Design and Optimization of Alkaline Flooding Formulations

Abstract The state-of-the-art for alkaline flooding technology is the injection of combinations of alkalis with synthetic surfactants. Surfactant-enhanced alkaline flooding formulations with low-pH alkaline agents have potential for increased oil recovery. They react less with reservoir minerals and facilitate the use of low concentrations of surfactants because surfactant adsorption is reduced in the presence of alkaline agents. Laboratory experiments and results from field projects have been evaluated and used to establish guidelines for designing optimum alkaline floods. For a reservoir to be a candidate for alkaline flooding, the reservoir should contain little or no gypsum, the divalent ion exchange capacity should be less than 5 meq/kg, and the in situ pH should be greater than 6.5. Two optimized alkaline systems which exhibit typical interfacial tension (IFT) behavior are described. One system was optimized with a very low acid (acid number = 0.13 mg KOH/g) crude oil from Delawâre-Childers (OK), field. The other system was optimized with an acidic (acid number = 1.59 mg KOH/g) crude oil from Wilmington (CA), oil field. A mixture of synthetic surfactant and low-pH alkali produced lower IFT and sustained low IFT longer than either reactant alone. This effect was observed with the acidic crude and the slightly acidic crude. It was concluded that surfactant-enhanced alkaline flooding with low-pH alkalis shows promise with both acidic and slightly acidic oils. Synthetic surfactant is an expensive component of low-pH alkaline flooding formulations. Dilute (0.1 – 0.2 %) concentrations of the added synthetic surfactant are being used in some current alkaline flooding field projects. Laboratory results showed that surfactant losses by adsorption are reduced under alkaline conditions. Losses by precipitation should also be reduced due to the lowering of divalent ion concentration by alkaline preflush. The final section of this paper is an example of how screening criteria, phase behavior tests, and corefloods can be used to design and optimize an alkaline flood for a specific oil field. The example is for the design of a low-pH, surfactant-enhanced alkaline flood in the Ranger Zone of the Wilmington (CA), field, where a high-pH alkaline flood was previously conducted. That project encountered severe problems with alkali consumption and scaling. A low-pH alkali (such as NaHCO3 or NaHCO3 + Na2CO3) in combination with synthetic surfactant should be effective for producing incremental oil in the Ranger Zone of Wilmington field. Problems associated with high pH floods, such as intractable scales and high consumption, will be mitigated.