Experimental Study of the Effect of Different Influencing Factors on the Interaction of CO₂, Water, and Basalt

Abstract

Basalt has been studied in recent years as a novel geological medium for CO₂ sequestration, thus enriching and expanding the carbon sink potential of geological bodies. In this study, four sets of CO₂–water–basalt interaction experiments were carried out for different ranges of temperature, pressure, particle size, and basalt type, while taking account of actual in situ temperature and pressure conditions in basalt strata. The aim was to provide a theoretical basis for the construction of future basalt CO₂ mineralization and storage projects. It was demonstrated that basalt begins to form calcitic minerals above 36 °C, with the highest carbonate mineral formation occurring at 66 °C accompanied by minor hydration of magnesium carbonate minerals. Below 26 °C, the dissolution of calcitic minerals showed a gradually increasing trend at higher pressures, with small amounts of hydrated magnesium carbonate minerals and calcite being formed at 9 MPa and 13 MPa. Maximum mineral dissolution occurred at 30–35 mesh particle size. The dissolution reaction in porous basalt was more intense for different basalt types (e.g., porous basalt and massive olivine basalt).

1. Introduction

The international community has recently reached a consensus that reducing excessive emissions of greenhouse gases, primarily CO₂, is an effective response to global warming and improves the conditions for living organisms [1,2,3]. With the growing adoption of ‘green’ and low-carbon principles and the imperative to advance ecological conditions, China is committed to peaking its CO₂ emissions by 2030 and achieving carbon neutrality by 2060 [4,5]. Carbon capture, utilization, and storage (CCUS) is a pivotal carbon reduction strategy and is regarded as the most promising carbon mitigation technology [6,7]. Potential sites for CO₂ geological storage include subsurface saline aquifers in sedimentary basins, depleted oil and gas fields, and non-exploitable coal seams [8,9,10,11,12]. Basalt, a novel geological material that has recently gained attention, has significantly expanded both the technical approaches and carbon sequestration potential for CO₂ storage [13]. Unlike other methods, this technology features permanent mineralization, large storage capacity, rapid response, and low leakage risk, making it a crucial pathway for future geological carbon sequestration [14].

Basalt is the most widely distributed rock on Earth with significant CO₂ sequestration potential. It has been extensively studied, and considerable potential has been demonstrated for submarine basalt, continental overflow basalt, and mid-ocean ridge basalt [15]. Laboratory experiments worldwide have confirmed the immense sequestration potential of basalt strata, as shown by demonstration projects such as Japan’s Nagaoka, Iceland’s Carbfix, and the USA Wallula Columbia River Basalt projects [4,13,14,16,17,18]. In particular, the mineralization rates of the latter two in real geological settings have negated the conventional understanding that a timescale of thousands of years is required, demonstrating the applicability and development prospects of this carbon storage technology [14,19]. For instance, the Carbfix project injected CO₂ solution into basalt, achieving a mineralization rate as high as 95% within two years. In the Wallula project in Washington State, USA, 60% of the supercritical CO₂ injected into basalt was mineralized and sequestered within 23 months [20,21,22,23,24,25]. Compared with international counterparts, China is still at the early stage, having conducted several feasibility studies [26]. In 2022, with Tencent investment, Chinese scientists collaborated with the Carbfix team to launch the first pilot project in China on basalt mineralization sequestration in the Leizhou Peninsula. The site has been selected and a preliminary analysis of theoretical mineralization sequestration potential has been completed [20].

Extensive research has yielded substantial insights into the mechanisms, reaction rates and influencing factors of basalt CO₂ storage [15,19]. The chemical reaction between CO₂ and rocks to form carbonates involves three key steps: CO₂ dissolved in aqueous solution (Reaction (1)); rock dissolution with release of divalent metallic cations (Reactions (2–4)); and the formation of stable carbonate minerals via carbonate ion reactions with these cations (Reaction (5) [15,27]) [15]. Free CO₂ is effectively sequestered in minerals such as calcite (CaCO₃), magnesite (MgCO₃), and siderite (FeCO₃) [28] (see Figure 1):

CO₂ + H₂O == H₂CO₃ == HCO₃⁻ + H⁺ == CO₃²⁻ + 2H⁺

(1)

Mg₂SiO₄ (olivine) + 4H⁺→2Mg²⁺ + 2H₂O + SiO₂ (aq)

(2)

MgCaSi₂O_{6 (}pyroxene) + 4H⁺→Mg²⁺ + Ca²⁺ + 2H₂O + 2SiO₂ (aq)

(3)

CaAl₂Si₂O₈ (anorthite) + 8H⁺→Ca²⁺ + 2Al³⁺ + 4H₂O + 2SiO₂ (OH)

(4)

(Ca, Mg, Fe)²⁺ + CO₂ + H₂O = (Ca, Mg, Fe)CO₃ + 2H⁺

(5)

Studies have shown that factors affecting reaction rates include salinity, temperature, pressure, pH, fluid flow rate, and mineral contact surface area [15,19,27,29]. Temperature affects the products of mineralization reactions. Ca²⁺ readily reacts with CO₂ to form calcite at temperatures below 300 °C (CaCO₃). Mg²⁺ readily precipitates as magnesite (MgCO₃) and dolomite [MgCa(CO₃)₂] at temperatures above 65 °C. At lower temperatures the precipitation of carbonates is kinetically inhibited, leading to the formation of less stable hydrated carbonate minerals such as brucite [Mg₅(CO₃)₄(OH)₂·4H₂O], dypingite [Mg₅(CO₃)₄(OH)₂·5H₂O], and nesquehonite [MgCO₃·3H₂O] [27,30,31].

For CO₂ mineralization and storage in basalt, experimental investigations on relevant influencing factors have been conducted via the single-factor experimental method. Under a pressure of 10 MPa and particle sizes ranging from 20 to 40 mesh, CO₂–water–basalt experiments were conducted at four different temperatures (100 °C, 150 °C, 200 °C, 250 °C) on both basalt and magnesite olivine rocks. Under the same temperature conditions (200 °C) and particle size range (20–40 mesh), experiments were performed at four different pressures (7 MPa, 10 MPa, 13 MPa, 16 MPa). Additionally, under 200 °C and 10 MPa pressure, experiments were carried out with four distinct particle size ranges (10–20 mesh, 20–40 mesh, 40–60 mesh, 60–80 mesh). The results show that higher reaction temperature, pressure and mineral particle surface area enhanced carbonate conversion and mineral carbonation. The basalt samples exhibited the formation of siderite-type carbonate minerals, while magnesia olivine samples developed dolomite and magnesite-type carbonate minerals [32]. This study focused on the influencing factors of temperature, pressure, particle size, and type of basalt, and also referred to the actual temperature and pressure conditions of the basalt strata. Four sets of CO₂–water–basalt experiments were designed for the porous basalt under a pressure of 5 MPa and a particle size of 60–65 mesh at temperatures of 26 °C, 36 °C, 46 °C, 56 °C, and 66 °C. Four different pressures (5 MPa, 7 MPa, 9 MPa, 11 MPa, 13 MPa) were designed for the porous basalt under a temperature of 26 °C and a particle size of 60–65 mesh. Three different particle sizes (16–18 mesh, 20–30 mesh, 60–65 mesh) were designed for the porous basalt under a temperature of 5 MPa and a pressure of 60–65 mesh. Two types of basalt (porous basalt and massive olivine basalt) were designed for the CO₂–water–basalt interaction experiments under the conditions of 26 °C, 5 MPa pressure, and 60–65 mesh particle size. The study aimed to investigate the differences and patterns of basalt CO₂ mineralization and storage under various influencing factors.

2. Design of Experiments

2.1. Experimental Technical Route

Based on the comprehensive collection of existing research results, this experiment focused on the influencing factors such as temperature, pressure, particle size, and type of basalt. Additionally, it referred to the actual temperature and pressure conditions of basalt strata to design CO₂–water–basalt reaction experiments with different temperatures, pressures, particle sizes, and types of basalt. The study aimed to investigate the differences and patterns of basalt CO₂ mineralization and sequestration under various influencing factors. The technical roadmap is shown in Figure 2 below.

2.2. Experimental Samples

Following preliminary sampling, the basalt samples used in the interaction experiments were porous basalt and massive olivine basalt collected from Yucheng Town, Yutai County, Jining City, Shandong Province. The porous basalt samples were collected at a depth of approximately 490 m; the massive olivine basalt samples were taken at a depth of approximately 560 m. The corresponding reservoir temperature was about 30 °C, and the formation pressure was approximately 5 MPa.

2.2.1. Porous Basalt

(1)

Petrological characteristics of the samples

The experimental samples of porous basalt were tested prior to the experiment for porosity and permeability (

Table 1. Porosity and permeability test results.

Serial Number	Identifier	Lithology	Length/cm	Diameter/cm	Permeability/mD	Porosity (%)
1	YX01 (V)	Porous basalt	4.108	2.426	1.5979	15.61
2	YX01 (H)	Porous basalt	3.646	2.400	4.0904	19.90

Note: V = core sampling parallel to the vertical core axis; H = sampling perpendicular to the core axis.

