Elastic Properties And Borehole Stress Concentration Of Anisotropic Rocks: Modelling Method And Experimental Study

The anisotropy of rocks is the dependence of rocks’elastic,mechanic,or otherproperties on the directions.This phenomenon has been widely observed from the deep core to shallow crust in the interior of Earth and is important for understanding the rock textures and deformation processes of Earthin the domain of geology and geophysics.Anisotropic rocks have some specific physical properties that will affect the study and interpretation of its properties.The studies referred to anisotropic rocks are relatively rare compared with those referred to isotropic rocks.This thesis aims at studying the dynamic elastic properties and borehole stress distribution in anisotropic rocks and is vital for the studies,such as the transmission of the elastic wave in anisotropic media and borehole stress concentration.In this study,based on the ray tracing theory,free-surface reflection theory and the distributed point sources method,a numerical modeling method is proposed to model the ultrasonic pulse transmission responses and help analyze the effect of beam skew and diffraction on the pulse transmission measurements.Basing on the numerical analysis,a modification is made to the experimental configuration that effectively reduces the effect of beam skew,confining pressure and diffraction.The modified experimental configuration is then applied to measure the dynamic elastic properties of eight anisotropic metamorphic rock samples.Finally,a numerical method is built to model the stress distributed around an inclined borehole in anisotropic formations and,based on this method,the effect of anisotropy on borehole stress concentration is analyzed.The numerical and experimental results demonstrate that the skew of beam affects the ultrasonic pulse transmission measurement leading the measured 45^o P-wave phase velocity being 3.6%slower and the elastic stiffness C₁₃being 100%smaller.The proposed experimental method effectively reduces the effect of beam skew and diffraction on the determination of the dynamic elastic properties of anisotropic media.The strength of anisotropy of the rocks in the study area could be as much as 30%at atmosphere pressure and remain more than10%under 200 MPa.Through the analysis,the anisotropy of the rocks in the study area is strong and is mainly caused by the crystal-preferred orientation of the micas and micro-fractures.V_p/V_sThe modeling method proposed for the stress concentration around the inclined borehole is proved to be effective.Compared to previous studies,the proposed method referred the geology angles to Euler rotation angles that make the method convenient for the geologist and also offered a solution to the sign issue of the reverse function of the Lekhnitskij mapping function that makes the method stable.This study is meaningful for the theoretical study of the wave transmission and borehole stress distribution in anisotropic rocks.