Damage-Based Microscale Model Explains Observed Two-Stage Shale Gas Depletion Profiles at Reservoir Scale

Production rates from shale gas reservoirs exhibit a rapidly decreasing initial rate, followed by a gradually decreasing tail. The heterogeneous mixture of inorganic and organic components comprising the ensemble matrix is key to explaining this response. We establish a multidomain multicomponent model, comprising inorganic minerals embedded within an organic matrix, to represent microscale composition and architecture of organic shales. Gas seepage and deformation within the aggregate are intrinsically coupled and control the evolution of microscale damage within the organic matter. The model is assembled, implemented, and solved by using finite elements (COMSOL Multiphysics). We used field observations from the Eagle Ford play to validate the model with near-perfect agreement. The initial rapidly decreasing flow rate stage is dominated by concurrent gas flow in both inorganic and organic matter before late-stage flow is confined to organic matter alone. This late-stage reduction in rate decline is supported by tensile failure in the organic matter resulting from local gas pressures and impacted by the heterogeneous mechanical and flow properties of the shale matrix. The lower deformation modulus of the inorganic matrix, together with the lower tensile strength of organic matter, focuses damage within the organic matter, elevates permeability, and arrests the precipitous decline in flow rate. Meanwhile, damage is easily determined with the larger transport ability of the inorganic matrix, where larger decline rates in both the first and second stages are observed. This work provides a new approach to evaluate the control of heterogeneous properties of the shale matrix on gas depletion at the reservoir scale.