before and after storage. The particle size retention of the mineralization accelerator (stability in a geological formation environment) is evaluated based on the ratio of the average secondary particle size of the silica particles after storage to the average secondary particle size of the silica particles before storage (DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage). In the mineralization accelerator of the present invention, the ratio <DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage> is preferably 0.9 to 3.0, 0.9 to 2.0, or 0.9 to 1.5. If the ratio <DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage> is too large, it means that silica particles will aggregate together, and in this case, when the mineralization accelerator is injected into an underground reservoir, it may cause blockage of gaps in the underground layer or pores in the rock. By making the ratio 0.9 to 3.0, even when the mineralization accelerator of the present invention is injected into underground rock, it can be expected to penetrate without blocking gaps, and ultimately the mineralization accelerator can be expected to reduce CO ２ It is expected that this will have the ability to promote mineralization.

[0016] <High-pressure salt resistance test (stability evaluation test in geological environment 2)> Also, supercritical CO ２ In anticipation of mineralization promotion in an environment, it is desirable that the particle size of the silica particles constituting the mineralization promoter of the present invention is maintained even in that environment. ２ and 2% by mass of NaCl, and the mineralization accelerator was added to a concentration such that the silica particle concentration was 1% by mass. ２The mixture is pressurized at 8 MPa and stored at 60°C and 8 MPa for 7 days. The average secondary particle size of the silica particles in the mixture is measured by dynamic light scattering before and after (high-pressure) storage, and the particle size retention of the mineralization accelerator under high pressure (stability in a geological formation environment) is evaluated based on the ratio of the average secondary particle size of the silica particles after storage to the average secondary particle size of the silica particles before storage, i.e., <DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage>. For the mineralization accelerator of the present invention, the ratio <DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage> is preferably 2.4 or less (change in average particle size: 140% or less) or 1.5 or less (change in average particle size: 50% or less) before and after storage under the above-mentioned high-pressure conditions. The lower limit of this ratio can be, for example, 0.9 or more. By having the ratio of 2.4 or less, when the mineralization accelerator of the present invention is injected into underground bedrock, it is possible to reduce the particle size retention of the mineralization accelerator under high pressure by supercritical CO 2 . ２ Since the silica particles remain dispersed even when the mineralization promoter comes into contact with CO, the mineral adsorption effect can be maintained. ２ It is expected that this will have the ability to promote mineralization.

[0017] As the silica sol containing the (a) silica particles and the (b) aqueous medium, for example, an aqueous silica sol can be used. The aqueous silica sol is a colloidal dispersion system in which an aqueous solvent (i.e., the (b) aqueous medium) is used as a dispersion medium and colloidal silica particles (i.e., the (a) silica particles) are used as a dispersoid, and can be produced by a known method using water glass (aqueous sodium silicate solution) as a raw material. The (b) aqueous medium in the present invention is usually water, and examples thereof include ordinary industrial water, deionized water, and distilled water.

[0018] In the silica sol containing (a) silica particles and (b) an aqueous medium, which is the mineralization accelerator of the present invention, the (a) silica particles have an average secondary particle diameter (DLS average particle diameter (also referred to as DLS particle diameter): Z-average particle diameter, harmonic mean particle diameter) in the silica sol as measured by dynamic light scattering (DLS) in the range of 5 to 200 nm, and can be, for example, 5 to 100 nm, 10 to 100 nm, or 15 to 80 nm. The average particle diameter as measured by dynamic light scattering (DLS) represents the average value of secondary particle diameters (dispersed particle diameters), and the DLS average particle diameter in a state where particles are completely dispersed in a medium is said to be about twice the average primary particle diameter (which is the specific surface area diameter obtained by measurement by nitrogen adsorption (BET) method or Sears method, and represents the average value of primary particle diameters), as described below. In the medium, silica particles are usually dispersed in the form of clumps of several particles (secondary particles), and if the DLS average particle size is about twice the average primary particle size, it can be determined that the particles are dispersed without agglomeration, and the larger the DLS average particle size, the more likely it is that the silica particles in the medium are in an agglomerated state. By making the DLS average particle size larger than 5 nm, the particles will be more stable in the silica sol without agglomeration, and by making the average particle size smaller than 200 nm, it can be expected that the particles will easily penetrate into gaps in underground layers and pores in rocks.

[0019] The DLS average particle size value is the (initial) value before the aforementioned <Mineral Adsorption Evaluation Test 1> and the like are conducted, but the silica particles in the mineralization accelerator according to the present invention are particles with excellent dispersion stability, and even after the above-mentioned salt tolerance test (Stability Evaluation Test 1 in a Geological Formation Environment) and high-pressure salt tolerance test (Stability Evaluation Test 2 in a Geological Formation Environment), the ratio of particle sizes before and after the test can be up to about 5, i.e., the DLS average particle size value can be approximately within the above-mentioned numerical range (range of 5 to 200 nm).

[0020] As mentioned above, particles with a large DLS average particle size are undesirable because they may clog gaps in the underground layer. Therefore, it is preferable to use a silica sol that does not contain coarse particles, for example, one in which the cumulative particle size distribution D90 of the average particle size (DLS average particle size) measured by dynamic light scattering of silica particles is 5 to 200 nm, 5 to 150 nm, 5 to 100 nm, or 5 to 70 nm. Furthermore, it is preferable that the silica particles contained in the silica sol have salt resistance such that, when they come into contact with saltwater contained in underground layers, the silica particles do not aggregate to an extent that exceeds the DLS average particle size or D90 range. The D90 is the particle size that represents the cumulative 90% of the cumulative particle size distribution from the fine particle side. The cumulative particle size distribution can be obtained, for example, by dynamic light scattering or image analysis. For example, the cumulative particle size distribution value can be measured using a dynamic light scattering particle size analyzer. Methods for analyzing the D value include the number distribution method and the volume distribution method. The number distribution method regards particles as perfect circles with the same area as the particle itself, and measures the percentage of particles with a specific particle diameter. The volume distribution method, on the other hand, assumes that if the particle density is constant, volume and weight are proportional, and measures the mass percentage of particles with a specific particle diameter in a given amount of sample. As an example, it is preferable to determine the D value (D90) using the volume distribution method.