). The results showed that the transverse permeability (4.0904 mD) was higher than the vertical permeability (1.5979 mD), with a porosity ranging from 15.61% to 19.90%.

The rock samples were then pretreated leading to the interaction experiments. Porous basalt was processed into three particle sizes: 60–65 mesh (0.25–0.3 mm), 30–35 mesh (0.6–0.65 mm), and 16–18 mesh (1.0–1.25 mm). All samples were rinsed with purified water, then oven-dried at 105 °C for 24 h and weighed on an electronic balance with a precision of ± 0.01 g for subsequent testing in the high-temperature and high-pressure reactors.

Prior to the experiment, the rock samples were photographed (Figure 3). The porous basalt particles exhibited intact surface morphology with a light brownish-red color.

Furthermore, the processed porous basalt particles were analyzed for their particle aperture size using the specific surface area and pore size analyzer. It was found that under the particle size range of 60–65 mesh, the pore diameter was 24.6929 nm.

(2)

The mineral and chemical compositions of the sample

Before the experiment, the mineral compositions of porous basalt were determined through X-ray diffraction (XRD) using a SmartLab SE X-ray diffractometer (CIQTEK Co., Ltd., Hefei, China).

Table 2. Mineral composition of massive olivine basalt and porous basalt.

Basalt Type	Quartz Content (%)	Calcium Feldspar Content (%)	Calcite Content (%)	Pyroxene Content (%)	Pyrite Content (%)	Clay Mineral Content (%)
Porous basalt	0.8	60	25.2	7.7	1.4	4.9

indicates that the porous basalt primarily consisted of calcium feldspar and calcite, with minor amounts of pyroxene, pyrite, quartz, and clay minerals. Given that secondary minerals such as calcite, quartz, and zeolite typically fill porous or amygdalitic structures, this porous basalt contained relatively higher calcite content.

The chemical compositions of porous basalt were determined by using the iCAP 7000 series plasma emission X-ray spectrometer (XRF) (Thermo Fisher Scientific Inc., Shanghai, China) and energy-dispersive X-ray spectrometer (EDX) (JEOL Ltd., Tokyo, Japan) before the experiment.

The XRF analysis results in

Table 3. Oxide content of porous basalt and massive olivine basalt.

Oxide content (%)	SiO2	Fe2O3	Al2O3	CaO	K2O	Na2O
Porous basalt	50.89	8.66	14.95	9.55	3.73	3.69

indicate that the main components of porous basalt are SiO₂, Fe₂O₃, Al₂O₃, CaO, K₂O, and Na₂O, with minor amounts of MgO, TiO₂, P₂O₅, and MnO.

According to the results of the EDX analysis (

Table 4. Elemental composition of porous basalt and massive olivine basalt.

Elemental mass percentage (wt%)	O	Si	C	Al	Ca	Na
Porous basalt	45.5	20.7	12.1	12.0	5.3	3.3

), the composition elements of porous basalt samples are mainly O, Si, C, Al, and a small amount of Ca, Na, Fe, K, and Mg.

2.2.2. Massive Olivine Basalt

(1)

Petrological characteristics of the samples

The experimental samples of massive olivine basalt were tested prior to the experiment for porosity and permeability (

Table 5. Porosity and permeability test results.

Serial Number	Identifier	Lithology	Length/cm	Diameter/cm	Permeability/mD	Porosity (%)
1	YX07 (V)	Massive olivine basalt	4.812	2.422	1.4793	14.61
2	YX07 (H)	Massive olivine basalt	4.133	2.416	4.9148	12.95

Note: V = core sampling parallel to the vertical core axis; H = sampling perpendicular to the core axis.

). The results showed that the transverse permeability (4.9148 mD) was higher than the vertical permeability (1.4793 mD), with a porosity ranging from 14.61% to 12.95%.

The rock samples were then pretreated leading to the interaction experiments. The massive olivine basalt was processed into 60–65 mesh (0.25–0.3 mm) particle size. All samples were rinsed with purified water, then oven-dried at 105 °C for 24 h and weighed on an electronic balance with a precision of ± 0.01 g for subsequent testing in the high-temperature and high-pressure reactors.

Prior to the experiment, the rock samples were photographed (Figure 4). The massive olivine basalt particles exhibited intact surface morphology with a light gray-black ~ gray-green color.

Furthermore, the processed massive olivine basalt particles were analyzed for their particle aperture size using the specific surface area and pore size analyzer. It was found that under the particle size range of 60–65 mesh, the pore diameter was 4.0616 nm.

(2)

The mineral and chemical compositions of the sample

Before the experiment, the mineral compositions of massive olivine basalt were determined through X-ray diffraction (XRD) using a SmartLab SE X-ray diffractometer.

Table 6. Mineral composition of massive olivine basalt and porous basalt.

Basalt Type	Quartz Content (%)	Calcium Feldspar Content (%)	Calcite Content (%)	Pyroxene Content (%)	Pyrite Content (%)	Clay Mineral Content (%)
Massive olivine basalt	/	88.5	2.0	/	/	9.5

indicates that massive olivine basalt mainly contained calcium feldspar, along with traces of calcite and clay minerals.

The chemical compositions of massive olivine basalt were determined by using the iCAP 7000 series plasma emission X-ray spectrometer (XRF) and energy-dispersive X-ray spectrometer (EDX) before the experiment.

The XRF analysis results in

Table 7. Oxide content of porous basalt and massive olivine basalt.

Oxide content (%)	SiO2	Fe2O3	Al2O3	CaO	K2O	Na2O
Massive olivine basalt	47.29	13.22	14.29	8.79	0.90	3.16

indicate that the main components of massive olivine basalt are SiO₂, Fe₂O₃, Al₂O₃, CaO, MgO, and Na₂O, with minor amounts of K₂O, TiO₂, P₂O₅, and MnO.

According to the results of the EDX analysis (

Table 8. Elemental composition of porous basalt and massive olivine basalt.

Elemental mass percentage (wt%)	O	Si	C	Al	Ca	Na
Massive olivine basalt	44.53	12.48	25.43	3.69	1.12	0.43

), the composition elements of the two basalt samples are mainly O, Si, C, Al, and a small amount of Ca, Na, Fe, K, and Mg. The mass percentage of Fe and Mg was higher in massive olivine basalt.

2.3. Experimental Scheme

Based on the basalt samples obtained in the preliminary stage, four groups of basalt CO₂–water–rock interaction experiments were conducted under different conditions (

Table 9. Statistical table of CO₂–water–basalt interaction experiments under different conditions.

Group Number	Serial Number	Basalt Type	Experimental Condition
1	1	Porous basalt	26 °C, 5 MPa, pH 8, particle size range 60–65 mesh
	2		36 °C, 5 MPa, pH 8, particle size range 60–65 mesh
	3		46 °C, 5 MPa, pH 8, particle size range 60–65 mesh
	4		56 °C, 5 MPa, pH 8, particle size range 60–65 mesh
	5		66 °C, 5 MPa, pH 8, particle size range 60–65 mesh
2	6	Porous basalt	26 °C, 5 MPa, pH 8, particle size range 60–65 mesh
	7		26 °C, 7 MPa, pH 8, particle size range 60–65 mesh
	8		26 °C, 9 MPa, pH 8, particle size range 60–65 mesh
	9		26 °C, 11 MPa, pH 8, particle size range 60–65 mesh
	10		26 °C, 13 MPa, pH 8, particle size range 60–65 mesh
3	11	Porous basalt	26 °C, 5 MPa, pH 8, particle size range 16–18 mesh
	12		26 °C, 5 MPa, pH 8, particle size range 30–35 mesh
	13		26 °C, 5 MPa, pH 8, particle size range 60–65 mesh
4	14	Porous basalt	26 °C, 5 MPa, pH 8, particle size range 60–65 mesh
	15	Massive olivine basalt	26 °C, 5 MPa, pH 8, particle size range 60–65 mesh

), with the experimental solution uniformly using Wahaha purified water (pH ~ 8). The CO₂–water–rock reaction experiment variables were as follows:

(1)

Porous basalt samples at 5 MPa pressure, pH 8, and particle size range of 60–65 mesh (0.25–0.3 mm) at temperatures of 26, 36, 46, 56, and 66 °C.

(2)

Porous basalt samples with particle size range of 60–65 mesh (0.25–0.3 mm) at 26 °C, pH 8, and pressures of 5, 7, 9, 11, and 13 MPa.

(3)

Porous basalt samples at 26 °C, 5 MPa, pH 8, and particle size range of 60–65 mesh (0.25–0.3 mm), 30–35 mesh (0.6–0.65 mm), and 16–18 mesh (1.0–1.25 mm).

(4)

Massive olivine basalt and pore basalt at 26 °C, 5 MPa, pH 8, and particle size range of 60–65 mesh (0.25–0.3 mm).

