An acoustic-parameter testing method and experimental study for in situ rock cores based on a composite waveguide-plug structure

Abstract

Accurate in situ acoustic parameters of deep reservoir cores are essential for the reliable inversion of key reservoir properties such as porosity and elastic modulus. The in situ pressure-preserved coring technology enables the acquisition of core samples from deep reservoirs under in situ conditions. Building on this capability, this study proposes an in situ ultrasonic testing method for cores inside a pressure-retaining corer. The method addresses the long-standing problem that the metallic wall of the corer causes strong acoustic interference and prevents effective wave penetration into the core. To overcome this limitation, a metallic waveguide module was developed and integrated into an opening in the simulated corer wall, and a waveguide plug was used to transmit acoustic energy into the core. Ultrasonic experiments were conducted on rock samples under different test conditions to systematically compare the acoustic transmission performance of three waveguide-plug geometries, namely planar, curved, and conical configurations. In parallel, a three-dimensional transient pressure acoustics model was established in COMSOL Multiphysics 6.3 to simulate wave propagation in the multilayer waveguide-core medium and to reveal the evolution of the wave field under different contact conditions. The results show that the composite waveguide-plug structure successfully establishes the intended acoustic transmission path and enables valid measurements of core P-wave velocity, thereby confirming the technical feasibility of the proposed method. Among the three contact modes, the curved-fit configuration performed best. For sandstone, the measured wave velocity was 3.889 km/s, with a deviation of only 2.21% from the reference value obtained by conventional direct testing (3.977 km/s), whereas the deviation for the planar contact reached 10.21%. The numerical results agreed well with the experimental data, with relative errors below 2.1%. The curved-fit configuration also exhibited the best wave-focusing performance and the highest signal clarity in the simulations, further confirming the validity of the numerical model and the reliability of the experimental conclusions. The proposed composite waveguide-plug structure provides a feasible technical route for ultrasonic testing of in situ pressure-preserved coring and offers a methodological basis for the in situ acoustic characterization of deep reservoir cores under pressure-preserved conditions.>

Keywords

waveguide plug / rock core / ultrasonic testing / P-wave velocity / numerical simulation

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