[0021] In the silica sol containing (a) silica particles and (b) an aqueous medium used in the present invention, the average primary particle diameter of the silica particles can be, for example, 5 to 100 nm, and can also be, for example, 5 to 70 nm, 5 to 60 nm, 5 to 50 nm, or 10 to 50 nm. Unless otherwise specified, the average primary particle diameter of the silica particles can be the specific surface area diameter or the Sears method particle diameter obtained by measurement using a nitrogen adsorption method (BET method). The specific surface area diameter (average particle diameter (specific surface area diameter) D (nm)) obtained by measurement using a nitrogen adsorption method (BET method) is calculated by multiplying the specific surface area S (m ２From the particle size (D / g), the particle size is given by the formula D(nm) = 2720 / S. The Sears particle size refers to the average particle size measured based on the method described in the literature as "A rapid method for measuring colloidal silica particle size" by G. W. Sears, Anal. Chem. 28(12), 1981, p. 1956. In detail, when 1.5 g of SiO ２ The specific surface area of ​​the colloidal silica was determined from the amount of 0.1N-NaOH required to titrate colloidal silica equivalent to 100 ppm from pH 4 to pH 9, and the equivalent diameter (specific surface area diameter) was calculated from this.

[0022] <Specific surface area by nitrogen adsorption (S Ｎ２ )> The silica particles according to the present invention have a specific surface area (S Ｎ２ ) is, for example, 25 to 550 m ２ / g, or 25 to 300 m ２ / g, or 25 to 250 m ２ / g, or 40 to 550 m ２ / g, or 40 to 250 m ２ The specific surface area (S Ｎ２ ) for 25m ２ / g or more, mineral components can be adsorbed onto the surface of the silica particles, thereby achieving a mineralization promoting effect. Ｎ２ ) for 550m ２ By setting the pore size at 1 / g or less, aggregation of silica particles can be reduced.

[0023] <Silanol Group Amount (mmol / g) Obtained by Thermogravimetric Analysis> The silica particles constituting the mineralization accelerator of the present invention can have a predetermined silanol group amount (mmol / g). That is, the mineralization accelerator containing the silica particles is ultrafiltered to remove components not bonded to the silica particles, and then the resulting powder is heated and dried to remove the aqueous medium. The resulting silica particle powder has a silanol group amount, calculated by the following formula from the mass loss upon heating from 25°C to 600°C in thermogravimetric analysis and the molecular weight of water molecules, of, for example, 3.5 to 20 mmol / g, 3.5 to 15 mmol / g, 3.5 to 10 mmol / g, or 3.8 to 10 mmol / g. [Mathematical Formula 2] Silanol Group Amount (mmol / g) = 2 × (M2 - M1) ÷ M Ｈ２Ｏ ÷ (M0) × 1000 where M0 is the mass of the silica particle powder subjected to thermogravimetric analysis at 25°C or less before heating, M1 is the mass loss amount when the temperature reached 200°C, M2 is the mass loss amount when the temperature reached 600°C, and M Ｈ２Ｏ By setting the silanol capacity (mmol / g) of the silica particles to a range of 3.5 to 20 mmol / g, it is possible to reduce the condensation and aggregation of the silanol groups on the silica particle surfaces between silica particles, while still achieving the adsorption effect of minerals.

[0024] To obtain silica particle powder by the above-mentioned heat drying, the target mineralization accelerator is first ultrafiltered, and the ultrafiltration is repeated as necessary to remove components unbonded to the silica particle powder. The component after ultrafiltration (filtered material: silica sol) is recovered and heated and dried, for example, on a hot plate or in an oven at 120°C to 150°C to remove the aqueous medium (water) that serves as the dispersion medium, thereby obtaining the desired silica particle powder. Note that silanol groups are present on the surface of the silica particles, and these silanol groups tend to adsorb water through hydrogen bonding. Therefore, to quantify the amount of silanol groups in the silica particles from the mass loss due to heating, it is preferable to heat the silica particles to approximately 200°C, at which the adsorbed water is desorbed, as described above.

[0025] < ２９Amount of silanol groups (%) obtained from Si-NMR spectrum> In this specification, ２９ The amount of silanol groups (%) obtained from the Si-NMR spectrum represents the amount of silanol groups present in all silicon atoms of the Q2 to Q4 structures present in the silica particles. ２９ The amount of silanol groups obtained from Si-NMR spectra indicates the amount of silanol groups present throughout the silica particle (particle surface and particle interior), i.e., the evaluation includes not only silanol groups deep within the particle that are thought not to contribute to the promotion of mineralization, but also isolated silanol groups on the particle surface that cannot be fully analyzed by thermogravimetric analysis (dehydration condensation does not occur because silanol groups are not adjacent). On the other hand, the amount of silanol groups obtained from the above-mentioned thermogravimetric analysis indicates the amount of silanol groups that are located in positions where water molecules can be released from the silica particle, i.e., it can be considered that the evaluation mainly includes silanol groups that are thought to contribute to the promotion of mineralization. Among the silicon atoms in silica particles, there are silicon atoms that are not bonded to a hydroxy group and silicon atoms that are bonded to one or two hydroxy groups. That is, the silicon atoms in silica particles have four structures, as shown in the following formula: a silicon atom bonded to two oxygen atoms and two hydroxy groups (Q2), a silicon atom bonded to three oxygen atoms and one hydroxy group (Q3), and a silicon atom bonded to four oxygen atoms (Q4). The amount of silanol (Si—OH) groups in the silica can be estimated by determining the proportions of Q2, Q3, and Q4 in the silicon atoms in the silica particles. The greater the amount of silanol groups (%), the greater the amount of minerals (Ca ２＋ The amount of silanol groups present on the silicon atoms of the Q2 to Q4 structures can be estimated by, for example, the amount of silanol groups present on the silicon atoms of the silica particles in the water-dispersed silica sol containing the silica particles to be investigated. ２９ It can be measured by Si NMR method (liquid NMR). Specifically, the silica particles (water-dispersed silica sol) to be measured ２９In the Si-NMR spectrum, the peak observed at a chemical shift of -136 ppm to -80 ppm is identified as originating from the Q structure (overall) of Si, the peak observed at -116 ppm to -106 ppm as originating from the Q4 structure, the peak observed at -105.5 ppm to -96 ppm as originating from the Q3 structure, and the peak observed at -95 ppm to -85 ppm as originating from the Q2 structure. The ratio of the area of ​​each peak originating from the Q2 to Q4 structures to the area of ​​the peak originating from the Q structure (overall) is the content ratio (mol%) of each structure (Q2 to Q4) in the silica particles being measured. Since the Q2 structure has two hydroxy groups, the Q3 structure has one, and the Q4 structure has zero hydroxy groups, the amount of silanol groups can be calculated by the following formula, taking into account the number of silanol groups in each structure. [Mathematical Expression 3] Silanol group amount (%) = ((PA2 / PA) x 100) x 2 + ((PA3 / PA) x 100) x 1 + ((PA4 / PA) x 100) x 0 In the above formula, PA represents the peak area from -136 ppm to -80 ppm, PA4 represents the peak area from -116 ppm to -106 ppm, PA3 represents the peak area from -105.5 ppm to -96 ppm, and PA2 represents the peak area from -95 ppm to -85 ppm. The silica particles constituting the mineralization accelerator of the present invention preferably have the silanol group amount (%) of 30% or less, and by setting it to 30% or less, it is possible to reduce the condensation and aggregation of silanol groups on the silica particle surface between silica particles. The lower limit can be 5% or 8% and by setting it to 5% or more, a mineral adsorption effect can be obtained. As will be described later, with respect to the silica particles constituting the mineralization accelerator of the present invention, ２９ The peak at chemical shifts of -80 to -85 ppm in the Si-NMR spectrum, i.e., the Q1 structure (Si(OH) ３ No peaks due to OSi) were observed.

