A positively charged calcite surface model for molecular dynamics studies of wettability alteration

A systematic and critical review of application of molecular dynamics simulation in low salinity water injection

Low Salinity Water Injection (LSWI) has been a well-researched EOR method, with several experimental and theoretical scientific papers reported in the literature over the past few decades. Despite this, there is still an ongoing debate on dominant mechanisms behind this complex EOR process, and some issues remain elusive. Part of the complexity arises from the scale of investigation, which spans from sub-pore scale (atomic and electronic scale) to pore scale, core scale, and reservoir scale. Molecular Dynamics (MD) simulation has been used as a research tool in the past decade to investigate the nano-scale interactions among reservoir rock (e.g., calcite, silica), crude oil, and brine systems in presence of some impurities (e.g., clay minerals) and additives (e.g., nanoparticles). In this paper, fundamental concepts of MD simulation and common analyses driven by MD are briefly reviewed. Then, an overview of molecular models of the most common minerals encountered in petroleum reservoirs: quartz, calcite, and clay, with their most common types of potential function, is provided. Next, a critical review and in depth analysis of application of MD simulations in LSWI process in both sandstone and carbonate reservoirs in terms of sub-pore scale mechanisms, namely electrical double layer (EDL) expansion, multi-ion exchange (MIE), and cation hydration, is presented to scrutinize role of salinity, ionic composition, and rock surface chemistry from an atomic level. Some inconsistencies observed in the literature are also highlighted and the reasons behind them are explained. Finally, a future research guide is provided after critically discussing the challenges and potential of the MD in LSWI to shed more light on governing mechanisms behind LSWI by enhancing the reliability of MD outcomes in future researches. Such insights can be used for design of new MD researches with complementary experimental studies at core scale to capture the main mechanisms behind LSWI.

The Influence of Oil Composition, Rock Mineralogy, Aging Time, and Brine Pre-soak on Shale Wettability

Experimental and field studies have indicated that surfactants enhance oil recovery (EOR) in unconventional reservoirs. Rock surface wettability plays an important role in determining the efficacy of this EOR method. In these reservoirs, the initial wettability of the rock surface is especially important due to the extremely low porosity, permeability, and resulting proximity of fluids to the solid surface. This study is designed to investigate the effect of oil components, rock mineralogy, and brine salinity on rock surface wettability in unconventional shale oil/brine/rock systems. Six crude oils, seven reservoir rocks, and seven reservoir brine samples were studied. These oil samples were obtained from various shale reservoirs (light Eagle Ford, heavy Eagle Ford, Wolfcamp, Middle Bakken, and Three Forks) in the US. SARA (saturates, aromatics, resins and asphaltenes) analysis was conducted for each of the crude oil samples. Additionally, this study also aims to provide a guideline to standardize the rock sample aging protocol for surfactant-related laboratory experiments on shale reservoir samples. The included shale reservoir systems were all found to be oil-wet. Oil composition and brine salinity showed a greater effect on wettability as compared to rock mineralogy. Oil with a greater amount of aromatic and resin components and higher salinity rendered the surface more oil-wet. Rock samples with a higher quartz content were also observed to increase the oil-wetness. The combination of aromatic/resin and the quartz interaction resulted in an even more oil-wet system. These observations are explained by a mutual solubility/polarity concept. The minimum aging time required to achieve a statistically stable wettability state was 35 days according to Tukey's analysis performed on more than 1100 contact angle measurements. Pre-wetting the surface with its corresponding brine was observed to render the rock surface more oil-wet.

Surfactant-induced wettability reversal on oil-wet calcite surfaces: Experimentation and molecular dynamics simulations with scaled-charges

HYPOTHESIS

Surfactant flooding is the leading approach for reversing the wettability of oil-wet carbonate reservoirs, which is critical for the recovery of the remaining oil. Combination of molecular dynamics (MD) simulations with experiments on simplified model systems can uncover the molecular mechanisms of wettability reversal and identify key molecular properties for systematic design of new, effective chemical formulations for the enhanced oil recovery.

