Low pH Manganese Assisted Waterflooding Processes for Enhanced Oil Recovery in Carbonates

Modifying the wettability of carbonate formations through divalent foreign metal incorporation can become a cost-effective practical method for enhanced oil recovery (EOR) applications. The addition of manganese ions to high salinity water (HSW) at dilute concentrations and optimized pH conditions is exploited in this study to mitigate compatibility issues at high temperature and promote water-wet conditions in carbonate reservoirs.

In this experimental investigation, the compatibility of manganese ions in high salinity water (HSW), ultra-high salinity connate water (CW) and low salinity SmartWater is studied at reservoir temperature. The results from compatibility tests showed that manganese brines prepared under low pH equilibrium conditions are homogeneous at high temperatures. The combination of maganese ions with low pH conditions was found to eliminate the aqueous stability issues by mitigating the preciptation of MnCO3 and manganese oxides that coud become a main challenge for practical field implementation. The contact angle and spontaneous imbibition tests were then carried out in carbonate chips and cores, respectively, using crude oil and low pH maganese brines at reservoir conditions. The oil-water interfacial tensions are also measured to understand the interactions of manganese ions at the oil/brine interface in low pH equilibrium.

The findings from contact angle measurments demonstrated the ability of manganese ions at low pH conditions (pH=5.5) to effectively decrease the contact angles from 162 to about 118o when added to HSW. Conversely, only marginal reductions in oil-water interfacial tensions were observed due to manganese ions. The manganese assisted spontaneous imbibition oil recoveries were increased by about 16% when compared to that obtained using high salinity injection water. By doubling the manganese ion concentration from 500 to 1,000 ppm further increased the oil recovery by about 36%. These results suggested that manganese ions at 1,000 ppm concentration and low pH conditions can become a practical recipe for both secondary and tertiary mode oil recovery in carbonates. The favorable wettability alteration towards water-wet conditions has been idenfied as the main mechanism responsible for incremental oil recovery in low pH manganese assisted water flooding processses proposed in this study.

This work for the first time identified the favorable impact of incorporating Mn+2 ions under optimized pH conditions to enhance the aqueous stability and promote the wetting transition in carbonate reservoirs. The new knowledge gained from this experimental study highlights the practical significance of Mn+2 ions as cheap wettability modifiers for EOR applications.