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An experimental study of the effects of partial saturation on elastic wave velocities in porous rocks Goertz, David


Elastic wave velocities in porous rocks containing air and water are sensitive to not • only the relative levels of fluid saturation but also to the distribution of the fluids within the pore space. Three factors that have significant control over-the relative distributions of fluids in multiphase saturated porous media are pore space microgeometry, saturation history and wettability. In this thesis, the effect of these factors on the form of dependence of velocities upon water saturation level in rocks is investigated experimentally. Ultrasonic elastic wave velocity and drying rate measurements were made as a function of water saturation in a limestone, a dolomite and two sandstone samples as saturation was reduced through evaporative drying. During the later stages of drying there is a reduction in drying rate that is associated with the transition from capillary transport to diffusive transport due to a loss of hydraulic connectivity of the liquid phase. For the rocks used in this study, this suggests that velocity variations below this point can be associated with the removal of disconnected water held in surface roughness and in crack-like porosity. Using these interpretations and simplified models of the pore spaces derived from thin section analysis, fluid distribution scenarios are proposed for the drying process in these rocks. A numerical modeling routine is then used to predict the form of the velocity saturation relationships for the rocks. The models were found to be in good agreement with the form of the experimental results. The effect of wettability on the relationship between velocities and saturation history was investigated in the sandstone samples by conducting imbibition and drainage experiments before and after treatment with a chemical that altered their surfaces from being strongly water-wet to being oil-wet. In the water-wet sandstones, the results indicate that grain contact regions are the last to drain of water and the first to fill with water. At high saturation levels, hysteresis is evident and is attributed to differences in the pore scale distribution of fluids that evolves in pore bodies during the imbibition and drainage processes. The results for the oil-wet samples during evaporative drying were found to be similar to those for the water-wet rocks: water was replaced by air first in the pore bodies and then in the grain contacts and cracks. In contrast, imbibition produced results that are consistent with water entering the pore bodies first and being excluded from the cracks and grain contacts until high saturation levels.

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