Pore pressure effects on fracture net pressure and hydraulic fracture containment: Insights from an empirical and simulation approach

Abstract

A B S T R A C TPore 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.

Fluid pressure drops during stimulation of segmented faults in deep geothermal reservoirs

“…As previously mentioned, Meyer et al (2017) concluded that pressure drops could be produced by the propagation of tensile fractures as a wing crack. This process could be interpreted in a similar way, as observed in breakdown tests and used to identify the minimum principal stress (Prabhakaran et al 2017). In these tests, the generation of a new hydrofracture produces a pressure drop because the fluid quickly migrates into the newly formed fracture, oriented normal to the minimum stress.…”

A Theoretical Model for Hydraulic Fracturing through Two Symmetric Radial Perforations Emanating from A Borehole

“…It can be seen from equations (16) and (17) that the value of K I-αp increases as the pore pressure increases, which has a positive e ect on the hydraulic fracture initiation and reduces the required pressure. According to the abovementioned discussion on the stress intensity factors under various cases, the mode-I stress intensity factor caused by the loading conditions in Figure 1 can be evaluated by the sum of equations (1), (7), and (17) as follows:…”

“…Figure 16(g) shows that the fracture initiation angle increases with the decrease of λ, which suggests the higher the fluid viscosity, the larger the fracture initiation angle. Figure 16(h) indicates that at a given perforation angle, the higher Biot's coefficient, the lower the fracture initiation angle since K I increases as Biot's coefficient increases according to equations (16) and (17). Biot's coefficient is an intrinsic property of rock and is not homogeneous [45]; this inhomogeneity leads to different fracture initiation angles under different stages of the hydraulic fracturing.…”

“…An accurate laboratory measurement of Biot's coefficient is recommended to accurately predict the fracture geometry, especially for the hydraulic fracturing applied in shale gas and oil formations. e variation of Biot's coefficient changes the effective pore water pressure, i.e., αp, according to equations (16) and (17). In practical hydraulic fracturing engineering, Biot's coefficient as well as the pore pressure surrounding the fracture is a time dependent parameter, which suggests it is important to develop a fully coupled poroelastic fracture model to study the hydraulic fracture initiation behaviors.…”

“…Sesetty and Ghassemi [16] developed a 2D numerical model based on the displacement discontinuity method (DDM) and the fictitious stress method (FSM) to investigate the effect of rock anisotropy on wellbore stresses and hydraulic fracture propagation. Additionally, the pore pressure is also a critical factor affecting the hydraulic fracture behavior [17,18]. Gholami et al [19] investigated the effect of pore pressure on the hydraulic fracture propagation and indicated that a higher than average pore pressure zone accelerates the fracture growth, whereas a lower pressure anomaly zone resists a growing fracture.…”