Can Anisotropic Rocks Influence Stress Directions from Boreholes?

Sarah J. Titus, Carleton College
Cassie Smith, Carleton College
Celine N. Smith, Carleton College
Jacqueline Reber, Iowa State University

Abstract

Boreholes are an important tool for characterizing stresses within the Earth and make up a large percentage of the data in the World Stress Map. Two complimentary structures provide information within boreholes: breakouts and hydraulic fractures. Their directions, which are typically mutually perpendicular, are interpreted to reflect the minimum and maximum horizontal stress directions, respectively. This interpretation hinges on the assumption that the rocks surrounding the borehole are homogeneous and isotropic.

We report preliminary investigations on whether anisotropy might influence the stress directions inferred from boreholes using two approaches. First, we conduct physical experiments using gelatin as the analog material for the elastic crust. We compress a block of gelatin, extract a cylinder of solid material from the center, observe structures that develop along the length of the cylindrical hole, and preserve the shape of the void using vegetable shortening. Our experiments produce consistent breakout directions for a single trial. Further, the results hint that varying the dip direction relative to the applied stress may affect the breakout directions. Second, we re-examine stress directions from boreholes in central California. The data from these boreholes was crucial in the development of the weak-fault hypothesis for the San Andreas fault. Many borehole breakouts occur nearly parallel to the San Andreas fault but they are occur in rocks whose bedding is nearly parallel to the San Andreas fault. Taken together, our experiments and analysis of field data indicate that bedding anisotropies may influence stress directions near boreholes. Thus, stress directions from boreholes in anisotropic rocks may reflect local structures rather than regional stress patterns.

Session

Session 6: Advances in Geology, Geochronology, Geophysics

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