Experimental and numerical analysis of hydraulic fracture propagation in naturally fractured granite cores

Abstract

Understanding hydraulic fracture (HF) propagation in naturally fractured granite reservoir is of great importance for deep geothermal energy exploitation. We first conducted laboratory hydraulic fracturing tests on granite outcrops containing natural fractures (NF) and observed deflection of HF toward the NF in a parallel-approach case, and arrested HF in a perpendicular-approach case. Then, a two-dimensional discrete element model is employed to analyze coupled hydro-mechanical processes in the experiments. Our modeling results reveal that, for a parallel-approach case, HF tends to deflect toward the NF when the normal distance between the HF propagation path and NF is less than 5 mm (equivalent to borehole diameter). HF crossing at perpendicular-approach case was found to occur only when two conditions are met: (i) the maximum principal stress at the intersection exceeds the tensile strength of rock, and (ii) no shear failure occurs in NF. Finally, we examined the combined effects of approach angle, NF friction coefficient, and differential principal stress under a constant minimum principal stress (σ_h) of 10 MPa. HF crossing occurred only when the differential principal stress (σ_H - σ_h) ≥ 3 MPa, and was further promoted by larger approach angles (e.g., θ > 50°) and higher NF friction coefficients (e.g., μ > 0.4).