[0026] Commercially available silica sols (aqueous silica sols) can be used. Aqueous silica sols with a silica concentration of 5 to 50% by mass are generally commercially available, which is preferred because of their easy availability. Aqueous silica sols include alkaline aqueous silica sols and acidic aqueous silica sols, and both can be used, although acidic aqueous silica sols are preferred. Commercially available acidic aqueous silica sols include Snowtex (trade name) ST-OXS, ST-OS, ST-O, ST-O-40, ST-OL, and ST-OYL, manufactured by Nissan Chemical Industries, Ltd.; and Adelite (trade name) AT-series, manufactured by ADEKA Corporation. Commercially available alkaline aqueous silica sols include Snowtex (trade name) ST-30 and ST-XL, manufactured by Nissan Chemical Industries, Ltd. The silica solids concentration in the aqueous silica sol used is preferably 5 to 55% by mass. Here, the silica solid content concentration is a value determined by a calcination method, specifically, the value obtained by dividing the mass of the calcination residue obtained by calcining an aqueous silica sol at 1000°C for 30 minutes or more in the atmosphere by the mass of the aqueous silica sol. The mass of the calcination residue is also referred to as the "silica solid content."

[0027] The concentration of (a) silica particles (silica solids concentration) in the mineralization accelerator according to the present invention is not particularly limited, but may be, for example, 0.01% by mass to 50.0% by mass, or 0.1% by mass to 50.0% by mass, or 1.0% by mass to 50.0% by mass, or 10.0% by mass to 40.0% by mass, or 15.0% by mass to 35.0% by mass, or 15.0% by mass to 25.0% by mass, or 0.001% by mass to 30.0% by mass, or 0.01% by mass to 30.0% by mass, or 0.1% by mass to 30.0% by mass, or 1.0% by mass to 30.0% by mass, based on the total mass of the mineralization accelerator.

[0028] In the mineralization accelerator of the present invention, the (a) silica particles may be at least partially coated with (c) a silane compound having a hydrophilic organic group.

[0029] In the present invention, "coated with a silane compound" refers to an embodiment in which the surface of a silica particle is coated with a silane compound, and also includes an embodiment in which a silane compound is bonded to the surface of a silica particle. "An embodiment in which a silane compound is coated on the surface of a silica particle" may refer to an embodiment in which a silane compound coats at least a portion of the surface of a silica particle, i.e., an embodiment in which the silane compound covers a portion of the surface of a silica particle, and an embodiment in which the silane compound covers the entire surface of a silica particle. This embodiment does not require bonding between the silane compound and the surface of a silica particle. Furthermore, "an embodiment in which a silane compound is bonded to the surface of a silica particle" may refer to an embodiment in which a silane compound is bonded to at least a portion of the surface of a silica particle, i.e., an embodiment in which the silane compound is bonded to a portion of the surface of a silica particle and covers at least a portion of the surface, and an embodiment in which the silane compound is bonded to the entire surface of a silica particle and covers the entire surface.

[0030] The (c) silane compound may be a silane compound having a hydrophilic organic group such as an epoxy group-containing organic group, an amino group-containing organic group, a hydroxy group-containing organic group, or a carboxy group-containing organic group. In addition to the hydrophilic organic group, the (c) silane compound preferably has a hydrolyzable group such as an alkoxy group, an acyloxy group, or a halogen group.

[0031] Specific examples of these (c) silane compounds include silane coupling agents having an epoxy group-containing organic group or an amino group-containing organic group. Examples of the silane coupling agent having an epoxy group-containing organic group include 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 2-(3,4-epoxycyclohexyl)propyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)propyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, 2-(3,4-epoxycyclohexyl)methyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)methyltriethoxysilane, 1-(3,4-epoxycyclohexyl)methyltrimethoxysilane, and 1-(3,4-epoxycyclohexyl)methyltriethoxysilane. Examples of the silane coupling agent having the amino group-containing organic group include 3-(2-(2-aminoethylamino)ethylamino)propyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrichlorosilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropylmethyldimethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, and N-phenyl-3-aminopropyltriethoxysilane.

[0032] The silica particles according to the present invention, at least a portion of which is coated with a silane compound having a hydrophilic organic group (also referred to as "silica particles surface-treated with a silane compound"), can be obtained, for example, by adding a silane compound to an aqueous silica sol, followed by heat treatment at 50 to 100°C for about 1 to 20 hours. In this case, the amount of silane compound added relative to the silica particles (silica solid content) in the aqueous silica sol can be, for example, a mass ratio of silane compound / silica particles = 0.1 to 10.0. The amount of surface treatment with the silane compound, i.e., the amount of silane compound bonded to the silica particle surface, is determined based on the amount of silane compound bonded to the silica particle surface within 1 nm of the silica particle surface. ２ It is preferable that the number of particles is, for example, about 0.1 to 12 per particle.

[0033] In addition, when particles coated with the (c) silane compound having a hydrophilic organic group (also referred to as surface-modified silica particles) are used as the (a) silica particles in the mineralization accelerator of the present invention, unreacted (c) silane compound may remain within the mineralization accelerator, or unmodified silica particles may remain, or the mineralization accelerator from which the unreacted (c) silane compound and unmodified silica particles have been removed may be used.

[0034] <Mineralization Promotion Method> The present invention also provides a method for producing a CO ２ The present invention also covers a mineralization promotion method, and more particularly, a mineralization promotion method including a step of injecting the mineralization promoter or a diluted solution of the mineralization promoter in an aqueous medium so that the silica particle concentration is 0.001 to 30 mass % into an underground layer. The step of injecting the mineralization promoter (diluted solution) into an underground layer includes the steps of: ２ The injection of the mineralization promoter and CO may be performed either before or after the injection. ２ The order of the above press-fitting is not particularly important, and the effects of the present invention can be obtained regardless of which order is performed.