Since each group used porous basalt under a temperature of 26 °C, a pressure of 5 MPa, and a particle size of 60–65 mesh as the control experiment, and the experimental samples, experimental conditions, and testing institutions were all the same, this experiment was not repeated. Excluding this repeated experiment, the total number of indoor experiments was 12. During the experiments, the temperature and pressure were kept relatively stable, and the experiment lasted for 15 days.

2.4. Experimental Apparatus

The experimental apparatus consists of a high-temperature and high-pressure reactor manufactured from GH4169 nickel-based superalloy, with a rated temperature tolerance of 400 °C, pressure capacity of 400 MPa, and a volume of 500 mL. For the CO₂–water–rock interactions experiments, the reactor was also equipped with a CO₂ gas cylinder, air compressor, and gas pressurization system (Figure 5 and Figure 6).

2.5. Analysis of Testing Methods

Varying influencing factors were employed in the CO₂–water–rock interaction experiments, producing a range of reaction intensities for the samples. Before and after the experiment, the experimental samples were sent to the testing units with corresponding qualification certifications for analysis and testing. Different testing instruments were used for detailed analysis. The samples were observed by polarizing microscope, and their mineral composition was analyzed using a SmartLab SE X-ray diffractometer. Chemical composition was analyzed by iCAP 7000 series plasma XRF spectrometer and an EDX spectrometer. The particle aperture size was measured with a specific surface area and pore size analyzer. The chemical composition of the post-reaction solution was analyzed to determine its pH and ion content. The specific analytical equipment and types are described in

Table 10. Analytical equipment and testing content.

Instrument Name	Instrument Model	Test Content
X-ray fluorescence spectrometer (XRD)	Smart Lab SE X-ray diffractometer (Thermo Fisher Scientific Inc., Shanghai, China)	Granular composition of sample particles
Plasma emission spectrometer (XRF)	Thermo Fisher Scientific iCAP 7000 Series Plasma Emission Spectrometer (CIQTEK Co., Ltd., Hefei, China)	Component of sample particle oxide
Desk scanning electron microscope (SEM)	Cold Field Emission Scanning Electron Microscope JSM-6700F (JEOL Ltd., Tokyo, Japan)	Microstructure of sample particles
Energy spectrometer (EDX)	X-ray energy spectrometer (accessory for scanning electron microscopy) (JEOL Ltd., Tokyo, Japan)	Elemental composition of sample particles
Burette	Tianke glass (Tianjin Tianke Glass Instrument Manufacturing Co., Ltd., Tianjin, China)	Ca2+, Mg2+, CO32−, HCO3− Ion concentration
Ion chromatograph	ECo IC (Metrohm China Ltd., Beijing, China)	Cl−, SO42−, Ion concentration
Ultraviolet spectrophotometer	SPECORD5OPLUS (Analytik Jena AG, Beijing, China)	Fe3+, Fe2+ Ion concentration
Atomic absorption spectrophotometer	A3AFG-12 (Beijing Puxi General Instrument Co., Ltd., Beijing, China)	K+, Na+, Ion concentration
Inductively Coupled Plasma-Emission Mass Spectrometer	iCAP RQ (Thermo Fisher Scientific Inc., Shanghai, China)	Al3+ Ion concentration
PH meter	PHS-3C pH meter (INESA SCIENTIFIC INSTRUMENT CO., LTD., Shanghai, China)	PH of the solution
Specific Surface Area and Pore Size Analyzer	3H-2000PS2 (BeiShiDe Instrument Technology (Beijing) Co., Ltd., Beijing, China)	Particle aperture size

.

2.6. Experimental Procedure

2.6.1. Blank Control Experiment

To minimize experimental errors and eliminate interference from possible equipment corrosion, a CO₂–water interaction experiment using purified water as the reaction medium was designed prior to conducting the formal experiments. Blank control experiments were performed without added rock samples at varying temperatures and pressures (

Table 11. Experiment of CO₂-water interaction at different temperatures/pressures.

Group Number	Serial Number	Reaction Medium	Experimental Condition
1	1	Purified water (Wahaha)	26 °C, 5 Mpa
	2		36 °C, 5 Mpa
	3		46 °C, 5 Mpa
	4		56 °C, 5 Mpa
	5		66 °C, 5 Mpa
2	1	Purified water (Wahaha)	26 °C, 5 Mpa
	2		26 °C, 7 Mpa
	3		26 °C, 9 Mpa
	4		26 °C, 11 Mpa
	5		26 °C, 13 Mpa

).

During the blank experiment procedure, 300 mL of purified water were introduced into the reactor, CO₂ gas was introduced at the specified pressure, and the temperature was raised to the designated level. Since no rock samples were included, in order to determine whether the experimental equipment had any impact on the results, the experimental duration did not need to be too long. A preliminary setting of 72 h has been made. Upon completion of the reaction, the reactor was opened to remove the reaction mixture, and the chemical composition of the post-reaction solution was analyzed using a pH meter and mass spectrometer.

In the blank control experiment, two approaches were designed. The interaction of CO₂ and water was carried out in the high-temperature–high-pressure reactor at five temperatures and five pressure values. The changes of pH and ion concentration of the post-reaction solution were observed at different temperatures and pressures to eliminate possible interference in the experiment.

The pH values and concentrations of anions and cations in the solutions obtained under different temperatures and pressures were analyzed (Figure 7 and Figure 8). The pH showed little variation, ranging from 4.65 to 6.56, which indicated that the dissolution of CO₂ in water resulted in an acidic solution. The concentrations of Na⁺, K⁺, Mg²⁺, Al³⁺, Fe²⁺, and other cations were less than 5 mg·L⁻¹, but the Ca²⁺ concentration exceeded those of the other metallic ions, in some cases being greater than 20 mg·L⁻¹. This was mainly due to the dissolved matter and impurities in the reaction vessel. The concentrations of anions such as Cl⁻ and SO₄²⁻ were less than 5 mg·L⁻¹, although the HCO₃⁻ concentration was higher than for the other anions, primarily due to the dissolution of CO₂ gas.

2.6.2. CO₂–Water–Basalt Interaction Experiments

Following the blank experiment, the CO₂–water–basalt interaction experiments were carried out for different conditions of temperature, pressure, particle size, and basalt type. The operational procedure is described in the following text:

(1)

The rock was ground into particles of different sizes according to the experimental protocol, then washed with purified water at 105 °C for 24 h, and dried for subsequent use.

(2)

Prior to the experiment, 300 mL of purified water was added to the reactor. The rock sample (solid-to-liquid ratio 1:15) was weighed, and 20 g of the sample was then sealed in the reactor.

(3)

The reactor air compressor was turned on and CO₂ was introduced. The CO₂–water–rock reaction experiment was performed according to the pre-set experimental protocol. The temperature and pressure were maintained under relatively stable conditions throughout the 15-day experiment.

(4)

After the reaction was completed, the reactor was opened to retrieve the rock samples and reaction solution.

(5)

After sampling, the rock specimens were rinsed with purified water, dried at 105 °C for 24 h, and weighed.

(6)

The rock samples and reaction solutions were tested. XRD was employed to identify the mineral types that had disappeared or had formed, while EDX and XRF spectroscopy were utilized to confirm the mineral types and their primary chemical compositions. SEM was used to observe the mineral morphology of the rock samples. The particle aperture size of the samples were analyzed using a specific surface area and pore size analyzer. The residual solution after the reaction was transferred to a solution bottle and the changes in pH before and after the reaction were recorded using a pH meter. The chemical composition of the solution was determined through ultraviolet spectrophotometry and mass spectrometry.

3. Data Analysis of Experiments

3.1. Effect of Temperature on CO_2–Water–Basalt Reaction Experiments

3.1.1. Changes in Sample Mass and Loss Rate

To consider the effect of temperature, the CO_2–water–rock reaction experiment was carried out on 60–65 mesh porous basalt particles at five temperatures (26 °C, 36 °C, 46 °C, 56 °C and 66 °C) and 5 MPa pressure. The pre- and post-experiment mass loss and loss rate (mass loss/pre-experiment mass) of the rock were analyzed using the balance and conversion rate calculation formula. The results are shown in

Table 12. Mass loss and loss rate of basalt samples after experiments at different temperatures.

Temperature	Pressure	Mass (g)		Loss Amount	Loss Rate
(°C)	(Mpa)	Before the Experiment	After the Experiment	(g)	(%)
26	5	20.05	19.81	0.24	1.20
36		20.08	19.35	0.73	3.64
46		20.06	19.4	0.66	3.29
56		20.04	19.43	0.61	3.04
66		20.01	19.41	0.6	3.00

and Figure 9.

