Heavy oils are known to contain relatively high proportions of native carboxylic acids. Thus, in alkaline waterflooding, the acids react with the alkalis present in the water forming surfactant soap anions that subsequently adsorb at the interface and thereby lower the interfacial tension. In order to design an effective caustic flood, we must be able to predict the interfacial tension at all possible caustic concentrations in the reservoir as the floodwater moves from the injection to the producing well. This movement is expected to occur under essentially equilibrium conditions and this is the reason why equilibrium models of such reacting systems are necessary.
One such equilibrium model has been proposed in this paper. The model applies to single acids dissolved in a known oleic phase and contacted with a spectrum of aqueous caustic concentrations. Two key parameters, namely, the acid ionization constant and the equilibrium constant governing the formation of inactive and non-dissociated soap species, determine the magnitude of the interfacial tension. Thus, in order to obtain precise estimates of these parameters, the model equations were solved with the aid of nonlinear regression analysis. Our results show that for oleic acid solutions in hexadecane, the pKa value increased from 6.2 to 6.9 as the acid concentration was raised from 0.10 to 0.6125 mM. For 0.02 M lauric acid solution in hexadecane. the pKa was about 9 at NaOH concentrations below 2.5 mM. but it rose to about 11 when higher caustic levels were employed. Values of KDNaX for the oleic acid systems increased marginally with the increase in acid concentration.
Heavy crude oils usually contain significant amounts of carboxylic acids. It is because of this relatively high acid content that heavy crude oil reservoirs are generally considered as prime candidates for caustic waterflooding. Hundreds of different types of acids have been positively identified in various crude oils. Jang et al. (1) found that a sample of Long Beach crude contained C8-C18 paraffinic acids in addition to benzene dicarboxylic acids. Seifert and Teete(2,3)carried out extensive analyses of Midway Sunset crude and reported that paraffinic, naphthenic and aromatic acids of different chain lengths were all present. These authors also noted that the interfacially active acids had molecular weights in the range of 200–700, with aromatic acids comprising the predominant species. Although a significant progress has been made in the qualitative identification of crude oil carboxylic acids, it is not yet possible to determine conclusively, the concentrations of the surface active acids in native crudes at reservoir conditions.
Several mechanisms have been proposed in an effort to rationalize the complex interactions between alkaline agents in flood water and crude oils acids. These mechanisms were recently reviewed by Castor et al. (4). They range from emulsification with entrapment through wettability reversal to emulsification with coalescence. Unfortunately, no one single mechanism can satisfactorily explain the often conflicting results obtained in laboratory tests employing various caustic reagents to displace different crude oils from natural and artificial cores.