[0035] The present invention will be described in further detail below based on synthesis examples, examples, and comparative examples, but it should be understood that the present invention is not limited to these examples in any way.

[0036] (Measuring equipment) DLS average particle size (dynamic light scattering particle size): A dynamic light scattering particle size measuring device, trade name Zetasizer Nano (manufactured by Spectris K.K., Malvern Division), was used. pH: A pH meter (manufactured by DKK-TOA Corporation) was used. Electrical conductivity: An electrical conductivity meter (manufactured by DKK-TOA Corporation) was used. ICP: A trade name Agilent 5110 ICP-OES (Agilent Technologies, Inc.) was used.

[0037] [Measurement of silica solids concentration (silica concentration)] The silica sol was placed in a crucible and dried at 150°C. The resulting gel was then calcined at 1000°C for 30 minutes in the atmosphere, and the calcination residue was weighed and calculated. [pH measurement] The pH of the silica sol was measured at 20°C using a pH meter (manufactured by DKK-TOA Corporation, product name: MM-43X). [Electrical conductivity measurement] The electrical conductivity of the silica sol was measured at 20°C using an electrical conductivity meter (manufactured by DKK-TOA Corporation, product name: CM-30R). [Measurement of average primary particle diameter (particle diameter by nitrogen adsorption method)] The specific surface area value (S Ｎ２ The specific surface area of ​​the silica sol was measured by removing water-soluble cations in the silica sol with an H-type cation exchange resin (manufactured by The Dow Chemical Company, trade name: Amberlite IR-120B), drying the silica sol at 290°C, and then pulverizing it in a mortar for 10 minutes to prepare a measurement sample. The specific surface area of ​​the silica sol was measured using a nitrogen adsorption method specific surface area measuring device (trade name: Monosorb, manufactured by Quantachrome Instruments Japan, LLC) to measure the specific surface area of ​​the silica sol. ２ The specific surface area was measured by the BET single-point method using a mixture of 30% nitrogen and 70% helium as the carrier gas. Ｎ２ (m ２ / g), the average primary particle size (nm) = 2720 / S Ｎ２The average secondary particle diameter was calculated using the following formula. [Measurement of Average Secondary Particle Diameter by Dynamic Light Scattering (DLS)] The average secondary particle diameter by the DLS method (also referred to as DLS average particle diameter) was measured using a dynamic light scattering particle diameter measurement device (trade name: Zetasizer Nano, manufactured by Malvern Panalytical). 0.1 g of the target silica sol was dispensed into a glass cell with an optical path length of 10 mm, and a 0.15 mass % NaCl aqueous solution was added to obtain a silica sol with a silica particle concentration adjusted so that the count rate at an attenuator of 7 was 200 to 400 kcps. The prepared silica sol was placed in the cell so that the height of the liquid surface from the bottom of the cell was adjusted to about 1 cm, and the DLS average particle diameter of the silica sol was measured using the attenuator of 7. This measurement device can also calculate the number average particle diameter and volume average particle diameter, but the Z average particle diameter was used in this application.

[0038] Example 1 A 2000 ml glass recovery flask was charged with 1000 g of aqueous silica sol 1 (Snowtex (trade name) ST-O, manufactured by Nissan Chemical Industries, Ltd., silica concentration = 20.5 mass%, BET average particle size 11.7 nm, DLS average particle size 18.6 nm), and then a magnetic stirrer was added. While stirring with the magnetic stirrer, 149.7 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KBM-403) was added. The recovery flask was then placed in an oil bath preheated to 60°C, and a cooling tube filled with tap water was placed on top of the recovery flask. The mixture was refluxed and maintained at 60°C for 3 hours, after which it was cooled. After cooling to room temperature, the aqueous sol was removed and filtered using a 460 mesh filter, yielding 1145 g of aqueous silica sol surface-treated with a silane compound. The aqueous silica sol of Example 1 was evaluated for pH, electrical conductivity, silica solid content, and DLS average particle size. The mineral adsorption performance of the aqueous silica sol of Example 1 was evaluated according to [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Tests> below. In addition, the stability of the aqueous silica sol of Example 1 in a CCS formation environment was evaluated according to [Stability Test in a CCS Formation Environment-1] and [Stability Test in a CCS Formation Environment-2].

[0039] Example 2 1,000 g of aqueous silica sol 1 (Snowtex (trade name) ST-O, manufactured by Nissan Chemical Industries, Ltd., silica concentration = 20.5 mass%, BET method average particle size 11.7 nm, DLS average particle size 18.6 nm) was added, followed by 228.2 g of lactic acid (manufactured by Kanto Chemical Co., Ltd., concentration 89 mass%), which was stirred with a magnetic stirrer. Except for adding 130.7 g of aminoethylaminopropylmethyldimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KBM-602) instead of 3-glycidoxypropyltrimethoxysilane, the same procedure as in Example 1 was repeated to obtain 1,352 g of aqueous silica sol. The pH, electrical conductivity, silica solids content, and DLS average particle size of the aqueous silica sol of Example 2 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 2 was evaluated according to [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Tests> below. Furthermore, the stability of the aqueous silica sol of Example 2 in a CCS geological formation environment was evaluated according to [Stability test in a CCS geological formation environment-1] and [Stability test in a CCS geological formation environment-2].

[0040] Example 3 The same procedure as in Example 1 was repeated, except that 1,000 g of aqueous silica sol 2 (Snowtex (trade name) ST-OL, manufactured by Nissan Chemical Industries, Ltd., silica concentration = 20.6 mass%, BET average particle size 45.4 nm, DLS average particle size 76.4 nm) was added instead of aqueous silica sol 1, and then a magnetic stirrer was added. 38.7 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KBM-403) was added while stirring with the magnetic stirrer. 1,038.0 g of aqueous silica sol of Example 3 was obtained. The pH, electrical conductivity, silica solid content, and DLS average particle size of the aqueous silica sol of Example 3 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 3 was evaluated according to [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Tests> below. Furthermore, the stability of the aqueous silica sol of Example 3 in a CCS geological formation environment was evaluated according to [Stability test in a CCS geological formation environment-1] and [Stability test in a CCS geological formation environment-2].

[0041] Example 4 A 500 mL glass recovery flask was charged with 195.2 g of aqueous silica sol 3 (Snowtex (trade name) ST-30, manufactured by Nissan Chemical Industries, Ltd., silica concentration = 30.6 mass%, BET average particle size 11.4 nm, DLS average particle size 19.3 nm) instead of aqueous silica sol 1, and 100.6 g of pure water was further added. While stirring with a magnetic stirrer, 13.8 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KBM-403) was added. The same procedure as in Example 1 was repeated to obtain 309.0 g of aqueous silica sol of Example 4. The pH, electrical conductivity, silica solid content, and DLS average particle size of the aqueous silica sol of Example 4 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 4 was evaluated according to [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Tests> below. Furthermore, the stability of the aqueous silica sol of Example 4 in a CCS geological formation environment was evaluated according to [Stability test in a CCS geological formation environment-1] and [Stability test in a CCS geological formation environment-2].

