In this study, we summarize and discuss the data reported from a series of multiscale experiments to explore the interactions of salinity and aqueous ions at fluid/fluid and rock/fluid interfaces and to understand the pore‐scale oil‐recovery mechanisms in controlled‐ionic‐composition waterflooding (CICW). Experimental data on various crude‐oil/brine/carbonate and crude oil/brine physicochemical changes/effects at elevated temperatures were obtained using a variety of static and dynamic techniques at different scales ranging through atomic/molecular/macroscopic scales. The techniques include surface‐force apparatus (SFA), cryo‐broad‐ion‐beam scanning electron microscope (BIB‐SEM), zeta-potentials, microscope‐based oil liberation, macroscopic contact angles, interfacial shear rheology, and integrated thin‐film drainage apparatus (ITFDA). The salinities of brines were varied from zero‐salinity deionized (DI) water to higher‐salinity injection water in addition to changing the individual ion compositions.

The integration of results obtained from different multiscale experiments showed that both salinity and individual aqueous ions play a major role not only in determining the oil release from the rock surface owing to the interactions at the rock/fluid interface but also in impacting released oil‐ganglion dynamics for efficient oil mobilization through the interactions at the fluid/fluid interface. The key findings can be summarized as follows: (1) at zero salinity, unfavorably much higher adhesion and stronger rigid films to adversely impact crude‐oil‐droplet coalescence were observed at rock/fluid and fluid/fluid interfaces, respectively; (2) an optimal lower salinity containing a sufficient amount of sulfate ions is necessary to cause nanoscale ion exchange at the rock/fluid interface that changes the surface charge/potential to favorably alter adhesion and microscopic contact angles for efficient oil release from the rock surface; and (3) an adequate salinity containing higher amounts of magnesium and calcium ions is desired to form less‐rigid films at the fluid/fluid interface that promote the coalescence of released oil ganglia for effective mobilization. On the basis of these novel findings, controlled-ionic-composition water can be defined as a tailored water containing certain salinity and selective composition of three key ions including: sulfates, magnesium, and calcium. It must contain lower amounts of monovalent ions and should have the right balance of the three key ions to enable favorable interactions at both fluid/fluid and rock/fluid interfaces in carbonates. The novelty of this work is that it systematically analyzes and consolidates all the multiscale (atomic/molecular/macroscopic scales) experimental data obtained using rock and crude‐oil samples from the same carbonate reservoir. Also, consistent trends were identified from different experimental techniques at both rock/fluid and fluid/fluid interfaces to establish a clear connection among multiscales and subsequently understand the causative pore‐scale mechanisms responsible for oil recovery in CICW processes. In other words, this work has successfully transmitted the physics behind individual mechanisms and their interplay through different length scales to directly address one of the key open questions raised by Bartels et al. (2019).

The analysis on multiscale aqueous‐ion interactions at the two interfaces performed in this study resulted in the major finding that the controlled‐ionic‐composition water effect in carbonates is a combination of two effects, one being related to the release of oil attached on rock surfaces (wettability change) and the other being related to improved oil‐phase connectivity and better oil mobilization (enhanced coalescence of oil ganglia). It also highlighted the important learning point that not every low‐salinity water can become a controlled‐ionic‐composition water for carbonates. These new learnings and the novel knowledge gained provide useful practical guidelines on how to design optimal injection‐water chemistries for waterflooding projects in carbonate reservoirs.

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