Experimental evaluation of interactions in supercritical CO₂/water/rock minerals system under geologic CO₂ sequestration conditions

Abstract

The hydrothermal autoclave experiments were conducted to simulate the interactions in the scCO₂/water/rock minerals (quartz, biotite and granite) reaction systems using a Hastelloy C reaction cell at 100 °C. The dissolution characteristics of rock minerals and their surface texture alternation after hydrothermal treatment were examined by ICP-AES and SEM/EDX investigation, respectively. The results suggested that the hydrolysis of plagioclase phase should be mainly responsible for the elements dissolved from the Iidate granite samples. The dissolution was encouraged by the introduction of CO₂ in the water/granite system, and generated an unknown aluminosilicate. No distinct chemical alternations occurred in the water-free scCO₂/granite system, which indicated that rock minerals should be chemically stable in the water-free scCO₂ fluids under the current mild experimental conditions. Both the highest concentration of Ca existing in the scCO₂/vapor/granite system and the SEM observation results of calcite deposit, suggested that a meaningful CO₂ minerals trapping process should be potential in the CO₂-rich field during a short physicochemical interaction period.

Introduction

Over the past several decades worldwide hydrocarbon consumption has lead to a substantial increase in emission of carbon dioxide (CO₂) [1]. Many evidences suggest that the burning of fossil fuels has contributed to the accumulation of CO₂ in the atmosphere. Global circulation models indicate that these increased CO₂ accumulations are an important contribution to warmer temperature, increased rainfall, and raising sea level [2]. In an attempt to reduce or at least slow the increase of atmospheric CO₂ concentration, thereby reducing the risk of global climate change, one important potential approach for C management is the capture of CO₂ emissions and subsequent injection of CO₂ into geologic media for sequestration of CO₂. And the presence of natural CO₂ reservoirs supports this idea [3]. The benefits of CO₂ sequestration in the subsurface are great. CO₂ injected into the subsurface can potentially be isolated from the atmosphere for long-term periods. It has been estimated that there is sufficient capacity in deep brine filled sedimentary rocks and depleted hydrocarbon reservoirs to store up to 1000 years worth of CO₂ emissions. During geologic sequestration, three trapping mechanisms exist to retain CO₂ in a saline aquifer: hydrodynamic, solubility, or mineral trapping [4]. Hydrodynamic trapping involves the storage of CO₂ as supercritical fluid beneath a low permeability cap rock. Solubility trapping involves the dissolution of CO₂ into a fluid phase, including both aqueous brines and oil. Mineral trapping involves CO₂ rich reservoir brine reaction with the reservoir rocks to participate carbonate minerals [5]. Permanent sequestration of CO₂ can be achieved by the third mechanism [6–8]. However, risks are present. CO₂ can leak from the subsurface returning at least some of the stored CO₂ to the atmosphere. Such leaks may be exacerbated by the dissolution of cap rock by acidic CO₂-rich fluids resulting from CO₂ injection [5]. In addition, precipitation of secondary minerals near the injection site may lead to lower permeability holding back further CO₂ injection. Therefore, the risk from the interaction in the supercritical CO₂/water/rock system needs to be considered carefully.

A few studies do examine reactions in scCO₂-brine-aquifer system or the rock (granite or sandstone)/H₂O/CO₂ system under the reservoir temperature and pressure conditions [9–11]. Additionally, the reactive behaviour of supercritical CO₂ (scCO₂) under the relevant geologic conditions is largely unknown [11]. Our previous experiments conducted in a broad temperature range up to 350 °C to understand granite and sandstone reactions in hot water in the absence or presence of excess CO₂, have been examined the significant effect of CO₂ on the dissolution of rock minerals and the deposition of aluminosilicate and calcium-aluminosilicate secondary minerals [10, 12]. Our results suggest that the alteration of granite under CO₂-saturated hydrothermal conditions has the potential to capture CO₂ when it is injected at moderate temperatures (150–250 °C) into granite-hosted rock masses.

