Oil recovery by water flooding is usually not effective because of capillary forces in fractured, oil-wet carbonate formations. Sodium carbonate/surfactant solution was used to enhance spontaneous imbibition between the fractures and the matrix by both wettability alteration and ultra-low interfacial tensions. Carbonate formations are usually oil-wet because the mineral-brine and oil-brine interfaces have zeta potentials of opposite sign. The resulting electrostatic attraction destabilizes the water film between the mineral and crude oil. The zeta potential of calcite can be reversed to be negative even at neutral pH, with dilute solutions of sodium carbonate and bicarbonate. Carbonate ion is a potential determining ion for carbonate formations. Thus sodium carbonate can promote water-wetness. Carbonate ion also sequesters calcium from the brine because of the small solubility product. The negative zeta potential and low calcium concentration greatly reduce the adsorption of anionic surfactants on the surface of carbonates. Another important effect of sodium carbonate is to generate natural soap in situ by saponifying organic acids in the crude oil. Optimal salinity was found to depend only on the soap-to-surfactant ratio. Below optimal salinity, sometimes a thin layer accumulated between the lower-phase microemulsion and excess oil. The IFT of excess oil with the equilibrated lower-phase microemulsion was high. However, ultra-low interfacial tension was observed when material from the thin layer was dispersed in the lower-phase or added to it. The existence of this thin layer made wide ultra-low interfacial tension possible. The alteration of wettability is graphically illustrated by observation with a polished marble plate. After aging in crude oil, the plate is strongly oil-wet in brine. When the brine is replaced with a sodium carbonate/surfactant solution, wettability can be altered to water-wetness to intermediate wetness. When sodium carbonate is the only salt, both drop size and contact angle are found to decrease with salinity to an equilibrium value. But when sodium carbonate concentration is fixed at one per cent, and sodium chloride is used to change ionic strength, contact angle is not found to change much with salinity, but drop size goes through a minimum at a salinity corresponding to the optimal salinity at the experimental condition. Oil in a narrow gap between two parallel plates remains in place when submerged in brine because of capillary forces. However, this oil is displaced by buoyancy when the brine is replaced with a sodium carbonate/surfactant solution. Displacement rate is found to be dependent on electrolyte concentration. No spontaneous imbibition occurred when a partially oil saturated reservoir core sample was placed in formation brine. Oil was spontaneously displaced when the brine was replaced with a sodium carbonate/surfactant solution. The recovery rate was found to scale with gravity drainage, not capillary imbibition
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