[0042] Example 5 300.3 g of aqueous silica sol of Example 5 was obtained in the same manner as in Example 4, except that 146.4 g of aqueous silica sol (Snowtex (trade name) ST-XL, manufactured by Nissan Chemical Industries, Ltd., silica concentration = 41.7 mass%, BET average particle size 45.4 nm, DLS average particle size 79.5 nm) was added instead of aqueous silica sol 1, 149.4 g of pure water was further added, and 5.1 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KBM-403) was added while stirring with a magnetic stirrer. The pH, electrical conductivity, silica solid content, and DLS average particle size of the aqueous silica sol of Example 5 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 5 was evaluated according to [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Tests> below. Furthermore, the stability of the aqueous silica sol of Example 5 in a CCS geological formation environment was evaluated according to [Stability test in a CCS geological formation environment-1] and [Stability test in a CCS geological formation environment-2].

[0043] Example 6 Aqueous silica sol 1 (Snowtex (trade name) ST-O, manufactured by Nissan Chemical Industries, Ltd., silica concentration = 20.5 mass%, BET average particle size 11.7 nm, DLS average particle size 18.6 nm) was used as the aqueous silica sol of Example 6, and the pH, electrical conductivity, silica solid content, and DLS average particle size were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 6 was evaluated according to [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Tests> below. In addition, the stability of the aqueous silica sol of Example 6 in a CCS geological formation environment was evaluated according to [Stability Test-1 in a CCS geological formation environment] and [Stability Test-2 in a CCS geological formation environment].

[0044] (Comparative Example 1) Sol-gel silica sol 1 (PL-2L (trade name), manufactured by Fuso Chemical Co., Ltd., primary particle size 17 nm, secondary particle size 26 nm, pH 7.3) was used as the aqueous silica sol of Comparative Example 1, and the pH, electrical conductivity, silica solid content, and DLS average particle size were evaluated. The mineral adsorption performance of the aqueous silica sol of Comparative Example 1 was evaluated according to [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Tests> below. The stability of the aqueous silica sol of Comparative Example 1 in a CCS geological formation environment was evaluated according to [Stability Test-1 in a CCS geological formation environment] and [Stability Test-2 in a CCS geological formation environment].

[0045] <Evaluation Test> [Mineral Adsorption Test-1] After placing a stirring bar in a 300 mL polystyrene bottle, CaCl ２ 133.3 g of brine-1 containing 12 mass% of CaCl and 3 mass% of NaCl was added. While stirring, the aqueous silica sol (mineralization accelerator) of Examples 1 to 6 or Comparative Example 1 was added so that the silica concentration was 1.0 mass% when the total solution amount was 200 g, and then 0.1 N hydrochloric acid (manufactured by Kanto Chemical Co., Inc.) was used to adjust the pH to 3.0. Then, pure water was added to adjust the CaCl ２200 g of mineral adsorption test solution-1 was prepared, containing 8% by mass of Ca, 2% by mass of NaCl, and a silica concentration of 1.0% by mass. The pH, conductivity, and DLS average particle size of the mineral adsorption test solution were evaluated. This mineral adsorption test solution-1 was stored at 60°C for 1 hour. 2 g of the mineral adsorption test solution-1 after storage was placed in a filter-equipped centrifugal cell (Merck, trade name Amicon Ultra-15, molecular weight cutoff 100,000, centrifugal ultrafiltration filter unit), and the centrifugal cell was then placed in a centrifuge (Tomy Kogyo Co., Ltd., trade name Supreme 21) and centrifuged at 5,000 rpm for 15 minutes to obtain filtrate-1 in which the silica particles had been removed by the ultrafiltration membrane. The silica particles were then filtered to remove the minerals (Ca ２＋ ) is adsorbed onto the filter, the adsorbed minerals are expected to be filtered out along with the silica particles, and the amount of minerals adsorbed onto the silica particles (absolute value) can be calculated as the difference between the amount of minerals in the mineral adsorption test solution-1 before storage and the amount of minerals in the filtrate-1 after storage.

[0046] The calcium concentrations in mineral adsorption test solution-1 before storage and in filtrate-1 after storage were quantified by ICP measurement, and the mineral adsorption rate-1 was calculated using the following formula. The calcium concentration was expressed on a volume basis (mg / L): Mineral adsorption rate-1 [%] = {[(Ca concentration in mineral adsorption test solution-1 before storage) - (Ca concentration in filtrate-1 after storage)] / (Ca concentration in mineral adsorption test solution-1 before storage)} x 100 The mineral adsorption rate-1 was evaluated according to the following criteria. <Criteria for determining mineral adsorption rate-1> A: Mineral adsorption rate-1 is 1% or more and 10% or less B: Mineral adsorption rate-1 is 0.1% or more and less than 1%, and more than 10% and 30% or less C: Mineral adsorption rate-1 is 0.01 or more and less than 0.1%, and more than 30% and 40% or less D: Mineral adsorption rate-1 is less than 0.01 and more than 40% The determination result for mineral adsorption rate-1 is A, which is the most preferable, followed by B, C, and D, with the exception that a C rating indicates that it is difficult to obtain the effect as a mineralization promoter, and a D rating is unpreferable because it does not obtain the effect as a mineralization promoter or may induce aggregation of silica particles.

[0047] [Mineral adsorption test-2] After placing a stirring bar in a 300 mL polystyrene bottle, CaCl２ 133.3 g of brine-1 containing 12 mass% of CaCl and 3 mass% of NaCl was added. While stirring, the aqueous silica sol (mineralization accelerator) of Examples 1 to 6 or Comparative Example 1 was added so that the silica concentration was 1.0 mass% when the total solution amount was 200 g, and then 0.1 N hydrochloric acid (manufactured by Kanto Chemical Co., Inc.) was used to adjust the pH to 3.0. Then, pure water was added to adjust the CaCl ２ 200 g of mineral adsorption test solution-2 was prepared, containing 8% by mass of Ca, 2% by mass of NaCl, and a silica concentration of 1.0% by mass. This mineral adsorption test solution-2 was stored at 25°C for 1 hour. After storage, the particle size of the silica particles in the mineral adsorption test solution-2 was measured using DLS, and this was taken as DLS average particle size-1 (nm). Note that DLS average particle size-1 is the average particle size of the silica particles with a mineral (Ca) on the surface of the silica particles. ２＋ ) are adsorbed, the particle size can be evaluated as including the adsorbed substances.

