During hydraulic fracturing operations, conventional pressure fall-off analyses (G-Function, Square Root of Time, and Diagnostic Plots) are the main methods for predicting fracture closure pressure. However, there are situations when it is not practical to determine the fracture closure pressure using these analyses. These conditions occur when closure time is long, such as in mini-frac tests in very tight formations, or waste fluid injection in reservoirs where there is low native permeability or where there is significant near wellbore damage. In these situations, it can take several days for the shut-in pressure to stabilize enough for conventional pressure fall-off tests analyses to be used. Thus, the objective of the present study is to attempt to correlate the fracture closure pressure to the early time fall off data using the field-measured Initial Shut-in Pressure (ISIP), rock properties and pumped / injection volumes.
A study of the injection pressure history of many injection wells with multiple hydraulic fractures in a variety of rock lithologies shows an interesting relationship between the fracture closure pressure and the initial shut-in pressure. An empirical equation has been created to calculate the fracture closure pressure as a function of the instantaneous shut-in pressure (ISIP) and the injection formation rock properties. Such rock properties include formation permeability, formation porosity, reservoir pressure, overburden pressure, Poisson's ratio, and Young's modulus. An empirical equation was developed using the injected volumes combined with data obtained from geomechancial models and core analysis of a wide range of injection horizons in terms of lithology type: Sandstone, Carbonate, and Shale.
The empirical equation was validated using different case studies by comparing the predicted fracture closure pressure calculated using the developed empirical equation to the measured fracture closure pressure value. The reported correlation predicted the fracture closure pressure with a relative error of less than 6%. Also, the empirical equation was used to predict the fracture closure pressure in a shale formation with less than 3% error.
The new empirical equation predicts the fracture closure pressure using a single point of falloff pressure data, the ISIP, without the need to conduct a conventional fracture closure analysis. This allows the operator to avoid having to collect pressure data between shut-in and until the actual fracture closure point which can take several days in highly damaged, very tight, and/or shale formations. Moreover, in operations with multiple batch injection events into the same interval / perforations, as is often found cuttings / slurry injection operations, the trends in closure pressure evolution can be tracked even if the fracture is never allowed to close.