Analysis of Acoustic Signals Generated by Proppant Slurry Flow Through Perforations: Experimental Study

Summary This study explores the acoustic signal generated when a water-sand slurry flows through perforations in the frequency domain. The goal is to identify the proppant/sand presence on distributed acoustic sensors (DAS) and therefore to allocate sand concentration during a multistage fracture treatment using DAS monitoring data. This study further explores whether sand-related signals can be distinguished from a water flow signal in the frequency domain, what the frequency components of the proppant signal are, and how proppant concentration or other parameters affect the proppant-related signals. In limited-entry design, water-sand slurry flows through perforations with a high injection rate and a high-pressure drop across the perforations. This high flow velocity generates the acoustic signal. It is believed that when solids (sand) pass the perforation, the frequency domain responds differently than when just liquid flow passes the perforation. To prove the concept, an experimental study simulating this process at the laboratory scale was conducted. The experimental setup consisted of a core with a hole drilled in the middle as a perforation. A steel plate placed at the inlet of the core served as perforated casing. The injection rate of single-phase flow (water only) and two-phase flow (water-sand slurry) was designed to mimic the field condition to generate enough pressure drop from the flow through the perforation. A microphone was used to provide acoustic measurements for the experiments. The apparatus used in this research had only one perforation, which allowed for building correlations between acoustic signals with known water flow rates and proppant concentrations. With acoustic signals collected, signal processing methods, including the fast Fourier transform method and Yule-Walker method, were applied to analyze the acoustic amplitude spectrum in the frequency domain. The spectra obtained for slurries with different concentrations were compared. Theoretical modeling started with understanding the acoustic signal generated by the collision of a single solid particle with a metal surface. Starting with the single collision model, signal processing was applied to the superposition of multiple signals in the time domain to explore the effect of sand concentrations on the spectrum function of the acoustic signal. The results from theoretical modeling were used to confirm the experimental data. From the experimental results, sand-related signals could be distinguished from water flow sounds in the frequency domain. A certain frequency band was detected when sand was introduced to the system. This characteristic of the frequency became higher in amplitude when the concentration of sand increased, but the characteristics of the frequency band remained the same. With regression analysis, sand concentration and acoustic signal amplitude were determined to be linearly related. This study provides a new correlation to interpret proppant placement in individual perforation clusters from high-frequency DAS measurements.