Many researchers refer to the injection of CO₂ into modeling aquifer at least 800 m deep where CO₂ is in supercritical state because of its critical point lying at 31 °C and 7.38 MPa [13]. Nevertheless, an excessive sequestration deep should not be welcomed by energy developers from the economical viewpoint in view of cost for CO₂ injection, and depth and relative temperature for a suitable aquifer should be in the range of 1000–2000 m and 50–110 °C. On the other hand, we noticed the fact that the permanent sequestration for CO₂ capture should be encouraged at moderate temperature [12]. Therefore, based on the comprehensive consideration, the relevant mild (100–150 °C) geologic conditions will be attractive. A typical geologic CO₂ sequestration reservoir should be located in saline aquifer or formations under cap rock, as shown in Fig 1. Because the solubility of scCO₂ into the formation water or saline water is very small within such a deep saline aquifer [14], the great amount of scCO₂ is immiscible in the liquid hot water in the aquifer or some formations; only a small amount of CO₂ partially dissolving in the hot water should react with rock minerals. For the low solubility of scCO₂ into the formation water or saline water, and for the density of scCO₂ far lower than that of the formation water, the scCO₂ fluid under cap rock tends to form a separated phase from the surrounding formation water or saline water, which is named as CO₂-rich filed. Based on the above consideration, the interactions in scCO₂/water/rock minerals systems in CO₂-rich field should occur potentially in four reaction systems: water-free scCO₂/rock reaction system, scCO₂/vapor/rock reaction system where a little of liquid water (pore water) diffuse in the state of vapour, CO₂/water/rock reaction system in the aquifer dissolving low concentration of CO₂ adjacent to CO₂-rich field, and water/rock reaction system far away from CO₂ filed, as shown in Fig. 1.

Fig. 1

Schematic diagram of the scCO₂/water/rock reaction systems under the geologic CO₂ sequestration conditions

Since the reactive behaviour of scCO₂ under relevant moderate (100–150 °C) geologic conditions has not been understood very well, the investigation of interactions mechanism among the scCO₂/water/rock minerals systems becomes essential to know the factors influencing this CO₂ trapping technology. Here we present experimental evaluation of interactions in the scCO₂/rock, scCO₂/vapor/rock, CO₂/water/rock and water/rock reaction systems under a simulated sequestration conditions at 100 °C suitable for underground storage of CO₂. Comparison is made among the dissolution of quartz, biotite and granite involving water or gas-rock reactions. The importance of scCO₂ pressure and the state of water is emphasized. The results of this study may be used to assess the hydrolysis behaviour of granite in the aquifer for CO₂ sequestration, and the validity of CO₂ trapping by means of the formation of the carbonate minerals as well as the further comprehend on the versatile problems of CO₂ sequestration under geological environment.

Experimental

Starting rock materials

In this experiment, granite specimen taken from the surface of Iidate granite strata in Fukushima Prefecture of Japan was chosen as the geochemical and mineralogical representation of the igneous rock as well as one of the components in the aquifer. Quartz and biotite specimens collected from pegmatite deposit in Nellore, Andhra Pradesh, India, were select as the individual minerals for further understanding of that which minerals in the granite should play an important role in the dissolution of granite under for the simulated geological environment. The chemical composition of SiO₂ in Quartz is larger than 99.5% with few impurities. The chemical oxides composition of biotite is presented in Table 1. The representative (igneous) granite specimen is composed of an interlocking mosaic of quartz, plagioclase and K-feldspar, with other minerals in a small quantity (including hornblende, biotite and opaque minerals). Its chemical oxides compositions and modal mineral abundance of Iidate granite are presented in Table 2.

