(590g) Effect of Brine & Oil Composition on Wettability of Carbonate Rocks

Carbonate rocks hold a majority of our existing oil reserves. However, little oil is recovered from these reservoirs due to their oil-wet nature. The presence of fractures and heterogeneity further reduces the oil recovery from these reservoirs. Recent studies have shown that the wettability of these rocks can be altered by changing the brine composition. Most studies have since focused on understanding the effect of brine composition on the wettability of carbonate rocks. However, little attention has been paid to understanding the role of organic acids, present in crude oil, on rock wettability. In this study, we investigated the effect of organic acid and brine composition on the wettability of carbonate rock. We prepared model oils containing different organic acids (stearic acid, oleic acid, heptanoic acid) and used calcite as the carbonate rock. Contact angle measurements were performed for samples aged in different brines and model oils. Thermogravimetric analysis was performed on crushed calcite samples, aged in different brines and organic acids, to understand the adsorption of different organic acids for different brines. Zeta potential measurements were performed for oil-brine and brine-rock interfaces for different oils and brines. Using this information, calculations were performed to understand the stability of the initially present thin brine film using the DLVO theory. Our studies show that brines rich in divalent cations made the rock surface oil-wet. Increasing the ratio of sulfate ions to divalent ions (calcium and magnesium ions) made the rocks water-wet. The TGA results attributed the oil-wet nature of carbonate rocks to the adsorption of organic acids of model oil. The adsorption amounts for different brines followed the observations made from contact angle measurements; that is, acid adsorption was highest for brines rich in divalent cations and reduced as the ratio of sulfate to divalent cations was increased. Zeta potential measurements and disjoining pressure calculations show that the brine film was unstable for all the samples. Different organic acids altered the rock wettability to different extent. Furthermore, the oil-wet rocks became water-wet when they were aged in brines containing a high amount of sulfate ions. These studies provide insights into the mechanisms resulting in making the rocks oil-wet and would help in identifying suitable reservoirs for application of wettability alteration techniques. Furthermore, the insights gained in this study would help in improving the capabilities of our existing reservoir simulations for better forecasting of wettability alteration.