Ultrasonic wave propagation characteristics in pore-fracture structure of deep coal under coupled thermo-pressure conditions

The elasticity and physical properties of coal and rock, along with their variation characteristics, underpin the prediction of coalbed methane reservoirs and delineating "sweet spots". Physical properties are significantly influenced by temperature and pressure. This study aims to address the unclear dynamic response mechanism of pore-fracture structures in deep coalbed methane reservoirs under coupled in-situ temperature and pressure conditions, coal and rock samples were taken from the Linxing Block in the Ordos Basin. Variable temperature and pressure ultrasonic pulse transmission tests (0~30 MPa, 11~99 ℃) were conducted to analyze the response patterns of wave velocity and waveform characteristics to pore-fracture evolution. An improved dual-pore model was employed to quantify the temperature-pressure coupling constraint mechanism, establishing a nonlinear mapping relationship between waveform similarity coefficient and porosity. Results indicate: Waveforms exhibited significantly higher sensitivity to confining pressure than wave velocity. When confining pressure increased from 0 to 30 MPa, both P- and S-wave shape similarity coefficients changed by over 60%, representing a 2.96-fold~5.99-fold increase relative to wave velocity. As confining pressure increased, coal-rock pore closure primarily resulted from the synergy effects of rigid pore linear compression and flexible pore exponential closure. Temperature elevation (62℃, 19 MPa) increased the total porosity of coal samples by 0.55% compared to 11℃, 19 MPa conditions through thermal expansion effects. We obtained experimentally the waveform similarity coefficient-porosity mapping model (R²=0.786) and porosity inversion constraints (waveform, dual-pore, temperature-pressure), subsequently establishing a dynamic temperature-pressure, dual-pore fracture model for multi-parameter prediction of coalbed methane "sweet spots". These findings provide theoretical references for refined characterization and efficient development of coalbed methane reservoir properties.