Mechanisms of water-in-crude oil emulsion formation, stabilization and resolution by electrostatic means.
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Oil reserves are declining all over the world. Nevertheless the age of oil is probably bound to last for several years still, even though the growth of the world energy consumption will lead to a faster depletion than the one we are experiencing today. The possibility of avoiding new oil and energy crises will depend upon the oil industry’s ability to find new fields and, above all, on mankind’s ability to achieve an optimum exploitation of the currently available resources. The search of new fields is very expensive and the probability of finding new highly productive formations is small. The development of efficient technologies for oil extraction and processing to apply to existing fields is very important to extend their profitability. After being extracted, crude oil is a mixture of gases and liquids which need to be separated and purified, before being injected into their respective pipelines. In general, those operations are carried out on site. Since approximately 40% of the world production of oil comes from the sea (a percentage expected to increase in the next years), the need to economize on the space of the equipment is important. In addition, the growing attention to heavy and extra heavy oils raises a question on how they can be efficiently produced. In the light of this, the electrostatic separation (often denoted as electrocoalescence) of crude oil emulsions is an important tool which, despite recent developments, still has a great potential. It is a cost-effective solution to guarantee the required export oil quality, and to increase the process efficiency on the platforms by allowing a quick discharge of the water in excess. This work was developed within the aims of the project “Electrocoalescence II – Criteria for an efficient process in real crude oil systems”, the continuation of a former project in which the fundamental mechanisms active in the electrocoalescence process were identified for model systems. The new purpose was to link this knowledge to realistic conditions and waterin- oil emulsions, and this thesis contributes to this challenge. Paper 1 addresses the rheological behaviour of different water-in-crude oil emulsions, under varying shear conditions, and exposed to a DC electrical field. Paper 2 investigates the reasons for the formation of an extremely stable emulsion on an off-shore field and discusses the advantages given by a commercial electrocoalescer (the VIEC, from Aibel AS) for its resolution and for reducing the demulsifier consumption. Solid particles are believed to play an important role in the stability behaviour of emulsions, not only enhancing the mechanical properties of the W/O interfaces, but also increasing the medium viscosity and thus hindering the sedimentation of water droplets. Paper 3 clarifies the influence of particle wettability and concentration on the electrostatic separation efficiency, highlighting the role played by the oil asphaltene fraction. Paper 4 demonstrates the great potential of electrocoalescence on the treatment of heavy oil emulsions. In fact, by using a commercial technology (the HTVIEC, from Aibel AS), the process performances can be maintained reducing the heat requirements. It is also shown that the electrocoalescer can be placed into the 1st stage separator, even in case of water continuous inlet conditions. Paper 5 aims to achieve a better understanding of the electrocoalescence process under AC fields, by a close comparison between experiments and theoretical predictions. Two additional reports were developed within the aim of the project. Report 1 investigates the possibility to apply electrocoalescence for the desalting operation on heavy crude oil emulsions. The experiments are carried out on a medium scale rig equipped with an electrostatic device, prototype of a commercial product (the CEC™ from Aker Kværner). Report 2 (not included in this thesis) describes the improvements made on the rheometer used in Paper 1. These were performed to eliminate the presence of electric field enhancements and to introduce electric diagnosis into the rheometer cup.