Study of liquid-coupled ultrasonic techniques to evaluate elastic wave propagation in rocks

The development of new methods to measure core acoustic properties will improve understanding of reservoir geomechanical properties. Conventional acoustic analysis under triaxial press is often infeasible or impractical to perform along the entire core. Unconsolidated core samples in liners or cores extracted with pressure coring techniques present additional challenges. This work explores ultrasonic analysis of core samples subject to the previously mentioned constraints by immersing samples in liquid (water) and placing transducers adjacent to the sample at a given distance. Modern pressure-coring ultrasonic sensors are capable of measuring P-wave velocity across the core diameter. This research explores this type of analysis with a focus on quality control effects of transducer-sample alignment. Further, this report studies a dual transducer array adjacent to the core sample to measure refracted P and S-wave velocities simultaneously. The methods are corroborated with forward models assuming a simplified 2D geometry to better understand the constraints of both systems and assist in data analysis. Experiments are performed on cylindrical samples of aluminum, Berea sandstone, and Texas Cream limestone, with the rock samples studied while dry and fully water saturated. The data for the cross-diameter P-wave analysis suggest that there is a minimum amount of fluid required to couple the energy to the sample and highlight the need to use transducers with small effective measurement areas to reduce the effects of sample curvature. Measurements on both dry and fully water saturated core samples agree with Gassmann’s fluid substitution theory within a 2% error margin. Results from the refracted wave tests agree with the forward model, with all P and S-wave velocity estimates below 6% error. A second receiver spaced further away from the source simplifies isolation of relevant wave modes. Overall, forward model predictions agree with experimental results and show the potential of simultaneous P and S-wave measurement in the laboratory.