Solubility and Exsolution Behavior of CH4 and CO2 in Reservoir Fluids: Implications for Fluid Compositional Evolution—A Case Study of Ledong 10 Area, Yinggehai

1. Introduction

The solubility and exsolution of CO₂ and CH₄ in formation water is an important part of global carbon cycle research, which occupies a significant position in the research of natural gas reservoir exploration, development and CO₂ geological storage [1,2,3,4,5]. Under high temperature and high pressure, CH₄ is distributed as dissolved state, dispersed free state and continuous free state, respectively. Solution gas will gradually precipitate and migrate to the high part with the attenuation of reservoir pressure, which will form a new free gas reservoir [6,7,8]. At present, the law of hydrocarbon-water solubility and exsolution under formation conditions is mostly obtained by physical simulation experiments [9,10,11] or thermodynamic calculations [12,13,14,15] and molecular dynamics simulations [16,17]. The solubility of natural gas in formation water has been carried out early by foreign scholars. In the 1960s, many scholars have measured the solubility of hydrocarbon gases in water and proposed the possibility of forming water-soluble gas reservoirs [18,19,20]. The effects of temperature, pressure and salinity were systematically analyzed by Yang et al. It was found that the increase of pressure significantly increased the solubility, while the effect of temperature on solubility was complex and nonlinear [21]. In recent years, the research on the solubility of natural gas has gradually shifted from atmospheric gas reservoirs to the solubility of natural gas in water under the conditions of high pressure and high temperature gas reservoirs [22,23,24,25]. Many scholars have also carried out research on the solubility characteristics of non-hydrocarbon gases [26,27,28,29]. By adjusting the chemical composition of temperature, pressure and water, Hemmati-Sarapardeh (2020) found that the increase of pressure significantly increased the solubility of carbon dioxide, but it did not further explore the multi-factor interaction [30]. Under the condition of high temperature and high pressure, hydrocarbons are miscible with water and organic-inorganic interaction occurs. The presence of CO₂ will promote a stronger degree of miscible between hydrocarbons and water. The main reason is that the CO₂ produced and exsolution when the chemical equilibrium of CO₂ moves will re-extract the excess CH₄, resulting in a significant solubility of CH₄ components and an increase in CH₄ concentration [31]. Xie et al. (2014) conducted a comprehensive analysis of hydrocarbon component dissolution-exsolution dynamics across distinct sedimentary facies, establishing critical correlations between lithological characteristics and gas phase behaviors. They found that the properties of formation water, rock mineral composition and other factors in the sedimentary environment will affect the solubility and exsolution behavior of natural gas components. The adsorption of heavy hydrocarbon components in natural gas in clay-rich strata is enhanced, which affects the solubility and solubility process of natural gas in water [32]. Although many reports on the solubility process of CH₄ and CO₂ under single-phase and mixed-phase conditions at different temperatures and pressures, there are few experimental models suitable for complex geological conditions such as ultra-high temperature and pressure [33,34]. Besides, due to the lack of ultra-high temperature and high pressure experimental device, there are still lack solubility and exsolution parameters of CO₂ and CH₄ in the reservoir of ultra-high temperature and pressure can not fully meet the demand the evaluation of natural gas reservoir geological reserves and CO₂ geological storage abundance. In this work, the fluid phase characteristics of LD10-X gas field, the effects of mixing ratio, mixing sequence, temperature and pressure on the solubility of CO₂ and CH₄, and the exsolution law of CO₂ and CH₄ were studied, respectively. At the same time, the solubility and exsolution law of CO₂ and CH₄ were employed to explain the reasons for the changes in CH₄ and CO₂ concentrations during the drill steam testing of wells LD10-X-10 and LD10-X-12 in the ultra-high temperature and pressure gas field. This study provides technical guidelines for the evaluation of natural gas reservoir geological reserves and CO₂ geological storage abundance.

