Abstract: The deep gas group of a certain gas field in the western part of South China Sea is a high-carbon gas reservoir, and the shallow gas group is a high-hydrocarbon gas reservoir. In order to effectively improve the recovery rate of high-hydrocarbon gas reservoirs and increase the economic benefits, the indoor experiments of replacing high-hydrocarbon gas with high-carbon gas were carried out, and the mechanism evaluation method was adopted, to obtain the changes of gas components and solubility after the replacement of high-hydrocarbon gas with high-carbon gas, as well as the effect of the high-hydrocarbon reservoirs on the recovery rate after the replacement and the influencing factors, and the study results were applied to the evaluation of the mine pilot experiments and the prediction of the effect of CO₂ storage in high-carbon gas reservoirs of a certain gas field in western part of South China Sea. The results show that after the replacement of the high-hydrocarbon gas with the high-carbon gas, the high-carbon gas can displace the high-hydrocarbon gas in formation irreducible water, free CH₄ increases, and the long core replacement was carried out under reservoir conditions such as low-injection and high-production, large dip and high permeability of the core, with a higher cumulative CH₄ recovery degree of up to 4.5%. This study provides theoretical references for the efficient development of high-hydrocarbon gas reservoirs in China , and the evaluation of CO₂ storage mode in high-carbon gas reservoirs.

Key words: high-hydrocarbon gas reservoirs, high-hydrocarbon gas enhanced recovery, solubility, replacement mechanism, gas field in the western part of South China Sea