Experiments and analysis on the influence of perforation mode on hydraulic fracture geometry in shale formation

Hydraulic fracture (HF) geometry is significant when evaluating simulated reservoir volume for shale gas reservoirs. The HF geometry is affected mainly by geological and engineering factors. The physical simulation experiment of hydraulic fracturing is an effective way to obtain the geometry and propagation criterion of HFs in a shale formation. However, the HF geometries created by the different perforation modes are not been well understood. To clarify this mechanism, several groups of true tri-axial hydraulic fracturing experiments were conducted on Longmaxi shale specimens to investigate the influence of perforation modes on the HF geometry. Results show that the HF geometry displays five basic patterns when intersecting with structural planes (SPs), namely, penetration/non-dilation, penetration/dilation, branching, derivation, and deflection. The diversities of HF geometries are the precondition for the formation of complex fractures. The HF geometries induced from oriented, helical, planar and combinatorial perforations show great differences, in which the combinatorial perforation fracturing has a certain promoting effect on the formation of complex fractures. The fluctuation degree of the pressure-time curve and the complexity of fractures show a positive correlation. Similar to previous studies, the HFs show a tendency for propagation beyond the natural fractures when there are higher stress differences. Higher cementing strength and roughness of SPs with weak points is favorable for forming the complex fracture network. The results provide some theoretical support and experimental basis for the optimization of fracturing parameters in the field.