Enhanced Alkaline Flooding

Abstract

Micellar/polymer technology tops the list of the many EOR projects labeledtechnical successes but economic failures. Although this process can becustom-designed for a specific rock/oil/water system. the cost of thesurfactant used is prohibitive. which was true even when oil prices exceeded$30/bbl [$189/m3]. One very promising modification of micellar/polymer technology is thesubstitution of cost-efficient materials, such as alkaline agents, for themajority of the surfactant. With alkaline agents, such as sodium carbonate, sodium hydroxide, andvarious silicates, surfactant concentrations as low as 0.1 wt% can be used. This concentration is less than 5 % of the concentration used in the micellarsolutions of the micellar/polymer technology. Another cost-saving modificationis the replacement of costly cosurfactants used for viscosity control in amicellar solution with water-soluble polymers. Also, the alkaline agent reactswith the rock, thus reducing the chemical adsorption of the surfactant andpolymer and allowing the chemical system to displace oil effectively. Although the alkaline flooding literature has identified numerous mechanismsfor oil displacement. the principal mechanism by which alkaline flooding worksis through reducing the oil/water interfacial tension (IFT). The chemicalreaction between the injected alkali and the in-place crude oil forms asurfactant that reduces the IFT. The level of IFT reduction is dependent on thetype and concentration of alkali and on the chemical makeup of the crudeoil. There is an optimum alkali concentration that must be identified to designthe system properly. As with classic micellar flooding, properly. As withclassic micellar flooding, the salinity of the system dramatically affects theIFT response. If the salinity is too low, surfactant concentrates in theaqueous phase; if it is too high, surfactant is driven primarily into the oilphase. The ideal situation is to have the surfactant concentrated at the oil/water interface, which may occur only over a very narrow range ofsalinities. In alkaline flooding, the added alkali is not only a surfactant precursor, but also, by nature, a salt. Therefore, as the alkali concentration isincreased, the salt level increases beyond optimum and the IFT increases. Amajor problem with some of the early flood designs was that the alkaliconcentration yielding the minimum IFT in the laboratory was translateddirectly to the field. In some cases this "optimum" alkaliconcentration was very low-often less than 0.1 wt%. Laboratory and fieldstudies have shown that alkali concentrations this low cannot survive very farfrom the injection wellbore. Alkali consumption caused by the many reservoirsystem interactions generally requires that alkali concentrations be higherthan those indicated by the laboratory IFT tests. To accommodate these higher alkali levels without sacrificing the low IFTvalues, a low concentration of surfactant can be added to the system to shiftthe optimal salinity window effectively to a higher level. Laboratory workdemonstrated that this combination of alkali and surfactant can be synergistic, even in cases where initial crude-oil/alkali testing is not encouraging. Table 1 illustrates the synergistic effect of the combinedalkaline/surfactant mixture for a 26 degrees API [0.9-g/cm3] -gravitycrude. Although the IFT between the solutions and the oil is very important in thedesign of an "enhanced alkaline system," it is also necessary that thislow IFT be maintained between the fluids moving through the reservoir rock orthe chemical system will be ineffective in displacing sufficient oil forprofitable operations. The chemical makeup profitable operations. The chemicalmakeup of the rock, including clays, and the fluids will determine whether aparticular reservoir is a viable prospect. Another important consideration in the design of an enhanced alkaline systemis mobbility control. If the oil mobilized by reducing the IFT, is displaced bywater alone, the water will probably bypass the oil and leave the mobilized oilin the reservoir. For this reason, it may be necessary to add polymer toimprove displacement efficiency. polymer to improve displacement efficiency. The design of the total chemical system should include displacement studies todetermine which chemical combinations offer the greatest economicpotential.