2. Geological Characteristics of LD10-X Gas Field

The LD10-X gas field is located in the southern part of the Yinggehai depression slope zone in the western part of the northern continental shelf of the South China Sea. The water depth within the gas field ranges from 87.0 m to 90.5 m. The Huangliu Formation in the LD10-X gas field represents a structural-lithologic gas reservoir. The burial depth of the central part of the gas reservoir is 3894.6m ~ 4273.3m. It is vertically divided into six gas-bearing layers: H₁IV, H₂I, H₂II, H₂III, H₂IV and H₂V gas groups. Planar analysis reveals that sand bodies have been truncated to form structural-lithologic gas reservoirs with varied gas-water systems (Figure 1). The formation pressure coefficient ranges from 2.174 to 2.305, indicating an abnormal high-pressure system. The original formation pressure spans 84.289 MPa to 93.598 MPa, with original formation temperatures ranging from 190.11 °C to 208.63 °C. What’s more, the geothermal gradient is 4.89 °C/100 m, which is an abnormal high temperature system. The gas reservoir drive type is mainly elastic drive, followed by weak edge water drive and individual bottom water drive [35,36,37]. In summary, the Huangliu Formation of LD10-X gas field is a structural-lithologic gas reservoir with abnormal high pressure elastic water drive.

The genesis of high-temperature and high-pressure fluids in the LD10-X area is associated with undercompaction overpressure caused by rapid sedimentation, fluid expansion overpressure formed after fluid injection, and late-stage deep thermal fluid activities. There were three main periods of significant fluid injections, with the first two periods involving high-pressure hydrocarbon fluids and the third period involving CO₂-rich high-pressure thermal fluids. Microfractures in mudstone interlayers opened during the injection of CO₂-rich high-pressure thermal fluids influenced by diapir structural activities, which lead to variations in natural gas composition, gas saturation, and the relative proportion of CO₂ among different gas groups. The relative density of natural gas in the LD10-X gas field ranges from 0.670 to 1.258. Overall, the methane concentration varies between 24.58% and 82.97%, and carbon dioxide concentration spans from 6.18% to 70.99%. The distribution pattern of natural gas properties among different gas groups indicates that the concentration of CO₂ increases with depth vertically, with the H₂III gas group serving as a distinct boundary. Above the H₂III gas group, the concentration of CO₂ is relatively low (6.18%-23.49%). In contrast, the the concentration of CO₂ in the H₂IV gas group and deeper layers below the H₂III gas group ranges from 43.43% to 70.99%. There is a gradual increase in the concentration of CO₂ from the lower structural parts to the higher structural areas and further to the elevated sections of the eastern branch channel on the slope.

3. Materials and Methods

3.1. Materials

The ultra-high temperature and high pressure reactor used in the experiment was produced by Dustec Hochdrucktechnik company, Germany. The Agilent 7890B gas chromatograph employed in the study was obtained from Agilent Technologies, America. The 2331-D gas measurement used in the experiment was produced by Jiangsu Lianyou Scientific Research Instrument Co., Ltd. China.

3.2. Methods

3.2.1. Measurement of Solubility of CO₂

The formation water solution of LD10-X was loaded into the ultra-high temperature and high pressure reactor, and the experimental temperature and pressure were adjusted to P₁ and T₁. The excessive CO₂ was injected into the formation aqueous solution and stirred for more than 24 h. After the gas-liquid equilibrium of the reactor was stable, the excess free gas was discharged. The concentration of each component was measured by the gas chromatograph, and the amount of CO₂ gas discharged was measured by gas measurement. Table 1 depicts the characteristics of formation water in X-1 reservoir. Fig 2 demonstrates the ion composition of the formation water of LD10-X gas field. The solubility of CO₂ is shown:

$$\mathrm{S}={\displaystyle \frac{{\mathrm{n}}_1-n_2}{m_1}}(1)$$

Where, S represents the solubility of CO₂ (m³/m³), n₁ and n₂ represent the mole fraction of injected and free gas of CO₂, respectively (m³). m1 is the initial volume of formation water (m³).

Figure 2. Experimental diagram of CO₂ solubility in formation water.

Figure 2. Experimental diagram of CO₂ solubility in formation water.

3.2.2. Measurement of Solubility of CH₄

The formation water solution from LD10-X was transferred into the ultra-high temperature and high pressure reactor, and the experimental conditions were set to the predetermined values of P₁ and T₁. A predetermined volume of excess CH₄ was then injected into the formation aqueous solution, followed by continuous stirring for at least 24 hours. Once the system reached gas-liquid equilibrium, the excess free gas was carefully discharged. The concentrations of all components were determined by the gas chromatograph, and the volume of CH₄ gas released was measured using gas measurement. Fig 3 depicts the experimental diagram of CH₄ solubility in formation water. The solubility of CH₄ is shown in equation 2.

$$\mathrm{S}={\displaystyle \frac{{\mathrm{n}}_1-n_2}{m_1}}(2)$$

Where, S is the solubility of CH4 (m³/m³), n1 and n2 are the mole fraction of injected and free gas of CH₄, respectively (m³). m1 represents the initial volume of formation water (m³).

