Qualitative assessment of improved oil recovery and wettability alteration by low salinity water injection for heterogeneous carbonates

Abstract

This study aims to evaluate the qualitative potential for low salinity water injection (LSWI) to alter the wettability and increase oil production in heterogeneous oil-wet coquina cores (one of the facies of Pre-Salt reservoirs) in relevant conditions of brine salinity, temperature, and initial wettability. We also aimed to verify the impact of micro-porosity fraction on oil recovery. Finally, we verified whether the presence of micro-dispersion in the oil phase upon contact with low salinity would have a positive correlation with improved oil recovery due to low salinity water injection.To achieve these objectives, we performed zeta-potential experiments to evaluate the charges at the oil-brine and brine-rock interfaces that affect rock wetting state and contact tests to test for the presence of micro-dispersions in the oil phase and potential for LSWI. A series of spontaneous imbibition and coreflood experiments were performed to calculate the Amott index and evaluate wettability of coquinas in core-scale as a function of brine salinity. Tertiary and secondary low salinity waterfloods were carried out to investigate whether a diluted desulfated seawater would increase oil production in coquina cores. The effect of predominance of macro-, meso-, and micro-porosity on secondary low salinity waterflood was also evaluated. Our results have shown that heterogeneity had a greater impact on recovery factor compared to brine salinity. Oil recovery for low salinity brine for a core with predominance of macro-pores was smaller than for cores with higher fraction of micro-pores flooded with FW and DSW. Low-salinity oil recovery was also smaller than high-salinity was for core with microporosity but with presence of dead-end pores. Secondary injection of low-salinity using the same core with large fraction of micro-porosity showed that it accelerated oil production compared to high-salinity brine. We have also observed that LSWI resulted in increased oil recovery in coquinas both in tertiary mode (average 10%) compared to FW/DSW and secondary mode using the same core (18% compared to FW) due to wettability alteration. This result was supported by spontaneous imbibition as well as zeta-potential results. The contact test between crude oil and low salinity brine resulted in a micro-dispersion ratio of 11.7, showing that a positive oil is likely to result in improved oil recovery due to low salinity injection. Despite the increased oil recovery seen during the tests, the results showed that oil production in tertiary mode was significantly delayed (after 4 PV), likely due to adverse oil/water viscosity ratio and water dispersion due to mixing between low- and high-salinity brines. We observed that for the shorter more oil-wet core significant oil production was measured during bump flow with both high-salinity and low-salinity brines, due to capillary end effects. This could mean that residual oil saturation was not achieved during high-salinity water injection and an overestimation of recovery factor due to low-salinity was seen.