Elastic wave velocities during evaporative drying

Laboratory measurements of drying rates and elastic wave velocities are made on limestone, dolomite, and sandstone samples during evaporative drying. The drying rate data are very similar in form. There is a constant rate period at higher saturations and a falling rate period below saturation levels of approximately 0.2. The falling rate period marks the transition in the sample from hydraulically connected to disconnected water. There is a strong link between elastic wave velocities and the drying process because different pore geometries drain at different stages in drying. The drainage of these different geometries results in specific changes in the moduli and velocities. Simple models of the pore geometries and the drying process are used to model the velocity data. The velocitysaturation relationship for each of the three samples is very different in form because of differences in porespace microgeometry. Of particular interest is the velocity response during the falling rate period of drying. In the limestone and the sandstone, there is a significant decrease in bulk and shear moduli and elastic wave velocities because of the drainage of crack-like pores and grain contacts. In contrast, the absence of these pore geometries in the dolomite results in essentially no changes in the moduli at low saturations. An understanding of the drying process and resulting pore-scale fluid distribution provides useful insights into the observed form of the velocity-saturation relationship.