Ali, Azeezat; (2024) Interfacial Properties of Complex Fluid Mixtures of Relevance to the Energy Industry. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The research presented in this thesis uses equilibrium molecular dynamics (MD) simulations to investigate some processes found in the energy industry. Interfacial fluids, such as water, CO2, hydrocarbons and H2, play a vital role in these processes because of the existence of distinct phases on mineral substrates. Therefore, it is important to understand and control the interfacial properties to achieve the process objectives. The studies were carried out at atomic scales, at relevant temperature, pressure, and salinity conditions. A key problem that has plagued crude oil exploration and extraction is the aggregation of asphaltenes which negatively affects oil production, transportation, and processing. Asphaltenes increase the viscosity of crude oil, and its colour changes from clear to dark brown. This effect is of great industrial interest for paving roads and for coating materials. Asphaltenes can also negatively impact the economic value of crude oil. However, their mere presence in crude oil does not lead to asphaltene-related production problems. For example, heavy oils with high asphaltene concentrations are usually stable during production and do not promote clogging. However, lighter oils containing minor percentages of asphaltenes tend to have asphaltene precipitation problems, which impede crude oil processing. In some circumstances, it may be advantageous to promote asphaltene agglomeration into small colloidal particles that can be removed by shear forces exerted by fluid flow. Therefore, to gain insights into the aggregation mechanisms, the effects of a hypothetical cyclohexane chain on the aggregation and deposition of asphaltenes on kaolinite clay surfaces were investigated. The results showed that the chemical composition of asphaltenes contribute to the complex aggregation mechanisms observed and accounted for the difference in aggregation observed near kaolinite and in the bulk. Emphasis has also been placed on geological CO2 sequestration as a method of alleviating the effects of global warming. Confining CO2 in 2–3 nm wide slit-shaped calcite nanopores filled with brine revealed that narrower calcite pores reduce the solubility of CO2 in brine, whereas monovalent (NaCl and KCl) and divalent (MgCl2) salts had varying effects on the solubility. These results complement the in-depth analysis of the structure of water on calcite, which showed that water forms distinct, well-organised hydration layers on calcite and that the presence of monovalent and divalent salt ions perturbs this organised structure to different extents. Hence, the interactions between water and calcite, which become stronger as the width of the pore decreases, could be responsible for the observed reduction in solubility. Finally, temporary underground hydrogen storage has been proposed to facilitate the large-scale use of hydrogen to store energy from intermittent renewable power sources. The performance of CO2 and CH4 as cushion gases that allow efficient injection and extraction of hydrogen was evaluated by quantifying the wettability of kaolinite clay surfaces and brine-gas interfacial tensions. The results suggest that these gases affect the sealing capacity of kaolinite for hydrogen storage in geological formations while potentially improving hydrogen recovery. Overall, the observations made in this thesis highlight the importance and consequences of multiphase interactions on several processes in the energy industry.

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