Multiscale coupled modeling of coalbed methane and water extraction from coal reservoirs with fractures and barriers

This study introduces a fully coupled two-phase flow and geomechanics model to simulate the extraction of coalbed methane (CBM) and water from coal reservoirs with conductive fractures and blocking barriers, characterized by high and low permeability, respectively. These fractures and barriers are modeled as low-dimensional objects. The accuracy of the proposed model is first validated against a reference case using the open-source simulator DuMux. Following validation, the model is applied to a coal reservoir with a discrete fracture network to simulate CBM and water production. The simulation results reveal that gas and water extraction decrease both nonwetting phase saturation and pressure, with pressure decreasing more rapidly. This leads to permeability changes due to depressurization and shrinkage-induced strain. Fracture permeability near the well decreases, while it significantly increases in the far-field region. Sensitivity analysis indicates that higher initial permeability and lower entry pressure of fractures enhance CBM productivity. The relationship between fracture stiffness, matrix elasticity, and production exhibits non-monotonic behavior. As fracture stiffness increases and matrix elasticity decreases, production may initially decline but increase again. These results highlight the complex interaction between fracture and matrix properties, where fracture permeability plays a key role in determining both production rates and cumulative production. Fracture angles influence the connectivity of the network, significantly affecting fluid flow, pressure changes, and production rates, even with similar initial permeability and fracture density. Overall, this study offers valuable insights for more accurate predictions of CBM production.