The Effect of Pore Pressure on Hydraulic Fracture Growth: An Experimental Study

Laboratory experiments are conducted in this study to investigate hydraulic fracture initiation and propagation in porous test specimens in the presence of multiple fluid injection sources and anisotropic far-field stresses. Numerous laboratory observations of fracture growth are documented via clear, high resolution images and digital image correlation analyses. It is clearly shown that injection-induced stresses can appreciably affect hydraulic fracture trajectories and fracturing pressures. We show that induced hydraulic fractures, under our laboratory conditions, are attracted to regions of high pore pressure. Induced fractures tend to propagate towards neighboring high pore pressure injection ports. The recorded breakdown pressure in the fracturing experiments decreases significantly as the number of neighboring injectors increases. When a hydraulic fracture is induced near an injection port that is maintained at a relatively high injection pressure, a secondary fracture can grow at a markedly lower fluid pressure than that of the main fracture. The influence of an adjacent fluid injection source on the hydraulic fracture trajectory can be minimized or suppressed when the applied far-field differential stress is relatively high. Multiple fractures induced from closely spaced injection ports tend to grow towards neighboring ports and perpendicularly to the applied maximum far-field stress. The experimental observations in this paper can serve as benchmark evidence for numerical hydraulic fracturing simulators that are used for modeling field applications.