When fluid injection is shut-in after a frac stage has been pumped, the sudden change in injection rate leads to a pressure pulse called a water hammer. With every instance of shut-in during field treatments, these pressure pulses are observed and available at no additional cost. This abundant field data has been commonly ignored. In this paper, we show that this data can provide diagnostic information on fracture geometry. The analysis of water hammer signatures has been applied to multi-stage hydraulic fracture treatments to show the effect of cluster spacing and stress interference between stages.

Our water hammer model numerically solves the wellbore continuity and momentum equations during the transient stage of a shut-in, which can last up to a minute. The fracture-wellbore system is represented by a circuit composed of resistance-capacitance-inertance (R-C-I) components. The amplitude and attenuation of the pressure pulse is a strong function of the compliance of the fracture (change in aperture with pressure), and the flow resistance associated with fluid moving in and out of the fracture. The R-C-I magnitudes which match pressure transients are converted to an effective fracture geometry including fracture height, width, and length. This method was applied to many stages of several multi-stage hydraulic fracture treatments.

Fracture geometry can accurately be obtained only with consistent net fracturing pressures and frictional pressure drops in the wellbore and at the fracture entrances. Magnitudes of minimum horizontal stress for each stage were critical data to make the calculated pressures reasonable. The trend in the minimum horizontal stress was observed to increase for later pumping stages representing stage-to-stage stress interference. This result was matched with ISIP trends recorded during fracturing operations and analytical models of stress changes.

The fracture diagnostics based on water hammer signatures allowed us to estimate the fracture geometry and its changes during a stage. The advantage of this method lies with that it can be obtained from readily available, which is essentially “free” field data. This does not require any additional expensive operations intended for diagnostics, but can be added to other diagnostic methods and augment the reliability of SRV estimation. The method also provides insights on stress changes along stages which can be used as a guide for future fracture designs.