Gas flow tightly coupled to elastoplastic geomechanics for tight and shale gas reservoirs: Material failure and enhanced permeability

Abstract

We investigated coupled flow and geomechanics in gas production from the extremely low permeability reservoirs such as tight and shale gas reservoirs. We accounted for coupling in pore volume and permeability dynamically during simulation, using the fixed-stress sequential method. The permeability was a step function of the status of material failure, updated every time step. We performed numerical simulations for the reservoirs that contained the vertical and horizontal fractures, employing the single and double porosity models. We modified the multiple porosity constitutive relations for modeling double continua for flow and singe continuum for geomechanics. From numerical results, production of gas caused redistribution of the effective stress fields, increasing the effective shear stress and resulting in plasticity. Shear failure occurred away from the original fractures as well as near the fracture tips, which indicated generation of secondary fractures. Secondary fractures increased permeability significantly, and changed flow, in turn, followed by geomechanics. When the double porosity model was used instead of the single porosity model, we obtained faster evolution of the enhanced permeability area than those from the single porosity because higher permeability of the fracture medium of the double porosity is used than that of the single continuum of the single porosity model. In conclusion, the complicated physics for stress sensitive reservoirs cannot be captured accurately by uncoupled or decoupled methods, and thus tightly coupled flow and geomechanics are highly recommended for gas production in tight and shale gas reservoirs. Copyright 2012, Society of Petroleum Engineers.