In May 1987, an innovative technique was presented to the industry by J. L. Castillo that introduced a new plot for fracture pressure decline analysis ("Modified Fracture Pressure Decline Analysis Including Pressure-Dependent Leakoff" (SPE 16417)).1 The plot is used to determine the pressure parameters (PC, P*) required in the Nolte leakoff calculations. The plot also set the stage for an analysis technique that enhances the square root of time plot to determine closure, and the Nolte type curves to determine P*.
Case histories demonstrate how the analysis technique is applied. The technique uses a combined analysis and simulator program to determine P*, total leakoff coefficient, closure pressure and closure time. These parameters are determined from a single calibration treatment. Advantages of the technique include speed and simple operation. An estimation of fracture height created during the calibration treatment is also made. Fracture height is a critical parameter that is not always available unless reliable surveys are performed after a calibration treatment. This is not to imply that surveys can now be eliminated. Surveys should always be run whenever possible to confirm height. There will invariably be discrepancies in determining fracture height, because surveys show the best case (inclined vs true vertical fractures), and calculated heights using three-dimensional (3D) simulators require more data than are usually available.
The case histories show the comparison between the heights determined by the analysis and the heights determined by surveys. The other calculated parameters are given for several formations and locations. The importance of the input and output parameters used in the case histories is discussed. Fracture height and total system leakoff are two critical variables needed for efficient fracture design. The method to obtain these variables and their value is presented.