Systematic wettability alteration by aging sandstone and carbonate rock in crude oil

A reproducible method for selectively altering the wettability of outcrop chalk has been established to obtain stable wettability conditions in the range from strongly water-wet to near-neutral-wet. Aging in different oils and the response to aging in different rocks are reported. The influence of different initial water saturations on the role of wettability alterations has been evaluated. Core plugs were aged in oil, at 90°C, for different time periods in duplicate sets. Oil recovery by spontaneous room temperature imbibition, followed by a waterflood, was studied for aged cores, to obtain Amott–Harvey wettability index to water. The procedure was also performed on cores containing aged crude oil, on cores where the aged crude oil was exchanged with fresh crude oil after aging was completed, but before imbibition testing and for cores where the crude oil was exchanged with decahydronaphthalene which was then displaced by n-decane. Imbibition in water-wet cores with n-decane was used as the baseline experiment. A consistent and reproducible change in wettability, from strongly water-wet to a near-neutral-wet state, with increased aging time was observed for five different outcrop chalks. The altered wettability was stable over several flooding cycles but when the aged crude oil was exchanged by fresh crude oil or decane after aging but before imbibition, results exhibited different but consistent Amott indices. Exchanging crude oil at temperature with decalin, which in turn was exchanged with decane, was found to be the best procedure for reproducible and stable wettability alteration.