Abstract
Analysts regard the fracture injection and falloff test known as the diagnostic fracture injection test (DFIT) as a reliable tool to quantify formation closure stress, leakoff coefficient, formation permeability, and pressure. The recent analytical DFIT model for before and after closure enables matching the pressure falloff for abnormal leakoff behaviors, and quantification of many more formation parameters than traditional DFIT models. However, the model design addressed only the falloff data after shut-in, and many express concerns that net pressure implied by the falloff is inconsistent with the injection pressure behavior. This paper provides a model capable of matching both injection and falloff pressure behavior.
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, minimum stress, and pore pressure. The early pressure response represents the dissipation of three kinds of friction, wellbore, perforation, and near-wellbore friction, each of which are quantified, and which together comprise the difference between the pressure at the end of injection and the ISIP. Presence of tip extension enables quantification of the minimum fracture propagation pressure. The minimum stress is consistent with the final closure stress. Subtracting the closure pressure and friction pressure losses from the recorded or calculated bottomhole pressure 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 pressure distribution along the wellbore, through the perforations and the near wellbore, and in the fracture 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 observed injection behavior. When the pressure difference between the final DFIT pick for closure stress and the pressure at the end of injection is very large, the reason may be large friction pressure losses and/or tip extension. Design for the main fracture treatments could consider an altered perforation strategy to reduce friction losses and/or reduce cluster spacing to address very low effective permeability likely consistent with tip extension and confirmed by the after closure match.
