EFFECTS OF CONFINING STRESSES ON FLUID FLOW ALONG THE SURFACES OF MECHANICAL DISCONTINUITIES IN LOW PERMEABILITY ROCKS

Changing the confining stress can modify rock properties such as permeability. If the rocks contain planar mechanical discontinuities such as faults, shear fractures, or tensile cracks, then there is a distinct possibility that the ability of fluid to flow along these surfaces can also be altered with variation of confining stress. Characterization of fluid flow along discontinuities requires an understanding of the surrounding subsurface stress field, the strength of the host rocks, and mode with which the equilibrium between stress and strength is achieved and maintained.

Stress magnitude and stress orientation appear to have first order control on the character of fluid flow along mechanical discontinuities. To constrain the horizontal stress magnitude, a stress-strength equilibrium approach is employed using overburden rock density estimation and insights to present day tectonic setting. Stress orientation can also be inferred from structural geology principles via interpretation of mapped features and wellbore information such as drilling history and image logs. Once information about stress magnitudes and orientation is available, one can calculate the shear and normal stress magnitudes acting on planer mechanical discontinuities of all possible strikes and dips. Furthermore, one can evaluate what magnitude of fluid pressure within each mechanical discontinuity would be required to encourage shear failure reactivation. Laboratory evidence indicates that high normal stress on fracture surfaces discourages fluid flow, while elevated shear stress appears to favor reactivation and flow. An example from the Barnett shale play is presented, offering various solutions to the likely orientations of fractures that contribute to fluid production.