Efficient Short-Process Treatment and Reinjection Solution for Wellhead Produced Liquid

Current Status and Challenges of the Conventional Treatment Mode

In conventional oilfield operations, produced liquid is typically treated in a centralized mode: the high-water-cut mixed liquid produced by each well is transported over long distances to a central treating station for oil-water separation. The separated crude oil is exported, while the produced water is purified at a water treatment station and, once it meets the required standards, conveyed to the water injection stations for reinjection into the formation. This mode mainly suits the treatment needs of produced liquid in the medium- and low-water-cut stages.

As oilfield development enters the high and ultra-high water-cut stages, the limitations of the conventional centralized treatment mode have become increasingly prominent. On the one hand, the volume of produced liquid keeps growing and the water cut keeps rising, so a large amount of electric energy is consumed in the ineffective transport of produced water, seriously affecting the overall benefits of oilfield development. On the other hand, the treating capacity of existing central stations is approaching saturation: some stations can expand capacity through reconstruction but at a very high investment cost, while others, constrained by urban expansion, have no room for expansion at all. Insufficient central-station treating capacity has become a key factor restricting oilfield profitability, and a new treatment system adapted to the characteristics of the high-water-cut stage is needed.

Current Development of Pre-separation Technology

The current mainstream solution is to install pre-separation devices at gathering stations or even further upstream — at wellheads and metering stations — to remove most of the water in advance and reduce the load on downstream treatment. Existing pre-separation technologies mostly adopt single or combined processes such as gravity settling, hydrocyclone separation, flotation, and thermochemical treatment; representative techniques include inclined separation, series hydrocyclones, high-efficiency three-phase separation, and low-temperature demulsification. However, systematic research on integrating cylindrical cyclones with hydrocyclone technology remains insufficient, and methods for coordinated parameter design of integrated units still need improvement.

SINOKLE's Innovative Short-Process Technology

With its patented CDFU cyclonic dissolved-air flotation oil remover as the core, complemented by filters of different precisions, SINOKLE has designed two wellhead produced-liquid short-process treatment and reinjection schemes; the process flow diagrams are shown below.

Scheme 1: After preliminary treatment in a two-phase separator, the water phase passes successively through two HAC units and two CDFU units for oil and suspended solids removal. The separated waste oil enters a collection tank, and the waste gas is routed to the flare system or the gas network. The purified water undergoes advanced treatment in two stages of activated-media filters and, after meeting standards, is stored in the reinjection water tank. Backwash water comes from the reinjection tank, and the generated sludge is dewatered and shipped off site.

Scheme 2: After pretreatment of the produced liquid, the water phase is purified through two HAC units and the CDFU system; the waste oil and waste gas are disposed of as described above. The effluent enters a circulation water tank, and the concentrate returns to the CDFU unit for treatment; the flotation scum is dewatered and shipped off site. The circulation tank effluent is finely filtered by a SiC membrane filter to meet standards before reinjection.

Process flow diagram of Scheme 2

Compared with conventional processes, the two produced-water treatment schemes above have the following features:

• Short process, high efficiency, strong shock resistance, and effluent that stably meets water quality requirements;

• No chemicals — a pure physical demulsification and separation technology, no aged oil, no secondary pollution, and good environmental benefits;

• Two-stage cyclone separators plus two-stage cyclonic dissolved-air flotation as pretreatment provide strong shock resistance and ensure stable, reliable operation of the subsequent process; advanced filtration technology delivers high filtration precision and good performance with strong shock resistance and long filter-media life, ensuring stable compliance of water quality;

• Skid-mounted design with a small footprint, suitable for space-constrained sites such as offshore platforms;

• Fully automated, information-based operation with remote monitoring, low labor requirements, and convenient, low-cost operation and maintenance.

Field Performance Verification

As shown in the figure, a produced-water reinjection treatment skid at a PetroChina central treating station achieves Class A1 effluent with oil content ≤5 mg/L and suspended solids ≤1 mg/L, and Class A2 effluent with oil content ≤6 mg/L and suspended solids ≤2 mg/L

Produced-water reinjection treatment skid at a PetroChina central treating station