Wettability alteration in high temperature and high salinity carbonate reservoirs
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The goal of this work is to change the wettability of a carbonate rock from oil wet-mixed-wet towards water-wet at high temperature and high salinity. Only simple surfactant systems (single surfactant, dual surfactants) in dilute concentration were tried for this purpose. It was thought that the change in wettability would help to recover more oil during secondary surfactant flood as compared to regular waterflood. Three types of surfactants, anionic, non-ionic and cationic surfactants in dilute concentrations (<0.2 wt%) were used. Initial surfactant screening was done on the basis of aqueous stability at these harsh conditions. Contact angle experiments on aged calcite plates were done to narrow down the list of surfactants and spontaneous imbibition experiments were conducted on field cores for promising surfactants. Secondary waterflooding was conducted in cores with and without the wettability altering surfactants. It was observed that barring a few surfactants, most were aqueous unstable by themselves at these harsh conditions. Dual surfactant systems, a mixture of a non-ionic and a cationic surfactant increased the aqueous stability of the non-ionic surfactants. One of the dual surfactant system, a mixture of Tergitol NP-10 and Dodecyl trimethyl ammonium bromide, proved very effective for wettability alteration and could recover 70-80% of OOIP during spontaneous imbibition. Secondary waterflooding with the wettability altering surfactant (without alkali or polymer) increased the oil recovery over the waterflooding without the surfactants (from 29% to 40% OOIP). Surfactant adsorption calculated during the coreflood showed an adsorption of 0.24 mg NP-10/gm of rock and 0.20 mg DTAB/gm of rock. A waterflood done after the surfactant flood revealed change in the relative permeability before and after the surfactant flood suggesting change in wettability towards water-wet.