[0048] Next, 2 g of the aqueous silica sol (mineralization accelerator) of Examples 1 to 6 or Comparative Example 1 was placed in a centrifugal cell with a filter (Merck, trade name: Amicon Ultra-15, molecular weight cutoff: 100,000, centrifugal ultrafiltration filter unit), and then 4 g of pure water was placed in the centrifugal cell. The centrifugal cell was placed in a centrifuge (Tomy Kogyo Co., Ltd., trade name: Supreme 21), and centrifuged at 3,000 rpm for 20 minutes. After recovering and quantifying the filtrate, pure water in an amount equal to the mass of the filtrate was again added to the centrifugal cell, and the cell was centrifuged again at 3,000 rpm for 20 minutes. This process was repeated five times, and then the silica sol remaining in the filter was recovered. The recovered silica sol was dried at 120°C using a hot plate to obtain silica gel, which was then pulverized in a mortar and further dried at 150°C for 3 hours to obtain a dry silica powder. The specific surface area (m ２ / g) was measured (BET method, i.e., nitrogen gas BET method). The specific surface area measured by the BET method divided by 2720 was taken as the BET particle size-1 (nm). Note that the BET particle size-1 represents the average primary particle size of silica particles (only). In the form of an aqueous silica sol (in water), silica particles are generally considered to be dispersed in the form of clumps of several particles (secondary particles), and the DLS average particle size is considered to be about twice the BET particle size. Therefore, when the DLS average particle size-1 is more than twice the BET particle size-1, it can be evaluated that minerals are adsorbed onto the particle surfaces of the clumped silica particles (secondary particles) (however, as will be described later, if this DLS average particle size is too large compared to the BET particle size, the silica particles are considered to be in an agglomerated state, which is undesirable).

[0049] The mineral adsorption capacity-2 was calculated from the DLS average particle size-1 and the BET particle size-1 using the following formula: Mineral adsorption capacity-2 = (DLS average particle size-1) / (BET particle size-1) Mineral adsorption capacity-2 was evaluated according to the following criteria. <Criteria for mineral adsorption capacity-2> A: 2.0 or more but less than 3.0 B: 1.5 or more but less than 2.0, and 3.0 or more but less than 5.0 C: 1.1 or more but less than 1.5, and more than 5.0 but less than 10.0 D: Less than 1.1 and 10.0 or more The mineral adsorption capacity-2 rating of A was most preferable, followed by B, C, and D, with A and B being desirable in consideration of the effect as a mineralization accelerator. Note that a mineral adsorption capacity-2 rating of more than 5.0 indicates excessive aggregation of silica particles, and is undesirable.

[0050] [Salt tolerance test: Stability test in a CCS geological environment - 1] After placing a stirring bar in a 300 mL polystyrene bottle, CaCl ２ 133.3 g of brine-1 containing 12 mass% of CaCl and 3 mass% of NaCl was added. While stirring, the aqueous silica sol (mineralization accelerator) of Examples 1 to 6 or Comparative Example 1 was added so that the silica concentration was 1.0 mass% when the total solution amount was 200 g, and then 0.1 N hydrochloric acid (manufactured by Kanto Chemical Co., Inc.) was used to adjust the pH to 3.0. Then, pure water was added to adjust the CaCl ２200 g of stability test solution-1 in a CCS geological formation environment was prepared, which contained 8 mass % of HCl, 2 mass % of NaCl, and a silica concentration of 1.0 mass %. This stability test solution-1 was also used as a test solution in the high-pressure salt resistance test described below.

[0051] 100 g of stability test solution-1 was placed in a sealed container, sealed, and stored at 60°C for 7 days. The pH, electrical conductivity, and DLS average particle size of the aqueous silica sol (silica particles) in the sample of stability test solution-1 were evaluated before and after storage, and the appearance of stability test solution-1 after storage was visually confirmed. The stability in a CCS geological formation environment was evaluated based on the ratio of the average secondary particle size (DLS average particle size) of silica particles in stability test solution-1 after storage to the average secondary particle size (DLS average particle size) before storage (average secondary particle size of silica particles after storage (DLS average particle size after storage) / average secondary particle size of silica particles before storage (DLS average particle size before storage)) and the appearance of the test solution after storage, according to the following criteria. <Judgment of Stability Test-1 in a CCS Geological Formation Environment> A: The ratio of DLS average particle size after storage / DLS average particle size before storage is 0.9 or more and 1.5 or less. B: The ratio of the DLS average particle size after storage to the DLS average particle size before storage is more than 1.5 and not more than 3.0. C: The ratio of the DLS average particle size after storage to the DLS average particle size before storage is more than 3.0 and not more than 8.0. D: The ratio of the DLS average particle size after storage to the DLS average particle size before storage is more than 8.0 and not more than 20.0. E: The ratio of the DLS average particle size after storage to the DLS average particle size before storage is more than 20.0 or the product is cloudy and has undergone solid-liquid separation. The results of Stability Test-1 in a CCS geological formation environment show that A is the most favorable, followed by B, C, D, and E, and considering the effect as a mineralization accelerator, A and B are desirable.