Figure 3. Experimental diagram of CH₄ solubility in formation water.

Figure 3. Experimental diagram of CH₄ solubility in formation water.

3.2.3. Measurement of Exsolution of CO₂

The CO₂ exsolution experiment was conducted based on the solubility of CO₂ experimental foundation. The experimental methodology was primarily grounded in the principle of mass conservation, focusing on the equilibrium of CO₂ and formation water before and after the experiment. Following the completion of the CO₂ solubility test under P₁ and T₁ conditions, the excess gas was carefully discharged to transition the fluid in the ultra-high temperature and high pressure reactor from a supersaturated to a saturated state. The system was then brought to P₂ and T₂ conditions by gradually reducing the temperature and pressure. Once the gas-liquid phase equilibrium of the ultra-high temperature and high pressure reactor was stabilized, the exsolution free gas was slowly released under constant pressure conditions, and the volume of released gas was measured by gas measurement. Fig 4 demonstrates the Experimental diagram of CO₂ exsolution in formation water. Equation 3 depicts the exsolution of CO₂ in formation water.

$$\mathrm{P}={\displaystyle \frac{{\mathrm{n}}_3}{m_1}}(3)$$

Where, P represents the dissolved amount of CO₂ (m³/m³), n₃ is the dissolved free gas under P₂ and T₂ conditions (m³), m₁ is the the initial volume of formation water (m³).

Figure 4. Experimental diagram of CO₂ exsolution in formation water.

Figure 4. Experimental diagram of CO₂ exsolution in formation water.

3.2.4. Measurement of Exsolution of CH₄

The solubility experiment of CH₄ was carried out on the basis of CH₄ solubility experiment. The experimental principle was mainly based on the principle of mass conservation of CH₄ and formation water before and after the experiment. After the solubility test of CH₄ was completed under P₁ and T₁ conditions, the excess gas was discharged to change the fluid in the ultra-high temperature and high pressure reactor from supersaturated to saturated. The temperature and pressure were reduced to P₂ and T₂ conditions. After the gas-liquid phase equilibrium of the ultra-high temperature and high pressure reactor was stable, the dissolved free gas was slowly discharged at constant pressure and the amount of gas was measured using gas measurement. Fig 5 depicts the experimental diagram of CH₄ exsolution in formation water. The exsolution of CH₄ is shown in equation 4.

$$\mathrm{P}={\displaystyle \frac{{\mathrm{n}}_3}{m_1}}(4)$$

Where, P is the dissolved amount of CH₄ (m³/m³), n₃ represents the dissolved free gas under P₂ and T₂ conditions (m³), m₁ represents the the initial volume of formation water (m³).

Figure 5. Experimental diagram of CH₄ exsolution in formation water.

Figure 5. Experimental diagram of CH₄ exsolution in formation water.

