Surfactant-Enhanced Low-pH Alkaline Flooding

Abstract

Summary Sodium bicarbonate was investigated as a potential alkaline agent in surfactant-enhanced alkaline flooding because it has very little tendency to dissolve silicate minerals. In experiments performed with Wilmington, CA, crude oil and three types of surfactants, the bicarbonate/surfactant combination caused a marked lowering of interfacial tension (IFT). Bicarbonate protected the surfactant against divalent cations and reduced adsorption of surfactant and polymer on various minerals. Coreflood tests confirm that sodium bicarbonate plus surfactant can be an effective alternative to the high-ph flooding process. Introduction Alkaline flooding is a potentially low-cost chemical EOR method. The main effect of alkaline chemicals is a reaction with the organic acids in crude oils to form natural surfactants that change interfacial properties and wettability. These natural surfactants are most effective in low-salinity brines (less than 0.1 eq/dm3), but alkali at low concentrations requires uneconomically large injection volumes to survive in most reservoirs because of rapid consumption by acidic and divalent ions in brine and on exchange sites. The combination of alkali with added low-concentration surfactant is effective at higher alkali concentrations. Strong alkali at high concentrations, however, is consumed by mineral dissolution reactions and reprecipitation, which also cause scale-formation problems. Efforts have recently been made to reduce alkali consumption and mineral dissolution by use of carbonate, a buffered alkali at lower pH. Its pH of 10.5 is within the range regarded as effective in reducing IFT; however, some mineral dissolution occurs even at this pH. This paper is part of an investigation into the potential of a still-weaker alkali, sodium bicarbonate (pH gout 8.5) for EOR. Bicarbonate alone does not produce ultralow EFT, but does generate distinct interfacial activity. We have found that the equilibrium IFT of Wilmington, CA, oil in contact with bicarbonate brine is less than 1 mN/m. Also, a transient minimum in IFT is seen within minutes of contact. Some investigators believe that low IFT improves oil recovery by inducing emulsification rather than by increasing the capillary number. For this purpose, the transient minimum IFT is probably more important than the equilibrium value. A study was made of emulsion on seven crude oils with acid numbers from 0.1 to 4.2 mg KOH/g oil. Sodium bicarbonate accelerated coalescence compared with NaCl alone with five of the oils, and for three of these, the coalescence rate was faster with bicarbonate than with sodium carbonate. Compared with emulsions in NaCl, coalescence was slowed by sodium bicarbonate for the other two oils, including the sample of Wilmington oil used in this study. This investigation focused on determining whether these indications of surface activity could be augmented by combining sodium bicarbonate with low-concentration surfactant. A study of the emulsion coalescence of Wilmington oil in sodium bicarbonate/surfactant brines has been reported elsewhere. This paper describes insults on phase behavior and IFT. With strong alkalis, combination flooding involves more than the synergistic effect on oil/water interfacial activity. The alkali plays an additional role of reducing surfactant adsorption and can also be a sacrificial agent to protect surfactant against divalent ions. Adsorption of and reaction with divalent ions were investigated to grate the extent to which bicarbonate has the same effect as stronger alkalis. Finally, the results of coreflood tests to assess oil recovery efficiency are presented.