Abstract:

Creating a fracture network by tensile and shear failure in rock, and keeping it conductive with proppants has been the commonly used conceptual model of designing a fracture job. The key to optimizing reservoir stimulation by shear slip is knowledge of how fracture permeability increases with slip and how the resulting unpropped conductivity evolves as the stresses (shear and normal) on the fractures change during production. Such data can then be used in numerical models that consider shear slip and time-dependent unpropped conductivity loss to help design stimulation jobs that are more effective in self-propping, and retain their conductivity, thus, maximizing its benefits of ultimately needing less proppants, additives, and less water (reduced stages and re-frac). Therefore, fracture's deformation properties such as shear strength and friction are needed. In this work, we performed laboratory experiments on several shale samples from the Barnett, Mancos and Pierre, to characterize their geomechanical properties. A multistage triaxial testing strategy was applied on saw-cut jointed shale samples to measure deformation properties, and then Mohr-Coulomb and Barton's shear strength criteria were used to determine shear strength envelope and friction properties.

Introduction

The mechanics behavior of rock is usually dominated by its discontinuities such as natural fractures, bedding planes, joints and faults, etc. As a result, fractures' deformation and failure properties are significant to engineering practices like wellbore stability and hydraulic fracturing. The natural fracture properties can be measured in two major experiments: direct shear test and triaxial shear test. Direct shear test is widely used to measure the strength and stiffness properties of rock fractures (Goodman, 1976; ASTM D5607-08, 2008), since it is easy to utilize large scale specimens at both laboratory and field site testing. However, the direct shear test has some disadvantages. For example, fractures are sheared without confinements, and the normal stress is limited by relatively low capacity of the shear apparatus, and displacement measurements are usually affected by grout deformation. The triaxial shear test can overcome some of these shortcomings as it can be conducted under confining pressure (Jaeger, 1959; Lane and Heck, 1964; Rosso, 1976; Li et al., 2012). In this test, a cylindrical specimen with an inclined fracture is subjected to a given confining pressure, and then the differential stress (s_diff = s₁- s₃) is increased until slip on the weakness plane is initiated. Thus, the fractures' deformation properties under confining pressure can be estimated from stress and displacement measurements.