Pressure Transient Analysis of Fracture Calibration Tests

Abstract

Summary Fracture-calibration pressure decline has been used for determination of the leakoff coefficient, a bulk variable describing the process of fluid influx into the reservoir, normal to the created fracture face. In this work, the fluid loss is modeled in terms of the controlling mechanisms: flow through the filter cake, the invaded zone, and the reservoir. A rigorous model describes unsteady-state fluid flow from fractures of varying area into the formation, with the filter cake considered as a time- and rate-dependent skin effect. The injection history is superposed on the pressure decline. This work provides a straight-line technique for determination of reservoir permeability and fracture-face resistance. Log-log diagnostic plots provide the means to recognize visually whether the transient response is dominated by flow in the reservoir or at the fracture face. We found that the pressure transient very frequently is dominated by the flow in the reservoir rather than through the filter cake. The reservoir permeability (an essential value for fracture design that is usually not available) can be estimated, while the model captures all trends of the falloff-pressure variation. Introduction The fracture-calibration treatment, also called an injection test or "minifracture," frequently is conducted before the main stimulation treatment. For the injection test, the fracturing fluid intended for the main treatment is pumped at a constant rate of a sufficient magnitude to achieve fracturing pressure. After several minutes (usually 20 to 30) the pumps are shut off and the bottomhole pressure declines as the fracture closes. During pumping, while the fracture volume grows, some of the fracturing fluid leaks off into the formation. After pumping ends, the fluid leakoff into the formation continues until the fracture is closed. Material balance, coupled with a model of propagation, permits estimation of the rate of fluid loss during pumping. The behavior of the pressure decline after the end of pumping has been used to estimate the leakoff (fluid-loss) coefficient, with techniques pioneered by Nolte. Castillo, using Nolte's G-function for modeling the pressure-decline behavior, developed the straight-line plot of the G-function vs. pressure. The slope of this curve is used for the computation of the leakoff coefficient that is independent of pressure.