Table 1 Major chemical oxide composition (wt.%) in the Nellore biotite

Table 2 Major chemical oxide composition (wt.%) and modal mineral abundance (vol.%) in the Iidate granite

The starting granite specimen was prepared by core boring of 16 mm in diameter from the mass of rock. Both of the coring granite samples and the quartz were broken into fragments approximately 3–5 mm in diameter to provide granular samples. The biotite sample was prepared by cutting the biotite specimen into slice with 3–5 mm in diameter. After washing by distilled water and impurity iron removal using magnetic separation, the rock samples prepared for subsequent experiments conducted in the scCO₂ fluid/water/rock interaction systems were vacuum dried preliminarily at 105 °C for 24 h in 0.1 kPa.

Experimental apparatus and methods

Laboratory scCO₂/water/rock interaction experiments were conducted in a hydrothermal autoclave apparatus consisting of a Hastelloy C reaction cell of 41 mL, as shown in Fig 2. A sample cup consisting of SUS316 mesh set to the middle section of reaction cell served as the sample holder of rock minerals. As a simulation of geologic CO₂ sequestration environment, three reaction systems (scCO₂/rock, scCO₂/vapor/rock and CO₂/water/rock reaction system) were achieved by addition of the rock minerals (quartz of 10 g, biotite of 5 g and granite of 10 g, respectively) to the sample holder, as well as the introduction of CO₂ and water or not (Fig. 3a, c, d) after air in the reaction cell being purged by CO₂ gas. The fourth reaction system was the water/granite system without introduction of CO₂ gas (Fig. 3b), which was selected as a reference system compared to the CO₂/water/rock reaction system so as to comprehend the contribution of CO₂ to the physicochemical interactions in the scCO₂/water/rock systems. The hydrothermal autoclave was treated at 100 °C for the desired time in an electric oven. After treatment, the hydrothermal autoclave was air cooled rapidly to room temperature. And then the residual CO₂ gas was absorbed by bubbling into purified water as well as the reacted rock samples and the reactor cell were washed by purified water.

Fig. 2

Schematic of the hydrothermal autoclave: (1) high-pressure triple valve; (2) well for thermocouple; (3) SUS316 mesh sample holder; (4) Hastelloy-C reaction cell; (5) cone packing; (6) holding nut

Fig. 3

Schematic of the four reaction systems in the reaction cell for simulation of the scCO₂/water/rock reaction systems under the geologic CO₂ sequestration conditions: (a) water-free scCO₂/rock system; (b) water/rock reaction system without CO₂; (c) scCO₂/vapor/rock system, water of 1 mL introduced to form a saturated steam in the reaction cell; (d) CO₂/water/rock system, rock mineral samples immersed into water of 30 mL, respectively

Dissolved element Na, K, Al, Si, Ca, Mg and Fe in the recovered solutions were determined using Optima 3300 SYS ICP-AES (PerkinElmer Ltd., Japan). The recovered solutions comprised the solution for the residual CO₂ gas absorption plus the solution for washing rock samples and the residual solution in the reaction cell; 30 mL, 30 mL, 1, 30 mL recovered solution for (i) scCO₂/rock, (ii) water/rock, (iii) scCO₂/vapor/rock, (iv) CO₂/water/rock reaction systems, respectively. The pH of the residual solution (at room temperature) was determined by a digital pH meter (Horiba pH meter D-52). The reacted rock sample was dried at 105 °C for 24 h in 0.1 kPa, reweighed and examined by SEM/EDX (HITACHI, S-4700T) to note any hydrothermal effects on the primary minerals, as well as the occurrence of any secondary minerals and their textures in the host rock.

Results and discussion

Effect of state of water on dissolution of rock minerals

A series of autoclave experiments to study the dissolution behavior of rock minerals (quartz, biotite and granite) were conducted in (i) scCO₂/rock, (ii) water/rock, (iii) scCO₂/vapor/rock, (iv) CO₂/water/rock reaction systems at 100 °C for 48 h, as shown in Fig. 4.

