Reinjection Water Quality: Why Emulsified Oil Between 1 and 50 Microns Decides It
As oilfields enter the mid-to-late stage of development, declining output becomes the norm rather than the exception, and waterflooding has become the most important tertiary recovery method available. That puts reinjection water quality in the critical path: it governs waterflood performance, production economics and environmental compliance at the same time.
In conventional blocks, reinjection water comes mainly from produced water treated to discharge standard, and it arrives with high oil content, high suspended solids and severe bacterial growth. When it fails to meet quality standards, the consequences are formation plugging, rising injection pressure and lower recovery — and in serious cases, shut-in wells.
Three technical difficulties
Oily wastewater is genuinely hard to treat. Produced water contains dissolved, emulsified and dispersed oil together. Emulsified oil forms a stable thermodynamic equilibrium with water, with droplet sizes mostly between 1 and 50 microns, and conventional gravity separation and filtration cannot remove that fraction effectively.
Suspended solids exceed limits. Mud and sand, corrosion products and bacterial metabolites carried in during production raise solids content, and the particles are fine enough to plug formation pores and reduce injection permeability.
Bacteria drive corrosion. Sulphate-reducing bacteria and iron bacteria metabolise into acidic substances that accelerate corrosion of injection equipment and pipelines and shorten service life.
Why the conventional train falls short
The standard arrangement is oil separation, flotation then filtration, and each element has a known limitation. Oil separation tanks remove emulsified oil poorly — they take out most free oil and leave the fine emulsified fraction untouched. Conventional flotation equipment generates larger bubbles of 50-100 microns, and the probability of those contacting fine oil droplets is low, so deoiling is limited. Quartz sand filters have only average precision, intercept suspended solids imperfectly, and compact easily, which makes backwashing poor.
Add large footprints, heavy chemical consumption and high O&M costs, and the conventional train works against exactly what oilfields need: lower cost and higher efficiency.
CDFU and KFM as an integrated answer
SINOKLE builds its solution around two units. The CDFU swirl dissolved-air flotation unit is a core product developed in-house, combining swirl centrifugal separation with dissolved-air flotation and using a CFD-optimised flow-field design to generate ultra-fine bubbles only 5-30 microns in diameter. Those micro-bubbles contact and attach to emulsified oil droplets, forming flocs lighter than water that separate readily. Compared with conventional flotation equipment, deoiling efficiency is more than 50% higher and effluent oil content is held below 5 mg/L.
The KFM modified filter media filter uses a high-quality activated carbon-based medium with well-developed pores and high adsorption capacity, with a specially modified surface for intercepting and adsorbing oil and suspended solids. Filtration precision is high enough to keep effluent suspended solids below 1 mg/L, and the medium backwashes thoroughly, resists compaction and lasts 5-8 years.
Matching the specification to the field
Not every conventional block needs the same effluent quality, and treating the specification as fixed over-engineers some sites while leaving others short. Blocks with tighter permeability demand lower suspended solids, because the particles that plug pores are the smallest ones. Heavier oil fields tolerate more oil in the reinjection stream but generate far more of it in the first place. Reading the reservoir first and the standard second usually produces the better design.
Water chemistry deserves attention as well. Where sulphate-reducing bacteria are active, corrosion takes hold in pipes rather than inside the treatment unit, so the improvement in water quality has to be sustained consistently — an intermittent gain in suspended solids does not stop the bacterial population re-establishing itself downstream.
Operating patterns matter too. Reinjection runs continuously while maintenance does not, and any arrangement that depends on constant operator attention will drift whenever that attention moves elsewhere. Automation and unattended operation are therefore not conveniences here; they are what keeps water quality inside the band the reservoir will accept, which is also why verification should be continuous rather than periodic. Because the failure mode is gradual plugging rather than a sudden break, sampling frequency has to be high enough to catch a trend before injection pressure starts to rise.
Footprint and cost
The combined process delivers stable, compliant effluent: oil below 5 mg/L and suspended solids below 1 mg/L, meeting the oilfield reinjection water quality standard SY/T 5329-2012. Equipment is skid-mounted, and a CDFU unit rated 200 m³/h occupies only 15 square metres, which is more than 60% less land than a conventional arrangement — a meaningful difference on an established field where space is already committed.
Because treatment is purely physical, there are no chemicals and no secondary pollution, and O&M costs drop accordingly. A high degree of automation allows unattended operation, which matters at remote well sites. The solution adapts across ordinary heavy-oil fields, conventional blocks and low-permeability oilfields, and it has already been applied in several oilfields, building up enough engineering history to make the economics predictable rather than theoretical.