A Geomechanics Approach to Evaluate Gas Shale Fracability: A Case Study with the Woodford Shale

In addition to the TOC, the brittleness of the shale formation has recently been used as a deciding factor for identifying prospect intervals for fracturing stimulation. This paper provides a geomechanics approach to quantify gas shale fracability taking into account the transversely isotropic nature of shales. In addition, geomechanics explanations are given for the field observations that hydraulic fracturing in clay-rich and finely laminated intervals produce less microseism than in quartz-rich and coarsely laminated intervals.

In this study, the shales fracability is evaluated based on groups of anisotropic elastic moduli that control the fracture gradient and fracture length growth in shale formations. The approach is illustrated with the results from geomechanics and geological characterizations of the Oklahoma Woodford shale. In particular, the preserved Woodford samples were prepared for laboratory characterizations including thin sections, XRD, UPV, UCS, tensile strength, and fracture toughness with microseism recorded during testing. The anisotropic geomechanics properties were modeled with correlations to mineralogy from XRD and well logs, then, ultimately used to estimate the fracture gradient and fracture length profiles.

Inverse proportional relations of elastic stiffnesses with clay and organic content were clearly observed with lab measurements and captured in the upscaling model. The results also showed that lower Poisson’s ratio and higher Young’s modulus gives lower fracture gradient and longer fracture length indicating a more brittle formation. Moreover, incorporate the shales anisotropic properties into modeling was shown to significantly increase the ability to resolve the brittle-ductile couplets at parasequence scale. Finally, finite element analyses for stresses distributions near the fracture opening in anisotropic and layered formations implied that the observed high microseism in quartz-rich and coarsely laminated intervals is the result of a combination of interlayer shearing and higher fracture shear stress concentration for the lower anisotropy shale.

The outcomes of this study finally explain the field observations that high quartz content or a combination of high Young’s modulus and low Poisson’s ratio may indicate a brittle formation. They also constitute a geomechanics consistent framework for quantitative evaluation of gas shale fracability at the parasequence scale taking into account the shales intrinsic anisotropy.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California