4. Results and Discussion

4.1. Study on fluid Phase Characteristics of LD10-X Gas Field

Within the temperature and pressure range of 20-210 °C and 0.1-100 MPa, CO₂ exhibits distinct phases, including gas, liquid, and supercritical states [38,39], while CH₄ primarily exists in gas and supercritical phases [40,41,42]. The basic physical properties of CO₂ and CH₄ fluids in different phase states will change significantly. The analysis of CO₂ and CH₄ fluid properties has confirmed that it is closely related to solubility. Therefore, the phase state of CO₂ and CH₄ significantly influences its solubility in formation water. The effects of varying component concentrations and temperature-pressure conditions on phase transitions were systematically investigated. In the CO₂ and CH₄ phase experiment, the fluids were configured according to the CH₄ mole fraction of 5 %, 30 %, 60 % and 85 %. The P-T-V relationship under different mixing ratios was tested, and the specific volume-pressure curves under different miscible ratios were plotted (Fig 6). When the temperature and pressure are constant, the specific volume of CO₂ and CH₄ increases with the increase of CH₄ mole fraction. In addition, the phase shifts to the gaseous state. The pure component CH₄ changes from gas phase to liquid phase with the decrease of specific volume at 25 °C. The 'platform' was used as the phase transition marker on the P-V phase diagram (Fig 6). When CH₄ was mixed with a small amount of CH₄ (10%), the phase transition platform disappeared immediately. At low pressure, CO₂ and CH₄ exists in a gas state, with specific volume decreasing linearly as pressure increases. Under high-pressure conditions, CO₂ and CH₄ transitions into a liquid-supercritical or liquid-supercritical-gas phase. After the phase is completely changed into a liquid-supercritical phase, the specific volume decreases linearly with the increase of pressure. Therefore, the addition of CH₄ at low temperature makes the phase transition marker in the mixed P-V phase diagram change from 'platform' to 'smooth curve'. CO₂ and CH₄ shows gas state at low pressure when the temperature is higher than the critical temperature of CH₄ (Figs 7 and 8). With the increase of pressure, CO₂ and CH₄ successively enter the critical region, and CO₂ and CH₄ will in the gas-supercritical phase. Due to supercritical CH₄ extraction, CH₄ and CH₄ form unstable 'polymeric macro-molecules' in the appropriate region with the increase of pressure and become a single phase. What’s more, both CO₂ and CH₄ enter the supercritical region and form a supercritical fluid in the high-pressure region.

Figure 6. Pressure-specific volume relationship CO₂ and CH₄ at different CH₄ mole fractions at 25 °C.

Figure 6. Pressure-specific volume relationship CO₂ and CH₄ at different CH₄ mole fractions at 25 °C.

Figure 7. Pressure-specific volume relationship CO₂ and CH₄ at different CH₄ mole fractions at 80 °C.

Figure 7. Pressure-specific volume relationship CO₂ and CH₄ at different CH₄ mole fractions at 80 °C.

Figure 8. Pressure-specific volume relationship of CO₂ and CH₄ at different CH₄ mole fractions at 205 °C.

Figure 8. Pressure-specific volume relationship of CO₂ and CH₄ at different CH₄ mole fractions at 205 °C.

For the gas-supercritical phase transition process (Fig 9), the phase transition conditions of CO₂ are lower than those of CH₄ at temperatures below 110°C. However, at temperatures above 110 °C, the supercritical phase transition conditions of CO₂ exceed those of CH₄. Within the gas-critical region, CO₂ consistently exhibits lower phase transition conditions compared to CH₄ throughout the experimental temperature and pressure range. The results indicate that as the CH₄ mole fraction increases, the critical phase transition point in the miscible system shifts toward higher pressures. For the H₂IV gas group of LD10-X gas and deeper formations, both CH₄ and CO₂ enter the supercritical region, which is a supercritical miscible fluid.

Figure 9. Comparison diagram of CH₄ and CO₂ gas-supercritical phase transition line.

Figure 9. Comparison diagram of CH₄ and CO₂ gas-supercritical phase transition line.

4.2. Study on the Solubility Law of CH₄ and CO₂ in Formation Water

The coexistence of CH₄ and CO₂ in mixed gas reservoirs is commonly encountered under actual geological conditions. It is of great significance to study the solubility law of CH₄ and CO₂ to avoid the risk of CO₂. The solubility of CH₄ and CO₂ under different mixing ratios and solubility sequences was studied. In addition, the change of the solubility of CH₄ and CO₂ phase with temperature and pressure was studied.

4.2.1. The Effect of CO₂ and CH₄ Mixing Ratio on Solubility

At 200 °C and 90 MPa, the mole fraction of CH₄ in the mixed phase of CO₂ and CH₄ was set to 5 %, 30 %, 60 % and 85 %, respectively. The experimental results revealed that there was only CO₂ in the solution and no CH₄ was found. When the molar fraction of CO₂ in the mixed phase was further reduced to less than 7 %, CH₄ was detected in the solution after solubility equilibrium (Fig 10). The main reason is that CO₂ is in the supercritical phase and in a multi-molecular aggregation state, which has a strong extraction ability for CH₄. When the CO₂ in the free phase is sufficient, CH₄ is completely bound to supercritical CO₂ and will no longer be dissolved in water. Conversely, there are free-moving CH₄ molecules in addition to the part of CH₄ extracted by CO₂ when the CO₂ in the free phase is insufficient, which can be dissolved in water. What’s more, some CO₂ will be dissolved in pure water due to the chemical equilibrium of CO₂ solubility will not constrain by its molecular morphology. The experimental results indicate that the extraction capacity of CO₂ to CH₄ is about 15 times at 200 °C and 90 MPa. In other words, 1mol CO₂ will extract about 15mol CH₄.

