High water cut and polymer flooding: can CDFU replace DAF in produced water treatment?
When a field reaches high water cut, the produced water stops being a by-product and becomes the main product of the surface facility. Most Chinese oilfields have now entered the mid-to-late stage of development, and to sustain crude output major fields such as Daqing and Shengli have widely adopted tertiary recovery technologies including polymer flooding and ASP flooding.
The consequence is a stream that is harder to treat than the crude it accompanies. Polymer content raises viscosity, emulsified oil becomes far more stable, and conventional processes struggle to meet reinjection standards. Produced water treatment has become a core bottleneck constraining oilfield surface engineering.
Why the two mainstream routes fall short
For a long time the dominant processes have been inclined-plate settling tank plus filtration, and conventional dissolved gas flotation (DAF) plus deep filtration. Neither fits modern oilfield development well.
Inclined-plate settling gives unstable results and poor tolerance to water quality swings, needs large doses of chemicals and readily generates oily sludge and other secondary pollution. Conventional DAF is somewhat more efficient but requires a long hydraulic retention time of 15-40 minutes, a large footprint and a low degree of automation, which is a poor match for facilities that are being debottlenecked rather than expanded.
Both routes share a structural limitation: they were designed for water whose emulsion breaks easily. Polymer flooding removed that assumption.
How the CDFU process differs in operation
Against this backdrop the CDFU (cyclonic dissolved gas flotation) process developed by SINOKLE emerged as a solution for oilfield produced water treatment. It relies on purely physical separation: a cyclonic dissolved-gas design combines centrifugal separation with micro/nano-bubble flotation to remove oil and suspended solids.
Produced water enters the unit through a lift pump and forms a stable flow field under cyclonic action. Free oil, emulsified oil and suspended solids collide rapidly with the 5-30 micrometre micro/nano bubbles generated by dissolved gas and adhere to them. Surface tension of the bubbles gradually ruptures the emulsified oil film, achieving physical demulsification without a reagent; the oil-laden bubbles and suspended solids gather and rise to the collection barrel at the top and are discharged under their own pressure into the slop tank. Treated water flows on to downstream processes by lift pump, off-gas is routed separately to the treatment system, and control is fully automatic and closed-loop.
Four comparisons that decide the choice
Efficiency and stability. Single-stage oil removal exceeds 90%, and emulsified oil in the 0.1-20 micrometre range is removed precisely, with no loss of performance when water quality fluctuates. A DAF unit facing the same swing typically needs additional chemical or a second pass.
Chemical footprint. No chemicals are added at any stage, oily sludge is eliminated at source, there is no secondary pollution, and the equipment runs fully enclosed under pressure with no fugitive odour or gas. This changes the waste management side of the plant as much as the water side.
Footprint and cost. The skid-mounted design cuts the footprint to one third of a conventional process. At a 180 m³/h project in Daqing, the unit occupies less than 30 m², and the treatment cost falls to RMB 0.2 per tonne of water, only one fifth to one third of the conventional cost.
Operational intensity. IoT-based remote control, a structure dominated by static equipment that greatly reduces maintenance needs, and long inspection intervals all deliver significant labour savings. Compared with a DAF train with scrapers and a long residence volume to manage, the difference in staffing is immediate.
Daqing field data
Practice at an oilfield project in Daqing confirmed the results. With influent oil content of 150 mg/L or less and suspended solids of 300 mg/L or less, a single-stage CDFU unit brought both indicators stably below 20 mg/L, fully meeting the client's reinjection requirements. The unit is quick to build and commission and needs only a little power and nitrogen, cutting operating costs while delivering environmental and economic benefits.
Field applications in North China oilfields have shown the same shock resistance and clear, bright effluent. That matters because a process that performs on a design feed and fails on a slug of polymer-laden water has not solved the problem the field actually has.
Where DAF still wins
Conventional dissolved gas flotation remains a defensible choice for large, steady streams with easily separated oil and where land is not a constraint, since the capital cost per cubic metre is lower. The CDFU process earns its place where the emulsion is stable, the water quality swings, the plot is tight or the chemical and sludge costs are unacceptable. As oilfield development becomes harder and environmental rules tighten, that set of conditions is becoming the norm rather than the exception.
One caution on the comparison. The 180 m³/h Daqing figure and the footprint ratio describe one project on one feed; they are not a general guarantee. A field with very high polymer concentration or heavy oil may need chemical assistance upstream before flotation can perform at all. Pilot work on the actual produced water, run long enough to include a water-quality excursion, remains the only reliable way to size the unit.