[0052] [High-pressure salt resistance test: Stability test in a CCS geological formation environment - 2] Supercritical CO ２To evaluate the stability in the environment, a test under high pressure was carried out. 100 g of the stability test solution-1 (hereinafter referred to as stability test solution-2 to distinguish the test type) was placed in a glass inner cylinder (manufactured by Taiatsu Glass Kogyo Co., Ltd., capacity 200 mL), and the glass inner cylinder was placed in a portable reactor (manufactured by Taiatsu Glass Kogyo Co., Ltd., product name TPR5 (TVS-N2 type portable reactor N2-300 equipped with a cross joint, a sampling tube, a valve, a pressure gauge, and a pressure gauge connection nozzle)). The portable reactor was sealed at 60 N m using a torque wrench, and then placed in a 60°C oven and maintained for 30 minutes. Thereafter, liquefied CO ２ The reactor was pressurized so that the pressure gauge of the portable reactor read 8 MPa, and after sealing, the internal pressure was maintained at 8-9 MPa at 60°C. When the internal pressure dropped to 8 MPa, the liquefied CO ２ The pressure was re-injected to 9 MPa and the solution was stored for 7 days. After storage, the temperature and pressure were released. The stability in a simulated high-pressure environment CCS geological formation environment was evaluated based on the ratio of the average secondary particle size (DLS average particle size) of silica particles in stability test solution-2 after high-pressure storage to the average secondary particle size (DLS average particle size) of the same particles before high-pressure storage (average secondary particle size of silica particles after storage (DLS average particle size after storage) / average secondary particle size of silica particles before storage (DLS average particle size before storage)) and the appearance of the test solution after high-pressure storage, according to the following criteria. <Judgment of Stability Test-2 in a CCS Geological Formation Environment> A: The ratio of DLS average particle size after (high-pressure) storage / DLS average particle size before (high-pressure) storage is 0.9 or more and 1.5 or less. B: The ratio of the DLS average particle size after (high pressure) storage to the DLS average particle size before (high pressure) storage is more than 1.5 and not more than 2.4. C: The ratio of the DLS average particle size after (high pressure) storage to the DLS average particle size before (high pressure) storage is more than 2.4 and not more than 8.0. D: The ratio of the DLS average particle size after (high pressure) storage to the DLS average particle size before (high pressure) storage is more than 8.0 and not more than 20.0. E: The ratio of the DLS average particle size after (high pressure) storage to the DLS average particle size before (high pressure) storage is more than 20.0 or the material is cloudy and solid-liquid separated. The results of Stability Test-2 in a CCS Formation Environment show that A is the most favorable, followed by B, C, D, and E, and considering the effect as a mineralization accelerator, A and B are desirable.

[0053] [Silanol Group Amount Evaluation-1] The amount of silanol groups (mmol / g) associated with mineral adsorption was evaluated using TG-DTA. 2 g of the aqueous silica sol (mineralization accelerator) from Examples 1 to 5 or Comparative Example 1 was placed in a filter-equipped centrifugal cell (Merck, trade name: Amicon Ultra-15, molecular weight cutoff: 100,000, centrifugal ultrafiltration filter unit), followed by 4 g of pure water. The centrifugal cell was placed in a centrifuge (Tomy Kogyo Co., Ltd., trade name: Supreme 21), and centrifuged at 3,000 rpm for 20 minutes. After recovering and quantifying the filtrate, pure water equivalent to the mass of the filtrate was added to the centrifugal cell, and the mixture was centrifuged again at 3,000 rpm for 20 minutes. This process was repeated five times to remove components unbonded to the silica particles, after which the silica sol remaining in the filter (ultrafiltration membrane) was recovered and dried at 120°C using a hot plate. Thereafter, the sample for TG-DTA measurement was prepared by drying at 150 ° C. in an oven. 10 mg of the sample for TG-DTA measurement was placed in an aluminum container and then placed in a TG-DTA (manufactured by NETZSCH Japan Co., Ltd., trade name STA 2500 Regulus). While flowing nitrogen gas at a flow rate of 100 cc / min, the temperature was increased by 10 ° C. per minute from 25 ° C., and heated to 600 ° C. The mass loss M1 at the time the temperature inside the apparatus reached 200 ° C. was the amount of water removed from the surface of the silica particles contained in the sample for TG-DTA measurement. The mass loss M2 at the time the temperature inside the apparatus reached 600 ° C. was subtracted from the mass loss M1 to determine the amount of water removed by dehydration condensation between silanol groups on the surface of the silica particles. The amount of silanol groups in the silica particles (mmol / g) was calculated using the following formula. [Equation 2] Silanol group amount (mmol / g) = 2 × (M2 - M1) ÷ M Ｈ２Ｏ ÷ (M0) × 1000 where M0 is the mass of the silica particle powder subjected to thermogravimetric analysis at 25°C or less before heating, M1 is the mass loss amount when the temperature reached 200°C, M2 is the mass loss amount when the temperature reached 600°C, and M Ｈ２ＯTable 3 shows the mass of the silica particles before heating (mass before heating), the mass of the silica particles at 200°C (mass at 200°C), and the mass of the silica particles at 600°C (mass at 600°C), as well as the silanol group amount (mmol / g) obtained from these measurements.

[0054] [Silanol Group Amount Evaluation-2] The amount of silanol groups (%) related to the mineral adsorption amount was measured using a liquid ２９ Evaluation was carried out using Si-NMR. 2 g of the aqueous silica sol (mineralization accelerator) of Examples 1 to 6 or Comparative Example 1 was placed in a centrifugal cell with a filter (manufactured by Merck, trade name Amicon Ultra-15, molecular weight cutoff 100,000, centrifugal ultrafiltration filter unit), and then 4 g of pure water was added. The centrifugal cell was placed in a centrifuge (manufactured by Tomy Kogyo Co., Ltd., trade name Supreme 21), and centrifuged at 3,000 rpm for 20 minutes. After recovering and quantifying the filtrate, pure water in an amount equal to the mass of the filtrate was added to the centrifugal cell, and the cell was centrifuged again at 3,000 rpm for 20 minutes. This process was repeated five times, and then pure water was added to the silica sol remaining in the filter portion so that the mass was 2 g, thereby removing the excess silane compound and preparing a sample for NMR measurement. 0.1 mL of heavy water was added to 0.5 mL of sample for NMR measurement, and the mixture was sealed in a 10 mm Teflon tube. The liquid was analyzed using an NMR (manufactured by JEOL, product name ECA500) with a 90-degree pulse, a waiting time of 120 seconds, and an accumulation count of 1000. ２９ Si-NMR was measured. The obtained spectrum was analyzed, and the peak area from -136 ppm to -82 ppm was determined to be the peak area derived from the Q structure of Si. The peak area from -116 ppm to -106 ppm was determined to be the peak area derived from Si(OSi) ４ The peak area from the Q4 structure derived from the SiO2, and the peak area from -105.5 ppm to -96 ppm is SiOH(OSi) ３ The peak area derived from the Q3 structure and the peak area from -95 ppm to -85 ppm were determined as Si(OH) ２ (OSi) ２The peak area derived from the Q2 structure derived from the above was used. The proportions of the Q4 structure, Q3 structure, and Q2 structure relative to the total Q structure were calculated as (peak area derived from the Q4 structure, Q3 structure, or Q2 structure) / (peak area derived from the Q structure). The proportion of the Q structure was analyzed from the integral ratio to evaluate the amount of silanol groups. The amount of silanol groups -2 (%) was calculated as ((proportion of the Q2 structure) x 2 + (proportion of the Q3 structure)). In all of Examples 1 to 6 and Comparative Example 1, ２９ The peak at chemical shifts of -80 to -85 ppm in the Si-NMR spectrum, i.e., the Q1 structure (Si(OH) ３ No peaks due to OSi) were observed.

