Effect of carbonate pore structure on dynamic shear moduli

Elastic shear moduli from ultrasonic measurements of carbonate rocks show an increase or decrease when saturated from the dry state with brine. The induced changes on the moduli have been attributed to several rock-fluid interaction effects, including viscous coupling, reduction in free surface energy, and dispersion caused by local flow with a subsequent effect on acoustic velocities. As many studies have recognized that acoustic velocity in carbonate rocks is dependent on pore structure, a relationship between the pore structure and observed changes must, to a certain extent, exist. We make use of quantitative digital image analysis parameters, such as dominant pore size and perimeter over area, to demonstrate how changes in dynamic shear moduli upon saturation relate to the geologic make up of the rock. Samples with a small dominant pore size and large perimeter over area show abnormal decrease in shear moduli with saturation from the dry state. This is attributed to the large surface area available for matrix-fluid interaction. Samples with large dominant pore size and small perimeter over area show an increase in shear moduli with saturation. This may be a dispersion effect, as high frequency and high permeability may cause the fluid in the saturated samples to move out-of-phase with the solid during propagation of the acoustic wave and thus cause a stiffening effect.