A Fully Coupled Multiphase Multicomponent Flow and Geomechanics Model for Enhanced Coalbed-Methane Recovery and CO2 Storage

Enhanced coalbed-methane (ECBM) recovery by the injection of CO2 and/or N-2 is an attractive method for recovering additional natural gas resources, while at the same time sequestering CO2 in the subsurface. For the naturally fractured coalbed-methane (CBM) reservoirs, the coupled fluid-flow and geomechanics effects involving both the effective-stress effect and the matrix shrinkage/swelling, are crucial to simulate the permeability change and; thus gas migration during primary or enhanced CBM recovery. In this work, a fully coupled multiphase multicomponent flow and geomechanics model is developed. The coupling effects are modeled by introducing a set of elaborate geomechanical equations, which can provide more fundamental understanding about the solid deformation and give a more accurate permeability/porosity prediction over the existing analytical models. In addition, the fluid-flow model in our study is fully compositional; considering both multicomponent gas dissolution and water volatility. To obtain accurate gas solubility in the aqueous phase, the Peng-Robinson equation of state (BUS) is modified according to the suggestions of Soreide and Whitson (1992). An extended Langmuir isotherm is used to describe the adsorption/desorption behavior of the multicomponent gas to/from the coal surface. With a fully implicit finite-difference method, we develop: a 3D, multiphase, multicomponent, dual-porosity CBM/ECBM research code that is fully compositional and has fully coupled fluid flow and geomechanics. It has been partially validated and verified by comparison against other simulators such as GEM, Eclipse, and Coalgas. We then perform a series of simulations/investigations with our research code. First, history matching of Alberta flue-gas-injection micropilot data is performed to test the permeability model. The commonly used uniaxial-strain and constant-overburden-stress assumptions for analytical permeability models are then assessed. Finally, the coupling effects of fluid flow and geomechanics are investigated, and the impact of different mixed CO2/N-2 injection scenarios is explored for both methane (CH4) production and CO2 sequestration.