Abstract
Pore pressure and its relationship with fracture net pressure has been reported qualitatively from both field and experimental observations. From a modeling perspective, the ubiquitously used pseudo 3D (P3D) models that are based on linear elastic fracture mechanics (LEFM) do not include the effect of reservoir depletion (or overpressure). Models that utilize effective stress as propagation criteria with a cohesive zone description, introduce the pore pressure directly into the simulation and hence can potentially capture the effect of pore pressure on fracture propagation. This work investigates the effect of pore pressure on hydraulic fracturing net pressure and geometry using empirical and numerical simulation approaches. We carried out an analysis of more than 400 datafrac injections spanning a wide range of geological ages and depositional environments in order to investigate the relationship between observed net pressure and reservoir pore pressure. The net fracture propagation pressure from the fracture treatment analysis was seen to be correlated with the effective stress in the reservoir. Fracture propagation simulations were performed using a coupled finite element – finite difference fracture simulator. The code uses a cohesive zone model (CZM) to describe fracture propagation. Four different effective stress scenarios were used to study the effect of effective stress on net pressure. The simulation results closely match the empirical relation between net pressure and effective stress as obtained from the analysis of actual frac treatment data. It is observed from the simulations that the magnitude of the effective stress also has an effect on the fracture geometry with a high effective stress leading to wider, shorter and more radial fractures. The derived empirical correlation is hence useful as a fracture design parameter. The datafrac net pressure diagnostics workflow in the pseudo 3D models can incorporate local tip pore pressure as a new pressure matching parameter. The pore pressure effect can thus explain high net pressures routinely observed in frac operations and also as a containment mechanism.
Highlights
Pore pressure changes affect the minimum stress, which results in well known effects on fracture propagation pressure and fracture height growth
It is seen that the median net pressure – effective stress ratio is higher in the case of minifracs than for breakdown injections and there are more treatments for a ratio greater than 1
From the strong empirical correlation between net pressure and effective stress, it is evident that the effective stress law for fracture propagation is valid over a wide range of effective stress
Summary
Pore pressure changes affect the minimum stress, which results in well known effects on fracture propagation pressure and fracture height growth. Field fracturing data had qualitatively shown that hydraulic fracturing net pressure observed from frac jobs performed in depleted reservoirs are much higher than was initially reported from treatments done in virgin reservoir condition (Shaoul et al, 2003). It was observed that the closure stress in the pay has decreased on depletion. Schmitt and Zoback (1989) conducted laboratory tests on the effect of pore pressure on tensile failure. Bruno and Nakagawa (1991) presented theoretical analysis based on Griffith strain energy release criteria that suggested that tensile fracture is controlled by general effective stress It was observed that the closure stress in the pay has decreased on depletion. Schmitt and Zoback (1989) conducted laboratory tests on the effect of pore pressure on tensile failure. Bruno and Nakagawa (1991) presented theoretical analysis based on Griffith strain energy release criteria that suggested that tensile fracture is controlled by general effective stress
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