Use of surfactants for crude oil-in-water emulsions demulsification

Abstract

Enhanced oil recovery (EOR), known as tertiary crude oil recovery, is an important technique applied by the petroleum industries to improve crude oil recovery. Over 30 to 60% of the original oil in place (OOIP) is improved by this technique as compared to primary and secondary techniques. However, the EOR technique results in the formation of stable waterin- oil and oil-in-water emulsions. Emulsions are undesirable in the petroleum industry as these add to the overall production cost and also to the loss of valuable amounts of crude oil. It is therefore essential for economic and environmental reasons to demulsify these emulsions in order to separate the oil from water. Chemical demulsification (use of demulsifiers to separate oil from water) is the most commonly practiced method to treat these emulsions. Nevertheless, no studies have been reported on the use of response surface methodology (RSM) to optimize the demulsification efficiency of crude oil-in-water emulsions resulting from the EOR technique. This study aims to investigate the effectiveness of two demulsifiers, namely cetyl trimethylammonium bromide (CTAB), and trimethyl-tetradecyl ammonium chloride (TTAC) on the demulsification efficiency of crude oil-in-water emulsions resulting from EOR techniques using response surface methodology (RSM). In order to achieve this aim, synthetic oil-in-water emulsions containing various oil to water ratios (15:85, 25:75, 35:65 and 45:55) were simulated with diesel as the oil phase. The emulsion stability index (ESI) of the simulated oil-in-water emulsions was investigated based on the emulsifier concentration, homogenization speed, brine salinity and oil to water ratio. The result showed that an increase in the homogenization speed and surfactant concentration helped to achieve better ESI. It was also observed that high brine salinity led to poor emulsion stability for this case study. In addition, the oil to water ratio did not have much influence on the emulsions’ stability index as long as the homogenization speed was as high as 24000 rpm. In this study, 2% surfactant concentration, 24000 rpm homogenization speed and 1000 ppm NaCl were the optimal conditions to formulate the emulsions. At this condition, the ESI of 15:85, 25:75, 35:65 and 45:55 was found to be 99.3, 92.5, 91.2, and 90% respectively. Central composite design (CCD) and response surface methodology (RSM) was used to design and optimize the settling time, demulsifier concentration and the oil to water ratio on the demulsification efficiency of CTAB and TTAC. The results showed that the settling time, demulsifiers concentration, oil to water ratio as well as the interaction between the demulsifiers concentration and oil to water ratio had significantly impacted the response (demulsification efficiency) because of their p-values < 0.0001. The quadratic model obtained was proven to be significant with the F-value of 16.03 and 16.63 for CTAB and TTAC demulsifiers respectively. The coefficient of determination (R2) was found to be 0.944 and 0.863 for CTAB and TTAC demulsifiers respectively. These high values validated the accuracy of the model. The results revealed that a maximum demulsification efficiency of 82.6% and 80% was achieved by adding 850 ppm of CTAB and TTAC demulsifier and settling for 10 h with 15 and 25% diesel in water (D/W) emulsions respectively. These were proven to be the optimum conditions for maximum oil recovery for this case study. It was also found that ageing the emulsion could greatly influence the performance of the demulsifier. This study showed ageing of the emulsions decreased the demulsification efficiency of CTAB from 86.66 for fresh emulsions to 50% after ageing the emulsions for 60 days.