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Effect of Layered Heterogeneity from Fracture Initiation and Containment in Tight Gas Shales

Roberto Suarez-Rivera
TerraTek, Salt Lake City, UT

Hydraulic fracturing is the methodology for completing nano-darcy matrix-permeability tight gas shales. Commercial success in producing these reservoirs depends to a large extent on successful hydraulic fractures. There is growing evidence that initiating hydraulic fractures from horizontal wellbores is often difficult, and requires abnormally high treating pressures. We postulate that the unusual combination of high stiffness, and high elastic anisotropy results in near-wellbore stress concentrations not observed in homogeneous, isotropic rocks.

In this paper we consider the dependence of wellbore pressure for fracture initiation on material anisotropy, in-situ stress, and lateral well orientation. Results show that the presence of strong elastic anisotropy facilitates the development of longitudinal hydraulic fractures and makes the initiation of hydraulic transverse fractures more difficult. The higher the elastic anisotropy, the stronger this effect is. There are combinations of anisotropy in Young’s modulus and Poisson’s ratio that reduce the overall effect. The collection of results provides a framework for understanding these phenomena and for anticipating potential problems based on measured values of shale elastic properties.

Results from this study also show that the origin of high treatment pressures is rock type dependent, and provides a methodology for selecting alternative lithologies for landing the horizontal wellbore, for minimizing these problems.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado