Effect of Brine and Organic Acids on Initial Wettability of Carbonate Rocks

摘要

Recent studies have shown that the wettability of carbonate rocks can be altered by carefully changing the brine composition. The initial oil-wet nature of carbonate rocks and wettability alteration, in the presence of low-salinity brines, are attributed to the adsorption and desorption of organic acids, respectively. However, few detailed studies are available in the literature on the adsorption of organic acids in different brines and its effect on rock wettability. Similarly, many surface-complexation-based models exist in the literature for wettability alteration. However, these models are not robust since the actual amount of acid adsorbed is not available experimentally. In this study, experiments were performed to understand the adsorption of organic acids in different brines and its effect on rock wettability. Contact angle measurements were performed on calcite chips aged in different brines and model oils. Thermogravimetric analysis (TGA) was performed to study the adsorption of organic acids on rock samples initially aged in different brines. Zeta potential measurements at oil-brine and brine-rock interfaces were performed to gain insights into the mechanisms affecting rock wettability. Finally, oil-wet calcite chips were aged in different brines to study the effect of brine composition on their wettability. Contact angle measurements showed that calcite chips initially aged in the formation brine (FB) were most oil-wet. TGA results showed that the acid adsorption was also highest for these rocks. Similarly, calcite chips initially aged in sulfate-rich seawater (SW) showed the least oil-wet nature and lowest acid adsorption. Furthermore, the amount of acid adsorbed was found to depend on the acid type; stearic acid showed the highest adsorption, whereas heptanoic acid showed the least adsorption. Zeta potential measurements and total disjoining pressure calculations showed that the initial water film was unstable in all the cases. Wettability alteration was observed on aging oil-wet calcite chips in SW, sulfate-rich SW, and their dilutions but not in the FB. This study provides detailed insights into the adsorption of different organic acids on carbonate rocks in different brines and their effect on rock wettability.