[0055]

[0056]

[0057]

[0058]

[0059] As shown in Tables 1 and 2, the aqueous silica sols (mineralization accelerators) of Examples 1 to 6 were rated A in the Mineral Adsorption Test-2, and were also rated A or B in the Mineral Adsorption Test-1, Stability Test in a CCS Geological Formation Environment-1 (salt tolerance test), and Stability Test in a CCS Geological Formation Environment-1 (high-pressure salt tolerance test), confirming that they have mineral adsorption capacity and good salt tolerance. The silica particles constituting the aqueous silica sols (mineralization accelerators) of Examples 1 to 6 are particles whose silanol group amount (mmol / g) by thermogravimetric analysis shown in Table 3 is in the range of 3.5 to 10 mmol / g, and the silanol group amount (mmol / g) shown in Table 4 is in the range of 3.5 to 10 mmol / g. ２９ The particles have a silanol group content (%) in the range of 8 to 28% according to Si-NMR spectrum analysis, and the silanol groups on the silica particle surface do not condense or aggregate between silica particles, and as confirmed by the mineral adsorption test described above, these particles are expected to exhibit a mineral adsorption effect. On the other hand, the aqueous silica sol of Comparative Example 1 was found to be stable in a CCS geological formation environment, but was evaluated to have almost no mineral adsorption capacity. As shown in Table 4 ２９Although the amount of silanol groups (%) measured by Si-NMR spectrum analysis was 31%, the amount of silanol groups (mmol / g) measured by thermogravimetric analysis shown in Table 3 was 3.4 mmol / g. In other words, even if the amount of silanol groups (%) present throughout the silica particles is large, the amount of silanol groups (mmol / g) that can contribute to mineral adsorption is small, and it was concluded that these particles are unlikely to have any mineral adsorption effect.

Claims

1. CO ２ The mineralization promoter of claim 1, wherein the silica sol contained in the mineralization promoter comprises silica particles (a) having an average secondary particle diameter of 5 to 200 nm as measured by a dynamic light scattering method, and an aqueous medium (b).

2. CaCl ２ 2. The mineralization accelerator according to claim 1, wherein in a mineral adsorption test-1, a mixture is added to a saltwater solution of pH 3.0 containing 8% by mass of calcium carbonate and 2% by mass of NaCl, at a concentration such that the silica particle concentration becomes 1% by mass, and the mixture is stored at 60°C for 1 hour, and the mineral adsorption rate is evaluated, wherein the mineral adsorption rate (%) calculated from the Ca concentration in the mixture before storage and the Ca concentration in the filtrate obtained by ultrafiltration of the mixture after storage using the following formula 1 is 0.1 to 30%. [Formula 1] Mineral adsorption rate (%) = {[(Ca concentration in the mixture before storage) - (Ca concentration in the filtrate of the mixture after storage)] / (Ca concentration in the mixture before storage)} x 100 3. CaCl ２ 2. The mineralization accelerator according to claim 1, wherein in a mineral adsorption test-2 in which the mineralization accelerator is added to a saltwater solution of pH 3.0 containing 8% by mass of HCl and 2% by mass of NaCl at a concentration such that the silica particle concentration becomes 1% by mass, the mixture is stored at 25°C for 1 hour, and the silica particle size in the mixture after storage is evaluated, the ratio (DLS average particle size / BET particle size) of the average secondary particle size (DLS average particle size) of the silica particles contained in the mixture after storage measured by dynamic light scattering to the specific surface area diameter (BET particle size) of the silica particles contained in the mineralization accelerator measured by nitrogen adsorption is 2.0 or more and 5.0 or less.

4. The silica particles have a specific surface area (S Ｎ２ ) is 25 to 550 m ２ The mineralization promoter according to claim 1, wherein the total amount of the mineralization promoter is 100% or more.

5. The mineralization accelerator according to claim 1, wherein the silica particle powder obtained by ultrafiltration to remove components not bound to the silica particles is heated and dried to remove the aqueous medium, and the amount of silanol groups calculated by the following formula 2 from the amount of mass loss when heated from 25°C to 600°C and the molecular weight of water molecules in thermogravimetric analysis is 3.5 to 20 mmol / g. [Formula 2] Amount of silanol groups (mmol / g) = 2 × (M2 - M1) ÷ M Ｈ２Ｏ ÷ (M0) × 1000 (where M0 is the mass of the silica particles subjected to thermogravimetric analysis at 25°C or less before heating, M1 is the mass loss of the silica particles when the temperature reaches 200°C, M2 is the mass loss of the silica particles when the temperature reaches 600°C, and M Ｈ２Ｏ represents the molecular weight of a water molecule.) 6. ２９ The mineralization accelerator according to claim 1, wherein the silica particles have a silanol group amount (%) of 30 or less, calculated by the following formula 3, where the peak area from -136 ppm to -82 ppm in a Si-NMR spectrum is PA, the peak area from -116 ppm to -106 ppm is PA4, the peak area from -105.5 ppm to -96 ppm is PA3, and the peak area from -95 ppm to -85 ppm is PA2. [Formula 3] Silanol group amount (%) = ((PA2 / PA) x 100) x 2 + ((PA3 / PA) x 100) x 1 + ((PA4 / PA) x 100) x 0 7. The mineralization accelerator according to claim 1, wherein the silica particles are at least partially coated with a silane compound (c) having a hydrophilic organic group, and the hydrophilic organic group is an epoxy group-containing organic group, an amino group-containing organic group, a hydroxy group-containing organic group, or a carboxy group-containing organic group.

8. CaCl ２ 2. The mineralization accelerator according to claim 1, wherein the mineralization accelerator is added to a saltwater solution of pH 3.0 containing 8% by mass of silica and 2% by mass of NaCl at a concentration such that the silica particle concentration is 1% by mass, and the mixture is stored at 60°C for 7 days. In a salt tolerance test, the ratio of the DLS average particle size of the mixture after storage to the DLS average particle size of the mixture before storage (DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage) is 0.9 to 3.

0.

9. CaCl ２ and 2% by mass of NaCl, and the mineralization accelerator was added to a concentration such that the silica particle concentration was 1% by mass. ２ 2. The mineralization accelerator according to claim 1, wherein in a high-pressure salt resistance test in which the mixture is pressurized at 8 MPa and stored at 60°C and 8 MPa for 7 days, the ratio of the DLS average particle size of the mixture after storage to the DLS average particle size of the mixture before storage (DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage) is 2.4 or less.

10. CO ２ 2. A method for promoting mineralization comprising the steps of: adding the mineralization promoter according to claim 1 or a diluted solution of the mineralization promoter adjusted to a solids concentration of 0.001 to 30% by mass with an aqueous medium to CO ２ into a subterranean formation before or after the underground injection of

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