Multiscale evaluation of the complex in-situ stress field in Yingxiongling shale and its influence on hydraulic fracture propagation

In the efficient development of continental shale oil, both macroscopic tectonic structures and microscopic mineral compositions/textures exert significant impacts on the in-situ stress field, thereby influencing fracturing strategies and outcomes. Guided by the geo-engineering integration concept, this study characterizes complex tectonic structures in typical areas by means of well logging data analysis and seismic surveillance, analyzing the effects of macroscopic structures and microscopic features on stress field variations. The propagation of hydraulic fractures under complex stress conditions is investigated via numerical simulation and field data analysis. At the macroscopic scale, the results indicate that the stress orientation variation diminishes and gradually stabilizes with increasing reservoir depth, while stress deflection and higher stress magnitudes occur near fault zones. Dense contours and open bedding planes readily induce stress deflection, with fracture density and carbonate content positively correlated with stress gradient. At the cyclic scale, reservoir properties and stress values within a single sedimentary cycle vary periodically with clay mineral content. Numerical modeling based on the multi-scale stress field reveals that, compared to slickwater three-stage fracturing, reverse hybrid three-stage fracturing reduces the maximum and minimum horizontal principal stress zones by 8.3% and 7.8%, respectively. Slug fracturing reduces the maximum horizontal principal stress zone by 8.3% but increases the minimum horizontal principal stress zone by 2.6%. The original reservoir stress field is macroscopically influenced by geologic factors (reservoir depth, fault structures, contours, bedding planes, and fracture density) and cyclically influenced by mineral content and laminated texture. In site treatment, reverse hybrid fracturing facilitates sufficient fracture propagation, while slug fracturing effectively conditions the fracture network and guarantees successful proppant placement. The evaluation method of multi-scale in-situ stress field can offer technical reference for fracturing design in similar continental shale reservoirs.