There is widespread interest in improved oil recovery by the low salinity effect (LSE) and a pressing need to better predict the likely response and its relation to wettability change. A LSE in kaolinite-bearing sandstones can arise from detachment of crude oil, by its peeling from rock surfaces due to increased oil/rock repulsion, and/or by detachment of mineral fines with adhering oil, due to increased fines/rock repulsion. In a mixed wet sandstone reservoir, oil is typically in close contact with an extremely small fraction of total rock surface, a key component of which are asperity tips such as at edges of kaolinite platelets. An Integrated pH Ion Surface Electrostatics (IpHISE) model is used to predict speciation and interactions of oil surfaces and kaolinite edges across NaCl and CaCl 2 solutions of variable pH in sandstones. At pH < 5, a LSE can arise by weakened oil adhesion due to fewer positively charged oil base groups adsorbed to kaolinite edges. At higher pH, the electrostatics is dictated by competition between negatively and positively charged acid groups produced by respectively deprotonation and calcium binding. The LSE is predicted to be strongest in a narrow range around pH 5−6 in which salinity reduction switches the oil/kaolinite edge interaction to repulsive. At pH > 6, the interaction becomes increasingly repulsive at all salinities. There, a LSE can only arise from the extended range of repulsion, both between oil and kaolinite edges and between the latter and the underlying rock. The existence and cutoff values of these pH ranges depend sensitively upon the oil's acid number/base number, salt concentrations, and the pH shift caused by injection of low salinity fluid.

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Oil adsorbs to carbonate reservoirs indirectly through a relatively thick separating water layer, and directly to the surface through a relatively thin intervening water layer. Whereas directly sorbed oil desorbs slowly and incompletely in response to changes in reservoir conditions, indirectly sorbed oil can be rapidly desorbed by changing the chemistry of the separating water layer. The additional recovery might be as much as 30% original oil in place (OOIP) above the ∼30% OOIP recovered from carbonates through reservoir depressurization (primary production) and viscous displacement (waterflooding). Electrostatic adhesive forces are the dominant control over carbonate reservoir wettability. A surface complexation model that quantifies electrostatic adhesion accurately predicts oil recovery trends for carbonates. The approach should therefore be useful for estimating initial wettability and designing fluids that improve oil recovery.

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