Consistent Model for Injection and Falloff Pressure Match of Diagnostic Fracture Injection Tests (DFITs)

Abstract

Summary The diagnostic fracture injection test (DFIT), a fracture–injection/falloff test, is a reliable tool for quantifying the formation of closure stress, leakoff coefficient, formation permeability, and pressure. The current analytical DFIT model (used before and after closure) enables one to match the pressure falloff of abnormal leakoff behaviors, and quantifies more formation parameters than a traditional DFIT model. However, this model design addresses only the falloff data after shut–in; thus, analysts have expressed concerns that the net pressure implied by the falloff is inconsistent with the injection pressure behavior. Therefore, this paper provides a model capable of matching both injection and falloff pressure behaviors. The pressure–falloff model is capable of quantifying essential pressure values including, in order of occurrence, instantaneous shut–in pressure (ISIP), minimum fracture–propagation pressure, one or more closure–stress values, formation minimum principal stress, and pore pressure. The early pressure response represents the dissipation of three kinds of friction—wellbore, perforation, and near–wellbore friction. Each of them is quantified, and, together, they comprise the difference between the pressure at the end of injection and the ISIP. Presence of tip extension enables the quantification of the minimum fracture–propagation pressure. The minimum principal stress is consistent with the final closure stress. Subtracting the closure stress and friction pressure losses from the recorded or calculated bottomhole pressure (BHP) provides the fracture net pressure. The model match for injection pressure behavior incorporates the same pressures and consistent values for 2D fracture geometry and leakoff coefficient. The global match confirms not only the estimation of formation and fracture properties from the pressure–falloff analysis, but also the friction losses along the wellbore, through the perforations, and in the near–wellbore tortuosity during and after injection. In particular, by matching both injection and falloff, the model incorporates friction pressure losses that can explain apparent excessive net pressure. The match with both injection and falloff pressure variation addresses concerns that the net pressure implied by the falloff model match cannot be consistent with the observed injection behavior. When the pressure difference between the final DFIT pick for closure stress and the pressure at the end of injection is large, the reason might be tip extension and/or large friction pressure losses, the latter of which can be addressed by the main treatment design.