Experimental investigation of methane dissolution and exsolution dynamics in gas reservoirs of the Ying-Qiong Basin at High-Pressures and High-Temperatures (HPHT)

• Methane solubility is measured as a function of pressure, temperature, and salinity. • Methane solubility is influenced by salinity and dissovled ion-methane molecular interaction. • Dissolved gas exsolves extensively from formation brine within core samples once a pressure threshold is reached. • The exsolution stage of methane in the core system is delayed with decreasing pore-throat size. With the development of ultra-deep natural gas resources from high-pressure and high-temperature (HPHT) reservoirs, the dissolved methane in geopressured aquifers and its impact on the production have attracted extensive attention. In this study, dissolved methane behaviour is investigated as a function of salinity in the solution and core systems over pressure of 5–90 MPa and temperature of 313.15–473.15 K, respectively, while microfluidic etched chips have been employed to visualize gas exsolution dynamics during depressurization. For the solution system, methane solubility is found to decrease as temperature is increased with its minimum at 333.15 K-353.15 K under pressures from 10 MPa to 90 MPa, and then exhibits a continuous increase with an increase in temperature. At identical salinity, pressure and temperature conditions, methane gas exhibits a lower solubility in formation brine compared with its solubility in NaCl aqueous solution because of ionic interactions and differences in ionic composition. For the core system, the influence of dissolved methane on gas recovery was assessed by conducting displacement experiments in long core samples with consideration of pressure–temperature conditions and free gas saturations. Not only has a significant increase in gas recovery been observed at pressures below its threshold, but also gas recovery in the core system exhibits a positive correlation with free gas saturation. Due to extensive exsolution of dissolved gas, more than 70% of the total gas recovery occurs during the pressure decline from 20 MPa to 2 MPa, which represents the most productive depletion stage. During depressurization, gas bubbles predominantly appear in large pores at the beginning, and then begin to form in small pores and gradually migrate and accumulate as pressure further decreases. These findings demonstrate that the combined effects of pressure–temperature, brine salinity, free gas saturation, and pore-size distribution dictate CH 4 dissolution-exsolution processes, providing essential experimental evidence for understanding gas exsolution behaviour and supporting more accurate assessment of production potential in geopressured gas reservoirs.