Impacts of matrix shrinkage and stress changes on permeability and gas production of organic-rich shale reservoirs

During reservoir depletion, effective stress is increased and permeability is reduced, while the organic- rich matrix might experience a shrinkage process that will boost the permeability. The main objective was to develop a mathematical simulator coupling gas flow process, geomechanics effects, and matrix shrinkage in order to evaluate their influences on reservoir permeability and production performance. The mesh was divided into three different continuums: Organic matter, inorganic matter, and natural fractures. Matrix shrinkage was only considered for organic matter because of gas desorption, and the stress- dependent permeability was considered for both inorganic matter and natural fractures. The flow and stress-equilibrium equations were solved by the fixed-stress sequential method, where the flow equations are solved first, followed by the mechanics equations. The displacements are solved for each grid node by finite element method, and the pressure is solved by the integral finite difference method. Different stress- dependent correlations are chosen to separately apply to the three porous media. Based on those correlations, the porosity and permeability are updated at end of each time step. A synthetic reservoir model was built, where the permeability change and the accumulative gas production is calculated at each time step. The results of permeability change and gas production rate are compared for three different cases: The coupled flow and geomechanics model without permeability change, the coupled model considering stress-dependent permeability, the coupled model considering both matrix shrinkage and permeability change. Additionally, the sensitivity analyses were investigated for total organic carbon (TOC), Young's modulus, matrix permeability, and bottom hole pressure. Results show that the stress-dependent permeability plays a large influence on the gas production performance, because permeability could be significantly reduced with the decrease of reservoir pressure. The matrix shrinkage on organic matter could provide an obvious rebound on accumulative production at the late producing stage, because the permeability is boosted by that media shrinkage at the late producing stage. That explains why permeability largely decreases at early stage and then gradually reduces in experimental data. However, the permeability and production loss are highly depended on the selected correlation, its coefficients, reservoir initial condition, and rock properties. Organic matter is the critical controller on matrix shrinkage: The higher the TOC, the larger the increase of permeability. Nevertheless, their overall impacts on production is quite limited. Young's modulus does not make obvious differences on the accumulative gas based on the numerical results. The large matrix permeability and higher bottom hole pressure can reduce the production loss caused by the effect of stress-dependent permeability. Overall, the triple-porosity coupled simulator can quantitatively interpret the impacts of matrix shrinkage and geomechanics effect on permeability and gas production performance for organic-rich shale reservoirs. This provides more realistic production performance evaluation and economic assessment when the stress-dependent permeability needs to be considered.