A new apparatus for coupled low-field NMR and ultrasonic measurements in rocks at reservoir conditions

Here, we report the design and performance of a novel NMR-compatible core holder system allowing for the measurement of both ultrasonic P-wave velocities and NMR relaxation parameters in rock cores at relatively high temperature and pressure conditions. To the authors' knowledge, this new apparatus represents the first documented example of coupled low-field NMR and ultrasonic measurements of rock cores at reservoir pressure and variable saturation conditions, and allows for a new approach to study pore-scale saturation effects on elastic-wave propagation in rocks. Saturation-wave velocity models usually require some description of fluid saturation and/or distribution and NMR provides a unique ability to measure the local physical and chemical environment of pore fluids. Hence, there are several advantages over other routine core saturation measurements often coupled with ultrasonic measurements, such as resistivity, X-ray tomography or simple fluid volume accounting. Successful validation of our apparatus against a conventional benchtop ultrasonic measurement system was performed using a dry Berea sandstone core, while demonstration of sequential NMR and ultrasonic measurements was performed on a Bentheimer Sandstone core as a function of variable brine and supercritical CO₂ saturation (coreflooding conditions).