Numerical simulation study on the influence of laminar characteristics on the fracture propagation law of hydraulic fracturing in oil shale

Hydraulic fracturing is one of the main technical means for oil shale reservoir development at present, and the difference of oil shale laminar characteristics plays a major role in influencing the post-pressure fracture morphology, and most of the current research focuses on the influence of the laminar development degree on fracture extension, ignoring the influence of the laminar thickness itself on hydraulic fracture extension. Taking the oil shale in Xunyi area of Ordos Basin as the research object, based on the theory of linear elastic fracture mechanics, we constructed a stress-damage-seepage model of hydraulic fracture fracture extension, and adopted the numerical simulation method of global FEM-CZM, to analyze the influence of the thickness of the laminae, the spacing of laminae, and the stress field on the extension of hydraulic fracture fracture, and to compare the type of damage of the fracture, length of the fracture, and the laminar communication area under different influencing factors. The results show that: (1) the thickness of the laminae affects the intercepting ability of hydraulic cracks on the laminae, when the thickness of the laminae is larger, it will lead to a stronger tendency of crack extension on the laminae, and more tensile damages will occur, which corresponds to a larger length of the cracks and laminae communicating area; (2) the spacing of laminae influences the time for hydraulic cracks to reach the laminae, a smaller spacing of laminae will lead to the cracks to penetrate through the laminae, and a larger spacing increases the extension of cracks, and the length of cracks will increase. Smaller spacing of the laminae will directly penetrate the laminae, larger spacing of the laminae will increase the resistance of crack extension, along with the larger spacing of the laminae, more tensile damage occurs, and the length of the hydraulic cracks and the laminae communicate with the larger area; (3) the ground stress field determines the direction of the expansion of the hydraulic cracks, the vertical stress will have a compaction effect on the laminae when there is a larger difference in the vertical ground stress, which will lead to the expansion of the laminae easier to penetrate the laminae. When the vertical ground stress difference is small, the hydraulic fracture extension at the layer surface will be bent and branched, and the corresponding fracture length and layer communication area will increase. It is suggested that the location of fracturing construction should be selected in the area with larger thickness of laminae, larger spacing of laminae and smaller vertical stress field, which is more favorable for the formation of highly efficient seepage and heat transfer channels, and this study can provide guidance for the hydraulic fracturing construction in oil shale in the Xunyi area.