A Method for Evaluating the Effects of Confining Stresses and Rock Strength on Fluid Flow along the Surfaces of Mechanical Discontinuities in Low-permeability Rocks

Abstract

Changing confining stress can modify not only rock properties, such as porosity and permeability, but can also affect the ability of fluid to flow along planar mechanical discontinuities, such as faults, shear fractures, tensile cracks, or bedding planes. The degree to which the flow of fluids can be altered with varying confining stresses depends on the spatial orientation of the mechanical discontinuity and the strength of the rock. Similarly, if hydraulic fracture stimulation occurs in the vicinity of a mechanical discontinuity and the pressurized fracture fluids enter the discontinuity, then the high-pressure fluids can alter the effective stress on the mechanical discontinuity. These changes can cause the mechanical discontinuity to reactivate in shear, possibly resulting in an increase in the ability of the mechanical discontinuity surface to experience fluid flow, potentially diverting the stimulation fluids in a direction other than anticipated. A key component in the characterization of fluid flow along mechanical discontinuities is an understanding of the surrounding subsurface stress field. To constrain the present-day horizontal stress magnitude, a stress-strength equilibrium approach can be taken using overburden rock density estimation and information on the present-day tectonic setting. Horizontal stress orientation and magnitudes can also be inferred from structural geology principles via the interpretation of mapped active 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 planar mechanical discontinuities of all possible orientations. Furthermore, one can evaluate what magnitude of fluid pressure within each mechanical discontinuity would be required to encourage shear failure reactivation. An example from the Barnett Shale play is presented here as an application of the method, offering various solutions to the likely orientations of fractures that could interact with hydraulic fracture treatment.