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

Area of Science:

• Geosciences
• Petroleum Engineering
• Materials Science

Background:

• Reservoir stability prediction and acoustic monitoring rely on understanding fracturing fluid impacts.
• Shale's complex pore structure and acoustic behavior are sensitive to environmental conditions.

Purpose of the Study:

• To investigate the multiscale evolution of shale's mineral composition, pore structure, and acoustic properties.
• To elucidate the "pore-acoustic" coupling mechanism under varying temperature and confining pressure.

Main Methods:

• Static immersion of Dawuba shale samples in fracturing fluid for 72 hours.
• Application of controlled confining pressures (5-15 MPa) and temperatures (20-50 °C).
• Analysis of mineral composition, pore structure (pore volume, fractal dimension), and acoustic properties (velocity, attenuation).

Main Results:

• At 20°C, clay swelling reduced pore volume and increased acoustic velocity.
• Increased pressure led to macropore evolution, microfractures, increased pore volume, and decreased acoustic velocity.
• Elevated temperatures accelerated carbonate dissolution, altering pore structure and increasing wave scattering.
• Combined temperature and pressure increased pore connectivity, weakened the shale skeleton, reduced acoustic velocity, and increased attenuation.

Conclusions:

• A temperature- and pressure-dependent "pore-acoustic" coupling mechanism was revealed.
• Low-pressure hydration causes pore compression, while high-pressure/temperature conditions induce pore transformation and crack development.
• Shale's pore structure evolution directly influences acoustic response, with strong correlations between pore volume and fractal dimension.