Adsorbed and free gas occurrence characteristics and controlling factors of deep shales in the southern Sichuan Basin, China

Abstract

Deep shale gas (3500–4500 m) will be the important succeeding field for the growth of shale gas production in China. Under the condition of high temperature and high pressure in deep shale gas reservoirs, its gas occurrence characteristics are markedly different from those of medium and shallow layers. To elucidate the gas occurrence characteristics and controlling factors of deep shales in the Wufeng-Longmaxi Formation, methane adsorption, low-temperature N2, and CO2 adsorption experiments were conducted. The results show that in deep shales, the mesopores provide approximately 75% of the total specific surface area (SSA) and 90% of the total pore volume (PV). Based on two hypotheses and comparing the theoretical and actual adsorption capacity, it is speculated that methane is adsorbed in deep shale in the form of micropore filling, and free gas is mainly stored in the mesopores. Correlation analysis demonstrated that TOC is the key material constraint for the adsorption capacity of deep shale, and micropore SSA is the key spatial constraint. Other minerals and mesopore parameters have limited effect on the amount of adsorbed gas. Moreover, the free gas content ranges from 2.72 m3/t to 6.20 m3/t, with an average value of 4.60 m3/t, and the free gas content ratio is approximately 58%, suggesting that the deep shale gas reservoirs are dominated by free gas. This ratio may also increase to approximately 70% when considering the formation temperature effect on adsorbed gas. Gas density, porosity, and gas saturation are the main controlling factors of free gas content, resulting in significantly larger free gas content in deep shale than in shallower formations.