Modelling of geomechanical response for coal and ground induced by CO2-ECBM recovery

Abstract

Carbon dioxide enhanced coalbed methane (CO2-ECBM) recovery is widely regarded as a premium choice for improving CH4 recovery and sequestrating CO2 in subterranean seams. Some geomechanical responses, such as the coal softening, the variety of reservoir stress state and the subsidence and uplift of ground, are induced by CO2-ECBM recovery. In this paper, a set of linear equations is established to characterize the impact of adsorptive gas and water in reservoir on mechanical property for coal. Then a fully coupled mathematical model comprised of the mechanical, hydraulic and thermal fields for coal reservoir as well as the mechanical and thermal fields for its overlying strata is constructed to estimate the geomechanical response of CO2-ECBM recovery. Based on the geometric model including coal reservoir and its overlying strata in Qinshui Basin, the changes of mechanical property for coal, effective stress for reservoir and vertical displacement for strata are evaluated with CO2 injection rates of 6, 8 and 10 t/d by numerical simulation. The simulation results indicate that: (1) Elastic modulus diminishes while Poisson's ratio raises because the softening effect induced by gas ad-desorption is stronger than that of water for the coal with large adsorption capacity and low water content, which generally weakens the ability of coal to resist shearing and compressing, (2) Inpouring CO2 into coal reservoir decelerates the compressive stress on the coal skeleton, and the risk of coal failure increases with the growth of CO2 injection rate, (3) Vertical displacement of coal reservoir top outstrips that of ground, which has little impact on ground buildings in the study area with CO2 injection rate not exceeding 10 t/d for 8000 d. This work analyzes the geomechanical response and safety for CO2-ECBM recovery and can provides guidance for the large-scale CO2-ECBM project.