Microfluidics-Based Low Salinity Wettability Alteration Study of Naphthenic-Acid-Adsorbed Calcite Surfaces

Abstract

Microfluidics is relatively a new area in the oil and gas industry and has great potential to be used as an effective tool to address various outstanding research questions where microscale investigation is necessary. Low salinity brine flooding has been known to enhance oil recovery in carbonate reservoirs through a wettability alteration mechanism. Wettability alteration is a molecular-scale phenomenon that is often characterized at a mesoscale through contact angle experiments. By using microfluidic chips, the low salinity effects involving solid–fluid interactions could be directly visualized, and the wettability alteration could be verified at microscale. However, to replicate solid–fluid interactions involved in carbonate reservoirs, the internal wetting surfaces must be transformed to carbonate surfaces. In this study, first, wetting surfaces of straight-channel glass microfluidic chips were functionalized using a metal-chelating silane coupling agent, and then, the surfaces were coated with CaCO3 using a layer-by-layer (LbL) deposition process. The calcite-coated straight-channel glass microfluidic chips were then used to investigate the effects of brine salinity and the role of divalent ions in the wettability alteration of oil-wet calcite surfaces at both room temperature and high temperature (70 °C). Naphthenic acid was used as the surface-active component in a model oil to render calcite surfaces oil wet. Six single-electrolyte-based brines were used in this study: high salinity 5 M NaCl, seawater equivalent 0.656 M NaCl, and 4-times diluted seawater-equivalent low salinity brines─0.164 M ionic strength NaCl, Na2SO4, MgSO4, and MgCl2. The low salinity brines were found to induce significant wettability alteration on naphthenic-acid-adsorbed calcite surfaces at ambient temperature. Low salinity Na2SO4 resulted in the highest wettability alteration followed by low salinity NaCl solution. Insignificant wettability alteration was observed in 0.656 M NaCl compared to 0.164 M NaCl where the wettability alteration was significant, suggesting the extent of wettability alteration increases with reduction in brine salinity. Magnesium ions were found to be unfavorable for wettability alteration in the presence or absence of sulfate ions. Exposure to higher temperature resulted in no or insignificant further wettability alteration in most of the brines.