A Microscale Thermo-Hydro-Mechanical (THM) Model for Thermal Recovery of Shale Gas

Abstract

ABSTRACT Studies have shown that up to 85% of total shale gas-in-place could be adsorbed gas. Thus, how much more adsorbed gas can be extracted 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 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 processes involved: (1) geomechanical deformation of heterogenous shale matrix; (2) gas sorption 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 matrix during thermal treatment and the impacts of operational and rock physical parameters on gas recovery. Simulation results indicate that a greater thermal treatment temperature enhances ultimate gas recovery; and that shale thermal properties and matrix permeability impact initial gas recovery but does not impact ultimate gas recovery. INTRODUCTION Gas shale is naturally tight and fine-grained sedimentary rock with methane trapped in its nanopores. Commercial extraction of gas in shale requires the application of horizontal drilling and hydraulic fracturing techniques (Kumar and Ghassemi, 2018; Gao et al., 2019). When hydraulically fractured, shale gas reservoirs can be divided into two sections. One section is the stimulated zone where massive hydraulic fractures are created with natural fractures being activated. The other section is unstimulated zone which mainly consists of unactivated natural fractures and matrixes. Once the production initiates, gas in hydraulic fractures will first flow out into the wellbore. Then, gas in natural fractures will migrate into hydraulic fractures. Finally, gas in matrix will transfer into natural fractures to sustain the production (Zhang et al., 2017). Although fracturing technique has been widely applied, the overall shale gas recovery rate still remains at a low level around 20% (Neil et al., 2020). The main reason is that up to 85% of shale gas-in-place could be adsorbed gas (Darishchev et al., 2013; Zhou et al., 2022) and most of the adsorbed gas is still unattainable after fracturing. Thus, stimulation techniques that can alter gas desorption behavior have the great potential to enhance shale gas recovery.