Frequency spectrum analysis of water hammer events during hydraulic fracturing and the associated diagnostic applications

Abstract

A “water hammer” is a hydraulic impulse created due to the shutdown of pumps at any point during fracturing operations. These hydraulic impulses oscillate between the wellhead and the isolating tools of the current treatment stage at the resonant frequencies of the wellbore, which are controlled primarily by hydraulic length. Analysis of the interaction of hydraulic impulses with the well and fracture network has the potential to provide diagnostic data cheaply and simply, facilitating design optimization. In this work, signal processing techniques are applied to pressure data recorded at high sample-rates at the wellhead, to identify the contributions to pressure oscillations at the wellhead on the frequency spectrum. The resonant frequencies of the wellbore are then identified as strong peaks in the frequency spectrum. By understanding the boundary conditions that exist downhole, the resonant frequencies can be normalized to accurately determine depth of isolation tools. The methods were applied to a sample dataset of water hammer events from 27 fracturing treatment stages. The frequencies identified through processing the recorded data were used to calculate the depth of impulse reflection, as a novel technique to confirm bridge plug depth after treatment stages in horizontal multi-stage completions. The predicted depth of the isolating plugs was identified and indicated an average error of 26’ off of the depth the plugs were set at according to the wireline counter. The maximum deviation between the hydraulic impulse measurement and the setting depth of a bridge plug was 137’, and was verified as an observed tool failure during drillout operations when no tag was recorded at the setting depth. The results of this work demonstrate frequency spectrum analysis as a promising technique for characterizing hydraulic impulse events.