Produced Water at High Water Cut: Sizing a CDFU Train for ASP Flooding at 180 m³/h

Most mature Chinese oilfields have stopped being oil fields in the simple sense. They are water-handling facilities that happen to recover some crude on the way through. Daqing, Shengli, Yumen and Dagang all sit in the middle-to-late stage of production now, and holding crude output steady has pushed operators toward tertiary recovery: polymer flooding first, then alkali-surfactant-polymer (ASP) flooding where the economics justify it.

That choice solves one problem and creates another. The chemicals that sweep residual oil out of the reservoir come back to surface with the produced fluid. Polymer concentration climbs, liquid viscosity climbs with it, and the oil that once separated in a settling tank now reaches the wellhead as a tighter emulsion. Conventional trains keep running, but the purified water they deliver increasingly misses reinjection quality. Once that happens, produced water treatment stops being a utility and becomes the constraint on how much the field can produce.

Why the Emulsion Tightens as a Field Ages

Three things happen at once. Polymer chains adsorb at the oil-water interface and thicken the film around each droplet, so two droplets that collide do not merge. Viscosity in the continuous phase rises, which slows the rise of everything suspended in it. And the droplet size distribution shifts downward, so a smaller share of the oil is present as free oil that gravity can catch on its own.

The result is a stream carrying free oil, dispersed oil, emulsified oil and suspended solids at the same time, in proportions that change week to week as injection patterns are adjusted. A process designed for one of those fractions will handle it well and handle the other three badly. That is the practical reason why field trials of settling tanks and conventional flotation units start out promising and end with someone dosing chemicals to stay compliant.

Starting from the Water, Not the Equipment

For a produced water purification project at an oilfield in Daqing, SINOKLE began with a full characterisation of the water rather than a predefined equipment list: oil speciation, solids loading, polymer residual, temperature and the seasonal swing in all of them. What came out of that work was a design built on the latest CDFU cyclonic dissolved gas flotation technology, applied as the single separation stage between the feed pump and the reinjection header.

The rationale is straightforward once the water is understood. Gravity separation needs a density difference and time; polymer-flooded produced water offers little of either. Bubble-driven separation needs a collision between a gas bubble and an oil droplet, and that collision probability rises sharply when the bubbles are small and the mixing field is violent enough to bring them together. A cyclonic field supplies the violence; a dissolved gas system supplies the bubbles.

Inside the CDFU Stage

Produced water is lifted by the feed pump into the vessel. Under strong cyclonic action, free oil, dispersed oil, emulsified oil and suspended solids contact and adhere to the micro and nano bubbles generated by the dissolved gas system. The surface tension effects at those bubbles are what break the emulsified oil film open, so the demulsification here is purely physical — no coagulant, no demulsifier, no pH adjustment.

Oil released from the film, oil adsorbed onto suspended solids, and the bubbles themselves form a multiphase mixture that gathers in the core zone and floats to the top of the vessel. It enters a built-in oil collection chamber and is drawn off through the oil outlet under its own pressure. Purified effluent leaves through the effluent pump to downstream units; separated slop oil is discharged by static pressure into the slop tank; off-gas goes to a dedicated waste gas treatment system. Effluent, oil discharge, sludge discharge, gas discharge and gas supply are all controlled automatically across the whole system, and the dissolved gas intake and surge tank venting are regulated the same way.

Seven Characteristics That Show Up in Operation

• Purely physical removal of free oil and scum, with a genuine demulsification effect. Separation efficiency stays high and does not collapse when the wastewater or the slop oil quality changes.
• Static equipment throughout. High automation, low maintenance, long inspection intervals, and reliability that comes from having almost nothing that rotates.
• No chemical agents, therefore no additional scum or oily sludge, no secondary pollution, and low operating plus secondary treatment cost.
• Small footprint and skid mounting, with a short manufacturing cycle and fast commissioning.
• Shock resistance. Units already commissioned show thorough oil-water separation, with effluent that is clear and bright and free of visible impurities.
• Fully enclosed pressurised operation, so no waste gas or odour escapes.
• Remote supervision. The automation layer supports IoT control, which cuts the labour hours a shift team has to spend on the unit.

What the Daqing Project Actually Delivered

The design capacity is 180 m³/h. Influent specifications are oil content of 150 mg/L or less and suspended solids of 300 mg/L or less. After treatment through a single-stage CDFU unit, both oil and suspended solids drop below 20 mg/L — which was the customer design target, met with margin rather than by a narrow pass.

The physical envelope matters as much as the water quality in a brownfield retrofit. The system occupies less than 30 m², which in an existing oilfield battery is often the difference between a project that can be built and one that cannot. Operation requires no chemical agents at all, only a small amount of electricity and nitrogen. Calculated treatment cost per tonne of water comes out as low as 0.2 yuan, and that figure is what makes the environmental benefit defensible to a cost engineer as well as to an environmental officer.

Where the Approach Needs a Companion Step

Bubble flotation removes what it can attach to. Dissolved hydrocarbons, some surfactants and fine colloids that stay in the water phase are not its target, so a specification written around those parameters will still need a filtration or adsorption stage downstream. Reinjection into a tight formation is the usual place where that shows up first. The sensible way to specify the train is to fix the reinjection quality first, then decide how much of the duty the CDFU stage should carry.

What the single stage buys in return is worth stating plainly: no chemical store on site, no oily sludge to classify and ship, no operator dosing a tank at three in the morning, and a footprint small enough to fit inside the battery it serves.