Hydraulic fracturing is integral to geothermal and hydrocarbon recovery. Rock tensile strength is overcome using a pressurized fluid during the initiation of a fracture. Tensile strength has historically been assumed to be a stress independent property of rock depending solely on the rock type. Previous studies on igneous rocks suggest otherwise. Our experiments on Berea and Tennessee sandstone show a dependency of tensile strength on the confining stress. Confined Brazilian tests were carried out to observe the tensile fracture initiation and propagation under confined stress conditions. Strain gauges recorded the deformation leading to and during failure. The measurements show a strong dependency of tensile strength on confining pressure; however, tests were limited to a maximum confining pressure of 1000 psi. As the strain data indicate, confining pressure hinders the microcrack growth and nucleation leading to higher tensile strengths with increasing confining pressure. Tennessee sandstone which has a higher tensile strength at atmospheric pressure than Berea sandstone shows slightly greater increased tensile strength with confining pressure.

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