Figure 10. Solubility of CH₄ under different mixing ratios.

Figure 10. Solubility of CH₄ under different mixing ratios.

4.2.2. The Effect of CO₂ and CH₄ Solubility Sequence on Solubility

The effect of different solubility sequence of CO₂ and CH₄ on the solubility was studied. In formation water, the equilibrium CO₂ is dissolved first and then CH₄ is dissolved at 200 °C and 90MPa. CO₂ was dissolved and balanced for 24 hours to keep the sampler stable. The free gas of CO₂ was discharged and injected into CH4 to reach the same experimental conditions. The solubility equilibrium was performed twice for 24 h, and the sample was tested. The experimental results show that CH₄ will no longer dissolved when CO₂ reaches the solubility equilibrium. At this time, there is no excess space in the CO₂ aqueous solution to accommodate CH₄. However, when the equilibrium CH4 is dissolved first and then CO₂ is dissolved in formation water at 200 °C and 90MPa. CO₂ will continue to be dissolved when CH₄ reaches the solubility equilibrium. There may be two reasons for this phenomenon. One is that the solubility equilibrium of CH₄ is only phase equilibrium, while CO₂ has chemical equilibrium in addition to phase equilibrium. Therefore, CO₂ will continue to dissolve through chemical equilibrium after CH₄ reaches the solubility equilibrium. On the other hand, the CO₂ in the free phase is in the supercritical state, which has the ability to extract the dissolved CH₄. At this time, the dissolved CH₄ through the phase equilibrium part will be returned to the free phase again, but it will be captured by supercritical CO₂ and cannot be returned to the liquid phase.

4.2.3. The Solubility of CO₂ and CH₄ with Temperature and Pressure

The solubility of CO₂ and CH₄ in LD10-X formation water was investigated under varying temperature and pressure conditions, with a fixed CO₂ mole fraction of 5%. The experimental results revealed that the solubility of both CH₄ and CO₂ in formation water was significantly influenced by pressure and temperature. Specifically, the solubility of both CO₂ and CH₄ increased with rising pressure. Temperature also played a role in enhancing solubility, though its effect was minimal below 100 °C. Above this threshold, the impact of temperature on solubility became more pronounced. Fig.11 indicates that CH₄ dissolves rapidly when the experimental pressure is lower than 40 MPa. When the pressure is higher than 40 MPa, the solubility of CH₄ almost no longer increases. The higher the temperature, the earlier the solubility of CH4 reaches the inflection point. This phenomenon is mainly determined by the extraction of supercritical CO₂ in the mixed phase. CO₂ will dissolve quickly when the pressure is lower than 20 MPa. When the pressure is higher than 20MPa, the solubility of CO₂ increases slowly with the increase of pressure. In addition, the solubility of CO₂ increases with the increase of temperature (Fig.12).

Figure 11. P-S-T diagram of CH₄ in formation water.

Figure 11. P-S-T diagram of CH₄ in formation water.

Figure 12. P-S-T diagram of CO₂ in formation water.

Figure 12. P-S-T diagram of CO₂ in formation water.

4.3. Study on the Exsolution Law of CO₂ and CH₄

The exsolution law of CO₂ and CH₄ in formation water was systematically investigated under initial saturation equilibrium temperatures ranging from 50°C to 210°C at 90 MPa. The experimental results demonstrate that, under constant temperature conditions, the exsolution amounts of both CO₂ and CH₄ increase proportionally with the pressure difference. Furthermore, when the pressure difference remains constant, the exsolution amounts of both components exhibit a positive correlation with temperature. For the CO₂ in the CO₂ and CH₄ phase, the exsolution amount increases slowly and then increases rapidly with the increase of the pressure difference. The inflection point of the exsolution law is near the pressure of 20 MPa (Fig 13). In contrast, the CH₄ component in the phase of CO₂ and CH₄ is almost insoluble when the pressure is higher than 60 MPa. However, when the pressure is lower than 60 MPa, CH₄ begins to exsolute rapidly (Fig 14).

Figure 13. P-S-T diagram of CO₂ exsolution in CO₂ and CH₄ miscible formation water.

