Study on the influence of drilling fluid soaking on rock acoustic and mechanical properties

ABSTRACT: The acoustic and mechanical properties of rock can reflect the characteristic information of rock pore structure and mechanics. During the drilling process, the borehole rock is soaked in the drilling fluid for a long time, and studying the influence of the drilling fluid on the acoustic and mechanical properties of the rock is of great significance to ensure the safety of drilling and increase the drilling speed. In this paper, acoustic testing was carried out on the original rock and the rock after soaking drilling fluid by ultrasonic transmission method. Based on the test results, the acoustic characteristics of the rock such as interval transit time, attenuation coefficient, time-frequency characteristics, and microstructure characteristics are studied by Fourier transform, wavelet transform and other methods. The changes in acoustic properties of rocks after soaking drilling fluid are analyzed. In addition, mechanical tests such as triaxial compression test and tensile test were carried out on the original rock and the rock after soaking drilling fluid. The mechanical properties of rocks are studied, and compare and analyze the uniaxial compressive strength, elastic modulus, Poisson's ratio, cohesion and internal friction angle of the rock before and after soaking drilling fluid. The changes in mechanical properties of rocks after soaking drilling fluid are analyzed. The results showed that the pore structure of the rock changed after the drilling fluid was soaked, the mudstone was broken, and the sandstone structure was damaged, which was affected by the rock mineral composition and cation exchange capacity. The sandstone interval transit time is decreased, and the attenuation coefficient is increased. The signal amplitude in the frequency domain is significantly reduced, high-frequency signals are absorbed, the trailing signal is greatly reduced, the proportion of low-frequency signals increases, and the main frequency of the acoustic signal shifts to the low-frequency direction. Based on the wavelet transform, from a microscopic point of view, the rock microstructure is damaged after soaking drilling fluid. Furthermore, the compressive strength, tensile strength, elastic modulus, and cohesion of the rock are significantly decreased, and the Poisson's ratio is significantly increased, which further indicates that the rock structure is damaged after the drilling fluid is soaked.