The thin brine film that wets rock surfaces governs the wettability of underground reservoirs. The ionic composition and salinity of this nanosized brine film influence the wetting preference of the rock pore space occupied by hydrocarbons. Despite numerous investigations over the last decades, a unanimous fundamental understanding that concerns the contribution of ions in the original wetting state of the reservoir is lacking and hence the mechanisms responsible for the wettability reversal of the mineral are still unclear. This wettability reversal is the main consequence of ion-tuned waterflooding. Although the method is widely accepted in practice, there is no universal consensus on the underlying mechanisms involved. Molecular dynamics simulation is an excellent choice to remove such ambiguities. This method can capture an atomic-level picture of the phenomena that affect reservoir wettability upon injecting low-salinity water. For the purpose, we performed simulations of brine films confined between a calcite substrate and a layer of an oil model. The films were at two different salinities to represent the initial state of high-salinity connate water and low-salinity brine. We found the development of ionic aggregates, mainly comprising Na+ and Cl–, within the high-salinity thin brine film. These aggregates act as pinning entities to localize polar oil components within oil/brine interface and connect the hydrocarbon phase to the calcite surface. This results in the adhesion of oil components to the rock surface though a high-salinity thin brine film. Simulation results suggest that the aggregates do not form after the change of the brine content to low salinity. From these observations, we concluded that diluting the brine content of the reservoir leads to the disintegration of such ionic aggregates. As a consequence, electrical double layers (EDLs) form at both rock/brine and brine/oil interfaces, which is supposed to be reflected by additional oil recovery at the macroscopic scale. Furthermore, we pointed out that EDL at an oil interface is established by negatively charged oleic polar species and cations around those compounds. Likewise, the EDL in proximity to calcite is composed of a positive Stern layer of Na+ cations and a negative diffuse layer of Cl– anions beyond that.

Viscous fingering is one of the main challenges that could reduce areal sweep efficiency during waterflooding in oil reservoirs. A series of waterflooding experiments were carried out in a Hele-Shaw cell at ambient temperature during which areal sweep efficiency was estimated and techniques to ease the fingering problem were examined. The onset and propagation of viscous fingers were monitored as a function of both injection rate and injection/production positions. Image processing techniques were utilized to quantitatively investigate the propagation of fingers. The experimental results show that, under specific conditions, increasing the number of finger branches could improve the areal sweep efficiency, whereas growth of a single narrow finger has a negative impact on oil displacement efficiency. According to the obtained results, increasing the injection rate improves the areal sweep efficiency up to a critical rate at which viscous fingers start to grow. The impact of heterogeneity of the medium on distributing the viscous fingers was also investigated by introducing two different arrangements of fractures in the model. The results show that fractures perpendicular to the direction of flow would distribute the displacing water more uniformly, while fractures in the direction of flow would amplify the unfavorable sweep efficiency.

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During water flooding of the oil reservoir, deposition of calcium sulfate on the pore surface causes formation damage and affects oil recovery efficiency. Thus, a clear understanding of this scale’s early crystallization stage is crucial to optimize and control the precipitation process. For the first time in this study, molecular dynamics simulation has been utilized to study the formation pathway of calcium sulfate in homogeneous and heterogeneous systems to address precipitation and deposition processes and the temperature influence on this phenomenon. We found four distinct steps in crystal evolution regardless of the temperature effect in both precipitation and deposition systems that confirmed the prenucleation theory. The results indicated that the induction time was strongly affected by temperature and solid surface presence at a specified supersaturation level. The influence of the solid surface is pronounced at higher temperature conditions in comparison to the bulk phase. The nucleation time varied between 2 and 12 ns at the high supersaturating level, depending on the temperature and crystal formation condition.

Inorganic scale deposition has been found to affect many industrial processes, including water injection into the oil reservoirs. The incompatibility of high sulfate ion content of seawater with formation water containing calcium ions results in formation damage and production decline. In this study, several simultaneous techniques are utilized for qualitative and quantitative analyses of calcium sulfate scale to get more insight into the formation damage during smart water flooding at micro and nanoscales. In the experimental section, calcium sulfate deposition due to the mixing of the formation water and seawater samples was investigated using the dynamic quartz crystal microbalance technique. The effect of sulfate and magnesium ions existing in the seawater on the amount of calcium sulfate deposition was studied, individually. The results showed that the sulfate concentration of seawater could significantly change the mass deposition in a specific range. Also, at an optimal concentration of the magnesium ions, the total amount of calcium sulfate deposition decreased by 60 percent. However, magnesium ions could decrease the time of the initial stage of deposition significantly. The results revealed the amount of deposition and the time of initial stage beyond 5 times dilution of seawater are not noticeable. In addition, the linear slope of the second stage of deposition for the mixture of formation water and 5-fold diluted seawater decreased by 92 percent compared to the original seawater. To verify the results for the magnesium effect, the molecular dynamics simulation method was used to compare the simulation results with the experimental data. Likewise, the results obtained from the simulation model showed that at an optimal concentration of the magnesium ions in the seawater, the amount of calcium sulfate deposition was noticeably decreased.

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