Microscale thermo-hydro-mechanical modeling of thermal recovery of shale gas

Abstract

The development of horizontal drilling and hydraulic fracturing technologies have made the commercial extraction of shale gas resources come true. However, significant amounts of shale gas are still unattainable even with these technical breakthroughs. One major reason is that up to 85% of total gas-in-place in shale reservoirs could be adsorbed gas. Thus, how much more adsorbed gas can be released will significantly impact the ultimate gas recovery. In recent years, the concept of using thermal stimulation to enhance shale gas recovery has been proposed as more gas desorbs under the higher temperature conditions. In this paper, a fully coupled thermo-hydro-mechanical (THM) model is developed to characterize gas transport and extraction in shale matrix from the microscopic perspective during thermal treatment. A set of partial differential equations are defined to model the coupled processes involved: (1) geomechanical deformation of heterogenous shale matrix; (2) gas adsorption/desorption and flow in heterogenous shale matrix; and (3) thermal transport in heterogenous shale matrix. All these processes are linked together through the porosity and apparent permeability models. This microscale THM model is verified against an analytical solution available in the literature. The verified model is then applied to investigate rock and fluid responses in shale during thermal treatment and the impacts of operation and rock physical parameters on gas recovery. Simulation results indicate that a greater thermal treatment temperature, a lower bottom hole pressure, a larger total organic carbon content and a larger Langmuir volume enhances ultimate gas recovery; and that shale thermal properties and matrix permeability impact initial gas recovery but does not impact ultimate gas recovery.