Abstract
While the effect of polar-oil component on oil-brine-carbonate system wettability has been extensively investigated, there has been little quantitative analysis of the effect of non-polar components on system wettability, in particular as a function of pH. In this context, we measured the contact angle of non-polar oil on calcite surface in the presence of 10,000 ppm NaCl at pH values of 6.5, 9.5 and 11. We also measured the adhesion of non-polar oil group (–CH3) and calcite using atomic force microscopy (AFM) under the same conditions of contact angle measurements. Furthermore, to gain a deeper understanding, we performed zeta potential measurements of the non-polar oil-brine and brine-calcite interfaces, and calculated the total disjoining pressure. Our results show that the contact angle decreases from 125° to 78° with an increase in pH from 6.5 to 11. AFM measurements show that the adhesion force decreases with increasing pH. Zeta potential results indicate that an increase in pH would change the zeta potential of the non-polar oil-brine and calcite-brine interfaces towards more negative values, resulting in an increase of electrical double layer forces. The total disjoining pressure and results of AFM adhesion tests predict the same trend, showing that adhesion forces decrease with increasing pH. Our results show that the pH increase during low-salinity waterflooding in carbonate reservoirs would lift off non-polar components, thereby lowering residual oil saturation. This physiochemical process can even occur in reservoirs with low concentration of polar components in crude oils.
Highlights
Carbonate reservoirs host roughly >60% of hydrocarbons in the world [1], but only up to 40% of the oil can be recovered from the reservoirs [2]
Our results show that increases of pH during low-salinity flooding in carbonate reservoirs lead to detachment of the non-polar oil components from carbonate surface
Wettability alteration is believed to be a major physicochemical factoroil in components) the application of reservoirs lead to detachment of the non-polar oil components
Summary
Carbonate reservoirs host roughly >60% of hydrocarbons (oil and gas) in the world [1], but only up to 40% of the oil can be recovered from the reservoirs [2]. The petroleum industry is constantly working to develop economically feasible techniques to enhance oil recovery in order to meet the growing global demand for energy [3]. Extensive research has been done to show that lowering injected brine salinity can improve oil recovery from carbonate reservoir at secondary and tertiary mode from 5 to 30% of the original oil in place (OOIP) [5,6,7,8,9,10,11,12]. Wettability alteration towards a more water-wet state is believed to be one of the main physiochemical processes behind the low salinity effect, which shifts oil relative permeability towards a lower residual oil saturation [9,13,14,15], . To understand the factors controlling the wettability alteration process in carbonate reservoirs, several explanations have been developed and proposed to predict and quantify the wettability alteration, such as electrostatic bridging [7,8,16,17,18], electrical double layer [13,19,20,21], and surface complexation modelling [22,23]
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