Impact of Effective Stress on Variations of Acoustic Velocities at Ultrasonic Frequencies in Ultramafic Rocks

Abstract The global energy transition effort is focused on decarbonization and greener sources of energy, of which hydrogen is an example. Geologic H2 production via the serpentinization reaction has emerged as an active research area with many open questions. Serpentinization refers to the oxidation of ferrous ions into ferric ions in iron-rich rocks, e.g., olivine, which releases hydrogen. Exothermicity and volumetric expansion associated with the reaction can have positive or negative feedback on the rock’s H2 generation potential. Here, we unravel such mechanisms at the core-scale via acoustic monitoring methods. We investigate the co-assisted impact of effective stress as an aftermath of the serpentinization process on the acoustic response of the selected ultramafic rocks. Specifically, we perform laboratory tests on a cylindrical rock sample under different distributions of axial and radial stresses. The measurements are carried out in a triaxial acoustic core holder with active seismic monitoring. For each effective stress distribution, we analyze compressional (Vp) and two shear waves (Vs and Vs*) in orthogonal directions at ultrasonic frequencies. The obtained velocities are used to determine dynamic elastic rock properties (Young modulus, Bulk modulus, Shear modulus, Poisson ratio). The results of our research study show that both types of velocities (Vp and Vs) increase with increasing effective pressure. This observation can be supported by the reduction of absolute permeability and porosity and the closeness of naturally occurring sub-core scale fractures. Furthermore, the decreasing difference between fast and slow shear waves (splitting) as a function of increasing confining pressure suggests that elastic anisotropy drops due to the closure of micro and nanofractures. The study also documents the discrepancy between acoustic velocities during loading and unloading cycles, which are relevant for hydraulic stimulation and post-stimulation stages. This clearly indicates the hysteresis effect in absolute permeability values. It can be assumed that once closed, fractures cannot be easily reopened and have the same dimensions again. A greater increase in the P-wave velocity is observed than the average S-wave velocities when the radial stress is increased, thus indicating that the rock samples are evolving toward isotropy. The derived rock mechanical properties are in good agreement with the values reported in the literature for similar types of igneous rocks. The results of our study are essential for understanding the acoustic response of ultramafic rocks subjected to serpentinization. This work is one of the first attempts to address elastic properties based on acoustic velocity monitoring in the context of changing stresses, thus mimicking in-situ conditions of real-field H2 production scenarios. The presented results can be used in the future as a point of reference, contrasted against well logs, and for 3D/4D seismic data interpretation.