Constructing a Geomechanical Model of the Woodford Shale, Cherokee Platform, Oklahoma; Effects of Confining Stress and Rock Strength of Fluid Flow.

Tyler Hair, Dr. Helge Alsleben, Dr. Milton Enderlin, and Dr. Nowell Donovan
Department of Geology, Texas Christian University, Ft. Worth Texas

An equilibrium relationship exists between the rock strength and the magnitude of present-day stresses, such that previously intact rock will break when the stresses acting on a rock diverge beyond the failure point. Reactivation is achieved when a preexisting mechanical discontinuity reaches its failure point. Both stress and strength data are used to construct a geomechanical model to determine the reactivation potential of planar mechanical discontinuities (faults, fractures, bedding planes) in the Woodford Shale. The contemporary stress state of the Cherokee Platform, central Oklahoma, is determined using a stress polygon approach, incorporating Anderson’s theory of faulting and available stress data to establish the active fault domain. A micro-indentation tool is used to estimate the strength of the Woodford Shale from whole core samples through the geometrical attributes (diameter and depth) of a ‘dimple’ produced by the tool on the rock’s surface. The measured dimples are correlated graphically with the unconfined compressive strength and internal friction angle of the Woodford and integrated with contemporary stress data from earthquake focal mechanisms and mapped active faults. Right-lateral, strike-slip motion on a deep, unnamed potential splay of the Wilzetta fault is representative of the contemporary stress state of the region. Vertical or near-vertical factures striking ~30° from SHaz (~87°) are the mechanical discontinuities most likely to be reactivated and allow fluids to flow along their surfaces. This reactivation will occur if the magnitude of pressure sources such as pore pressure or fluid pressure exceeds the reactivation pressure for that fracture surface.

AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012