Effect of initial pore pressure on the hydraulic fracturing of tight sandstone: an experimental study

Abstract

Pore pressure is a crucial factor affecting reservoir stimulation, especially in unconventional reservoirs, because it can control crack initiation and propagation. However, studies on the effect of initial pore pressure on hydraulic fracturing in low-permeability reservoirs are lacking, and fracturing mechanism remains unclear. Therefore, we conducted hydraulic fracturing tests under initial pore pressure, and proposed an improved fracturing initiation criterion to elucidate the fracturing mechanism of tight sandstone. The results demonstrated that the breakdown pressure increased with the initial pore pressure by 8 to 24%, and its evolution was identified approximately by the improved initiation criterion, which resulted in the complicated, invisible and intense features of the macroscopic fracture morphology and microscopic crack. The finding was mainly due to the viscous stress induced by fracturing disturbance under initial pore pressure and represented the dual interaction between rock structure and work fluid viscosity. As a result, the change in microporous diameter in micropores and a high pressure to failure can be obtained. Accordingly, the initial pore pressure in tight sandstone may not only induce n effective stress on the skeleton but also be responsible for the double effect on the fracturing mechanism with high pressure and intense failure features, which should be given attention in unconventional reservoir stimulation.