Occurrence Characteristics of Initial Free and Adsorbed Methane under Pressure-Preserved Coring Conditions and the Influence of Initial Free Methane on Deep Coalbed Methane Production

Differing from shallow coalbed methane (CBM), higher initial free methane content in deep coal reservoirs results in excellent gas production, exceeding 10000 m3/d. However, the occurrence characteristics of initial free and adsorbed methane and the influence of the initial free methane content on deep CBM production remain elusive. To explore these questions, the initial free and adsorbed methane contents, pore structure, macerals, adsorption potential, and surface free energy of samples from the No. 8 coal seam, the Ordos Basin, were obtained through pressure-preserved coring, methane isothermal adsorption, and pore structure experiments. The performance of deep CBM production under different initial free methane contents was investigated through Eclipse simulation. The results indicate that the initial adsorbed methane content ranges from 5.3 to 26.7 cm3/g, accounting for 73% to 94% of the total methane, while the initial free methane content ranges from 0.3 to 7.3 cm3/g, comprising 6% to 27%. The initial adsorbed methane content increases with rising porosity, micropore development, and fixed carbon content (FCad), but it decreases with increasing ash yield (Aad). The maximum gas production (6000 m3/d) at an initial free methane content of 3 m3/t is approximately five times higher than that at a free methane content of 1 m3/t (1200 m3/d). An excessive free methane content causes a sharp increase in the reservoir pressure during the initial drainage stage, leading to the highest maximum and average gas production rates. In the early stage (before 300 days), gas production is primarily driven by free methane. These findings provide essential theoretical support for evaluating and predicting deep CBM production.