Interaction Of Aqueous Caustic With Acidic Oils
Authors: Chiwetelu Chris I. , Hornof Vladimir , Neale Graham H. 
Published: March 2010

By virtue of their relatively high content of native carboxylic acids, heavy oils may be regarded as acidic solutions, While the identity of these crude oil acids is yet to be well-established, it is generally accepted that they react with caustic reagents present in floodwater, resulting in the in situ formation of surface active soap anions. When the soap anions adsorb at the oil-water interface, they can drastically lower the interfacial tension to the point where residual oil is mobilized. The most intriguing aspect of these acid/caustic interactions is the dynamic nature of the interfacial tension. Given a sufficiently long time the dynamic tension would attain an equilibrium slate. Thus, equilibrium analysis provides us with a good starting basis of dynamic modelling. Moreover, equilibrium tensions would be important at the latter stages of the flood following caustic breakthrough at the producing well.

In this work, the authors measured equilibrium interfacial tensions of acidified oleic phases contacted with a wide range of caustic concentrations in the aqueous phase. Tension measurements were made by means of the novel technique of photomicropendography and the analysis of the data was carried out by non-linear regression utilizing an appropriate model. Unlike earlier models of acidic crude-caustic systems, the present approach utilizes pre-selected fatty acids in a de/med oleic phase. This enabled them to quantify relevant model parameters, thereby facilitating the evaluation of the key variables influencing interfacial activity. Consequently, the acid ionization constant and the equilibrium constant governing the formation of inactive soap species were found to be the most important parameters for systems of oleic and lauric acids dissolved in hexadecane. The regression analysis showed that the pKa of oleic acid ranged from 6.2 to 6.9 and was dependent on the working acid concentration. For lauric acid, the pKa values were in the range of 9 to 11 depending on the caustic concentration of the aqueous phase. Species aggregation in both the oleic and aqueous phases is mostly responsible or the variations in pKa values.

Introduction

Heavy crude oils usually contain significant amounts of carboxylic acids. It is because of this relatively high acid content that heavy oil reservoirs are generally considered as prime candidates for caustic waterflooding. Efforts have been made in order to identify the surface active acids present in crude oils. Jang et al.(1) found that a sample of Long Beach Californian crude contained C8-C18 paraffinic acids in addition to benzene dicarboxylic acids. Seifert and Teeter(2, 3) carried out extensive analysis of Midway Sunset crude and found paraffinic, naphthenic and aromatic acids to be present. These authors noted that the interfacially active acids had molecular weights in the range of 200 to 700; At the moment it is not possible to determine conclusively the identity of crude oil acids or more importantly, their concentrations at reservoir conditions. It is therefore assumed that a mixture of acids is responsible for the over-all response of the crude to a caustic flood. In this investigation, paraffinic acids, namely oleic and lauric that have been identified among the crude oil acids by Jang et al. were used.