Stress-Permeability Relationships in Low Permeability Systems: Application to Shear Fractures

Abstract

For tight or low matrix permeability reservoirs, such as geothermal fields, faults and fractures are the main conduits for flow. Parallel plate models are typically used to model heat and mass transport in fractures. Using such a model, the cubic law specifies that the permeability (k) is equivalent to b2/12, where b is the average fracture aperture. However, the limitation of this approach is that it fails to take into account the roughness within the fracture plane that results to permeability variation. An important component of understanding fracture surface roughness is identifying its source. There are several mechanisms by which rough fractures can be created, through chemical effects, mechanical effects, inelastic effects, and stress. In this study, the fracture generation mechanism considered was the application of stress. The spatial variation of crack opening depended on the following: applied stress values and direction, stress interactions, and initial shape of the fracture or fault. Analytical models for stress-dependent permeability were evaluated. The displacement discontinuity boundary element method (DDM) integrated with the complementarity algorithm was used as a consistent model that can simulate crack opening and slip from imposed stress conditions under elastic behavior. For future work, it is recommended that results from the DDM method be compared to the analytical models of stress-permeability relationships.