Figure 8 shows the temperature effects on the CO₂–water–basalt reaction experiment. The blue bars represent the mass loss of the basalt sample during the experiment and the orange bars show the conversion rate changes throughout the process. The order of mass loss relative to temperature was 26 °C < 66 °C < 56 °C < 46 °C < 36 °C, for the reason that the basalt particles were gradually dissolved in the CO₂ solution at low temperature, then the rate of dissolution of minerals in the basalt increased with rising temperature; the concentration of metal ions in the solution increased accordingly. Meanwhile, beyond 36 °C the metallic ions in the solution reacted with HCO₃⁻ and H⁺ to form a precipitate of carbonate mineral, leading to the decreasing loss of basalt particles.

3.1.2. Changes in Mineral Composition of Samples

The SEM images in Figure 9 show that the feldspar was slightly dissolved and a few pores appeared at 26 °C (Figure 10B). At 56 °C (Figure 10C) the feldspar was further eroded leaving the surface rough and uneven, and a few irregular mineral crystals were formed on the surface.

To further validate the phenomena observed by SEM, whole-rock XRD and XRF analyses were performed on basalt particle samples before and after the experiment, with results compared (

Table 13. Mineral composition of basalt particles after experiments at different temperatures.

Temperature	Quartz (%)	Anorthite (%)	Calcite (%)	Pyroxene (%)	Common Pyrite (%)	Clay Mineral (%)
Before the experiment	0.8	60	25.2	7.7	1.4	4.9
26 °C	/	78.8	6.7	5.4	2.5	6.6
36 °C	0.8	80.3	4.3	5	3.7	5.9
46 °C	0.8	82.3	4	4.1	2.4	6.4
56 °C	0.8	74.2	4.4	11.1	1.8	7.7
66 °C	1	65.7	20.4	5.6	1.9	5.4

and

Table 14. Chemical composition of basalt particles after experiments at different temperatures.

Temperature	SiO2 (%)	Fe2O3 (%)	Al2O3 (%)	CaO (%)	K2O (%)	MgO (%)	Na2O (%)	TiO2 (%)	P2O5 (%)	MnO (%)
Before the experiment	50.89	8.66	14.95	9.55	3.73	1.85	3.69	1.32	0.6	0.11
26 °C	56.95	7.57	19.2	5.24	3.6	1.45	3.5	1.31	0.6	0.05
36 °C	56.49	7.32	18.95	4.73	3.48	1.37	3.38	1.27	0.6	0.04
46 °C	53.33	7.65	19.54	5.34	3.61	1.49	3.57	1.32	0.62	0.05
56 °C	52.74	7.79	20.16	5.31	3.69	1.48	3.66	1.35	0.61	0.05
66 °C	51.65	7.78	17.87	8.46	3.84	1.76	3.84	1.41	0.66	0.1

). Notably, the percentage contents of calcium feldspar and calcite changed significantly, corresponding to variations in the chemical compositions of SiO₂, Al₂O₃, and CaO.

The percentage of the calcite mineral component firstly showed a decreasing trend followed by an increase, which indicates that a certain degree of dissolution occurred at the beginning of the experiment. With increasing temperature, the degree of dissolution increased, and reached a minimum value at 46 °C. Then, with further temperature increase, the dissolved Ca²⁺ reacted with the CO₂ solution to form a CaCO₃ precipitate, peaking at 66 °C. Referring to existing studies, the Arrhenius equation directly expresses the relationship between the mineral dissolution rate constant k and temperature T, namely, the higher the temperature, the greater the dissolution rate constant [28]. In this study, after the experiments conducted at 26–56 °C, the calcite mineral content had largely dissolved compared to before the experiments, whereas at 66 °C it increased to 20.4%. This further confirms that the calcite mineral formed after the 66 °C experiment was generated through the precipitation reaction between Ca²⁺ and CO₂ solution.

The percentage of calcium feldspar showed an increasing and then decreasing trend because it increased at the early stage with the sharp decrease in the percentage of calcite, then decreased with the increase of calcite beyond 46 °C.

Due to the presence of the CO₂, the crystals of feldspar, pyroxene, and calcite in basalt dissolved in water, whereas other cations reacted with the CO₂ solution to form carbonate minerals. EDX was employed to further analyze and study the elemental composition of the generated mineral particles (

Table 15. Chemical elements in the basalt particles after experiments at different temperatures.

Temperature	O (%)	Si (%)	C (%)	Al (%)	Ca (%)	Na (%)	Fe (%)	K (%)	Mg (%)	Ti (%)	Mn (%)
Before the experiment	45.5	20.7	12.1	12.0	5.3	3.3	0.6	0.4	0.3	/	/
26 °C	55.74	8.81	24.8	2.80	2.38	1.35	1.02	0.10	2.85	0.14	/
36 °C	38.62	21.95	18.93	10.08	4.96	3.18	1.32	0.65	0.32	/	/
46 °C	50.76	28.36	/	10.26	0.44	4.47	0.27	5.10	/	0.34	/
56 °C	50.2	16.9	17.3	4.1	0.9	1.0	3.0	1.3	5.3	/	/
66 °C	53.1	0.3	19.9	0.2	25.7	/	0.3	/	/	/	0.5

). At 66 °C, the carbonate mineral crystals were clearly detectable, with elemental composition mass percentages primarily consisting of 19.9% O, 53.1% C, and 25.7% Ca. The mineral type was inferred to be calcite, with the molecular formula CaCO₃.

3.1.3. Changes in the Concentration of Ionized Solution and pH Value

Chemical composition and pH tests were performed on the post-experiment solution (

Table 16. Chemical composition and pH of the solution after experiments at different temperatures.

Analysis Project		26 °C	36 °C	46 °C	56 °C	66 °C
Cation concentration (mg/L)	K+	3.66	3.15	3.25	6.18	3.47
	Na+	3.48	6.01	5.34	5.11	5.35
	Ca2+	268.54	196.59	265.53	114.73	215.43
	Mg2+	78.25	67.92	91.73	100.54	116.64
	Fe3+	<0.08	<0.08	<0.08	<0.08	<0.08
	Fe2+	<0.08	<0.08	<0.08	<0.08	<0.08
	Al3+	0.04	21	9.50	5.58	0.01
anion concentration (mg/L)	Cl−	23.88	1.8	7.26	3.74	0.93
	SO42−	3.36	2.54	3.34	3.24	7.47
	HCO3−	1156.33	970.83	1313.46	869.54	1308.88
	CO32−	/	/	/	/	/
TDS		959	763	1033	668	1004
PH value		6.55	6.59	6.67	7.01	6.93

).

As the temperature increased, the pH value of the post-experiment solution (Figure 11) first rose and then declined. The reason for this was that during the initial stage of the experiment, CO₂ dissolved in water to form carbonic acid H₂CO₃⁻, resulting in a decrease in the pH value of the solution. As the temperature increased, metallic cations in the minerals gradually dissolved due to the acidic conditions, consuming H⁺ ions in the solution and causing a progressive increase in pH, which peaked at 56 °C. Concurrently, the metallic cations combined with the CO₂ solution to form carbonate minerals, which further promoted the dissolution of the CO₂ gas into the water, leading to a further decrease in pH.

The Ca²⁺ in the solution was mainly derived from the dissolution of calcite, calcium feldspar, and pyroxene minerals. With increasing temperature, it showed a decreasing–increasing–decreasing–increasing trend (Figure 11). This may have been due to the dissolution of calcite and calcium feldspar at the beginning of the experiment, together with the increase of Ca²⁺ concentration in the solution and the combination of Ca²⁺ with CO₂ solution to form carbonate minerals, which would have resulted in the decreased Ca²⁺ concentration in the solution. With the increase of temperature and the formation of carbonate minerals, Ca²⁺ precipitation in the acidic condition increased to a peak at 46 °C, then it combined with the CO₂ solution to form carbonate minerals to reach a minimum value at 56 °C. Following this, the dissolution of calcite and pyroxene continued, and the Ca²⁺ content in the solution continued to increase.

The primary source of Mg²⁺ in the solution was the dissolution of pyroxene and clay minerals. As temperature increased, the Mg²⁺ concentration in the solution first decreased and then increased (Figure 11). The reason may be that pyroxene minerals dissolved at the beginning of the experiment. The data at 36 °C indicated a decrease in both pyroxene and clay minerals, and the Mg²⁺ content also decreased. It is speculated that the Mg²⁺ combined with CO₃² to form unstable hydrated magnesium carbonate minerals (e.g., Brucite [Mg₅(CO₃)₄(OH)₂·4H₂O], dypingite [Mg₅(CO₃)₄(OH)₂·5H₂O], and nesquehonite [MgCO₂·3H₂O]), which reduced the concentration of Mg²⁺ in the solution. With increasing temperature and the formation of carbonate minerals together with the further dissolution of pyroxene and clay minerals, the content of Mg²⁺ in the solution gradually increased.

The source of Al³⁺ in the solution was mainly from the dissolution of calcium feldspar and clay minerals (montmorillonite), showing an increasing and then decreasing trend (Figure 11), with the peak at 36 °C. The reason is that the solubility of calcium feldspar and clay minerals was the greatest at 36 °C, after which the Al³⁺ in the solution reacted with OH⁻ to form precipitates.

The primary source of Fe²⁺ in the solution was the dissolution of pyrite. As the concentration was below the detection limit, it was not analyzed in this study. Additionally, the sources of K⁺ and Na⁺ in the solution were mainly the dissolution of clay minerals. Due to their low concentrations, they were also not analyzed in this study.

The concentration of HCO₃⁻ in the solution showed a decreasing trend, then increasing, decreasing, and increasing with the increase of temperature (Figure 9), which is consistent with the trend of Ca²⁺ concentration. The reason may be that the mineral dissolved at the beginning of the experiment, and with the introduction of CO₂, the concentration of HCO₃⁻ in the solution increased, and the metallic cations (Ca²⁺, Mg²⁺) reacted with the CO₂ solution to form carbonate minerals, thus decreasing its concentration in the solution. Then, as the temperature rose and carbonate minerals formed, CO₂ continued to dissolve in water, generating HCO₃⁻ ions, which increased their concentration. The further dissolution of calcium feldspar and calcite at elevated temperatures promoted the binding of Ca²⁺ with CO₂ solution to form carbonate minerals, reaching a peak at 46 °C. Subsequently, the continued dissolution of CO₂ into the water produced HCO₃⁻ ions, leading to a sustained rise in their concentration.

In addition, the concentrations of CL⁻ and SO₄²⁻ in the solution were low, so they were not analyzed in this study.

3.1.4. Changes in Particle Aperture Size

The particle samples after the experiment were analyzed for their pore size using a surface area and pore size analyzer. The pore size data are presented in

Table 17. Particle aperture size of basalt samples after experiments under different temperatures.

Temperature	Before the Experiment	26 °C	36 °C	46 °C	56 °C	66 °C
Particle aperture size/nm	24.6929	33.7079	37.5877	34.8477	33.5583	24.8513

below.

The data of sample particle aperture size shows a trend of increasing first and then decreasing. The reason for this is that minerals gradually dissolved at the beginning of the experiment. The particle aperture size reached their peak at 36 °C, and then began to decrease. Carbonate precipitates gradually formed thereafter, and the particle aperture size were the smallest at 66 °C, indicating that the maximum amount of carbonate precipitates was generated at this temperature.

Section 3.1.1 indicates that the mass loss of samples at 26 °C is less than that at 36 °C, while Section 3.1.3 shows that the Ca²⁺ concentration in the solution at 36 °C is lower than that at 26 °C, both suggesting the formation of calcium carbonate precipitate at 36 °C. Regarding Section 3.1.2, the calcite percentage at 36 °C is lower than that at 26 °C, and Section 3.1.4 reveals that the pore size at 36 °C is larger than at 26 °C. The analytical reason lies in the inflection point within the 26–36 °C range, which shifts the calcite mineralization reaction from dissolution to precipitation.

The final conclusion is that calcite mineral precipitation begins to form at 36 °C, with the highest amount of calcite minerals formed at 66 °C.

3.2. Geothermal and Hydrogeological Characteristics

3.2.1. Changes in Sample Mass and Loss Rate

The CO₂–water–rock reaction experiments were carried out on 60–65 mesh pumice granules at 26 °C subjected to five different pressures (5, 7, 9, 11, and 13 MPa). The mass loss and conversion rate of the rock before and after the reaction were analyzed by the balance and the conversion rate calculation formula. The results are shown in

Table 18. Loss and loss rate of basalt samples for different pressure conditions after experiments.

Temperature (°C)	Pressure (Mpa)	Mass (g)		Loss Amount (g)	Loss Rate (%)
		Before the Experiment	After the Experiment
26	5	20.05	19.81	0.24	0.01
	7	20.07	19.79	0.28	0.01
	9	20.00	19.17	0.83	0.04
	11	19.98	19.00	0.98	0.05
	13	20.00	19.73	0.27	0.01

and Figure 12.

Figure 12 shows the effect of pressure in the CO₂–water–basalt reaction experiment. The blue bars represent the mass loss variation of the basalt sample during the experiment; the orange bars indicate the changes in the conversion rate throughout the process. The post-experiment basalt mass had decreased, with the amount of loss varying with applied pressure from 5 MPa to 11 MPa. At low pressures, basalt particles were gradually dissolved in the CO₂ solution. As the pressure was increased, the dissolution rate accelerated, leading to higher concentrations of metallic ions. These reacted with the CO₂ solution at elevated pressures to form carbonate mineral precipitates.

3.2.2. Changes in Mineral Composition of Samples

SEM analysis revealed that at 5 MPa (Figure 13B), the feldspar exhibited slight dissolution with minor pore formation. At 13 MPa (Figure 13C), the dissolution was intensified, resulting in the formation of small mineral crystals on the surface.

To further validate the SEM observations, whole-rock XRD and XRF pre- and post-experiment analyses were performed on basalt particle samples, with results compared in

Table 19. Mineral composition of basalt particles at different pressures.

Pressure	Quartz (%)	Anorthite (%)	Calcite (%)	Pyroxene (%)	Common Pyrite (%)	Clay Mineral (%)
Before the Experiment	0.8	60	25.2	7.7	1.4	4.9
5 Mpa	/	78.8	6.7	5.4	2.5	6.6
7 Mpa	1.8	79.6	5.1	5	2.2	6.3
9 Mpa	0.7	80.3	7.1	4.7	2.5	4.7
11 Mpa	/	91.4	4.5	/	/	4.1
13 Mpa	/	92.4	1.6	/	/	6.0

and

Table 20. Chemical composition of basalt particles at different pressures.

Pressure	SiO2 (%)	Fe2O3 (%)	Al2O3 (%)	CaO (%)	K2O (%)	MgO (%)	Na2O (%)	TiO2 (%)	P2O5 (%)	MnO (%)
Before the Experiment	50.89	8.66	14.95	9.55	3.73	1.85	3.69	1.32	0.6	0.11
5 Mpa	56.95	7.57	19.2	5.24	3.6	1.45	3.5	1.31	0.6	0.05
7 Mpa	56.23	7.66	19.47	4.8	3.62	1.36	3.51	1.35	0.64	0.04
9 Mpa	51.96	8.32	19.76	6.13	3.65	1.59	3.57	1.38	0.61	0.07
11 Mpa	54.32	7.28	15.02	8.36	3.37	1.26	4.46	1.28	0.59	0.11
13 Mpa	62.38	6.03	13.01	5.04	3.03	0.90	4.07	1.16	0.55	0.05

. Notably, the percentage contents of calcium feldspar and calcite showed significant changes, corresponding to variations in the chemical compositions of SiO₂, Al₂O₃, and CaO.

The percentage of the calcite component showed a decreasing—increasing—decreasing trend, which indicates that some dissolution occurred at the beginning of the experiment, then increased with pressure increase. At 9 MPa pressure, the dissolved Ca²⁺ reacted with the CO₂ solution to form a small amount of carbonate mineral. Then, with further pressure increase, the calcite continued to be dissolved. The percentage of calcium feldspar shows a gradually increasing trend as pressure increased, attributable to the progressive dissolution of minerals such as calcite and pyroxene, leading to a gradual rise in the percentage of calcium feldspar.

The elemental composition of the sample particles was further analyzed through EDX (

Table 21. Elemental analysis of basalt grains at different pressures.

Pressure	O (%)	Si (%)	C (%)	Al (%)	Ca (%)	Na (%)	Fe (%)	K (%)	Mg (%)	Ti (%)
Before the Experiment	45.5	20.7	12.1	12.0	5.3	3.3	0.6	0.4	0.3	/
5 Mpa	38.62	21.95	18.93	10.08	4.96	3.18	1.32	/	0.32	/
7 Mpa	52.63	17.81	12.65	9.18	2.18	3.82	0.5	1.23	/	/
9 Mpa	56.38	14.98	14.11	7.62	1.58	3.91	0.39	0.93	/	0.11
11 Mpa	64.29	12.41	11.41	1.50	0.75	/	7.57	0.11	1.94	/
13 Mpa	61.47	18.84	/	17.37	0.10	/	1.07	0.90	0.27	/

). Below 13 MPa, the mass composition was mainly 61.47% O, 18.84% Si, and 17.37% Al. The mineral type was mainly feldspar, and no new carbonate mineral was formed. It was also found that the percentage content of Fe increased significantly at 11 MPa. Given that the formation temperature of siderite is above 100 °C [31], the possibility of siderite formation was excluded in this study, so the inferred reason for the increase in Fe content was the lower content of elements such as Ca, Al, and Na.

3.2.3. Changes in the Concentration of Ionized Solution and pH Value

Chemical composition and pH tests were performed on the post-experiment solution, with the results presented in

Table 22. Chemical composition and pH of solutions after experiments at different pressures.

