Formation strength and elastic wave velocities in rift basin mudstones: effects of stress, frequency, and lithology

Accurate knowledge of formation strength is essential for geomechanical modeling, borehole failure and in situ stress analysis. Laboratory strength measurements are limited in scale and spatial extent, posing significant challenges for extrapolating strength measurements, especially in highly heterogeneous formations such as fluvial and lacustrine sedimentary rocks. Acoustic/sonic P-wave velocity (Vp) provides one of the best proxies for formation strength, but it is stress- and frequency-dependent. Vp-strength relationships, therefore, can be very sensitive to experimental conditions. In this study, we investigate Vp-strength relationships for Mesozoic rift basin formations based on measurements on over 70 samples from the Newark Rift basin, a candidate site for geologic CO₂ storage and one of the largest in a series of the Mesozoic rift basins on the eastern North-American coast. Elastic wave velocity measurements were obtained for a range of confining pressures from ~2 to ~41 MPa, roughly corresponding to in situ confining pressure range. Although, overall, Vp values tend to increase with increasing pressure, the degree of Vp response to stress varies dramatically from sample to sample, and does not appear to correlate directly to lithology or porosity. Laboratory Vp measurements agree well with corresponding sonic logs; therefore, a systematic frequency-dependent core-log difference is not observed, but accounting for Vp dependence on confining pressure is important. We quantify the Vp-pressure dependence using laboratory acoustic measurements and develop depth-dependent Vp-strength relationships. These could be used with sonic logs to improve geomechanical analysis, and could be potentially applied in similar Mesozoic rift basin formations.