Deep Coalbed Methane Production Potential Based on Isothermal Adsorption Curves in the Ordos Basin of China

Abstract

The adsorption/desorption features of coal seams affect the productivity of coalbed methane (CBM) wells. In light of the gas content and isothermal adsorption data of coal in different depth sequences in the Daning–Jixian (DJ) block, the adsorption characteristics and control factors of coal seams in the study area as well as the vertical variation characteristics of gas content and gas saturation are analyzed. Based on the connotation of isothermal adsorption curves, multiple indicators, such as the movable total gas content (MTGC) and desorption efficiency (DE), are proposed to evaluate the mobility and gas production potential of coal reservoirs under the constraint of depth effects. The findings reveal that increasing geo temperature, pressure, and coal rank play a role in the adsorption traits of coals, while the gas content and saturation tend to increase for shallow to deep depths. For deep seams in the western slope zone, the gas saturation is greater than 100%, forming an oversaturated gas reservoir rich in free gas. Under the same abandonment pressure conditions, the proportion and content of effective movable total gas gradually increase as the depth increases, while the proportion and content of residual adsorbed gas gradually decrease. The pressure at the key desorption nodes increases as the depth increases. However, the production case analysis shows that the initial desorption efficiency (IDE) of the adsorbed gas in the oversaturated reservoirs was low, but its gas production time was long, the total gas production was large, and the gas production slowly increased to the peak of the adsorbed gas, meaning that the gas production potential is high, which is more conducive to the development in general.