On the Optimum Aging Time: Magnetic Resonance Study of Asphaltene Adsorption Dynamics in Sandstone Rock

Wettability is a key factor defining ultimate hydrocarbon recovery. Extensive experimentation is required to replicate the wetting state of reservoir rocks. This involves a rock sample wettability restoration procedure, including an aging step and a concept of an optimum aging time, in which asphaltene chemistry plays a major role. There are numerous reports on significance of various crude oil components and elements of asphaltene structure to their tendency to interact with the solid phase, though subject evidence is contradictory. We investigate a possible relationship between the composition of oil, kinetics of an aging process, and a change of sandstone rock wettability. Wettability state of the cores was monitored using low-field NMR relaxometry. The composition and key components of accumulated deposits were established by matching H-1 solution-state NMR and X-band EPR spectra of deposits to the spectra of SARA (saturates-aromatics-resins-asphaltenes) fractions of aging fluids. We determined adsorption rate as a function of aging fluid type, monitored free radical and vanadyl content of deposits, and wettability state (through surface relaxivity) over extended aging time interval. EPR data suggest no correlation between the concentration of free radicals in deposits and wettability of the cores. We observe two distinctive periods in the aging process: (i) early-time adsorption best described by a pseudo-second-order kinetic model similar for all aging fluids used in this study, suggesting a common reaction-limited process; (ii) a late-time adsorption process of a faster rate proportional to a bulk diffusion coefficient of aging fluids approximated by an intraparticle diffusion kinetic model. The transition time interval between the two can be used as a definition of an optimum aging time in special core analysis. Results provide a link between oil chemistry and wettability phenomena in rocks and may contribute to the development of a model predicting wettability reversal and more accurate reservoir modeling.