Carbonate rock reservoirs comprise approximately 60% of the world's oil and gas reserves. Complex flow mechanisms and strong adsorption of crude oil on carbonate formation surfaces can reduce hydrocarbon recovery of an oil-wet carbonate reservoir to as low as 10%. Low salinity waterflooding (LSW) has been confirmed as a promising technique to improve the oil recovery factor. However, the principal mechanism underpinning this recovery method is not fully understood, which poses a challenge toward designing the optimal salinity and ionic composition of any injection solution. In general, it is believed that there is more than one mechanism involved in LSW of carbonates; even though wettability alteration toward a more desirable state for oil to be recovered could be the main cause during LSW, how this alteration happens is still the subject of debate. This paper reviews different working conditions of LSW, previous studies, and field observations, alongside the proposed interfacial mechanisms which affect the colloidal interactions at oil-rock-brine interfaces. This paper provides a comprehensive review of studies on LSW in carbonate formation and further analyzes the latest achievements of LSW application in carbonates, which helps to better understand the challenges involved in these complicated multicomponent systems and potentially benefits the oil production industry.

The influence of different types of salts (NaCl, CaCl 2 , MgCl 2 , NaHCO 3 , and Na 2 SO 4 ) on the surface characteristics of unconditioned calcite and dolomite particles, and conditioned with stearic acid, was investigated. This study used zeta potential measurements to gain fundamental understanding of physico-chemical mechanisms involved in surface charge modification of carbonate minerals in the presence of diluted salt solutions. By increasing the salt concentration of divalent cationic salt solution (CaCl 2 and MgCl 2 ), the zeta potential of calcite particles was altered, resulting in charge reversal from negative to positive, while dolomite particles maintained positive zeta potential. This is due to the adsorption of potential-determining cations (Ca 2 + and Mg 2 + ), and consequent changes in the structure of the diffuse layer, predominantly driven by coulombic interactions. On the other hand, chemical adsorption of potential-determining anions (HCO 3 - and SO 4 2 - ) maintained the negative zeta potential of carbonate surfaces and increased its magnitude up to 10 mM, before decreasing at higher salt concentrations. Physisorption of stearic acid molecules on the calcite and dolomite surfaces changed the zeta potential to more negative values in all solutions. It is argued that divalent cations (Ca 2 + and Mg 2 + ) would result in positive and neutral complexes with stearic acid molecules, which may result in strongly bound stearic acid films, whereas ions resulting in negative mineral surface charges (SO 4 2 - and HCO 3 - ) will cause stearic acid films to be loosely bound to the carbonate mineral surfaces. The suggested mechanism for surface charge modification of carbonates, in the presence of different ions, is changes in both distribution of ions in the diffuse layer and its structure as a result of ion adsorption to the crystal lattice by having a positive contribution to the disjoining pressures when changing electrolyte concentration. This work extends the current knowledge base for dynamic water injection design by determining the effect of salt concentration on surface electrostatics.

Fundamental understanding of the wettability of carbonate formations can potentially be applied to the development of oil recovery strategies in a complex carbonate reservoir. In the present study, surface energies of representative carbonate samples were evaluated by direct quantitative force measurements, using scanning force microscopy (SFM) at sub-micron scale, to develop a reliable method to predict reservoir wettability. Local adhesion force measurements were conducted on appropriate calcite and dolomite samples and performed in air as well as in the presence of polar and nonpolar fluids. This study demonstrated that, by comparing measurements of adhesion forces between samples of the same mineral in different fluids, it is feasible to determine the surface energy of a given mineral as well as its polar and nonpolar components. The derived values are in agreement with literature. A proof-of-principle protocol has been established to quantify surface energy using SFM-based adhesion measurements. This novel methodology complements the conventional contact angle measurement technique, where surface energy can only be examined at large length scale. The reported methodology has great potential for further optimization into a new standard method for fast and accurate surface energy determination, and hence provides a new tool for reservoir rock wettability characterization.

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