Effects of temperature and pressure on the wave responses of deep carbonate reservoirs

Deep carbonate reservoirs are subject to in-situ conditions of high temperature and high pressure. We consider six water-saturated dolomite specimens from these reservoirs and perform ultrasonic experiments to obtain the P- and S-wave waveforms and velocities under different pressure and temperature conditions. The P-wave attenuation is estimated using the spectral-ratio method. The results show that with the increase of temperature, the velocities slightly decrease and attenuation increases. At effective pressures <40 MPa, the P-wave velocities increase sharply with pressure and then approximately linearly at higher pressures, while attenuation decreases gradually with pressure. The crack porosity as a function of pressure is obtained from the experimental data. The P-wave velocities decrease with this porosity while attenuation shows an opposite behavior. Then, a multiscale poroelasticity model considering micro-, meso- and macro-scale fluid-flow mechanisms is proposed to analyze the effects of the fluid properties, temperature and crack content on the wave responses. The model results agree well with the experimental data at different pressures, which provides a theoretical basis for the analysis of broadband wave velocity dispersion and attenuation phenomena of carbonate reservoirs and underground porous media in general.