An influence of pore pressure gradient on hydraulic fracture propagation

One of the hydraulic fracturing problems relates with interaction between the fractures and the neighboring wells. It is important to understand how the pore pressure variation can affect the fracture propagation. Another problem is related to unstable fractures which can appear when the produced well is switched to injection. The paper presents the results of laboratory experimental study of the hydraulic fracture propagation under various angles between the main stress axis and the pore pressure gradient. A special laboratory setup was developed to study fracturing under the variation of both pore pressure gradient and the main stress axis directions. The setup allows one to obtain true 3D loading of the samples, and is equipped by the pore pressure transducers. The sample diameter is large enough for placing several boreholes: the fracturing borehole in the centre and the injection and production boreholes on sides of the central well. The hydraulic fractures were produced under constant fluid rate. Special experiments were conducted to study a possibility to grow the fracture by the pressure, which is lower than the pressure of fracturing. It was found that the influence of the pore pressure resulted in turning the fracture towards the injection well. Fracturing fluid injection under the low pressure resulted in fracture growth. The results allow better understanding of the hydraulic fracture behavior under complex conditions of both stress and pore pressure change due to oil field development.