A Microfluidic Study of Droplet And Particle Retention: A Path from the Rationale to Pore-Scale Experiments
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Produced water is the major byproduct of oil and gas production, containing a number of components that can be harmful to the environment, e.g., dispersed particles, dispersed oil, dissolved organics. Re-injection is an environmentally sound approach for the management of produced water. However, the dispersed components of produced water damage the formation during the injection process by clogging the pores, which causes a decline in the injectivity of wells. The uncontrolled injectivity decline of injection wells is one of the challenges that hinders the achievement of “zero harmful discharges” to the sea. To date, there are several gaps in the knowledge about the retention of the dispersed components in porous media, limiting the understanding of the phenomenon. This thesis has consequently been dedicated to (1) the review of the factors affecting the retention and the methods that are and can be employed to investigate the injectivity decline; (2) the development of a microfluidic technique in order to systematically investigate the retention of droplets and particles at pore-scale, something that cannot be achieved by the traditional methods; (3) to the expansion of the image analysis toolbox available at the Ugelstad Laboratory by employing neural networks, implemented to enlarge the capacity and the resource-effectiveness of the analysis process. A methodology to inject and follow dilute dispersions in porous media using microfluidics was established. The method enables consistent injection of emulsions, suspensions, and dispersion containing both droplets and particles and provides a comprehensive image analysis pipeline for recovering information regarding the retention events from microscopy images. At the same time, a number of factors influencing the transport of dispersed components in porous media were investigated. Firstly, the effect of the drop-to-pore size ratio, the flow rate, the dispersed phase concentration and the polydispersity on the retention of droplets was examined using monodisperse and tailored polydisperse emulsions. The results demonstrated that drop-to-pore size ratio is the main parameter governing the capture of droplets. Droplets that were larger than pore throats showed severe retention with every single droplet being retained, while droplets smaller than pore throats demonstrated limited retention governed by the bridging mechanism. The flow rate influenced the flow re-entry mechanism of the droplets that were larger than pore throats. The tests on polydispersity showed that large droplet facilitated the retention of smaller sized droplets. Secondly, retention of particles was investigated by varying the chemistry of the continuous phase. Water salinity and chemical additives such as flocculants and a non-ionic surfactant were to mimic the chemistry of produced water, which can contain production and Enhanced Oil Recovery chemicals. The investigation involved not only the retention but also the release of the particles under various conditions. The results showed that the water salinity play a crucial role in the retention of particles as the retention rate increased significantly with an increase in salinity. The same can be said about the release of particles; the release of particles was more pronounced as the salinity difference increased between the water used to deposit particles and to release the particles. On the other hand, there was a greater retention of particles in the presence of surfactant molecules than without; moreover, the particles retained in synergy with the surfactant demonstrated greater release when exposed to surfactant free systems. The effect of flocculants was largely dependent on their charge. While a polyanionic surfactant had a very limited influence on the retention and the release of negatively charged particles on a glass surface, the introduction of a polycationic surfactant into the system resulted in the most severe retention across all the tested systems. Thirdly, the influence of particle concentration on the retention of oil droplets was scrutinized. Overall, the amount of retained droplets was comparable across four systems with various particle concentrations and a particle-free system. However, it was observed that the particles affected the mechanisms of droplet retention. The results showed that droplets were more likely to be retained individually when the concentration of particles increased. This caused droplet capture in the areas of the pore network where a very limited retention happens in the case of particle-free systems.