Fracture propagation induced by water injection in poorly consolidated rocks

Fracture propagation induced during water flooding operations of weak, poorly consolidated oil reservoirs is not well understood. Indeed, the theoretical framework developed for conventional hydraulic fractures breaks down, as the treatment efficiency is virtually zero and pore pressure perturbations in the reservoir occur over a length scale that is large compared to the fracture length. This paper focuses on understanding the unusually high breakdown pressure observed during water flooding operations in poorly consolidated and highly permeable reservoirs, within the framework of a plane strain, KGD-type, hydraulic fracture model. In contrast to classical models of hydraulic fractures, this study tracks the propagation of the fracture from its initiation at the borehole and takes into account the partitioning of the injected fluid between the borehole and the fracture as well as the large scale perturbation of the pore pressure caused by injection. The model consists of a set of equations encompassing linear elastic fracture mechanics, porous media flow, and lubrication theory. Three asymptotic solutions for the different time regimes are found theoretically, and numerical results are obtained from the discretized governing equations. These results show that the borehole pressure continues to increase after fracture initiation to eventually drop off. According to this model, the peak pressure does not correspond to a breakdown of the formation, but rather to a transition between two regimes of porous media flow.