Alcohol-free micellar solutions which yield high solubilization ratios (15-30 cc/cc), high final oil recoveries and eliminate the use of alcohol, a costly chemical, have been formulated for application in chemical floods.
A major difficulty was to preclude gels, liquid crystals, macroemulsions and precipitates along the composition path during a chemical flood. A screening strategy was applied to select among combinations of quite different surfactant structures. Corefloods were run with a blend of pure surfactants (C18OCH2·CH2-SO3Na, 8fC16SO3Na and 7fC14SO3Na), an ethoxylated sulfonate (6C16(OCH2·CH2)2SO3Na and a blend of an internal olefine sulfonate and a petroleum sulfonate. A slug size-efficiency curve obtained from corefloods with screened alcohol-free formulations, was compared to previous data with a conventional petroleum sulfonate/alcohol system (TRS 10-410/IBA) at 65°C taken as our basecase.
An equimolar blend of an internal olefin (C20-C24) sulfonate and a petroleum sulfonate gave a final oil recovery of 94% with a 13% Pvi slug size, 3v% surfactant concentration, at 80°C. When the slug size was reduced to 3% Pvi, the oil recovery still was 80%. Final results are compared to the basecase and other combinations tested and reported under comparable conditions.
A surfactant flood with an alcohol-free slug has the appeal of yielding a high final oil recovery for a small amount of surfactant injected, thanks to the high solubilization ratios they produce compared to formulations with cosurfactant alcohols(1-3,6,18,19,21,22). Corefloods without alcohol are time consuming and demand important amounts of surfactant not always available in enough quantity when dealing with experimental surfactants! Screening tests (phase behavior) help to quickly select the best conditions to design a surfactant flood avoiding the formation of gels (G), liquid crystals (X), emulsions (E) or precipitates, in general known as condensed phases, here abbreviated as CP. Efforts have been addressed to build surfactant structures which give high solubilization ratios(2-4,6,18-22). The inclusion of ethoxylated groups, double bonds, aromatic rings, branching of alkyl chains, break the viscous aggregates allowing the micelles to accommodate water or oil, working like a "built in" cosurfactant. Conversely, longer alkyl chains (more paraffinic), less branching, fewer aromatic groups all lead to lower interfacial tensions. The negative salinity gradient design(11,16) promotes a composition path from an overoptimum phase behavior (upper or II+ type microemulsion) beyond the middle phase region to end with an underoptimum (lower or II- type) microemulsion. In a II+ type microemulsion the surfactant is mainly in the oil phase, mobilizing the residual oil. However it also causes surfactant retention, jeopardizing the integrity of a finite slug and the oil mobilization process. A II- type gives a good mobility control at the rear edge of the slug. Thus, the negative salinity gradient design maintains the effectiveness of the slug by combining the benefits of both II+ and II- with the high solubilization ratio (s*) of the III type. On the other hand CP are mostly found at the boundaries of three phase region, at minimum surfactant concentration. If clean microemulsions can not be obtained in those critical regions, what means surfactant retention and pore plugging all the benefits coming from high s* might be lost.
