During hydraulic‐fracturing operations, conventional pressure‐falloff analyses (G‐function, square root of time, and other diagnostic plots) are the main methods for estimating 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‐fracture tests in very tight formations, or in slurry‐waste‐injection applications where the injected waste forms impermeable filter cake on the fracture faces that delays fracture closure because of slower liquid leakoff into the formation. In these situations, applying the conventional analyses could require several days of well shut‐in to collect enough pressure‐falloff data during which the fracture closure can be detected. The objective of the present study is to attempt to correlate the fracture‐closure pressure to the early‐time falloff data using the field‐measured instantaneous shut‐in pressure (ISIP) and the petrophysical/mechanical properties of the injection formation.
A study of the injection‐pressure history of many injection wells with multiple hydraulic fractures in a variety of rock lithologies shows a relationship between the fracture‐closure pressure and the ISIP. An empirical equation is proposed in this study to calculate the fracture‐closure pressure as a function of the ISIP and the injection‐formation rock properties. Such rock properties include formation permeability, formation porosity, initial pore pressure, overburden stress, formation Poisson's ratio, and Young's modulus. The empirical equation was developed using data obtained from geomechanical models and the core analysis of a wide range of injection horizons with different lithology types of sandstone, carbonate, and tight sandstone.
The empirical equation was validated using different case studies by comparing the measured fracture‐closure‐pressure values with those predicted by using the developed empirical equation. In all cases, the new method predicted the fracture‐closure pressure with a relative error of less than 6%.
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 the time when the actual fracture closure occurs, which can take several days in highly damaged and/or very tight formations. Moreover, in operations with multiple‐batch injection events into the same interval/perforations, as is often the case in cuttings/slurry‐injection operations, the trends in closure‐pressure evolution can be tracked even if the fracture is never allowed to close.