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 1H 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.