Radial Wellbore Cross-Layer Fracturing in Multi-Lithologic Superimposed Shale Oil Reservoirs: A Laboratory Study

Abstract

Medium-high maturity continental shale oil is one of the hydrocarbon resources with the most potential for successful development in China. Nevertheless, the unique geological conditions of a multi-lithologic superposition shield the vertical propagation of hydraulic fractures and limit the longitudinal reconstruction in reservoirs, posing a great challenge for large-scale volumetric fracturing. Radial wellbore cross-layer fracturing, which transforms the interaction between the hydraulic fractures and lithologic interface into longitudinal multilayer competitive initiation, could provide a potential solution for this engineering challenge. To determine the longitudinal propagation behaviors of fractures guided by radial wellbores, true triaxial fracturing experiments were performed on multilayer shale–sandstone samples, with a focus on the injection pressure response, fracture morphology, and cross-layer pattern. The effects of the radial borehole length L, vertical stress difference K_v, injection rate Q, and viscosity ν of the fracturing fluid were analyzed. The results indicate that radial wellbores can greatly facilitate fracture initiation and cross-layer propagation. Unlike conventional hydraulic fracturing, there are two distinct fracture propagation patterns in radial wellbore fracturing: cross-layering and skip-layering. The fracture height guided by a radial wellbore is positively correlated with K_v, Q, and ν. Increasing these parameters causes a shift in the fracture initiation from a single root to an asynchronous root/toe end and can improve the cross-layer propagation capacity. Critical parameter thresholds exist for fracture propagation through and across interlayers under the guidance of radial boreholes. A parameter combination of critical cross-layering/skip-layering or alternating displacement/viscosity is recommended to simultaneously improve the fracture height and degree of lateral activation. The degree of correlation of different parameters with the vertical fracture height can be written as L > Q/ν > K_v. Increasing the radial wellbore length can effectively facilitate fracture cross-/skip-layer propagation and reduce the critical threshold of injection parameters, which is conducive to maximizing the stimulated reservoir volume.