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

The lack of ultra-high temperature and ultra-high pressure (U-HTHP) experimental devices makes the data of CO2-CH4 solubility and exsolution insufficient under U-HTHP conditions, which leads to an unclear competitive solubility-exsolution mechanism of CH4-CO2 miscible natural gas. This study systematically investigates fluid-phase characteristics in the LD10-X gas field, the impacts of mixing ratio, sequence, temperature, and pressure on CO2/CH4 solubility, and the CO2/CH4 exsolution patterns. Mixing ratio experiments showed that CH4 does not appear in the mixed solution when CO2 mole fraction exceeds 7%. Solubility sequence tests revealed that CH4 is no longer dissolved when CO2 reaches solubility equilibrium. However, CO2 continues to dissolve when CH4 reaches the solubility equilibrium. Solubility with temperature and pressure experiments showed that solubility of both CO2 and CH4 increased with rising temperature and pressure. In addition, the exsolution amount increased slowly and then increased rapidly with the increase in the pressure difference for the CO2 in the CO2 and CH4 phase. In addition, these laws were employed to explain the changes in CH4 and CO2 concentrations during the drill steam testing of wells LD10-X-10 and LD10-X-12, mainly because the extraction capacity of CO2 decreased after pressure reduction. Additionally, CO2 produced by chemical equilibrium movements extracted excess CH4 again. This study provides guidelines for the design of CO2 storage schemes and enhanced CH4 recovery.