Through-layer propagation laws of hydraulic fractures in continental shale reservoirs

Continental shale reservoirs are characterized by strong vertical heterogeneity, large inter-layer lithological and stress differences and developed weak interfaces, which make through-layer propagation of hydraulic fractures difficult, leading to poor fracturing stimulation effects. In this paper, a fluid-solid coupling model of through-layer propagation of hydraulic fractures in continental shale was established based on finite element + cohesive unit method, and its accuracy was verified by comparing the analytical solution with the laboratory experimental result. Based on this model, a case study was carried out by means of single factor and orthogonal experiment analysis method, and the control mechanisms and influence laws of geological and engineering parameters on through-layer propagation behaviors of hydraulic fractures in continental shale reservoirs were clarified. It is indicated that the main mechanisms hindering the through-layer propagation of hydraulic fracture are that interface shear slide changes the vertical propagation path of hydraulic fracture, so as to restrict the increase of fracture height, and hydraulic fracture is wider, leading to the weakening of fracture height expansion capacity. High interface bonding intensity, vertical stress difference, and fracturing fluid viscosity and injection rate, and low inter-layer stress difference, tensile strength difference and elastic modulus difference are favorable for the through-layer propagation of hydraulic fracture, and their influence degree is ranked as interface bonding intensity > inter-layer stress difference > tensile strength difference > fracturing fluid viscosity/injection rate > vertical stress difference > elastic modulus. The research results further improve the basic theories on through-layer propagation of hydraulic fractures in continental shale reservoirs, and provide a theoretical basis for well and layer selection and construction scheme optimization design of hydraulic fracturing of continental shale reservoirs.