Analysis of Acoustic Signals Generated by Proppant Slurry Flow Through Perforations
Authors: Chen Jiayue , Zhu Ding , Hill A. D. 
Published: January 2025

This experimental study explores the acoustic signal generated when water/sand slurry flows through perforations in the frequency domain. The goal is to identify the proppant presence on distributed acoustic sensors (DAS), and possibly to estimate proppant concentration distribution during a multi-stage fracture treatment using DAS monitoring data. We have studied whether sand-related signals can be distinguished from water flow signals in the frequency domain, what are the frequency components of signals for proppant, and how do proppant concentration or other parameters affect the proppant-related signals. Field data interpretation includes developing a method of proppant distribution between clusters with observations from the experiments. We applied this method to a field case of DAS measurements to determine proppant distribution and validate the results with the information of downhole imaging.

In limited entry design, proppant/water slurry flows through perforations with high injection rates and high-pressure drop across the perforations. This high flow velocity generates acoustic signal. It is believed that when solids (proppant) passing the perforation, the frequency domain responds differently than when it is just liquid flowing. To prove the concept, we conducted an experimental study simulating this process at the lab scale. The experimental setup consists of a 4-inch diameter core with a hole drilled in the middle simulating a perforation. A steel plate placed at the inlet of the core serves as the perforated casing. The injection rate of single-phase flow (water only) and two-phase flow (water-proppant slurry) was designed to mimic the field conditions to generate enough pressure drop from the flow through perforation. A microphone was used to provide acoustic measurements for the experiments. The apparatus used in this research has only one perforation, which allows us to build correlations between acoustic signals with known water flow rates and proppant concentrations.

With acoustic signals collected, signal processing methods including the Yule-Walker method were applied to analyze the noise amplitude spectrum in the frequency domain. The spectra obtained for slurries with different concentrations were compared.

To test the methodology for field data interpretation, the time domain waterfall plot for one stage was transferred into the frequency domain waterfall plot at each cluster location. Several patterns were observed and interpreted on the frequency-domain waterfall plots. Proppant-related frequencies were identified by comparing the frequency domain response under different injection periods. Proppant distributions between clusters were estimated with two methods (energy summation, and filtered energy summation), and were compared to the proppant distribution results from other erosion analysis.

Sand-related signals can be distinguished from water flow sounds in the frequency domain. Certain frequency band picks up when sand is introduced to the system. This characteristic of the frequency becomes higher in amplitude when the concentration of sand increases, but the characteristics of the frequency band remain the same. With regression analysis, sand concentration and noise amplitude were determined to be linearly related. From the field data interpretation, both methods have reasonable agreement at with some discrepancy when compared with perforation erosion by downhole camera.

This study provides a new correlation to interpret proppant placement in individual perforation clusters from high frequency DAS measurements.