Numerical simulation of hydraulic fracture extension patterns at interfaces of coal-measure rock strata and a new theoretical prediction model

Abstract

Coal-measure gas (CMG), a unique form of unconventional natural gas located within coal seams and surrounding rock layers, represents a potential avenue for enhancing energy consumption patterns while mitigating carbon emissions. The geological structure of CMG reservoirs exhibits pronounced variability in layer strength and contains vertical discontinuities at lithological boundaries. These boundaries, shaped by sedimentation and tectonic forces, possess varying adherence strengths and inclinations, influencing the behavior of hydraulic fracturing fluids. Upon reaching such interfaces, fracturing fluids might either halt or propagate along them, thereby affecting the vertical expansion of hydraulic fractures. In this study, nine modes of hydraulic fracture extension at the interface of coal-measure rock strata were obtained by simulation using ABAQUS embedded in a 0-thickness Cohesive element.This study highlights how the disparity between the interface bond strength and adjacent rock mass primarily dictates fracture propagation direction under low vertical stress difference coefficient (k). A minimal disparity facilitates vertical fracture continuation through the interface. Conversely, with increasing k values, the influence of interface strength on fracture direction diminishes. Additionally, lower interface dip angles β correlate with heightened vertical stresses, promoting fracture vertical propagation. Furthermore, it fully considers the combined effect of tension and shear in the process of hydraulic fracture extension, and establishes a prediction model for the extension trajectory of hydraulic fracture at the interface of coal-measure rock strata based on the theory of maximum energy release rate and the mixed fracture energy criterion. This research offers novel insights into the mechanisms governing hydraulic fracture expansion at coal-measure rock strata interfaces and the critical factors influencing cross-interface propagation.