The prediction of hydraulic fracture growth is controlled in part by the subsurface confining stress. Previous work has been based upon a simple relationship between vertical overburden stress and horizontal stress as a function of Poisson's ratio. Field measurements of closure stress have failed to consistently verify the relationship between vertical and horizontal stress, so that additional calibrations are required. Alternatively, consideration of an anisotropic Poisson's ratio results in a different relationship between vertical and horizontal stress. A portion of the observed difference between theoretical and calculated closure stresses is probably due to such an anisotropic effect. The azimuthal direction of induced hydraulic fracture growth is controlled by anisotropy in both tensile strength and Poisson's ratio. Field measurements of closure stress do not necessarily determine the minimum horizontal stress in anisotropic rocks. Geophysical measurements of anisotropy can greatly affect the engineering of hydraulic fracture treatments.

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