Analysis Project		5 Mpa	7 Mpa	9 Mpa	11 Mpa	13 Mpa
Cation concentration (mg/L)	K+	3.66	0.68	3.36	3.06	3.26
	Na+	3.48	3.1	4.78	4.87	3.28
	Ca2+	268.54	252.5	406.81	638.78	292.58
	Mg2+	78.25	92.34	35.36	98.72	62.57
	Fe3+	<0.08	<0.08	<0.08	<0.08	<0.09
	Fe2+	<0.08	<0.08	<0.08	<0.08	<0.08
	Al3+	0.04	0.04	<0.01	<0.01	0.04
anion concentration (mg/L)	Cl−	23.88	1.3	1.18	2.36	1.58
	SO42−	3.36	3.24	5.7	2.56	11.3
	HCO3−	1156.33	1188.64	1412.61	2623.86	1195.99
	CO32−	/	/	/	/	/
TDS		959	956	1163	2062	973
pH value		6.55	6.72	6.23	6.53	6.43

.

As pressure increased, the pH of the post-experiment solution exhibited a rise–fall–rise–fall pattern (Figure 14). This was attributed to the initial dissolution of CO₂ in water, which lowered the pH. When the pressure was increased, metallic cations in the minerals gradually dissolved in acidic conditions, consuming H⁺ ions in the solution and causing the pH to rise. Subsequently, the dissolved metallic cations reacted with the CO₂ solution to form small amounts of carbonate minerals, promoting further dissolution of the CO₂ gas in the water. This led to a decline in the pH of the solution to reach its lowest value at 9 MPa pressure. Then, minerals such as calcite continued to dissolve, consuming H⁺ ions in the solution, again raising the pH. The metallic cations dissolved at pressures above 11 MPa then reacted with the CO₂ solution to form a small amount of carbonate minerals, causing the CO₂ to continue to dissolve in the water, again reducing the pH.

The Ca²⁺ in the solution was mainly derived from the dissolution of calcite and pyroxene minerals. With the increase of pressure, the Ca²⁺ content showed a decreasing–increasing–decreasing trend (Figure 14) for the reason that the Ca²⁺ concentration in the solution was slightly decreased when the Ca²⁺ and CO₂ solution combined to form a small amount of calcite. With increased pressure and the formation of calcite, the calcite and pyroxene minerals were further dissolved and the Ca²⁺ gradually increased, peaking at 11 MPa. The content of Ca²⁺ subsequently decreased, indicating that Ca²⁺ reacted with the CO₂ solution to form a small amount of calcite precipitation.

The Mg²⁺ in the solution was mainly the result of the dissolution of pyroxene and clay minerals and showed an increasing–decreasing–increasing–decreasing trend with increasing pressure (Figure 14), possibly for the reason that the pyroxene and clay minerals dissolved at the beginning of the experiment, increasing the Mg²⁺ content in the solution. The contents of both pyroxene and clay minerals then decreased with the increase of pressure, lowering the of Mg²⁺ content. At 9 MPa, both pyroxene and clay mineral contents decreased, accompanied by a reduction in Mg²⁺ content. This suggests that Mg²⁺ and CO₃²⁻ formed less stable hydrated magnesium carbonate minerals, such as brucite [Mg₅(CO₃)₄ (OH)₂·4H₂O], dypingite [Mg₅(CO₃)₄(OH)₂·5H₂O], and nesquehonite [MgCO₃·3H₂O], which reached their maximum values at 9 MPa. With the increase of pressure and the formation of carbonate minerals, the pyroxene and clay minerals were further leached, and the content of Mg²⁺ in the solution gradually increased. At 13 MPa, the content of Mg²⁺ again decreased, which is speculated to be due to the reaction of Mg²⁺ and CO₃²⁻ to form a small amount of hydrated magnesium carbonate minerals.

The Fe²⁺ in the solution was mainly derived from the dissolution of pyrite. The Fe²⁺, together with K⁺, Na⁺, and Al³⁺ primarily derived from the dissolution of clay minerals, were not analyzed in this study due to their low concentrations.

The concentration of HCO₃⁻ in the solution firstly increased, then decreased with rising pressure (Figure 14), largely consistent with the trend of Ca²⁺ concentration. The analysis suggests that initially, as CO₂ was introduced, the HCO₃⁻ concentration gradually increased to peak at 11 MPa. Subsequently, metallic cations (Ca²⁺, Mg²⁺, Fe²⁺) in the minerals combined with CO₃²⁻ to form a small amount of carbonate minerals, leading to a decrease in the ion concentration in the solution.

In addition, the concentrations of Cl⁻ and SO₄²⁻ in the solution were low, and were not analyzed in this study.

3.2.4. Changes in Particle Aperture Size

The particle samples after the experiment were analyzed for their pore size using a surface area and pore size analyzer. The pore size data are presented in

Table 23. Particle aperture size of basalt samples after experiments under different pressures.

Pressure	Before the Experiment	5 Mpa	7 Mpa	9 Mpa	11 Mpa	13 Mpa
Particle aperture size/nm	24.6929	33.7079	34.6216	37.6706	39.4143	36.6208

below.

The data of sample particle aperture size shows a trend of increasing first and then decreasing. The reason for this is that minerals gradually dissolved at the beginning of the experiment. The pore diameter reached its peak at a pressure of 11 MPa, and decreased at a pressure of 13 MPa, indicating that a small amount of carbonate precipitates were formed under this pressure.

Section 3.2.1 indicates that the mass loss of samples at 13 MPa is less than that at 11 MPa, and Section 3.2.3 shows that the Ca²⁺ concentration in the solution at 13 MPa is lower than at 11 MPa, both suggesting the formation of a small amount of calcium carbonate precipitate under 13 MPa pressure. Regarding Section 3.1.2, the calcite percentage at 13 MPa is lower than at 11 MPa; this is attributed to an inflection point between the 11 MPa and 13 MPa pressure ranges, which shifts the calcite mineral from a dissolution reaction to a precipitation reaction.

The final conclusion is that as pressure increases, the existing calcite minerals primarily exhibit a gradual dissolution trend, with only a small amount of calcite precipitate forming at 13 MPa.

3.3. Effect of Particle Size on CO_2–Water–Basalt Reaction Experiment

3.3.1. Changes in Sample Mass and Loss Rate

To evaluate the effect of particle size on the experiment, CO₂–water–basalt reactions were tested on three pore-bearing basalt particle samples (16–18, 30–35, and 60–65 mesh) at 26 °C and 5 MPa. The mass loss and conversion rate of the rock were analyzed using a balance and conversion rate calculation formula (results in

Table 24. Loss and loss rate of three particle-size basalt samples.

Temperature (°C)	Pressure (Mpa)	Particle Size (Mesh)	Mass (g)		Loss Amount (g)	Loss Rate (%)
			Before the Experiment	After the Experiment
26	5	16–18 mesh (1–1.25mm)	20.03	19.82	0.21	0.01
		30–35 mesh (0.6–0.65mm)	20.06	19.78	0.28	0.01
		60–65 mesh (0.25–0.3mm)	20.05	19.81	0.24	0.01

and Figure 15).

Figure 15 shows the effect of particle size on the CO₂–water–basalt reaction experiment. Blue bars show the variation in mass loss of the basalt sample during the experiment; orange bars show the change in conversion rate of the sample throughout the reaction. The reduction of post-experiment basalt mass was found to be in the order 16–18 mesh < 60–65 mesh < 30–35 mesh, since finer rock particles with greater specific surface area experienced greater contact with CO₂ solution, although excessively fine particles (i.e., 60–65 mesh) impeded solution penetration and flow, thereby reducing internal reaction rates. Consequently, the greatest dissolution occurred for 30–35 mesh size particles.

3.3.2. Changes in Mineral Composition of Samples

SEM analysis showed that feldspar was strongly eroded in 16–18 mesh particles (Figure 16A) with a rough and uneven surface, and was slightly dissolved in 60–65 mesh particles (Figure 16B) in which a only small numbers of pores were evident.

To further illustrate the phenomena observed through SEM, the mineral and chemical compositions of solid-state samples were analyzed through whole-rock XRD and XRF before and after the experiments (

Table 25. Mineral composition of different size basalt particles before and after experiments.

Particle Size	Quartz (%)	Anorthite (%)	Calcite (%)	Pyroxene (%)	Common Pyrite (%)	Clay Mineral (%)
Before the experiment	0.8	60	25.2	7.7	1.4	4.9
16–18 mesh	/	81.9	3.6	5.8	2.5	6.2
30–35 mesh	/	82.9	4.4	3.1	2.9	6.7
60–65 mesh	/	78.8	6.7	5.4	2.5	6.6

and

Table 26. Chemical composition of different size basalt particles before and after experiments.

