Low-salinity Water-alternate-surfactant in Low-permeability Carbonate Reservoirs

Low-salinity water injected into carbonate cores, which have undergone sea-water injection, can produce additional oil more economically if a low-concentration non-ionic surfactant is added to the low-salinity water and injected as chase fluid. One major reason for the additional oil recovery is that low-concentration surfactant effectiveness favors the low-salinity environment. Several coreflooding, contact angle, and IFT experiments were performed to assess the proposed process. The core flooding sequence includes seawater, low-salinity water, and low-concentration non-ionic surfactant. However, for field application, we proposed low-salinity water-alternate-surfactant injection. The surfactant concentration in low-salinity water was 1,000 and 5,000 ppm. Phase behavior and cloud point measurements were conducted prior to surfactant injection. The core permeability is 0.5 to 1.5 md, and porosity ranges from 0.18 to 0.25. Cores were aged for eight weeks at reservoir pressure and temperature. The pendant drop oil-brine IFT and captive oil-droplet contact angle measurements were performed at variable brine salinity in the presence of surfactant. Seawater and low-salinity waterflooding corefloods yielded ultimate oil recoveries of up to 57 percent. Up to 10 percent additional oil recoveries was obtained from low-concentration non-ionic surfactant in low-salinity waterflood. With decreasing salinity, in presence of 1,000-ppm surfactant, favorable wettability alteration from intermediate-wet to water-wet was observed by contact angle measurements. Moreover, addition of small concentration of surfactant decreased the IFT and altered the wettability of several one-inch diameter, crude-aged, discs to water wet.