Activation of natural fractures during hydraulic fracturing in elastoplastic jointed rocks

Abstract

Complexity of fracture development in low permeability oil and gas reservoirs is of interest to many field operators for production enhancement. In highly jointed shales such as Marcellus and Sanmin, fluid-induced shear slip deformation along joint junctions coupled with soft nature of shale fabrics make fracturing operations more complicated. Sophistication includes widely reported occurrences of seismic tremors. Understanding of slip phenomena and fracture network development in such scenarios is never straightforward and curtailed with limited lines of evidence. In this study, we propose a globally embedded cohesive zone modelling within an elastoplastic three-dimensional framework to provide an understanding of the complex fracture development in soft shale formations. Shear strength of the global pore pressure cohesive elements is obtained as a function of field variable (stress) through parallel computation with FORTRAN language. Drucker Prager frictional behavior is included in the model with different friction coefficients to ascertain the joint slip phenomena when shear strength of the joints is reduced under the drenched effect of fracturing fluid. Results indicate that when the coefficient of friction is high, the total slip of the rock is reduced compared to the scenario with low friction coefficients. Other parametric analyses indicate that an isotropic stress regime leads to a more complex fracture development, while increased injection rates favour the opening of more compressed fracture wings. The model is validated against field data from layered shale reservoir as well as against Kristianovich-Geertsma-de Klerk (KGD) model with excellent agreements in both cases. This study is important for understanding fracture development complexity in soft jointed rocks together with associated slip phenomena.