Figure 13. P-S-T diagram of CO₂ exsolution in CO₂ and CH₄ miscible formation water.

Figure 14. P-S-T diagram of CH₄ exsolution in CO₂ and CH₄ miscible formation water.

Figure 14. P-S-T diagram of CH₄ exsolution in CO₂ and CH₄ miscible formation water.

The difference in the exsolution of CO₂ and CH₄ in the phase of CO₂ and CH₄ is mainly caused by the extraction of CO₂. Initially, under the equilibrium conditions, CO₂ exists in a supercritical state while CH₄ remains dissolved in the solution. As the pressure decreases, a portion of the dissolved CO₂ is released into the free phase, which shifts the chemical equilibrium toward further CO₂ generation. This process results in the continuous production of free CO₂, which is subsequently removed from the system. Notably, CH₄ remains in the dissolved state throughout this stage and does not undergo exsolution. Consequently, during the high-pressure exsolution process, CO₂ exsolution dominates while CH₄ exsolution is negligible. The extraction capacity of CO₂ in the solution began to decrease when the pressure was further reduced, and the CO₂ produced by the chemical equilibrium of CO₂ and dissolved will extract the excess CH₄ again. At this time, the exsolution amount of CH₄ began to increase significantly. However, the exsolution law of CO₂ has not changed obviously at this stage, mainly because it is from the chemical equilibrium movement. In the process of pressure reduction, the compression coefficients of CH₄ and CO₂ components become larger, and the free phase CO₂ also begins to dissolve. At this time, the exsolution rate is higher than that under high pressure due to the change of CO₂ concentration in the liquid phase system. CH₄ and CO₂ will exsolution simultaneously when the pressure is lower than the critical state phase transition pressure at this temperature. It has little influence on each other and mainly depends on the change law of compression coefficient.

4.4. Application Analysis

Natural gas is distributed in solution gas and free gas under the condition of high temperature and high pressure. As the reservoir pressure decays, the solution gas will gradually precipitate and migrate to higher parts. Table 2 depicts the characteristic parameters of different wells in LD10-X gas field.

Figure 15 illustrates the variations in CH₄ and CO₂ concentrations during the drill stem testing in the LD10-X-10 well. The results demonstrate that as water output increased, the CO₂ concentration exhibited an upward trend, while the CH₄ concentration decreased correspondingly. This phenomenon can be attributed to the higher solubility of CO₂ in water under supercritical extraction conditions. Conversely, in the LD10-X-12 well, the trend was reversed (Figure 16). During the testing period, CO₂ concentration decreased and CH₄ concentration increased. The reason can be attributed to that the mixed gas exsolution from water, and the extraction capacity of CO₂ in the solution decreases after the pressure is reduced. In addition, the CO₂ produced by the chemical equilibrium movement of CO₂ and exsolution will extract the excess CH₄ again. CH₄ exsolution leads to the increase of CH₄ concentration.

5. Conclusions

A series of experimental investigations have been conducted to study solubility and exsolution law of CO₂ and CH₄ and its influence on the fluid composition in the ultra-high temperature and pressure gas field of LD10-X. The results are followed: Fluid phase characteristics experiments showed that the critical phase transition point in the miscible system shifts toward higher pressures with the increase of the CH₄ mole fraction. For the H₂IV gas group of LD10-X gas and deeper formations, both CH₄ and CO₂ enter the supercritical region, which is a supercritical miscible fluid. Mixing ratio experiments depicted that when the CO₂ mole fraction exceed 7%, CH₄ will not appear in the mixed solution due to the high extraction ability of supercritical CO₂ for CH₄, which was about 15 times greater. The solubility sequence demonstrated that CO₂ continued to dissolve even after CH₄ reached solubility equilibrium, while CH₄ became insoluble when CO₂ reached its solubility equilibrium. Both CO₂ and CH₄ solubility increased with rising temperature and pressure. The exsolution amount of CO₂ in the CO₂ and CH₄ phase was increased slowly at first and then rapidly near the pressure of 20 MPa, whereas CH₄ remained almost insoluble above 60 MPa.Drill stem testing of LD10-X-10 well showed an upward trend in CO₂ concentration, while CH₄ concentration decreased due to higher solubility of CO₂ in water under supercritical conditions. In contrast, CO₂ concentration decreased and CH₄ concentration increased in LD10-X-12 well.