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Investigation of the Governing Liquid-Liquid Interactions at the Oil-Brine Interface in Low-Salinity Waterflooding using Molecular Dynamics Simulations

Seif, Arya | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57009 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Mahani, Hassan; Ayatollahi, Shahaboddin; Pourkhiabani, Nahid; Koleini, Mohammad Mehdi
  7. Abstract:
  8. The characteristics of the brine-crude oil interface in hydrocarbon reservoirs are very important, especially during water-based enhanced oil recovery (EOR) operations such as low-salinity waterflooding. At the interface of the two aforementioned fluids, the interaction mechanisms between the molecules of these two non-miscible fluids would affect capillary forces hence the efficiency of the EOR method. However, based on the published literature, there are still many open questions and uncertainties about the interaction of oil and brine molecules at the interface and their roles in the interfacial tension (IFT) changes for different oil and brine compositions. In this regard, molecular dynamics simulation is a practical tool to identify, investigate and interpret the mechanisms at the molecular scale. Therefore, this research aims to investigate the physicochemical interactions between oil-brine, and the effect of the concentration of the polar group in oil as well as water salinity, as the two key parameters, on the properties of the water-oil interface. To this end, molecular-scale simulations were performed using LAMMPS software for different oil and brine samples in contact with each other. In this study, a model oil containing normal-decane molecules and acidic polar groups such as benzoic acid molecules was used, and the acid concentration was increased up to 240 mg KOH/g Oil. Also, the aqueous phase included water molecules and sodium chloride salt ions with two, high (175000 ppm) and low (35000 ppm) concentrations. All the simulations were carried out at the temperature of 300 K and pressure of 1 atm to assess the mechanisms involved including the distribution and arrangement of polar species at the interface. The simulation results showed that the interfacial tension (IFT) of water and pure decan is about 50 dyne/cm, which is consistent with numerous laboratory results in this field. At a given salinity, by increasing the concentration of the oil polar groups, the thickness of the contact surface between the oil-brine increases, which is indicative of higher interaction between the two immiscible phases. However, at high concentrations (192-240 mg KOH/g oil), it starts to level off. Also, with the increase in the number of polar molecules in the bulk oil, the IFT between oil and water decreases to a minimum point, such that for the low salinity water system and at the concentration of 144 mg KOH/g oil of benzoic acid, the IFT reaches a minimum value of 34 dyne/cm, followed by IFT increases for higher benzoic acid concentration. For the high salinity water system, a similar non-monotonic trend is observed however, the minimum IFT is higher (41 dyne/cm) which occurs at a lower concentration of benzoic acid molecules (48 mg KOH/g oil). One of the important findings of this research is that the site density at the oil-brine interface follows a nonlinear and polynomial function, which means that the concentration of polar acids at the interface does not follow a linear trend with their bulk concentration in the oil phase. This showed that the interface was saturated with polar molecules after a threshold concentration and beyond that site density increased very slightly. In this regard, the findings were utilized to present a mathematical relationship between the surface density of polar molecules, oil acid number and water salinity for the first time. Furthermore, interaction energy measurements between species showed that the energy ratio between acid-water and acid-normal-decane at a concentration of 144 mg KOH/g oil of benzoic acid for low salinity water has a maximum value of 3.35. In comparison, the maximum value of this ratio for high salinity water, at the concentration of 48 mg KOH/g oil of benzoic acid, is equal to 2.33. These maximum values indicate at what concentration of acidic groups the attraction of acids to be placed on the interface was the highest. Besides, the measurement of the orientation parameter demonstrates that with the increase in the concentration of acid molecules at the interface, the orientations are more angular, so that for the low-salinity water system, the parameter increased from 0.08 to 0.17. Based on the obtained results and the presented mathematical correlation, an optimal water salinity can be determined at which the surface density of the oil polar groups can be maximized during the waterflooding process. This finding would help to formulate the best EOR agent, here the injection water, such that the oil recovery factor can be increased. Also, the correlation can be useful in optimizing the existing relationships in geochemical software such as PHREEQC and GWB to estimate DLVO forces, changes in wettability, and proposed mechanisms for low-salinity waterflooding technique
  9. Keywords:
  10. Low Salinity Water Flooding ; Molecular Dynamic Simulation ; LAMMPS Software ; Enhanced Oil Recovery ; Oil Polar Groups ; Acid Number ; Two Phase Liquid-Liquid Flow ; Two Phase Water-Oil

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