Fig. 4

Major elements in the recovered solution from quartz (a), biotite (b1) and (b2), and granite (c1) and (c2) systems treated at 100 °C for 48 h, respectively. scCO₂; H₂O(l), scCO₂/H₂O(g) and scCO₂/H₂O(l) in X axis title representing the following reaction system: (i) water-free scCO₂/rock system, scCO₂ of 10 MPa; (ii) water/rock reaction system, water of 30 mL introduced without CO_2; (iii) scCO₂/vapor/rock system, water of 1 mL introduced to form a saturated steam in the reaction cell filled with scCO₂ of 10 MPa; (iv) CO₂/water/rock system, rock minerals sample immersed into water of 30 mL without CO₂, respectively

Quartz (SiO₂) is one of main important components of granite in the aquifer, holding 37.1% (vol%) of mineral abundance in Iidate Granite. Figure 4a shows that the concentrations of Si both in the water/quartz and the CO₂/water/quartz reaction system were approached to about 20 μmol/L (5 ppm) and about 3 μmol/L in the scCO₂/vapor/quratz system, which suggests that the introduction of scCO₂ into the water/quartz system has no significative influence on the dissolution of quartz in the hot water at 100°C. In the scCO₂/quartz system, the dissolution of Si in the scCO₂ fluid was hardly to determine. Therefore, quartz phase is present chemical stably in the scCO₂/rock system because quartz could not dissolve into the scCO₂ fluid without liquid water or vapor.

Biotite ((K, Na) (Mg, Ca, Mn, Fe²⁺)_2.4 (Al,Fe³⁺)_2.2 (Si,Ti)_3.2O_11.8 (OH)_1.6) is one of the main components of the granite despite its lower mineral abundance (6.3 vol%) in the Iidate Granite, and is also the most important resource of Fe²⁺. The concentration analysis for the recovered solution was conducted in the four reaction systems of using biotite as the starting rock minerals (Fig. 4b1 and b2). Si, Mg and Ca are the major elements in the recovered solution in the related reaction system compared with Na, K, Al and Fe of low concentration. In the scCO₂/bitiotite reaction system without water, elements in trace quantities were detected; no descried chemical interaction proceeded similar to that in the scCO₂/quartz system. Compared to that in quartz reaction system, the elemental concentrations in scCO₂/biotie system with water or vapor were far greater than that in the water/biotite system. In addition, the higher Si, Mg and Ca concentration (950, 300, 270 μmol/L, respectively) found in the scCO₂/vapor/bitiote system instead of that in the scCO₂/water/biotite system or the water/biotite system shows a surprising impact on the function of vapor. The result implies the difference of biotite dissolution mechanism attributing to the state of water.

Compared to other granite systems, the elemental concentrations were greatest in the scCO₂/vapor/granite system (Fig. 4c1 and c2); the Ca concentration took an unimaginable maximum of 13650 μmol/L (550 ppm) in a short reaction time of 48 h; In addition, Si, Na and Mg were major elements from the dissolution of granite, with concentrations of about 3400, 3300 and 1700 μmol/l (95, 76 and 40 ppm) respectively. In the scCO₂/granite without water, only a trace elemental concentrations less than 0.05 ppm were detected in the recovered solution, suggests no effective dissolution of granite in the scCO₂ fluid without water. In addition, introduction of scCO₂ fluid into the water/granite system shows a more severe dissolution of granite for Ca Mg and Si, nevertheless there is no distinct alternation for Na, K and Al.

In all the scCO₂/rock (quartz, biotite and granite) systems without water, no evident chemical alternation occurred contributing to the dissolution of rock minerals in the scCO₂ fluids. Consequently, rock minerals exist chemical stably in the scCO₂ fluids without water under the current experimental conditions for CO₂ sequestration.

