Experimental and numerical study on the non-planar propagation of hydraulic fractures in shale

Abstract

To better understand the non-planar propagation and geometry of fracture networks, hydraulic fracturing simulation tests on shale specimens were conducted using an established true triaxial hydraulic fracturing simulation test system. A two-dimensional numerical model of hydraulic fracturing was developed to clarify the evolution of hydraulic fractures and their non-planar behaviour at bedding planes. The numerical model with the alternating rock matrix and bedding planes considered the coupling effect of stress, fluid flow, and damage. The mechanisms by which hydraulic fractures penetrate or become arrested/deflected at bedding planes were then revealed as per the physical and numerical simulation results. The results revealed that the typical severe fluctuation of the injection pressure, which is closely related to the growth of sub-fractures along bedding planes or natural fractures, is a clear characteristic of the non-planar propagation of hydraulic fractures. The mechanical properties of the bedding primarily determine whether a hydraulic fracture penetrates or is deflected at the bedding. Strong beddings favour hydraulic fracture penetration; whereas, weak beddings favour hydraulic fracture deflection along the beddings. The hydraulic fractures typically extend along the path of least resistance, and therefore, induce the non-planar propagation behaviour at bedding planes. Tensile failure is a major fracture mechanism of the hydraulic fractures that penetrate or deviate at bedding planes. The experimental and numerical findings provide a basis for the optimum design of hydraulic fracturing and control of the fracture growth geometry in shale gas reservoirs.