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Low salinity water flooding (LSWF) is increasingly seen as a cost-effective and environmentalfriendly way to enhance oil recovery from sandstone reservoirs. The controlling mechanism for LSWF appears to be wettability alteration through oil and mineral surface interactions, but the specifics of the process, and the effects of temperature and pressure remain unclear. Weshow that water chemistryhas a much larger impact onLSWF than reservoir temperature and pressure, and that low salinity water flooding can be applied to high temperature and high pressure reservoirs. Contact angles were measured betweena sandstone andan oil (AN=3.98 mg KOH/g, BN=1.3 mg KOH/g) from a reservoir in the Tarim Field in western China in the presence of various water chemistries. Salinity (formation brine, 100X diluted formation brine, and softened water), temperature (60, 100 and 140 o C) and pressure (20,30, 40, and 50 MPa) were varied.Oil-water and water-rock zeta potentials were measured and used to calculateoil-rock disjoining pressures. A surface complexation model was developed to interpret contact angle measurements and compared with DLVO theory predictions.Contact angles were greatest in formation water, followed by the softened water, and low salinity water at the same pressure and temperature. Contact anglesincreased slightly with temperature, whereas pressure had little effect.DLVO and surface complexation modelling predicted similar wettability trends and allow reasonably accurate interpretation of core-flood results.

Wettability alteration appears to be the main mechanism of low salinity water flooding in carbonate reservoirs. However, what factor(s) controls the wettability alteration is not clearly defined. We hypothesized that zeta potential at interfaces of oil/brine and brine/rock controls the wettability alteration in carbonate reservoirs. To test our hypothesis, we removed SO4 2– ions from the aqueous ionic solutions because SO4 2– ions likely adsorb at the pore surface, triggering desorption of carboxylic materials. We examined the zeta potential of interfaces of crude oil/brines and brines/rock. We also measured the contact angle and conducted two core-flooding experiments. Moreover, we performed a geochemical study to examine the potential of calcite dissolution by low salinity water using PHREEQC software. Our work demonstrates that contact angle strongly correlates with polarity of the zeta potential of interfaces of oil/brine and brine/rock, showing that the same polarity of zeta potential for oil/brine and brine/rock triggered a strongly water-wet carbonate surface in the presence of formation brine. The opposite polarity of zeta potential of oil/brine and brine/rock, on the other hand, yielded an intermediate or slightly oil-wet surface in the presence of 10 times diluted formation brine. However, incremental recovery of 5.8–18.1% was observed under tertiary mode, implying that low salinity water likely shifts the reservoir wettability from strongly water-wet to intermediate-wet. In addition, geochemical modeling revealed a negligible amount of calcite was dissolved in the presence of low salinity water, confirming that calcite dissolution is not a contributing factor to the low salinity effect. Moreover, we demonstrate that the contact angle on the carbonate substrate decreases linearly with increasing Z parameter, Z p = (|Zeta oil/brine + Zeta brine/rock |). We therefore argue that manipulating the Z parameter (polarity of zeta potential of brine/rock and oil/brine) is of vital importance to shift the reservoir wettability and improving oil recovery. We suggest that the Z parameter may be used as an interpolant to model the low salinity effect, rather than using salinity level or individual ions in the solution.

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