Prediction Method of In-situ Stress and Anisotropic Rock Mechanics Parameters in Strong Tectonic Compression Strata

ABSTRACT: There are some differences in rock mechanical properties among different directions, therefore, the predictive accuracy of the isotropic rock mechanics parameter calculation model is limited when confronted with high-stress, strongly tectonically compressed formations. The existing anisotropic models rely on Sonic Scanner well logging and ultrasonic testing experiments, high costs and low returns hinder its large-scale application in the field. In order to accurately determine the horizon where tectonic compression occurs, clarify the change of in-situ stress distribution, and improve the prediction accuracy of rock mechanics parameters in complex geological structures, this study proposed a simplified analogy model of ultrasonic testing and a microelement model of sand-mud layer system, under the assumption of transverse isotropy. With the help of these models, the formation anisotropy was evaluated by using traditional logging data. the wave velocities propagating along various angles of formation were fitted by using the Thomsen formula, the anisotropic rock mechanical parameters were calculated through the stress-strain constitutive equation. The results were verified by triaxial compression experiments and Kaiser acoustic emission stress test experiment: (1) Gypsum rock and mudstone have poor resistance to tectonic compression, while sandstone has strong resistance and can maintain the stability of mechanical properties. (2) The linear transformation relationship established by keeping the same direction of dynamic and static rock mechanics parameters is more suitable in structural compression strata (3) Compared with the traditional isotropic model, the anisotropic rock mechanics parameter calculation model can identify the change of in-situ stress distribution and improve the prediction accuracy by 9%.