Pore Structure–Acoustic Property Coupling of Shale under Fracturing Fluid Thermopressure Dynamics

Understanding the impacts of fracturing fluids on the shale pore structure and acoustic behavior under varying temperature and confining pressure conditions is crucial for reservoir stability prediction and optimization of acoustic monitoring techniques. In this study, the multiscale evolution characteristics of the mineral composition, pore structure, and acoustic properties of the Dawuba shale were systematically investigated. This research used static immersion, with shale samples immersed in fracturing fluid for 72 h under three pressures (5, 10, and 15 MPa) and two temperatures (20 and 50 °C). The results indicate that at 20 °C under confining pressure, hydration-induced clay swelling leads to a reduction in pore volume (PV), an increase in the fractal dimension <i>D</i>₁, and a significant rise in acoustic velocity (Vp). The increase of confining pressure leads to the evolution of micro- and mesopores to macropores and induces microfractures, which increase PV and decrease <i>D</i>₁ and Vp, and the acoustic attenuation coefficient α increases. Under elevated temperature conditions, carbonate minerals exhibit accelerated dissolution, triggering pore structure reconstruction and significantly enhancing wave scattering and energy loss. When temperature and pressure act together, pore connectivity increases, and the shale skeleton weakens, further reducing Vp and increasing α. These results reveal a temperature- and pressure-dependent "pore-acoustic" coupling mechanism: The low-pressure hydration phase is dominated by pore compression, whereas high-pressure and high-temperature conditions drive the transformation of micropores to mesopores and induce crack development. This pore structure evolution directly influences acoustic response parameters, with a strong correlation observed between pore volume and fractal dimension <i>D</i>₁.