Simulation of hydraulic fracturing in quasi-brittle shale formations using characterized cohesive layer: Stimulation controlling factors

Abstract

Economic production from shale gas cannot be achieved by natural mechanisms alone; it requires technologies such as hydraulic fracturing in multiple stages along a horizontal wellbore. Developing numerical models for hydraulic fracturing is essential since a successful fracturing job in a shale formation cannot be generalized to another due to different shale characteristics, and restricted access to the field data acquisition. The cohesive zone model (CZM) identifies the plastic zone and softening effects at the fracture tip in a quasi-brittle rock such as shale, which leads to a more precise fracture geometry and injection pressure compared to those from linear elastic fracture mechanics. The incorporation of CZM in a fully coupled pore pressure–stress, finite element analysis provides a rigorous tool to include also the significant effect of in situ stresses in large matrix deformations on the fracturing fluid flow components, for instance leak-off. In this work, we modeled single and double-stage fracturing in a quasi-brittle shale layer using an improved CZM for porous media besides including the material softening effect and a new boundary condition treatment, using infinite elements connecting the domain of interest to the surrounding rock layers. Due to the lack of experimental data for the cohesive layer properties, we characterized the cohesive layer by sensitivity study on the stiffness, fracture initiation stress, and energy release rate. We demonstrated the significance of rock mechanical properties, pumping rate, viscosity, and leak-off in the pumping pressure, and fracture aperture. Moreover, we concluded that the stress shadowing effects of hydraulic fractures on each other majorly affects not only fractures’ length, height, aperture, and the required injection pressure, but also their connection to the injection spot. Also, we investigated two scenarios in the sequence of fracturing stages, simultaneous and sequential, with various fracture spacing and recommended the best scenario among them.