EXPERIMENTS/SIMULATIONS

Wettability reversal by a series of surfactant solutions was studied experimentally using contact angle measurements on aged calcite chips, and a novel MD simulation methodology with scaled-charges that provides superior description of the ionic interactions in aqueous solutions.

FINDINGS

The MD simulation results were in excellent agreement with the experiments. Cationic surfactants were the most effective in reversing the calcite wettability, resulting in complete detachment of the oil from the surface. Some nonionic surfactants also altered the wettability, but to a lesser degree, while the amphoteric and anionic surfactants had no effect. From the tested cationic surfactants, the double-tailed one was the least effective, but the experiments were inconclusive due to its poor solubility. Contributions of specific interactions to the wettability reversal process and implications for the design and optimization of surfactants for the enhanced oil recovery are discussed.

A review of wettability alteration using surfactants in carbonate reservoirs

The wettability of carbonate rocks is often oil-wet or mixed wet. A large fraction of oil is still remained in carbonate reservoirs, it is therefore of particular significance to implement effective methods to improve oil recovery from carbonate reservoirs. Altering wettability from oil-wet to more favorable water-wet has been proven successful to achieve this goal. Surfactants are widely investigated and served as wettability modifiers in this process. Yet a comprehensive review of altering wettability of carbonate reservoirs with surfactants is ignored in literature. This study begins with illustration of wettability evolution process in carbonate reservoirs. Techniques to evaluate wettability alteration extent or to reveal behind mechanisms are also presented. Several surfactant systems are analyzed in terms of their wettability alteration mechanisms, influential factors of performance, applicable conditions, and limitations. Mixture of different types of surfactants could obtain synergic effects, where applicable conditions are extended, and final performance is improved. Gemini surfactants have many desirable properties, which warrants further investigation for understanding their wettability alteration mechanisms and performance. At the end, this review discusses strategies related with surfactant cost, surfactant adsorption, and challenges at high temperature, high salinity reservoirs conditions. Also, some unclear issues are discussed.

Wettability Reversal on Dolomite Surfaces by Divalent Ions and Surfactants: An Experimental and Molecular Dynamics Simulation Study

Due to the importance of the dolomite mineral in carbonate reservoirs, the wettability characteristics of dolomite surfaces were studied with both experiments and molecular dynamics simulations. Contact angle measurements confirm that the dolomite surface can be rendered oil-wet by carboxylates (acidic components of crude oil) and that the cationic surfactant can reverse the oil-wetness more effectively than the anionic surfactant used in this study. The oil-wetness of an aged dolomite chip was reduced when treated with MgSO₄ solution at 80 °C, while CaCl₂, MgCl₂, and Na₂SO₄ solutions did not produce any significant wettability alteration. The effects of surfactants and divalent ions, Ca²⁺, Mg²⁺, and SO₄^2- (also referred to as Smart Water ions), were simulated with two model dolomite surfaces containing point defects and step vacancies, respectively. The results indicate that the cationic surfactant can weaken the attraction between the oil phase and the carboxylates, while the anionic surfactant tends to maintain the oil-wetness of the dolomite surface by replacing the carboxylates through competitive adsorption. All Ca²⁺, Mg²⁺, and SO₄^2- ions can act as potential determining ions, and the detachment of carboxylates is due to the repulsion from SO₄^2- ions drawn close to the surface in the presence of adsorbed Mg²⁺.

Effects of Surfactant Charge and Molecular Structure on Wettability Alteration of Calcite: Insights from Molecular Dynamics Simulations

Wettability alteration of oil-wet calcite by surfactants was studied by means of molecular dynamics (MD) simulations. The simulations use the recently developed model for positively charged calcite surface, whose oil-wet state originates from binding of negatively charged carboxylate molecules contained in the oil, consistently with the bulk of the available experimental data. The ability to alter the surface wettability, which can be directly quantified by the release of the surface-bound carboxylates, is tested for nine different surfactants of all charge types-cationic, anionic, nonionic, and zwitterionic-and compared to that of brine. It was found that only the cationic surfactants are able to detach the organic carboxylates more efficiently than brine, while the neutral and anionic surfactants do not seem to have any measurable effect on the wettability. The outperformance of the cationic surfactants is generally consistent with the majority of previously published experimental observations. The data also point toward a consistently better performance of single-tailed cationic surfactants over the two-tailed structure. Molecular mechanism of the wettability alteration by different types of surfactants is discussed, along with the implications of the results for the design of new surfactant formulations for the enhanced oil recovery.