In the case of introduction of water or vapor, elements dissolved from both quartz and biotite were too low to be responsible for the occurrence of high concentration elements dissolved in the residual solution in the granite system. Under the acid scCO₂ hydrothermal conditions of 100 °C, the dissolution of quartz is not active because the dissolution is a releasing process of acid (H₄SiO₄). The high concentrations of Ca and Na with low concentrations of K and Fe in the granite system suggest most of Ca, Si and Na leached from granite were originated from the hydrolysis of plagioclase phase (Na, Ca) (Al, Si)₄O₈ in Iidate granite. The presence of Al with very low concentration less than 8 μmol/L in the residual solution implies the possibility of generation of secondary minerals consisting of some aluminosilicates in the scCO₂/granite/system.

The presence of highest concentrations elements in the residual solution from the scCO₂/vapor/granite system reveals that the vapor provides stronger reactivity for the granite hydrolysis process than do the liquid water under the same experimental hydrothermal conditions of temperature and CO₂ pressure. If taken into account the volume of water introduced into the scCO₂/vapor/granite system was 1 mL (about 1 g), the dissolved amount of elements in the vapor system were a several times less than that in the scCO₂/water/granite with addition of 30 mL water. However, in the scCO₂/vapor/granite system, introduction of about 0.025 g water could generate a saturated steam in the reaction cell of 41 mL. Therefore, large amount of 1 mL water only exist in the liquid state in the bottom of the reaction cell, and the minute quantity of water (0.025 g) in the form of the vapor could diffuse into the scCO₂ fluid above. We infer that in the scCO₂/vapor/granite system, the vapor in the scCO₂ fluid was absorbed and formed a very thin water film on the porous surface of granite for surface absorption. At the same temperature the free energy of reaction of the vapor was relatively higher than that of the liquor, and the diffusion of CO₂ in the thin water film should be easier than that in the scCO₂/water/granite system which had low solubility of CO₂. Moreover, the dissolution of granite should be enhanced by the diffusion of CO₂ which was discussed in the next section. Therefore, the vapor shows greater reactivity with granite than the liquid water does.

Effect of CO₂ pressure on the dissolution of rock minerals

Figure 5a shows a slight reduction for the Si concentration in the residual solution from the CO₂/water/quartz system with the increase of CO₂ pressure ( ${\text{P}}_{{{\text{CO}}_{{\text{2}}} }}$). Release of Si comes from the hydrolysis of quartz:

$$ {\text{SiO}}_{2} + 2\,{\text{H}}_{2} {\text{O}} \Rightarrow {\text{H}}_{4} {\text{SiO}}_{4} $$

(1)

Fig. 5

Effect of the pressure of the CO₂ introduced on the major elements concentrations in the water/rock minerals (quartz (a), biotite (b1) and (b2), and granite (c1) and (c2)) system

This reaction gives acid H₄SiO₄ and releases [SiO₄]⁴⁻ anion, that is inactive under the current experimental conditions of 100 °C. Obviously, the solubility of CO₂ in the water/quartz system was enhanced with the increase of CO₂ pressure in the reaction cell [10, 12], and made the solution become more acidic and holdback the formation of H₄SiO₄. Thus, in the scCO₂/water/granite system, quartz phase was not responsible for the increasing dissolved Si in the residual solution from Iidate granite.

As shown in Fig. 5b1, b2, c1 and c2, the concentrations for most of elements except for Al were encouraged by the introduction of CO₂ into the water/granite system, as well as depended on the pressure of CO₂. Of interest is the dissolved behavior of Al, which shows a greater concentration (about 23 μmol/L) in the residual solution from the water/granite system than that (less that 9 μmol/L) in the CO₂/water/granite system. It strongly suggests that Al was fixed on the granite samples in the presence of CO₂, which was attributed to the deposition of some aluminosilicates (i.e., kaolinite) based on the following reactions:

$$ \begin{aligned}{} & 2({\text{Na}},{\text{Ca}})({\text{Al}},{\text{Si}})_{4} {\text{O}}_{8} \,({\text{plagioclase}}) + 2{\text{CO}}_{2} + 2{\text{H}}_{2} {\text{O}} \Rightarrow {\text{CaCO}}_{{\text{3}}} \,({\text{calcite}}) + \\ {\text{Al}}_{2} {\text{Si}}_{2} {\text{O}}_{5} & ({\text{OH}})_{4} \,({\text{kaolinite}}) + 3{\text{SiO}}_{2} + {\text{NaAlCO}}_{3} ({\text{OH}})_{2} \,({\text{dawsonite}}) \\ \end{aligned} $$

