Low Dose Surfactant-based Waterflood: An Effective Method to Improve Oil Recovery in Tight Dolomite-rich Formations

Oil-wet and mix-wet carbonate formations exhibit high negative capillary pressure. Oftentimes, the injected fluid flows through high-permeability zones and bypasses the oil in the low-permeability matrix. The invading water has a low chance to get imbibed into the matrix from those high permeable channels due to capillary pressure resistance. To enhance oil recovery from such reservoirs, it is essential to reduce the high capillary pressure resistance by either wettability alteration and/or interfacial tension (IFT) reduction.
Experiments showed that formulation PROE 1 (amphoteric) showed excellent wettability alteration ability by significantly reducing the contact angle. PROE 2 (anionic/amphoteric) demonstrated intermediate wettability alteration and low IFT. The other two anionic/amphoteric mixtures, PROE 3 and 4, showed only IFT reduction without imposing any wettability alteration. Imbibition tests showed that surfactants demonstrating either wettability alteration or IFT reduction recovered additional 13%-35 % of original in place (OOIP) compared to the brine only case. However, PROE 2 with low IFT and intermediate wettability alteration outperformed other PROEs, resulting in additional recovery of 23%-40% OOIP. This implies that a combination of IFT reduction and wettability alteration would promote a better imbibition of water in this specific oil-wet carbonate matrix. One interesting finding noteworthy is that PROE 4 exhibited a higher oil recovery and recovered 6% OOIP more than PROE 3 although its IFT reduction (10-1 mN/m) was less than PROE 3 (10-2 mN/m). This suggests that, unlike conventional surfactant flooding in sandstone reservoirs, lower IFT does not necessarily lead to higher recovery in tight carbonate-rich formation.
Designing a suitable surfactant-based EOR methods is a challenge. Understanding the underlying mechanisms of such treatment based on specific reservoir conditions is essential for a successful field application. One of the tested surfactant formulations in this study demonstrated an optimization of surfactant EOR mechanisms for specific carbonate reservoirs in Texas.