Produced Water Reinjection Schemes for Ageing Oilfields: Matching CDFU Stages to Formation Permeability

As oilfields move into mid-to-late production, produced water stops being a by-product and becomes the constraint on everything else. The formation water lifted alongside crude oil carries high oil content, awkward suspended solids and poor chemical stability, and the conventional processes built decades ago no longer clear the environmental bar. The new-generation scheme developed by SINOKLE around CDFU (cyclonic dissolved air flotation) was designed for exactly this stage of field life.

Water cut only moves one way in a mature field. As the oil fraction shrinks year over year, the same wells deliver more water per tonne of crude, so a produced water system sized for early-life flow finds itself running at the limit of its hydraulics precisely when the discharge rules tighten. Planning the treatment train against end-of-life water volumes is the cheaper decision.

What makes produced water awkward

Produced water is a by-product of extraction whose composition shifts with the reservoir: crude oil, suspended solids, dissolved salts and whatever chemical additives the production chemistry has introduced. Oil content runs to 1,000-2,000 mg/L with suspended solids up to 500 mg/L, and the mixture is stabilised enough that simple settling barely dents it.

Why the conventional trains fall short

The familiar inclined-plate settling tank followed by walnut shell or fibre-ball filtration produces unstable results, tolerates shock poorly, swings widely in effluent quality and needs chemical dosing that breeds aged oil and oily sludge. The richer alternative, settling tank plus centripetal flotation plus fibre-ball or multi-media filtration plus ceramic membrane, performs somewhat better but pays for it with a complex flow sheet, high cost, difficult operation and maintenance, and a large footprint. Neither adapts well to what a modern oilfield actually needs.

Reinjection targets are set by the formation, not by preference. High-permeability zones accept water with modest oil and solids content because their pore throats are wide, while ultra-low-permeability formations plug at concentrations that would pass unnoticed elsewhere, which is why the industry distinguishes several classes of injection water quality. A treatment train that can be tuned to each class, rather than built once to the loosest target, is what keeps a mature field flexible as water cut rises year over year.

The CDFU principle and the three reinjection schemes

CDFU integrates centrifugal separation, dissolved air flotation and ultra-fine bubble generation in one hybrid device. A stable vortex flow field inside the unit applies strong centrifugal force for rapid splitting, while bubbles fine enough to attach to micro-droplets finish the job. The recommended flow is short: produced fluid through a three-phase separator, then first-stage CDFU, second-stage CDFU, and an activated filter media filter, with three treatment schemes matched to reinjection duty. Scheme 1, for high-permeability formations, targets effluent oil at 10 mg/L or below and suspended solids at 5 mg/L or below. Scheme 2, for low-permeability formations, tightens to oil of 6 mg/L or below and solids of 2 mg/L or below. Scheme 3, for ultra-low-permeability formations, demands oil of 5 mg/L or below and solids of 1 mg/L or below.

The supporting claims are specific rather than promotional. Single-stage oil removal exceeds 90% against the 60%-85% of conventional processes, and the minimum removable particle size reaches 2 μm. Demulsification is purely physical, so oily sludge never forms in the first place. Hydraulic retention time is 1-5 minutes, the footprint comes in at one third of conventional plant, the skid-mounted design deploys fast, and operation is fully automatic. Operating and maintenance costs drop accordingly, and the recovered oil, being high quality, goes straight back into the value chain.

Operationally, the train asks little of its crew. No chemical dosing means no reagent procurement, storage or quality control, and the oily sludge problem is eliminated at source rather than managed. The hydraulic retention time of 1-5 minutes lets the equipment absorb flow swings that would swamp a settling tank, and skid construction means capacity can be added by adding skids, which suits the stepped expansion typical of waterflood projects.

Three field projects, three different duties

The record covers the main scenarios. At the CNOOC Suizhong terminal project in 2023, a 260 m³/d installation used two-stage CDFU to treat power-fluid separator produced fluid with oil removal stably above 80%. The PetroChina Changqing oilfield project from 2017 runs 20 m³/h with inlet oil of 1,500 ppm or less; after CDFU the effluent oil sits at 30 ppm or below, and after polishing in an AFM filter it reaches 10 ppm or below. The Yanchang oilfield project, also 2017, treats 15 m³/h with inlet oil of 150 ppm or less, delivering effluent oil at 10 ppm or below and suspended solids at 10 ppm or below, every indicator stably on standard.

For operators planning reinjection upgrades, the selection logic is straightforward: define the formation's permeability class, pick the matching scheme from the three targets above, and size the CDFU stages to the peak, not the average, produced water flow. Efficiency, stability and environmental friendliness have made this train a reference point in produced water treatment, and as fields deepen and rules tighten, it is the pattern more operators are adopting.