Experimental study on P-wave response characteristics of hot dry rock under high temperature and high confining pressure

Abstract

In the exploration and development of hot dry rock reservoirs, accounting for lithology, temperature, and confining pressure is vital when predicting rock mechanical properties. Acoustic logging data is essential for accurate information in hot dry rock fracturing. However, under high-temperature and high-confining pressure conditions, logging signals in these reservoirs become more susceptible to interference, resulting in significant errors in rock mechanics data. This study employs the in-house CCZU-HTHP logging system to assess P-wave velocity in granite and sandstone over a temperature range of 25–400 °C and a pressure range of 0.1–110 MPa. Findings indicate that under normal temperature and pressure conditions, granite exhibits an average P-wave velocity 1.3 times higher than sandstone, with sandstone having a variation range 1.1 times that of granite. Furthermore, as temperature increases, both granite and sandstone experience logarithmic and linear reductions in P-wave velocity, with respective decreases of 24.3% and 29.67%. Notably, P-wave velocity and pressure curves demonstrate pronounced nonlinear characteristics as confining pressure rises. Under temperature-confining pressure coupling, P-wave velocity initially increases, then decreases. Sandstone's P-wave velocity increases by 12%, compared to 6.25% in granite. Conversely, P-wave velocity decreases by 12.04% in sandstone and 9.38% in granite. Importantly, the response threshold of temperature-confining pressure in granite exceeds that of sandstone.