Insights into asphaltene stability, aggregation, deposition and molecular structure
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Asphaltenes are the heaviest, most polar, and most surface-active species of crude oils which are fairly stable in the oil; however, a small variation in the pressure, composition, and temperature can cause asphaltene phase instability and alteration in their solubility parameter and can precipitate and aggregate out of the crude oil, leading to expensive deposition problems in pipelines, well, valves, and porous media. The overarching aim of this body of work is to depict the fundamental structure and behaviour of asphaltenes for ultimate application in different operating conditions. In this treatise, asphaltenes are studied over a wide range length scale, ranging from the macro to the molecular scale. Following a literature review, the dissertation begins by reporting the results of a study on the destabilization and deposition of asphaltenes using various experimental techniques. Asphaltenes were destabilised owing to addition of a normal alkane (n-alkane) to the crude oil, and the influence of amphiphilic molecules on asphaltene stabilisation was also illustrated. It is shown that the current techniques that are employed to select the most appropriate asphaltene inhibitor based on their efficiency should be revisited to provide a better methodology for choosing the most suitable strategy for inhibitor/solvent injection. In the study of asphaltene deposition, a new High Pressure-High Temperature Quartz Crystal Microbalance (HPHT-QCM) rig was designed and developed to determine the rates of asphaltene deposition onto the solid surfaces. Also, a reliable procedure is proposed for selection of chemical additives for remediation/prevention strategies to handle gas-induced asphaltene deposition problems. The factors that can play a role in controlling the effect of chemistries on asphaltenes at various conditions are also investigated in this thesis. Furthermore, the differences between the molecular structures of n-alkane and gas induced asphaltenes is explored. Based on the results, it was denoted that the gas induced asphaltenes are structurally, morphologically, and compositionally different from n-alkane precipitated asphaltenes which lead to have different interactions between the asphaltene and inhibitor molecules and diverse rankings of chemistries based on the utilised evaluation techniques. In this thesis, a new two-dimensional dynamic model was developed and validated to simulate asphaltene precipitation, aggregation, and deposition at isothermal and non-isothermal conditions. The effect of the aggregate size on the rate of aggregation and deposition was studied through this simulation study, and it was inferred that the rate of asphaltene deposition increases as a function of concentration of nanoaggregates in the well column. The tendency of smaller aggregates to deposit onto the surfaces could be explained because of the increase in the diffusion coefficient of asphaltene aggregates. For the first time, experimental results of the effect of water with different salinities on gas induced asphaltene aggregation and deposition at elevated pressure and temperature conditions were attained. The roles of ion type on formation of asphaltene stabilised water in oil micro-emulsions, asphaltene deposition, and respective water wettability alteration of solid surface at micro scale were also investigated. Finally, the effects of oil composition changes owing to different gas injection scenarios and addition of paraffin waxes on asphaltene destabilisation and deposition under real field conditions were thoroughly illustrated.