Influence of wettability on water retention curves in unconsolidated porous media.

Abstract

Controlled laboratory experiments were carried out using the hanging column method. Prior to the experiments, three uniform silica sands, which were originally water-wet, were aged in contact with crude oil until they were moderately oil-wet. Five fractionally wet sands were obtained by mixing the water-wet sands with oil-wet sands containing 25, 50 and 75 vol% oil-wet sands. A total of 11 tests formed the basis for the present study. The measured water retention curves showed that the capillary pressure heads were greatly reduced in sands that were oil-wet or fractionally wet. Changes in the wettability of the sands also affected their irreducible water saturation: The higher the proportion of oil-wet sand in the sand mixtures, the lower the irreducible water saturation. To quantify the characteristics of the measured water retention curves, the Van Genuchten model was used. The two optimized parameters seem to indicate a general trend: The higher the volume fraction of oil-wet sand, the higher is α and the lower is n. For the three unaged sands and the aged medium-sized sand, it was found that each of the two branches of the measured retention curves can be suitably scaled to a unique curve if, in addition to the petrophysical parameters (intrinsic permeability, porosity, surface tension, gas-water contact angle), the irreducible residual water saturation and the residual air saturation are taken into account. To quantify the observed deviations of the other two aged sands from the unified Leverett J-function, a theoretical fit function was used to match the experimental data of the three unaged sands. The experimental data sets for P2040ag and P100ag were found to be overestimated overall by the fit function. However, when the petrophysical parameters of the unaged sands were used instead of the actual measured parameters, the individual experimental Leverett J-functions came closer to the uniform J-curve. Based on this, it could be concluded that the apparent differences in pore structure between aged and unaged sands in addition to wettability, expressed by the cosine of the contact angle, may have contributed to a further reduction in the capillary pressure plateaus of the aged sands, which was particularly visible and significant in the P2040 and P100 sand. Using the measured static contact angles for two-phase gas-oil and oil-water systems and the measured interfacial tensions when the porous medium is either water-wet or moderately oil-wet, it was shown that the Bartell-Osterhof equation overestimates the measured gas-water contact angles. Reasonable agreement was achieved when a calibrated roughness factor of the solid surface was considered in the Young's contact angle.