Effect of pressure and fluid on pore geometry and anelasticity of dolomites

The acoustic properties of rocks depend on porosity, pressure, and pore fluid and also on pore geometry. Anelasticity (attenuation and velocity dispersion) is more affected by crack aspect ratio and fraction (soft pores) than by equant (stiff) pores. To study this fact, we have performed ultrasonic measurements on two dolomite samples under variable pressure and fluid content, and used the EIAS (equivalent inclusion-average stress) model to obtain the crack aspect ratio and fraction from the bulk and shear moduli of the rock. The theory has an excellent agreement with the experimental data, and the results show that the crack attributes decrease with increasing differential pressure and are higher for a stiffer fluid. In fact, the interpretation of the experiments with the model shows that crack fraction and aspect ratio increase with the bulk modulus of the fluid (water and oil). Then, by extending the theory to all frequencies, using the Zener mechanical model, we obtain the phase velocities and quality factors as a function of frequency. Our findings reveal the importance of considering differential pressure and fluid type to analyze pore geometry and rock anelasticity.