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Investigation of Ionic Diffusion and Mixing Phenomena in Polymer-Enhanced Low-Salinity Waterflooding Using Molecular Dynamics Simulation

Abdolmaleki, Amir Hossein | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56927 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Ayatollahi, Shahaboddin; Mahani, Hassan; Esmaeilbeig, Mohammad Amin; Pourkhiabani, Nahid
  7. Abstract:
  8. Various studies have confirmed that water salinity and its composition significantly affect the efficiency of the waterflooding process. Field-scale operation of low-salinity water injection has been proven to be a cost-effective enhanced oil recovery (EOR) method which is also compatible with the environmental regulations. Although this method has satisfactory results, it faces some challenges, such as in-situ mixing of the injection low-salinity water with the saline water in the host reservoir. The salinity of the injected water increases, as it contacts the high-salinity reservoir brine in the pores. This phenomenon significantly impairs the efficiency of this technique, and increases the operational costs. Recent studies in our research group at the core- and micro-scales have clearly shown that if a small amount of polymer, such as HPAM (partially Hydrolyzed Polyacrylamide) is added to the injection low-salinity water, it can significantly reduce the mixing effect by suppressing the diffusion and dispersion phenomena, and increasing the integrity of the fluid-fluid front. This method is so-called polymer-enhanced low-salinity waterflooding (or PELS). To gain a more in-depth understanding of this process, molecular dynamics simulations were conducted using LAMMPS software to study the mixing phenomenon in this system at the atomic scale. In this study, sodium chloride was used as a representative of the dissolved salts in water, and HPAM polymer was employed to investigate the polymer effect. In the first stage of the simulation, two separate compartments (or simulation boxes) for high-salinity and and low-salinity brines were created and were brought separately to thermodynamic equilibrium. Then, they were placed in proximity to each other to start mixing. As such, non-equilibrium simulations were performed under no flow conditions. Polymer molecules were introduced into the low-salinity water and sensitivity analysis was performed on the main factors affecting the mixing phenomenon, including the presence/absense of polymer molecules, the effect of polymer concentration, the salinity of low-salinity water, and the salinity of resident high-salinity water. To interpret the results, radial distribution function (RDF), density profile, energy values, as well as viscosity and diffusion coefficients were determined. The simulation results show that the polymer molecules and their chemical interaction with the brine ions would act as a diffusion barrier and reduce the diffusivity of low-salinity water, increase the viscosity, and slow down the ionic diffusion phenomenon; thereby reducing the growth rate of the mixing zone. In all cases the mixing zone growth is linear with the square root of time; indicative of diffusive mixing or Fickian diffusion. We find that, among different parameters affecting the ionic diffusion, the polymer concentration, salinity of the resident brine, and salinity of the low-salinity water have the most significant impact. For example, in the presence of 50 strands of polymer in low-salinity water, the ionic diffusion coefficient was reduced by up to 37.5 percent. Once the diffusivity is reduced, the salinity profile becomes sharper which leads to a more effective low-salinity water effect. Thus, less volume of injection water would be required to establish low-salinity condition at large scales
  9. Keywords:
  10. Low Salinity Water Flooding ; Molecular Dynamic Simulation ; Enhanced Oil Recovery ; LAMMPS Software ; Polymer-Enhanced Low-Salinity Waterflooding ; Ionic Diffusion ; Mixing Phenomena

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