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 influence of supercritical CO2 on natural gas dissolution-exsolution mechanisms under high-temperature and high-pressure (HTHP) reservoir conditions remains insufficiently investigated, affecting reserve evaluation accuracy. This study systematically investigates fluid-phase characteristics in the LD10-X gas field, impacts of mixing ratio, sequence, temperature, and pressure on CO2/CH4 solubility, and CO2/CH4 exsolution patterns. Mixing ratio experiments showed CH4 will not appear in mixed solution when CO2 mole fraction exceeds 7%. Solubility sequence tests revealed CH4 will be no longer dissolved when CO2 reached solubility equilibrium. However, CO2 will continue to be dissolved 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, exsolution amount increased slowly and then increased rapidly with the increase of the pressure difference for the CO2 in the CO2 and CH4 phase. Besides, 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 will extract excess CH4 again. This study provides guidelines for the evaluation of CO2 geological storage abundance.