Principle and Application for Thermal Exploitation of Coalbed Methane Recovery

Coalbed methane (CBM) is strongly adsorbed to coal, with over 95% existing in an adsorbed state, making it difficult to effectively extract using traditional borehole drainage or drainage gas methods, particularly in low permeability and under pressure CBM reservoirs. This study proposes thermal-enhanced CBM extraction by superheated water injection. Laboratory experiments were first conducted to examine deformation and temperature change during coal adsorption/desorption, to quantify adsorption heat at low (room temperature-150 ?) and high (150-240 ?) temperature ranges, and to elucidate the potential physical mechanisms of temperature modulation on coal adsorption of methane. Further experiments studied two-phase methane-water flow under elevated temperature and thermal-induced methane drainage affected by the water lock effect. Pilot field tests then validated the feasibility and effectiveness of this thermal extraction technology based on the theoretical findings. The results demonstrate that (1) energy conversion occurs during coal adsorption/desorption of methane, absorbing heat and causing matrix shrinkage during desorption. (2) Heterogeneous coal adsorption potential requires increased temperature to desorb methane from deep wells into the free state. Under isobaric conditions, adsorption heat is significantly higher at 150-240 ? versus room temperature-150 ? due to potential well depth effects. (3) The gas-driving-water process involves liquid flow, gas-liquid two-phase flow, and gas flow stages. Elevated temperature facilitates water and gas flows, accelerates gas breakthrough, and improves the production rate. (4) High temperature eliminates the water lock effect and increases the desorption rate to around 80% regardless of adsorption pressure, greatly shortening equilibrium time. (5) Further field tests show that the drainage period after thermal injection is only 1/46-1/31 that of the conventional method given the same production volume. Thus, thermal-enhanced CBM extraction is disruptive, with this study providing theoretical and engineering basis.