In-situ assessment of stress-dependent dynamic elastic properties: concept and modeling results

ABSTRACT: Accurately capturing the dynamic elastic properties of rock and their relation with stress and strain is crucial for interpreting time-lapse seismic surveys from projects including hydrocarbon exploration and geological carbon sequestration. Traditionally, these relations have been analyzed in the laboratory on centimeter-scale core samples extracted from the subsurface. In addition to the difficulties of correctly upscaling such results from the laboratory to the field, rock cores can suffer from significant disturbance induced during core extraction and subsequent handling. These disturbances not only lead to inaccurate representation of the in-situ conditions but also make the samples unsuitable for testing in many cases. To address these challenges, we propose the development of a downhole testing concept for the quantification of the stress-dependent dynamic elastic properties. The testing measures the sensitivity of acoustic waves propagating longitudinally through a borehole interval under controlled pressurization. Combining geomechanical, rock physics, and acoustic modeling, we showed that the stress perturbation can alter the recorded wavefield close to the ends of the loading interval. A tailored analysis of the synthetic full waveform sonic log data revealed that P wave attributes, namely amplitude and arrival time, varying systematically with the offset to the source. This analysis suggested that a comparison between perturbed and unperturbed scenarios can be used to quantify the stress sensitivity of the formation.