Effects of Shear Planes and Interfacial Slippage on Fracture Growth and Treating Pressures

Abstract

Abstract The equations used in current hydraulic fracture simulators are based on plane-strain solutions, or use a complete surface integral solution for fracture width. Assumptions inherent in these solutions control the stress field surrounding the fracture tip and the stress intensity developed at the tip which, in turn, controls the rate of fracture growth and containment and the predicted net pressure. The overriding assumption made in these solutions is that the entire rock mass is elastically coupled so that all stresses and deformations interact. Many reservoirs that are hydraulically fractured are susceptible to complex fracturing which can invalidate the assumption of elastic coupling. Microseismic monitoring of fracture growth indicates that energy is lost to shear failures around the fracture. During hydraulic fracturing high fluid pressures, often exceeding both the minimum and maximum horizontal stress (fissure opening pressure), result in the reduction of the normal stress acting across natural fissures. This allows free shear or slippage along natural fracture planes in reservoirs or cleats in coal. When shear or slippage occurs elastic coupling in the rock mass is lost and each shear block deforms as a separate unit. This shear decoupling results in tremendous reduction in created fracture width and leads to high frictional pressures (low transmissibility) and difficulty in placing proppant, especially large proppant. The purpose of this work is to suggest that current fracture models are missing what could be a dominant containment mechanism in the fracturing of fissured reservoirs, coals, and soft rocks and that further work is required to fully understand the implications of slippage and shear failure on treatment designs. P. 11