Experimental Analysis of Hybrid Fracture in Berea Sandstone

Jennifer Bobich
Texas A&M University Department of Geology and Geophysics
College Station, TX
jbobich@geo.tamu.edu

The distribution of dilational and shear failure modes of fracture in rock is important to understand fluid transport and storage in the Earth’s crust. Recent experimental study has shown the existence of hybrid fracture in Carrara Marble, an isotropic, low porosity, polycrystalline, calcite rock. Hybrid fractures bridge the transition from extension to shear fractures, form under mixed tensile and compressive stress states, and display evidence of both shear and opening modes of failure. The occurrence and characteristics of hybrid fractures in porous, granular aggregates have not been experimentally determined, and are expected to differ from polycrystalline calcite due to the presence of high porosity, Hertzian grain contacts, and different micro-mechanisms of deformation. The goal of this project is to determine the occurrence and characteristics of hybrid fractures in Berea Sandstone through triaxial extension experiments on notch-cut samples of Berea Sandstone at mixed compressive and tensile stress conditions. Analyses will include describing 1) the macroscopic fracture surface morphology, 2) the occurrence of acoustic emissions prior to macroscopic failure, 3) the orientation of fractures, and 4) the failure strength with confining pressure across the extension to shear fracture transition. The data will be compared to results of Carrara Marble and used to test published models of fracture development and failure criteria.