(2)

Therefore, the introduction of CO₂ accelerated the hydrolysis of plagioclase phase in the Iidate granite.

$$ {\text{Ca}}^{{{\text{2}} + }} + {\text{CO}}_{{\text{2}}} {\text{(g)}} + {\text{H}}_{{\text{2}}} {\text{O}} \Leftrightarrow {\text{CaCO}}_{{\text{3}}} {\text{(s)}} + {\text{2H}}^{ + } $$

(3)

$$ {\text{K}} = \frac{{[H^{ + } ]^{2} }} {{[Ca^{{2 + }} ] \cdot P_{{CO_{2} }} }} $$

(4)

According to Eq. (4), the calcite generation was dependent upon the hydrogen ion concentration, CO₂ pressure and Ca ion concentration. In fact, the formation of calcite and the dissolution reactions of H₂CO₃ (HCO ⁻₃ , CO ²⁻₃ ) in Eq. (3), and the release of Ca from the dissolution of the plagioclase were concurring or/and competing reactions. In the water/granite system, the release of Ca was encouraged by the introduction of CO₂ as suggested in Eq. (2). In the initial dissolution of the granite, the release rate of Ca from the dissolution of plagioclase was dominant which made the equilibrium of Eq. (3) tend to right side and promoted the formation of calcite. Therefore, the concentration of Ca was increased with the increase of CO₂ pressure even if the solution became acidic, as shown in Fig. 5c2. On the other hand, Fig. 6 shows that with the increase of CO₂ pressure the pH for the residual solution tended to a sequential decrease which should contribute to the dissolution of calcite in the acidic solution. Finally, the competing reactions between formation and dissolution of calcite reached an equilibrium state, and then the concentration of Ca and the pH of solution showed slightly modification at CO₂ pressure of 15 MPa (Figs. 5c2, 6). Therefore, an endless formation of calcite could not be expected in such acid conditions in the case of the CO₂-rich /water/granite system [15].

Fig. 6

Effect of the scCO₂ pressure on the pH of the residual solution in the water/granite system

Deposition of secondary minerals and CO₂ mineral trapping

In the case of the scCO₂/water or vapor/quartz or biotite and the water/granite system, any evident alternation of the rock surface texture had not been seen by our exclusive SEM investigation, which could attribute to their sluggish hydrolysis process under the current mild hydrothermal conditions.

For the scCO₂/water/granite system, our SEM/EDX observation reveals the presence of an aluminosilicate (kaolinite?) that was not present in the starting granite when granite was hydrothermal treated at 100 °C and the scCO₂ fluid of 10 Mpa for 96 h (Fig. 7a). The aluminosilicate phase was characterized by randomly oriented, fine flocculent platelets. Nevertheless, no distinct carbonate minerals were found on the granite sample in the system during such a short reaction period, which attributed to the relatively lower concentration of Ca in the scCO₂/water/granite system. As suggested by Eq. (4), lower Ca and acidic solution was not beneficial to the stable formation of calcite. Many researchers focusing on the mineral trapping process under the reservoir conditions suggested that times for precipitation of the various carbonates should be on the order of hundreds of years, and mineral trapping conversion of CO₂ to carbonate minerals may contribute significantly to CO₂ sequestration within saline aquifers but only in the very long term [8, 15, 16]. Our results from the scCO₂/water/granite system also verify carbonate minerals not present in a short term under the mild experimental conditions.

