Effects of High Concentration of Mn2+ on Wettability of Tight Sandstones : Experimental and Molecular Insights

ZL Cheng and SK Tong and S Zhang and ZJ Jia and F Lyu and WT Zhang and H Gao, SPE JOURNAL, 30, 7690-7702 (2025).

To enable the quantitative evaluation of oil recovery and mobilization across different pore types using the nuclear magnetic resonance (NMR) transverse relaxation time (T-2) technique, high-concentration manganese(II) chloride (MnCl2) brines are frequently used to suppress the hydrogen signal from water. However, the potential effects of MnCl2 on the interfacial properties of the crude oil/brine/rock system remain largely uncharacterized. In this study, we combine contact angle measurements with molecular dynamics (MD) simulations to investigate the impact of high-concentration MnCl2 on wettability at both macroscopic and molecular scales. Our findings demonstrate that the influence of high-concentration MnCl2 is minimal, as indicated by the contact angle change obtained from both the contact angle measurements and molecular simulations. High-concentration MnCl2 reduces the density of water molecules near the quartz surface while simultaneously increasing the local concentration of crude oil, particularly its acidic components, which results in a slight increase in the contact angle across different brine compositions, suggesting a trend toward reduced water affinity. However, this change is insufficient to induce a wettability alteration. Compared with the systems without MnCl2, manganese(II) (Mn2+) ions exhibit a negligible impact on the ionic distribution near mineral surfaces. Moreover, the strong hydration energy of Mn2+ disrupts hydrogen bonding, both among water molecules and between water molecules and surface hydroxyl groups on quartz, thereby contributing to the observed slight increase in contact angle. Overall, the findings suggest that high-concentration MnCl2 exerts only a minor effect on wettability and is unlikely to significantly influence imbibition-driven oil recovery.

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