Determining Failure Behavior at Hydraulic Fracturing Conditions Through Experimental Rock Deformation

Abstract

Production of shale reservoirs through hydraulic fracturing techniques has fundamentally changed the U.S. energy landscape. The induced fracture systems are the primary source of transmissivity from the reservoir to the wellbore, so it is vital to understand and predict the extent of the induced fracture network. Geologic observations of natural fluid-pressure assisted fracture networks and microseismicity associated with induced fracturing demonstrate that the resulting geometries are complex. Typical explanations invoke reactivation of preexisting fractures; while natural fractures are important, we suggest that complex fracturing at multiple scales is characteristic of failure at the mixed tensile and compressive stress states associated with hydraulic fracturing. Previous experimental work has demonstrated that the conventional Griffith and modified Griffith failure criterions are inaccurate in predicting the failure strength and fracture angle for mixed stresses; fracture in these conditions involves both opening and shear modes with characteristic fracture morphologies and damage accumulation important in understanding hydraulic fracture networks. We report an experimental rock deformation study to develop a failure criterion for fracture in mixed stress states appropriate to hydraulic fracturing. Triaxial extension experiments employing necked (dogbone) samples were performed on four different rock types representing different porosity, grain structure, and composition. The results demonstrate a characteristic failure envelope for the transition from opening-mode fracture at very low mean stresses, to Coulomb shear fracture at high mean, compressive stress states. Fracture mode and orientation vary systematically across the transition similarly for all the rock types. The results support the hypothesis of a universal failure criterion scalable by rock strength. The results show a constant shape to the failure envelope, such that the ratio of unconfined compressive strength to tensile strength decreases with increases in absolute strength. Additionally, fracture orientation (angle between the fracture and maximum compressive stress) increases linearly with mean stress across the transitional regime. We suggest that the empirical failure envelope can be used to predict failure modes and fracture characteristics for a given reservoir by scaling the failure criterion to the tensile strength of the reservoir and considering in situ stress states.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015