Overview

Oil extraction at the seabed generates a byproduct called produced water (PW), which contains compounds that pose a danger to marine life and the overall environment. These compounds can be in the form of either dissolved or dispersed organics, dissolved inorganics, and solid particles. The European Commission [1] has set a target of achieving zero discharge of such compounds to the sea at new oil production facilities. Thus, new or improved methods are needed to remove these pollutants from the produced water under subsea conditions. Performing this step subsea will also contribute to reducing the energy intensity of water treatment. In this context, different stages of water treatment need to be investigated. Presently, induced gas flotation (IGF), dissolved gas flotation (DGF), and compact flotation units (CFU) are commonly used. Therefore, this project will focus on gas flotation as a subsea water treatment method, and we plan to use microfluidic experiments to improve our understanding of gas flotation mechanisms. Ultimately, our goal is to use this new knowledge to increase the efficiency of water treatment to reach the target of zero discharge to sea.

Background and aims

Improving gas flotation requires systematic experimental studies that help to understand gas flotation phenomena at the microscale. Towards this goal, there are some challenges to tackle in gas flotation. The gas flotation stage of the current produced water treatment technologies struggles with the removal of dissolved oil and oil droplets smaller than 20 µm. Also, industry-scale rigs are costly to operate experimentally to understand the oil removal behavior for combinations of various PW compositions and environmental conditions. Lab-scale rigs are an important tool for conducting low-cost experiments. However, they cannot track individual coalescence events and still have drawbacks on the rate of performing experiments, given the number of parameters that are involved in PW treatment.

Microfluidics systems are convenient tools to realize the necessary conditions to study and facilitate our understanding of the microscale phenomena behind macroscale fluidic applications. Experimenting with microfluidic systems is beneficial in terms of low chemical use, ease of repeatability, and precise control over the working parameters. In addition, transparent microfluidic chips allow for the in-situ observation of the phenomena under an optical microscope, which makes them good candidates for the automation of the experimental system for certain parameters. Digital images of these experiments showing bubble–droplet interactions further allow for performing data analysis on the tasks, which aligns with the ability of microfluidic systems to produce large amounts of experimental data. Therefore, microfluidics can be considered a testing ground for the number of parameters that influence the efficiency of water treatment via gas flotation, including: temperature, salinity, pH, existence and concentration of surfactants, size and dispersity of oil droplets, chemical composition of the pollutant hydrocarbons, means of introducing air bubbles etc. During the PhD work, a better understanding of surface effects in gas flotation will be achieved. New microfluidic chips will be designed and manufactured to investigate the phenomena and further development methods may be aimed at the industry scale-up to determine if the outcome of the research can compete and surpass the performance of the existing systems.

The work will build on a SUBPRO project (“Gas Flotation for Subsea Produced Water Treatment”; PhD Martina Piccioli [2–4]), where microfluidic techniques were introduced as a method for visualizing and quantifying, for example, the frequency of bubble-droplet attachment. It will also be linked to the SUBPRO-Zero project “Gas Flotation for Subsea Produced Water Treatment II”; Bahar Forouzeshrad), in which a laboratory-scale gas flotation setup will be used to investigate flocculation with comparison to microfluidic results.

Innovation potential

Microfluidic methods are still an emerging technology within the field of produced water treatment, with significant impact and innovation potential specifically for gas flotation. For example, by systematically understanding the microscale phenomena on a bubble and droplet level with precise control over experimental conditions, optimal water treatment conditions can be identified as a function of which compounds are present in the aqueous phase and of processing conditions such as temperature and pressure. The microfluidic techniques developed in this PhD work will transfer directly to and complement the research done by the companion SUBPRO Zero PhD project wherein a lab-scale gas flotation unit will be partly used to determine the effect of different flocculants on oil removal efficiency.