Particle Size	SiO2 (%)	Fe2O3 (%)	Al2O3 (%)	CaO (%)	K2O (%)	MgO (%)	Na2O (%)	TiO2 (%)	P2O5 (%)	MnO (%)
Before the experiment	50.89	8.66	14.95	9.55	3.73	1.85	3.69	1.32	0.6	0.11
16–18 mesh	56.59	7.53	19.14	4.41	3.48	1.39	3.43	1.33	0.6	0.04
30–35 mesh	56.82	7.62	19.47	4.57	3.6	1.37	3.58	1.37	0.62	0.04
60–65 mesh	56.95	7.57	19.2	5.24	3.6	1.45	3.5	1.31	0.6	0.05

). Notably, the percentage contents of calcium feldspar and calcite showed significant changes, corresponding to variations in the chemical compositions of SiO₂, Al₂O₃, and CaO.

The percentage content of calcite in the mineral composition shows a gradually increasing trend. Under the particle size condition of 16–18 mesh, the percentage content of calcite is the smallest, and under the particle size condition of 60–65 mesh, the percentage content of calcite is the largest. The analysis of the reason is the co-dissolution of calcite, pyroxene, and calcium feldspar under different particle size conditions. This led to the percentage content of calcite not being consistent with the overall degree of dissolution.

The percentage of the mineral components of the calcic feldspar showed a trend of firstly rising then falling. The reason was that the percentage of calcite decreased significantly and the calcic feldspar and pyroxene also dissolved to a small extent, leading to the change of the percentage of calcic feldspar.

The application of EDS for further analysis (

Table 27. Elemental analysis of different size basalt particles.

Particle Size	O (%)	Si (%)	C (%)	Al (%)	Ca (%)	Na (%)	Fe (%)	K (%)	Mg (%)	Ti (%)
Before the experiment	45.5	20.7	12.1	12.0	5.3	3.3	0.6	0.4	0.3	/
16–18 mesh	53.86	11.73	23.44	4.80	0.84	3.18	0.47	1.08	0.6
30–35 mesh	55.17	14.80	11.68	6.41	2.54	2.98	2.75	2.63	0.64	0.41
60–65 mesh	55.74	8.81	24.8	2.80	2.38	1.35	1.02	0.10	2.85	0.14

) revealed that the percentage of Ca element was the least in the 16–18 mesh particle size material, indicating that the degree of calcite dissolution was the greatest at that particle size.

3.3.3. Changes in Concentration of Ionized Solution and the pH Value

Chemical composition and pH tests were performed on the post-experiment solution, with the results presented in

Table 28. Chemical composition and pH of post-experiment solutions for different basalt particle sizes.

Analysis Project		16–18 Mesh (1–1.25 mm)	30–35 Mesh (0.6–0.65 mm)	60–65 Mesh (0.25–0.3 mm)
Cation concentration (mg/L)	K+	2.89	3.02	3.66
	Na+	7.34	4.31	3.48
	Ca2+	214.43	320.64	268.54
	Mg2+	54.68	89.3	78.25
	Fe3+	<0.08	<0.08	<0.08
	Fe2+	<0.08	<0.08	<0.08
	Al3+	<0.01	0.02	0.04
anion concentration (mg/L)	Cl−	4.1	2.88	23.88
	SO42−	5.8	3.02	3.36
	HCO3−	933.61	1354.64	1156.33
	CO32−	/	/	/
TDS		758	956	959
PH value		6.44	6.9	6.55

.

The pH of the solution after the experiment (Figure 17) firstly increased then decreased. The pH reached its maximum with the 30–35 mesh particles, indicating the greatest mineral dissolution and highest consumption of H+ ions. The pH was lowest for the 16–18 mesh particles, suggesting lower mineral dissolution and the highest H+ ion content in the solution.

The primary source of Ca²⁺ ions in the solution was the dissolution of calcite and calcium feldspar minerals; their concentration firstly increased then decreased with particle size variation (Figure 17). This is attributed to the fact that calcite and calcium feldspar minerals dissolved most effectively in the 30–35 mesh particles, resulting in the highest Ca²⁺ content. Conversely, the dissolution of calcite and pyroxene minerals was relatively limited in the coarser 16–18 mesh particles, which produced the lowest Ca²⁺ concentration.

The primary source of Mg²⁺ in the solution was the dissolution of pyroxene and clay minerals. The Mg²⁺ concentration firstly increased then decreased with particle size variation (Figure 17), also attributable to the fact that pyroxene and clay minerals in 30–35 mesh particles dissolved most completely, giving the highest Mg²⁺ content. Conversely, their dissolution in 16–18 mesh particles was relatively limited, leading to the lowest Mg²⁺ ion concentration.

Fe²⁺ ions in the solution were mainly derived from the dissolution of pyrite, but was below the detection limit and was not analyzed in this study. Additionally, the K⁺, Na⁺, and Al³⁺ ions in the solution were primarily derived from the dissolution of pyroxene minerals. Due to their low concentrations, they were also not analyzed in this study.

Figure 17 also shows that the concentration of HCO₃⁻ anions in the solution firstly increased then decreased with particle size variation, largely consistent with the trends observed for Ca²⁺ and Mg²⁺. This may be attributed to the fact that minerals such as calcite and pyroxene in 30–35 mesh particles dissolved most completely, resulting in the highest consumption of H⁺ ions. Consequently, more CO₂ gas was introduced to achieve the maximum dissolved HCO₃⁻ concentration. Conversely, the dissolution of pyroxene minerals was relatively limited for the coarser 16–18 mesh particles, leading to the lowest dissolved HCO₃⁻ concentration.

In addition, the concentrations of Cl⁻ and SO₄²⁻ anions in the solution were relatively low and thus were not analyzed in this study.

3.3.4. Changes in Particle Aperture Size

The particle samples after the experiment were analyzed for their pore size using a surface area and pore size analyzer. The pore size data are presented in

Table 29. Particle aperture size of basalt samples after experiments under different particle sizes.

Particle Size	Before the Experiment	16–18 Mesh (1–1.25 mm)	30–35 Mesh (0.6–0.65 mm)	60–65 Mesh (0.25–0.3 mm)
Particle aperture size/nm	24.6929	33.5079	37.3995	33.7079

below.

Based on the data of sample particle aperture size, the particle aperture size is the largest at 30–35 mesh, indicating that the dissolution degree is the highest under this particle size condition, while the particle aperture size is the smallest at 16–18 mesh, suggesting that the dissolution degree is the lowest under this particle size condition.

Section 3.3.2 explains that under varying particle size conditions, the simultaneous dissolution of calcite, pyroxene, and calcium feldspar results in the percentage content of calcite not correlating with the overall dissolution degree.

Therefore, the final conclusion is that the dissolution degree is the highest when the particle size is between 30 and 35 mesh, and the lowest when it is between 16 and 18 mesh. The reason for this is that overly fine particles are not conducive to the penetration and flow of the solution, thereby reducing the reaction rate within the sample particles.

3.4. Effect of Different Types of Basalt on CO₂–Water–Rock Interaction Experiments

3.4.1. Changes in Sample Mass and Loss Rate

To consider the effect of different types of basalt, the CO₂–water–rock reaction experiments were carried out on the 60–65 mesh porous basalt and massive olivine basalt at 26 °C and 5 MPa. The pre- and post-experimental mass loss and conversion rate of the two basalt types shown in

Table 30. Loss and conversion rate of samples after experiments with different basalt types.

Basalt Type /	Temperature (°)	Pressure (Mpa)	Mass (g)		Loss Amount (g)	Loss Rate (%)
			Before the Experiment	After the Experiment
Porous basalt	26	5	20.05	19.81	0.24	0.01
Massive olivine basalt			19.52	19.30	0.22	0.01

and Figure 18 were obtained through the balance and conversion rate calculation formula.

Figure 18 shows the effects of different basalt types on the CO₂–water–rock interaction experiment. The blue bars represent the mass loss variation of basalt samples in this experiment, while the orange bars show the conversion rate changes during the process. The results show that the mass loss of basalt was larger in porous basalt than in massive olivine basalt. The reason is that the rate of mineral dissolution minerals was more rapid in the porous basalt.

3.4.2. Changes in Mineral Composition of Samples

SEM analysis revealed that the porous basalt particles exhibited slight dissolution with minor pore formation (Figure 19A), and the massive olivine basalt particles showed relatively weaker dissolution (Figure 19B).

To further investigate the SEM observations, whole-rock XRD and XRF analyses were conducted on basalt particle samples before and after the reaction, with results compared in

Table 31. Mineral composition of basalt-type particles after experiments.

Basalt Types	Quartz (%)	Anorthite (%)	Calcite (%)	Pyroxene (%)	Common Pyrite (%)	Clay Mineral (%)
Massive olivine basalt before the experiment	/	88.5	2.0	/	/	9.5
After the experiment with massive olivine basalt	/	92.2	1.1	/	/	6.7
Porous basalt before the experiment	0.8	60	25.2	7.7	1.4	4.9
After the experiment with porous basalt	/	78.8	6.7	5.4	2.5	6.6

and

Table 32. Chemical composition of basalt-type particles after experiments.

