A fully coupled multidomain and multiphysics model considering stimulation patterns and thermal effects for evaluation of coalbed methane (CBM) extraction

Abstract

A deep understanding of the process of coalbed methane (CBM) extraction is of significance for both the unconventional energy supply and mine safety. The effects of stimulation patterns and thermal effects (including thermal shrinkage, non-isothermal sorption, and gas density change) significantly affect the CBM extraction process. Although thermal effects on CBM extraction have been investigated in previous studies, these effects have not been coupled with stimulation patterns, leading to an insufficient evaluation accuracy of previous models and a limited understanding of gas extraction in stimulated CBM reservoirs. This study developed a multidomain and multiphysics (thermal-hydraulic-mechanical, THM) model to fully couple different physical processes (including gas flow, coal deformation, gas desorption, and heat transfer) within a CBM reservoir framework considering various stimulation patterns to improve the simulation accuracy and obtain insights into CBM extraction. Three domains with distinct properties, i.e., the stimulated reservoir domain (SRD), non-stimulated reservoir domain (NSRD), and radial primary fracture (PF), were integrated into the CBM reservoir model to characterize the stimulation pattern. A group of partial differential equations (PDEs) was derived to characterize CBM transport within each domain and across different domain boundaries. A stimulated coalbed was defined as an assembly of three interacting porous media: coal matrix, continuous fractures (CF), and PF. The matrix and CF constituted a dual-porosity dual-permeability system, while the PF was simplified as a 1-D fractured medium. The finite element method (FEM) was employed to numerically solve the above PDEs. The proposed model was verified against two sets of field-measured CBM production data and compared to three previously published numerical models to reveal its advantage. The verified model was applied to investigate multidomain effects of stimulation treatment and evaluate the thermal effects of gas depletion on gas extraction in stimulated CBM reservoirs. The simulation results suggested (1) that the distinct properties of the different domains result in different permeability evolutions, which in turn influences CBM production; (2) that increasing the fracture complexity, enlarging the SRD size, and improving the SRD permeability with suitable stimulation techniques constitute effective approaches to enhance the CBM recovery; and (3) that ignoring thermal effects in CBM extraction can either overestimate or underestimate production, which mainly depends on the net effects of the thermal shrinkage strain and non-isothermal sorption during different production periods.