Simulation study on the deflection and expansion of hydraulic fractures in coal-rock complexes

The physical difference and structural weak surface of coal-rock complexes greatly influence the morphology of hydraulic fractures (HFs). This study aims to explore the law of deflection and expansion of HFs in coal-rock complexes under different stress conditions. First, a theoretical model for HF deflection in coal-rock complexes was derived. Besides, a numerical model for HF expansion in coal-rock complexes was established based on the traction–separation criterion, the cohesion embedding method and the finite element method for fracture expansion. With the aid of the established model, the laws of morphology and induced stress field evolutions during HF expansion were studied. The following conclusions were drawn. When the HF is closer to the interface, the effect of physical difference is more notable, and the HF is more likely to expand along the coal seam. As the difference between the maximum horizontal principal stress and the vertical ground stress decreases from 8 MPa to 0 MPa, the coal-rock complexes experience HF expansion in the horizontal direction, secondary fracture generation in the Z-axis direction, and HF deflection to the interface in the -Z-axis direction in turn. During HF expansion, a stress difference of about 19 MPa exists between the fracture tip section and the interface, which is favorable for HF initiation and expansion. The effect of physical difference between coal and rock leads to the formation of an induced stress field, which causes a 3–4 times difference in the displacement values in different areas during HF expansion. The abovementioned main causes of HF deflection can provide reference for the design and application of the hydraulic fracturing technology for complex formations with different lithologies.