Abstract
Shale gas reservoirs have risen in importance in China’s new power source exploration and development program. The investigation of the propagation of ultrasonic waves in shale forms the basis for the full waveform application of acoustic logging data to the exploration of shale gas. Using acoustic wave theory, initial conditions, vibration source conditions, and stability conditions are developed in combination with experimental background of ultrasonic wave transmission. With improved boundary conditions, we performed numerical simulations of the ultrasound transmission experiments in shale using the high-order staggered-grid finite difference method (second-order in the time domain and fourth-order in the space domain). With programs developed within MatLab, the results obtained from numerical simulations agree well with experimental results based on physical models. In addition, using snapshots of the wave field that give a microscopic perspective, the propagation laws for ultrasonic waves can be analyzed. Using this method, human error is avoided, transmission experiments costs can be reduced and efficiency improved. This method extends the scope of experimental investigations regarding the transmission of ultrasonic waves in a shale gas reservoir with increasing stratification, and thus has great theoretical value and practical significance.
Highlights
For some time, it has been understood that the acoustic characteristics of rocks are complicated by stratification
Kuila et al (2011)[2] investigated the effects of wave velocity on the magnitude and direction of stress applying various stress field conditions, and found magnitude values to be related to the microporosity of shale
They concluded that the direction of anisotropy changed the stress field and greatly affected the variation in wavelength
Summary
It has been understood that the acoustic characteristics of rocks are complicated by stratification. Based on the physical models of pores with different shapes and patterns, such as rods, pieces, spheres, columns, and fractures, Wang et al (2008)[12] designed the seismic model with the same vertical scales and fillers but different horizontal scales They conducted many numerical simulations and analyzed the seismic response characteristics induced by variations in pore structure. With breakthroughs in exploration and development of shale gas, experts are increasingly involved in studying physical data of rock in shale strata (Vernik et al, 1992; Deng et al, 2004; Deng et al, 2015; Chen et al, 2013; Dewhurst et al, 2011).[15–19] further research advances in the anisotropy of ultrasonic wave velocity in shale are still expected, in view of the difficulties in preparation of shale samples in laboratory experiments. This paper focuses on shale with mature bedding structures and presents numerical simulations on the transmission of ultrasonic wave based on wave theory
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