Abstract
Produced Water Treatment (PWT) technologies have evolved significantly over the past 15 years. In 2001, the first Compact Flotation Unit (CFU) was introduced to the oil and gas industry on the Norwegian Continental Shelf. Today, this well-proven technology, which separates residual oil from produced water (PW), operates worldwide. CFUs function either in the facility's PW train as a final step downstream from a separator in various configurations, or as a standalone treatment system in the slop/reject treatment process system.
Our analysis shows that it is possible to substantially improve conventional CFU oil separation performance. A portion of small gas bubbles that have been in contact with oil droplets will never rise to the top of the CFU vessel due to countercurrent water flow. Instead, they exit at the bottom of the unit. Removing a larger portion of these small bubbles in flotation technology will achieve an additional decrease of oil-in-water (OiW). Because of their high surface-to-volume ratio, removing small gas bubbles helps move more oil away from the PW discharge.
Schlumberger development of next-generation CFU technology included theoretical study, benchmark lab testing and field trials. To improve separation efficiency, we implemented several new internal designs within the same external CFU size and design, and analyzed them in a computational fluid dynamics (CFD) model. For manufacturing and further lab- and field-testing, we selected a final design that considerably improved oil-separation efficiency. The new technology delivers two-stage oil separation in a single vessel. Each stage uses a different mechanism to improve oil separation.
Results from lab testing show that the new CFU increases oil-separation efficiency up to 45% compared with existing technology. Key benefits of the new design include:
Less impact on the environment through improved OIW discharge figures
Lower skid weight through fewer vessels (one instead of two)
Reduced facility footprint through smaller skid size
Retrofittable technology
We based the new CFU design on CFD analysis, testing in a PW flow loop, and an offshore field trial which also verified better oil separation performance. Results from the field trial indicated the new CFU improved performance 27 % compared with conventional technology. This was achieved while maintaining the outer CFU dimensions and design.
