Investigation Parameters for Sound Induced by Fluid Displacement in Rock Samples

Abstract

We conducted laboratory experiments to measure the sound produced during water‐by‐gas and kerosene‐by‐water displacements in samples of Indiana limestone and sandstones with different permeabilities. In water‐by‐gas displacement experiments, intensive sound was detected during the injection of hundreds of pore volumes and even when mobile water saturation was small (~1%). The sound was produced in a wide frequency range starting with frequencies less than 1 kHz; the upper frequencies varied from approximately 15 kHz for high‐permeability sandstone to almost 50 kHz for low‐permeability sandstone. In contrast to the experiments with gas flow through the dry rock samples, where sound appears only at high gas rates corresponding to the Forchheimer flow regime, the displacement produces sound at lower rates, corresponding to Darcy regime. Both the drop rate of sound emission and the sound intensity at the same mobile water saturation were determined to inversely depend on the rock permeability. We found that sound produced by sandstones shows a power law relationship between the number of events and their amplitudes, where the absolute value of the exponent also inversely depends on the rock permeability. In kerosene‐by‐water displacement experiments, the signal was observed at lower frequencies (up to 5 kHz) and only during injection of the first several pore volumes. A model describing the sound generation was suggested; the model considers acoustic events as a train of random pressure pulses corresponding to a Poisson point process. Simulation results demonstrated that the model reasonably reproduces the general trend of sound spectra produced by sandstones.