Fig. 7

SEM micro-photographs of granite in the (a) scCO₂ of 10 MPa/water/granite system after reaction at 100 °C for 96 h, (b) scCO₂ of 10 MPa/vapor/granite system after reaction at 100 °C for 48 h

A calcium-rich carbonate mineral (calcite?) formed in the scCO₂/vapor/granite system experiment undertaken at 100 °C and CO₂ of 10 MPa for 48 h, as shown in Fig. 7b. The secondary mineral phase occurred as grained on plagioclase and biotite phase in the granite samples. The result is far away from the previous option that conversion of CO₂ to stable carbonate minerals was expected to be slow and was a long-term reaction process. Obviously the remarkable phenomenon of carbonate minerals occurring during such a short reaction period has not been understood yet. In the residual solution of 1 mL, the Ca took the maximum concentration with 13650 μmol/L (550 ppm); concentrations of Mg and Fe were about 1700 and 45 μmol/L (40 and 2.5 ppm), respectively. The entire M²⁺ (Ca, Mg and Fe) originated from the hydrolysis of granite. We considered the fact that in the scCO₂/vapor/granite system only a very small proportion of water in the form of vapor could diffuse into the scCO₂ fluid, and then was absorbed to form the very thin water film on the porous granite surface. At the same temperature the free energy of reaction of the vapor was relatively higher than that of the liquor, and the diffusion of CO₂ into the thin water film should be easier than that in the scCO₂/water/granite system which had low solubility of CO₂. Consequently, the vapor shows greater reactivity with granite than the liquid water does. Therefore, it is conceivable that the concentrations of M²⁺ on the granite surface should be one or two even more orders of magnitude greater than that in the residual solution. Based on the Eq. (4), higher concentrations of M²⁺ were beneficial to the formation of carbonate minerals. Additionally, the presence of accumulational elements dissolved from the granite sample made the ionic strength of the aqueous water film layer on the granite surface larger, which contributed to the aqueous layer becoming more basic and promoted the formation of carbonate minerals. Since the concentrations of Mg and Fe in the residual solution were too low and far less than that of Ca, calcite (CaCO₃) should precipitate instead of MgCO₃ and FeCO₃ based on their solubility product constants, K_sp (MgCO₃ > CaCO₃ > FeCO₃) [17]. Thus, a meaningful CO₂ minerals trapping process should occur in the scCO₂/vapor/granite system under the geologic CO₂ sequestration conditions during a short reaction period. The injection of CO₂-rich fluid into such a mild geothermal system (100 °C) should be certainly encouraged by electric energy developers and investors. Nevertheless, the system was lack of long-term experimental valuation. The risk is never neglectable that the deposition of carbonate and secondary minerals near the injection site may lead to lower the permeability of the granite in the aquifer and hold back further CO₂ injection. Additionally, the long-term stability and quantity of carbonate minerals need to be carefully accessed.

Conclusions

Our experiments were conducted to understand the physicochemical interactions in the scCO₂/water/rock reaction systems using a hydrothermal reaction cell at 100 °C. The results suggest that the hydrolysis of plagioclase phase should be mainly responsible for the elements dissolved from the Iidate granite samples. The hydrolysis of the plagioclase was encouraged in the presence of CO₂ in the water/granite system, and generated an unknown aluminosilicate. No distinct chemical alternations occurred in the scCO₂/granite system without water, which indicates that rock minerals exist chemical stably in the water-free scCO₂ fluids system under the current mild experimental conditions. The highest concentration of Ca occurred in the scCO₂/vapor/granite system combining with the SEM investigation results of calcite deposit, suggest that a meaningful CO₂ minerals trapping process should be potential in this system under the geologic CO₂ sequestration conditions during a short reaction period.

Our work is ongoing; it aims to identify the optimum geologic conditions including reasonable organization of aquifer and cap rock that will allow for safe and reliable subsurface disposal of CO₂.