Understanding the Effect of Rock Fabric on Fracture Complexity for Improving Completion Design and Well Performance

Abstract

Abstract In the past, containment of hydraulic fracture height growth has been evaluated based on an assumption of rock formation layers with contrasting conditions of minimum horizontal stress, and to a lesser extent, Young's modulus, leak off rates, and fracture toughness between adjacent rock layers. Most recently, large-block hydraulic fracturing experiments in the laboratory, and observations of fracture propagation (natural or induced) in core, have provided evidence that the rock fabric plays a significant role in arresting fracture height growth and also in promoting fracture complexity. In addition, unconventional reservoirs are often over-pressured. And, as the pore pressure increases, the stress contrast tends to be reduced, and the role of rock fabric becomes dominant. In this paper, we investigate the effect of weak interfaces on fracture geometry and height containment by conducting hydraulic fracturing tests on large blocks from tight shale outcrops, under simulated effective stress conditions. We define rock fabric as the presence, orientation and distribution of bed boundaries, lithologic contacts, mineralized fractures, and other type of weak interfaces. This rock fabric creates discontinuities in the stress and strain fields and affects the way the rock deforms and fails. Continuous monitoring of acoustic emissions and using acoustic transmission during fracturing, allows understanding the process of fracture initiation and fracture interaction with the weak interfaces. Post-test CT x-ray scanning and detailed dissection and photographic imaging provide a good record of the fractures. In addition, these post fracture measurements allow comparing the fractures created with results from acoustic emissions localization. The experimental results clearly demonstrate the importance of rock fabric to understand and predict fracture complexity and fracture height containment. Introduction The application of geomechanics to petroleum-industry problems of well construction, reservoir stimulation, long term production, and others has been traditionally based on strong assumptions of homogeneous media, and often elastic, isotropic behavior. These assumptions have helped the industry develop useful models and reasonable predictive capabilities for conventional reservoirs. However, they limit the applicability of geomechanics to heterogeneous tight shale reservoirs. A homogeneous material exhibits a uniform spatial distribution of properties, from location to location. This implies that the stress-strain relationship is constant throughout the material and changes in stress and strain are reasonably smooth and gradual. In contrast, a heterogeneous material exhibits non-uniform distribution of properties and their stress-strain relationships change from location to location. Here, one needs detailed information of the material structure and the properties of each constituent, to understand the distribution of stresses and strains in the material. This also means that abrupt and often discontinuous changes in stress or strain are possible, and that preferential directions of weakness may emerge along these discontinuities.