This document is an expanded abstract.
Summary
Naphthenic acids are important constituents of crude oil. Their adsorption on calcite, the dominant mineral surface in reservoirs, can have implications for altering the wettability of the surface and thus on the extraction/recovery of the hydrocarbon. While calcite is nominally polar, in the presence of an adsorbed layer of water, the surface shows characteristics that are expected of apolar surfaces. We find that both the acid group of naphthenic acid and the hydrocarbon tail of the acid play important roles in the adsorption. Longer-chain acids, for example, bind better to the surface than shorter-chain acids. The adsorption is also entropically driven, reminiscent of association between hydrocarbons in water.
Introduction
The wettability of reservoir rock is an important factor in the efficiency of water-flooding oil recovery process. With carbonate reservoirs dominating the world discovered oil reserves [1], especially the Middle-East petroleum fields, understanding its wettability is of interest. An oil-wet rock matrix will prevent water/brine from displacing the oil, hence oil recovery can be very poor in these systems [2]. Although carbonate surface is originally water-wet, around 80% of carbonate reservoirs are preferentially oil-wet at reservoir conditions. This is attributed to the adsorbents such as organic acids attached to the surface [2-4]. It is commonly recognized that the electrostatic interaction between the positively charged surface of carbonate and negatively charged acid group of the carboxylates contributes to the adsorption of the same. The tail group behaves as an anchor for the adsorption of oil molecules [5], hence there is a change in wettability from water-wet to preferentially oil-wet. As a result, considerable effort has been put in desorption of carboxylates.
It was found that some cationic surfactants were able to desorb the adsorbed carboxylates due to strong affinity between the acid and the surfactant [4]. Additionally, it was also noted that potential determining ions such as carbonates dramatically decrease the adsorption of anionic surfactants. It can be inferred that they can also reduce the adsorption of anionic carboxylic acids. Nonetheless, a molecular level understanding of the adsorption of carboxylic acids on carbonate surface is still lacking in the literature and will be the subject of this work.