Adsorption mechanisms of fatty acids on fluorite unraveled by infrared spectroscopy and first-principles calculations

HYPOTHESIS

The adsorption mechanisms of fatty acids on minerals are largely debated from years, and their understanding is now required to improve flotation processing in the critical context of raw materials. Three wavenumbers have been observed in the literature for the asymmetric stretching vibration of COO^- after the adsorption of fatty acids on mineral surfaces. They have been interpreted as different adsorbed forms, such as a precipitate formation, an adsorption of sole or bridged carboxylates, an anion exchange, or adsorbed modes, such as monodentate or bidentate configurations.

EXPERIMENTS/THEORY

Diffuse reflectance infrared Fourier transform spectroscopy was combined with ab initio molecular dynamics simulations and simulation of infrared spectra. Fluorite and sodium octanoate - or longer-chain fatty acids - were used as prototypical materials for all the investigations.

FINDINGS

At low fatty acids concentration, the asymmetric stretching vibration of COO^- peaks at 1560 cm^-1 while, at higher concentration, this infrared band converts into a doublet peaking at 1535 and 1575 cm^-1. Using simulations, we assign the band at 1560 cm^-1 to the adsorption of a carboxylate molecule bridged on a sodium counter-cation and the doublet at 1535 and 1575 cm^-1 to the adsorption of the sole carboxylate anion under a monodentate or a bidentate binuclear configuration, respectively. The formation of an adsorbed layer on the mineral surface is initiated by the adsorption of a sodium carboxylate and followed by the adsorption of mixed sole anionic forms. The role of the carboxylate counter-cation is highlighted for the first time, which was totally ignored in the literature beforehand. This particularly opens the path to the development of innovative strategies to enhance the separation contrast between minerals, which is of uttermost importance for the recovery of critical raw materials.

Wettability of Calcite Surfaces: Impacts of Brine Ionic Composition and Oil Phase Polarity at Elevated Temperature and Pressure Conditions

The interactions among the polar components of oil, aqueous phase ions, and carbonate minerals, as well as their subsequent effects on surface wettability, can significantly impact the fluid distribution and recovery in a hydrocarbon reservoir. In this study, we investigate the adsorption/desorption of molecules from oils with different levels of polarity on calcite surfaces during different displacement processes under elevated pressure and temperature conditions. We measured dynamic contact angles (CA) on untreated and aged calcite substrates using brines with different salinities and compositions and model oils, that is, mixtures of varying concentrations of stearic acid (SA) and n-decane. In particular, the impacts of the concentrations of Ca²⁺, SO₄^2-, and OH^- ions on the adsorption phenomena were explored. For the nonpolar oil, increasing brine salinity or removing Ca²⁺ ions from the aqueous phase impacted the potentials of oil-brine and brine-mineral interfaces and shifted the wettability of calcite surface toward more water-wet conditions. In the presence of polar oil, the adsorption of the polar components controls the surface wettability. Higher concentrations of Ca²⁺/SO₄^2- could facilitate/obstruct the polar component adsorption and thus increase/decrease the dynamic oil-water CAs. It is also observed that the brine salinity does not impact the wettability if excess SA is added to the oil phase, that is, if the oil phase is strongly polar. Moreover, the adsorption of SA on the calcite surface under experimental conditions is found to be reversible during the displacement events. The surface energy calculation for the adsorption process indicates that the surface coverage of calcite by SA is more sensitive to the presence of Ca²⁺ in brine than the concentration of polar components in oil. We also conducted several experiments on calcite substrates aged with SA. The measurements demonstrate that the adsorbed SA molecules are detached when the aged mineral surface is exposed to a lower-salinity brine at high temperatures, and the SA molecules could be adsorbed back on the surface once the displacement is halted.