Complex thermal coal-gas interactions in heat injection enhanced CBM recovery

摘要

Heat injection into coal seams can stimulate coalbed methane reservoirs to enhance the recovery of coalbed methane. However, the effects of temperature change induced coal-gas interactions on gas production are still unclear, although several studies have taken the temperature sensitive coal-gas interactions into considerations. In this study, thermal coal-gas interactions, such as thermal expansion, alteration of the gas sorption capacity, thermal fracturing and thermal volatilization, were firstly observed and validated using experimental data from the literature. Secondly, these interactions were conceptualized into the porosity and permeability models of a dual-porosity coal seam. Thirdly, a fully coupled thermo-hydro-mechanical finite element model was developed to consider comprehensively the interactions among coal deformation, gas transport in the coal matrix and the fracture network, thermal volatilization and heat conduction/convection. Finally, the stimulation effects of heat injection into a coal seam reservoir with one heat injection well and one production well were numerically investigated and compared with the effects observed with the conventional coalbed methane recovery method. These results indicate that (1) heat injection can enhance the cumulative gas production by 70% over 30 years; (2) with heat injection, the gas composition decreased preferentially in the vicinity of the production well and the temperature affected area between the production well and the heat injection well; (3) the porosity and permeability of the coal matrix were greatly increased by both temperature enhanced gas desorption and thermal fracturing but slightly decreased by thermal expansion; (4) the porosity and permeability of the fracture network were linearly increased by temperature enhanced gas desorption but slightly decreased by thermal fracturing and thermal expansion; (5) the enhancement of gas production by thermal fracturing was due to the great increases of the porosity and permeability in the coal matrix, but the porosity and permeability in the fracture network decreased slightly. Therefore, heat injection induced enhancement was due to the increased coal matrix permeability for gas transport. (C) 2016 Elsevier B.V. All rights reserved.