Laboratory quantification and detection of pre-existing fractures and stress-induced microfracturing through combined ultrasonic and triaxial-stress testing

Simultaneous triaxial stress testing and ultrasonic wave propagation were utilized to quantify natural fractures and microfracturing in Berea Sandstone and Silurian Dolomite. Experimental results indicate that the presence of fractures distinctly decreases wave velocities, with calculated dynamic elastic moduli decreasing by up to 7.5% in artificially fractured sandstone. Wave analysis of intact and artificially fractured Berea Sandstone reveal the nonlinear mechanical and geophysical response or fractured rocks subject to isotropic and deviatoric stress loading paths. Specifically, fractures increase hysteretic stress-strain behavior, and tend to amplify the stress dependence of wave attenuation and the filtering of high-frequency wave components. Additional deviatoric loading tests of Berea and Silurian samples provide evidence for the onset of stress-induced microfracturing, detected at a threshold of 1% shear wave anisotropy called the “shear wave crossover” (SWX). The SWX and subsequent increases in shear wave anisotropy evidence microstructural damage development well before quasi-static indicators such as the volumetric strain positive point of dilatancy (PPD) and yield/failure in all samples. Specifically, Berea and Silurian samples exhibit up to 5% and 7% shear wave anisotropy at the PPD, respectively. Additionally, stresses at the SWX and PPD were compared to peak axial stress to understand linkages between damage at several scales and ultimate rock strength. The SWX occurs at an average of 27% lower axial stresses, and 5% less shear wave anisotropy than the PPD, indicating that samples undergo irreversible microstructural changes earlier than previously thought. The SWX and PPD both provide meaningful estimates of failure stress, however samples must be subjected to higher stresses and strains to reach the PPD, making it less favorable for sample preservation. Furthermore, correlation between the SWX and peak stress under several different radial stresses, present a viable technique for using dynamic measurements to predict static rock failure properties, while also preserving sample competence for future tests. Linking the dynamically measured SWX to static rock failure properties provides an additional avenue for developing accurate transforms for several rock types. Therefore, the SWX can add value across industries for predicting rock behavior and maximizing the value of expensive samples and rock testing.