Investigation of Stress-Dependent Fracture Permeability in Naturally Fractured Reservoirs Using a Fully Coupled Poroelastic Displacement Discontinuity Model

摘要

Fractures are the main channels of production/injection in naturally fractured reservoirs; therefore the fracture permeability is a key parameter to production optimization and reservoir management. Pressure depletion in naturally fractured reservoirs can result in effective stress change that, in turn, can change fracture permeability. We apply a fully coupled poroelastic displacement discontinuity method to model the fracture permeability change during production in naturally fractured reservoirs. The fully coupled poroelastic displacement discontinuity method employs a boundary element displacement discontinuity model for the behavior of rock discontinuities and fractures under changes in stress from pore pressure variations due to fluid flow in both the matrix and fractures. The nonlinear Barton-Bandis joint mechanical medel is applied to represent the fracture deformation including normal and shear deformation. Sensitivity studies using the numerical model show that fracture permeability decreases with pressure depletion during production in fractured reservoirs under isotropic stress conditions. However, in critical stress conditions the fracture permeability may increase with pressure depletion during production because of the fracture dilation caused by fracture slide, which may explain near well permeability improvement observed in shale gas reservoirs. Simulations of pressure transient behavior show that behavior characteristic of stress dependent fracture permeability can be identified in conventional pressure buildup data, and over a longer time frame the model can be used for production data analysis. Matching with field data enables quantification of external stress contrast and fracture stiffness, and the result can provide a more rigorous forecast of production decline and primary oil or gas recovery efficiency for wells in naturally fractured reservoirs.