Gas Flotation for Subsea Produced Water Treatment II

Short summary of the project

The goal in this project is to utilize the gas flotation and microfluidic procedures developed in the ongoing gas flotation project in SUBPRO (Gas Flotation for Subsea Produced Water Treatment; Martina Piccioli) to study how a broader range of fluid properties influence the removal efficiency of oil and solids during high temperature high pressure gas flotation. Addition of flocculants to improve the cleaning efficiency, and determination of the working mechanisms of the flocculants will also be included. The project will be linked to the project “Complete subsea separation – operation of water treatment systems”.

Background and motivation for the project

Gas flotation is a common process for treatment of produced water. It relies on dispersion or nucleation of gas bubbles in the water phase and attachment of oil droplets or solids to the bubbles to enhance the rising velocity of droplets. Following first stage separation, it can reduce the oil in water concentration below the discharge limit or achieve required water quality for re-injection.

Subsea gas flotation is performed under considerably higher pressure and usually also at higher temperature than topside separation. This implies differences when it comes to the conditions of the process and the fundamental, microscopic phenomena associated with it. One example is the size of the dispersed gas bubbles, where higher pressure will reduce the size of the bubbles and increase the gas solubility in both the oil and water phases. Another example is differences in the water and feed gas composition, with higher oil in water inlet concentration and significant presence of dissolved gases in the water phase. There is a need for better knowledge of how the combination of these effects can impact the separation process.

In the ongoing SUBPRO project, a gas flotation rig has been re-designed to allow studies of oil and solids removal at pressures and temperatures up to 80 bar and 80oC, respectively. The ability of using this setup for studying oil removal at high temperatures and pressures has been demonstrated using a light North Sea crude oil [1]. Currently, studies of how the presence of solids will influence the oil removal is carried out, while parallel experiments (i.e. similar conditions and chemistries) for studying drop-bubble interactions is carried out in a microfluidic setup to investigate the process in the microscopic scale, with the goal to identify the scaling potential between the methods. Furthermore, the working mechanisms of various flocculants (cationic/anionic/nonionic, various charge densities and molecular weights) is elucidated in an associated master project. The goal in this project is to utilize the developed gas flotation and microfluidic procedures to study how a broader range of fluid properties influence the removal efficiency of oil and solids during high temperature high pressure gas flotation. The working mechanisms for flocculants will also be determined for a broader range of fluid properties.

Innovation potential

Innovative microfluidic methods have and are being developed in related SUBPRO projects, and the combination with high-pressure lab-scale gas flotation is novel. The project will provide experimental data about fundamental microscopic processes during HP gas flotation. Systematic studies of the influence of solids and production chemicals have not been carried out before. Detailed understanding of flocculation mechanisms during gas flotation. Altogether, this can facilitate novel designs or solutions for subsea CFUs.