In Situ Gas Content and Extraction Potential of Ultra-Deep Coalbed Methane in the Sichuan Basin, China

Abstract

China’s deepest coalbed methane (CBM) exploratory well was drilled to over 4000 m in the Sichuan Basin, providing an effective database for analyzing the depth effect on methane accumulation. The results indicate that the in situ gas content generally continues to increase with depth, from 11.61 m³/t at 453.75–1829.6 m to 23 m³/t at 2467.98–4324.29 m. For coals at depths less than 2000 m, poor preservation conditions that are unable to seal free gas and adsorbed gas are dominant, with gas saturation ranging 45.79–97.61%. At depths deeper than 2000 m, the total gas content tends to be greater than in situ gas adsorption capacity, with gas saturation of 137.9–150.72%, indicating the coexistence of free gas (4.54–10.22 m³/t) and adsorbed gas (11.04–21.98 m³/t) as better preservation conditions. Similar to shale gas, deep coal seams exhibit excellent productivity potential as free gas can be extracted without the necessity of a drainage-pressure reduction stage for desorption, while a large amount of adsorbed gas can support long lifecycle of production wells. This research fills the gap in understanding the gas-bearing characteristics of ultra-deep CBM and is of great significance in guiding the exploration and development of deep CBM.