Solvent Assisted-Steam Flooding (SA-SF) was developed to address the environmental and economic challenges associated with steam generation. The solvent composition can greatly affect its interaction with both the asphaltenes present in the oil as well as the clays in the reservoir; therefore solvent selection plays a major role in the process. This study explores the wettability alteration mechanism of oil recovery, and how the solvent selection impacts its performance.
Five experiments were conducted; two chemical flooding (E1 and E2), one steam flooding (E3) and two SA-SF (E4 and E5) experiments. The solvents tested were n-hexane (E1 and E4) and a commercial solvent (CS) (E2 and E5). The wettability alteration caused by the different enhanced oil recovery methods was established through contact angle measurements. The asphaltene and clay content of the produced oil was determined using standard methods. The residual oil content was measured by washing the postmortem samples with toluene, an asphaltene soluble solvent capable of removing all the oil present in the rock. The oil recovery rate was used to measure the performance of each experiment.
Although all experiments had a final wettability that is considered oil-wet, they varied among each other. Experiment E4 showed the least oil-wet surface, while E5 showed the most. At the same time E4 yielded the highest oil recovery of the SA-SF experiments. Lowest clay content and highest asphaltene content was observed in the produced oil of E5, while the opposite was seen in E4. Asphaltene-clay-water interactions in the pores affect the wettability of the rock while the solvent used directly impacts the quantity of asphaltene and clay present. Asphaltenes as insoluble in n-hexane, therefore they precipitate out of the oil onto the rock, interacting with the clay and water to produce a less oil-wet surface, the CS however is composed of heavy dearomatized hydrocarbons which range from C11-C16 and enable more asphaltene production, as all produced oil samples showed.
This study provides a data set for wettability alteration during steam-solvent processes. Our finding suggests that solvents control the wettability alteration due to their interaction with asphaltenes. Wettability varies according to the precipitated asphaltenes polarity and the distance from injection well.