Modeling of hydraulic fracturing in ultra-low permeability formations: The role of pore fluid cavitation

Hydraulic Fracturing (HF) refers to the process of nucleation and growth of tensile fractures in a reservoir formation by means of flow-induced pressurization. The processes in the fracture process zone (FPZ) of a fluid-driven fracture involve a non-linear coupling between fracturing-fluid flow, rock deformation and diffusion of pore fluid. Identifying all the key physical processes is critical for reliably modeling and simulating fluid-driven fractures. The role of cavitation and subsequent alteration in pore fluid saturation is often ignored in hydraulic fracturing simulations, i.e., the pore fluid is modeled to be able to sustain arbitrarily large negative pressures without undergoing cavitation. Using multi-physics Finite Element Analyses (FEA), we show that ignoring cavitation may lead to spurious outcomes in FEA simulations of fluid-driven fractures in ultra-low permeability formations. The FEA simulations, in the absence of cavitation, predict an unrealistically large suction (negative pressure) ahead of the crack tip, which grows without bound upon refinement of the FEA mesh. Owing to such a large suction at the crack tip, the breakdown pressure obtained from the FEA simulations is anomalously large and lacks objectivity (i.e., progressively increases upon a continued refinement of the FEA mesh). Mechanistic insights gained from FEA simulations suggest that the negative pressure ahead of the crack tip is likely to cause cavitation of the pore fluid, resulting in creation of a partially-saturated region around the crack tip. This means that irrespective of the initial saturation of the rock, inclusion of cavitation and subsequent alteration in pore fluid saturation in FEA simulations is necessary for objectively modeling the fluid-driven fractures in ultra-low permeability formations. The revised FEA simulations of hydraulic fracturing show that the inclusion of cavitation and subsequent alteration in pore fluid saturation in FEA simulations eliminates the unrealistically large suction at the crack tip, regularizes the breakdown pressure, and removes the noted lack of objectivity.