Influence of Rock Wettability Alteration on Dielectric Dispersion Data

摘要

Rock surface wettability controls the fluid distribution in the pore space of the formation and thus, has an influence on its electrical properties. The variation of dielectric dispersion data with the variation of formation wettability is not well understood at present time. In this study, we investigated the influence of wettability alteration on dielectric dispersion data by laboratory experiments using different dielectric data interpretation models. We used a reflection open- ended coaxial probe to measure dielectric dispersion of a 1.5-in. core plug in a frequency range of 10 MHz to 1 GHz. The studied rock wettability was varied in a coreflooding apparatus during a drainage experiment. We used low-polarity oil for the initial drainage to preserve the rock in the water-wet state and added different concentrations of surface-active stearic acid to the oilflood during later drainage stages to change the sample wettability. The used dielectric data interpretation models, in addition to standard interpretation answers, solve for geometrical parameters of the fluids and pore space, which allow us to evaluate wettability variation. Our results showed a decrease in dielectric dispersion at the low-megahertz frequency range as we inject higher acid concentrations into the rock. This decrease in the dispersion also corresponds to an increase of the inverted MN-parameter, which characterizes water phase tortuosity. The results of the inversion, using the textural model, showed a consistent decrease of the depolarization factor of water inclusions as we increase the acid concentration in the injected oil. At the highest acid concentration used (1 wt%), this depolarization factor drops down to 0.36, which is very close to the depolarization factor of spherical inclusions. It follows from our findings that water films, which are characteristic for water-wet rocks, are transformed to more spherical droplets due to the change of wettability to oil-wet state. NMR T2 measurements on the same plug and contact angle measurements on reference samples of same lithology support our conclusions on wettability variation. This study illustrates how to interpret the rock wettability state from dielectric dispersion data based on geometrical models and how to use the inverted geometrical parameters as a wettability indicator.