Basalt Types	SiO2 (%)	Fe2O3 (%)	Al2O3 (%)	CaO (%)	K2O (%)	MgO (%)	Na2O (%)	TiO2 (%)	P2O5 (%)	MnO (%)
Before the experiment with massive olivine basalt	47.29	13.22	14.29	8.79	0.90	7.27	3.16	1.60	0.21	0.15
After the experiment with massive olivine basalt	46.66	14.42	13.65	8.28	0.91	7.26	2.99	1.67	0.20	0.15
Before the experiment with porous basalt	50.89	8.66	14.95	9.55	3.73	1.85	3.69	1.32	0.6	0.11
After the experiment with porous basalt	56.95	7.57	19.2	5.24	3.6	1.45	3.5	1.31	0.6	0.05

. Notably, the percentage contents of calcium feldspar and calcite showed significant changes, corresponding to variations in the chemical compositions of SiO₂, Al₂O₃, and CaO.

The percentage of calcite in the mineral composition of massive olivine basalt was slightly decreased, the percentage of plagioclase was slightly increased, and the percentage of clay minerals was slightly decreased. The reason is that the calcite and clay minerals were dissolved in the CO₂ solution, which led to the slight increase in the percentage of plagioclase. The analysis of the porphyritic basalt showed a relatively significant decrease in calcite content, a relatively substantial increase in calcium feldspar content, a slight reduction in pyroxene content, a slight increase in pyrite content, and a slight rise in clay minerals. These suggest that the dissolution of calcite and pyroxene minerals in the CO₂ solution was relatively extensive, leading to significant variations in their mineral percentage content.

The elemental composition of the samples was further analyzed through EDX (

Table 33. Chemical element analysis of basalt particles after experiments with different basalt types.

Basalt Types	O (%)	Si (%)	C (%)	Al (%)	Ca (%)	Na (%)	Fe (%)	K (%)	Mg (%)	Ti (%)
Massive olivine basalt before the experiment	62.38	9.14	15.75	2.58	0.30	/	1.36	/	8.50	/
After the experiment with massive olivine basalt	57.48	13.18	12.27	3.48	0.39	/	3.50	/	9.70	/
Porous basalt before the experiment	45.5	20.7	12.1	12.0	5.3	3.3	0.6	0.4	0.3	/
After the experiment with porous basalt	55.74	8.81	24.8	2.80	2.38	1.35	1.02	0.10	2.85	0.14

), which showed that, both before and after the experiment, the percentage of Ca in massive basalt was relatively small, but was relatively large in porous basalt. This substantiated the indication that the dissolution reaction of porous basalt in the CO₂ solution was more intense.

3.4.3. Changes in Concentration of Ionized Solution and pH Value

Chemical composition and pH tests were performed on the post-experiment solution, with the results presented in

Table 34. Chemical composition and pH of post-experiment solutions for different basalt types.

Analysis Project		Porous Basalt	Massive Olivine Basalt
Cation concentration (mg/L)	K+	3.39	3.66
	Na+	10.37	3.48
	Ca2+	115.23	268.54
	Mg2+	99.02	78.25
	Fe3+	4.31	<0.08
	Fe2+	0.09	<0.08
	Al3+	<0.01	0.04
Anion concentration (mg/L)	Cl−	1.46	23.88
	SO42−	6.53	3.36
	HCO3−	896.99	1156.33
	CO32−	/	/
TDS		689	959
PH value		6.94	6.55

.

Figure 20 shows that the pH of the massive olivine basalt solution after the experiment was higher than for the porous basalt solution. Analysis revealed that at the start of the experiment, as the CO₂ dissolved in water, the H⁺ concentration in the solution increased, causing a drop in pH. Subsequently, although the soluble minerals in both the porous and massive olivine basalt solutions gradually dissolved, it was most pronounced in the porous basalt. Thus further CO₂ was incorporated into the water, ultimately leading to a decrease in the pH of the porous basalt solution.

The Ca²⁺ content in the porphyritic basalt solution was higher than in the massive olivine basalt solution (Figure 20). The reason is that the Ca²⁺ in the massive olivine basalt solution mainly stemmed from the dissolution of calcite crystals and clay minerals; the Ca²⁺ content was relatively low because the dissolution of massive olivine basalt was relatively low. The Ca²⁺ source in the porous basalt solution was mainly the dissolution of calcite crystals and pyroxene minerals. Because the dissolution of the porous basalt was greater than for the massive basalt, the Ca²⁺ content was higher.

The Mg²⁺ content in the massive olivine basalt solution exceeded that of porous basalt (Figure 20) since it mainly resulted from the dissolution of clay minerals, whereas the Mg²⁺ in the porous basalt solution mainly came from the dissolution of pyroxene minerals. Because the Mg²⁺ content of massive olivine basalt was higher than in porous basalt, the Mg²⁺ content in its solution was accordingly higher than in the porous basalt solution.

In addition, the source of Fe²⁺, K⁺, Na⁺, and Al³⁺ in the solution was mainly from the dissolution of pyroxene minerals or clay minerals, which were not analyzed in this study because of their low content.

The experiment revealed that the HCO₃⁻ concentration in the solution was higher in the porous basalt than in massive olivine basalt (Figure 20). Subsequent analyses indicated that the primary source of HCO₃⁻ in the solution was CO₂ gas dissolved in water. Since the pore-bearing basalt dissolved more readily, it consumed more H⁺ ions, thereby promoting the incorporation of more CO₂ gas into the water. Consequently, the porous basalt solution contained higher levels of HCO₃⁻.

It is noted that, due to the low concentrations of Cl⁻ and SO₄²⁻ in the solution, they were not analyzed in this study.

3.4.4. Changes in Particle Aperture Size

The particle samples after the experiment were analyzed for their pore size using a surface area and pore size analyzer. The pore size data are presented in

Table 35. Particle aperture size of basalt samples after experiments under different basalt types.

Basalt Type	Massive Olivine Basalt Before the Experiment	After the Experiment with Massive Olivine Basalt	Porous Basalt Before the Experiment	After the Experiment with Porous Basalt
Particle aperture size/nm	4.0616	4.2529	24.6929	33.5079

below.

Based on the data of sample particle aperture size, the particle aperture size of porous basalt increased significantly, while that of massive olivine basalt increased slightly. This further indicates that the degree of dissolution of porous basalt is greater, while that of massive olivine basalt is smaller.

4. Conclusions, Limitations, and Prospects

4.1. Conclusions

(1)

The transverse permeability of both porous basalt and massive olivine basalt was greater than that of the vertical permeability, and both the transverse and vertical porosity of the porous basalt were greater than those of massive olivine basalt The main mineral components of both types of basalt are the same: plagioclase, calcite, pyroxene, pyrite, quartz, and clay minerals. Because the filling material in the pore or almond structure usually includes secondary minerals such as calcite, quartz, and zeolite, the porous basalt contained the higher calcite content. The massive basalt contained a higher mass percentage of Fe and Mg, and the mass percentage of Ca was higher in the porous basalt.

(2)

According to the experimental data, the carbonate minerals formed in the porous basalt after the experiment were mainly calcite (CaCO₃), with a small amount of hydrated magnesium carbonate minerals such as magnesite [Mg₅(CO₃)₄(OH)₂·4H₂O], anhydrite [Mg₅(CO₃)₄(OH)₂·5H₂O], and magnesite hydrate [MgCO₃·3H₂O].

(3)

The experimental CO₂–water–basalt reaction under four different conditions found that calcite mineral began to form in basalt above 36 °C, and carbonate mineral was most abundant at 66 °C, accompanied by a small amount of hydrated magnesium carbonate mineral. When different pressures were applied at 26 °C, the original calcite minerals primarily exhibited a gradually greater dissolution trend as the pressure was increased. A small amount of hydrated magnesium carbonate mineral was generated at pressures above 7 MPa, and a small amount of calcite was formed at 13 MPa. For different particle sizes, the dissolution degree was most pronounced at 30–35 mesh. The dissolution reaction was more intense in porous basalt than in massive olivine basalt.

4.2. Limitations

(1)

There were few basalt types investigated in the CO₂–water–basalt experiments, and no experiments were conducted on basalt rich in calcium, magnesium, and iron and that free from filling impurities.

(2)

The results of the four groups of CO₂–water–basalt experiments, which were carried out by controlling the variables, failed to reflect the results of the mixed changes of temperature, pressure, particle size, and basalt type, with the conditions of 26 °C, 5 MPa and 60–65 mesh granular porous basalt as the benchmark.

(3)

Based on the mineral mass loss and changes in mineral composition, it is impossible to calculate a quantitative estimate of CO₂ storage capacity due to the excessive number of variables.

4.3. Prospects

The results of this study provide the basis for the construction of basalt CO₂ mineralization storage in the next step. When selecting drilling sites for basalt CO₂ storage, priority should be given to basalt reservoirs with high formation temperature, substantial pressure, and abundant Ca, Fe, and Mg content, while maintaining reasonable cost control. Fracturing tests can be conducted to explore the enhancement of contact area between basalt and the CO₂ solution.