Petrohysical characteristics of ultra-deep carbonate reservoirs under coupled temperature-pressure variations

Abstract

Understanding the petrophysical response of carbonate reservoir under high-temperature and high-pressure (HTHP) conditions is essential for ultra-deep oil and gas exploration. However, the coupled influence of temperature and pressure on elastic properties and pore-structure evolution remains insufficiently quantified. We investigate nine Ordovician limestone samples from the Tarim Basin (7203.96–7444.56 m) using controlled laboratory experiments under temperatures up to 200°C and confining pressures up to 137.9 MPa. P- and S-wave velocities and corresponding elastic moduli are measured and inverted using the Sun model to quantify pore-structure evolution under coupled thermo-mechanical conditions. Results show that increasing temperature reduces wave velocities and elastic moduli due to enhanced lattice vibrations in calcite and weakened grain contacts. Inversion results indicate a reduction in pore aspect ratio and systematic variations in pore-structure parameters γ and γμ, accompanied by decreased shear resistance. In contrast, increasing pressure enhances stiffness by closing microfractures, increasing pore aspect ratio and restoring shear strength. A key observation is that elastic properties are jointly controlled by temperature and effective pressure through their regulation of pore structure, with pressure exerting the dominant influence under ultra-deep conditions. Temperature effects are partially reversible and more pronounced in low-porosity rocks, whereas pressure effects exhibit clear stage dependence, transitioning from fracture closure to matrix compaction with increasing stress. This study establishes a quantitative framework linking elastic response to pore-structure evolution in ultra-deep carbonates, providing a theoretical basis for seismic inversion, pore‑type identification and pore‑pressure prediction.