Experimental study on the effect of hydration and temperature synergism on acoustic wave propagation and mechanical strength of Longmaxi deep shales

Deep shale exhibits high water sensitivity and high-temperature characteristics, and the coupled effects of hydration and temperature have a significant impact on the physical and mechanical properties of shale. Using deep shale samples from the Longmaxi Formation in the Sichuan Basin as the research subject, fracture parameters,ultrasonic parameters, and mechanical parameters of the shale after immersion under different temperature conditions were obtained through experiments involving X-ray diffraction (XRD), scanning electron microscopy (SEM),hydration-temperature coupled immersion tests, ultrasonic testing, and laboratory mechanical compression. The experimental results reveal that, the energy of ultrasonic waves decayed with increasing immersion time under both ambient temperature (24°C) and high-temperature (120°C) conditions. The reduction in longitudinal and transverse wave velocities in the high-temperature environment was 1.41 and 1.71 times greater, respectively, than that under ambient conditions. Hydration caused changes in the fracture parameters of the shale, showing a phased behavior. The ultrasonic attenuation coefficient and hydration-induced structural damage coefficient increased rapidly in the early stages of hydration, increased gradually in the middle stages, and tended to stabilize in the late stages. The ultrasonic attenuation coefficient and hydration damage coefficient at high temperatures were 1.72 and 2.98 times higher,respectively, than that under ambient conditions. Additionally, the mechanical properties of the shale deteriorated after immersion, with compressive strength, elastic modulus, internal friction angle, and cohesion showing a staged decline.Under high-temperature conditions, the reduction in these mechanical parameters was 1.24, 1.42, 2.06, and 1.39 times greater, respectively, than that under ambient conditions. The findings of this study provide a theoretical basis for optimizing geological designs for horizontal drilling in deep shale formations and for adjusting drilling fluid densities.