Water Treatment

At the pilot stage (with only a small percentage of injecting wells under polymer flooding), there is generally no need to assess in detail the effect of the back-produced polymer since the dilution effect in the reservoir will lead to negligible concentration of polymer within the water treatment facilities.
When dealing with a full field project, two different aspects have to be considered in the treatment processes. The first one is related to the separation of the crude and the produced water. The next one deals with the treatment of the produced water once it has been separated from the crude, in order to be re-used or disposed of. Both aspects are discussed in the following paragraphs.


Impact on the Separation of Oil and Water

Influence on the separation process

At the production side, depending on the design of the production facilities and the type of injected fluid (presenceof surfactant or alkali or both) an emulsion is produced. De-emulsifiers or “breakers” are needed to break this emulsion. The presence of polymer can interact with the breaker and it may result in altered separation. Skilled service companies generally do the selection of the most  appropriate and compatible breaker through bottle tests. Since polyacrylamide polymers are water soluble, they are considered absent from the oil phase.

Presence of suspended solids

Anionic polyacrylamides are efficient flocculating agents of mineral suspended solids. Thus, if the TSS (Total Suspended Solids) is high in the produced fluid, some agglomerates should appear in the water phase in presence of polymer. These precipitates can deposit on the surface of specific equipment such as heater treater used for the separation of oil and water, which contributes to the creation of hot points, which in turn can lead to accelerated corrosion issues. These deposits have to be removed on a regular basis.

Treatment of Produced Water

Once the main part of the oil has been recovered from the produced fluid, a low amount of oil and TSS still remain in the produced water. Devices for produced water treatment are usually sized to operate with water viscosities below 4 mPa.s. Polymer breakthrough at the production side will contribute to an increase of viscosity and will reduce the efficiency of the treating devices to separate the oil and suspended materials wetted by it. With produced water viscosities of 10 mPa.s for example, the residence time required is five times longer and devices required are five times larger.
Different methods can be applied to decrease the viscosity of the fluid containing polymer in order to improve water treatment efficiency. The principle is to decrease drastically its Mw and thus, the associated viscosity of the fluid by means of mechanical and/or chemical degradations.

Mechanical Degradation of the Polymer

Mechanical degradation can be performed to decrease the Mw of the polymer and thus decrease the viscosity of the solution. A shearing pump or a choke valve can be used to achieve the degradation and decrease the viscosity of the polymer solution

Chemical Degradation of the Polymer

Polyacrylamides are chemicals that can experience chemical degradation in presence of oxidizers. Oxidizers generate radicals that react with the backbone chain of the polymers, resulting in a decrease of the Mw and a viscosity drop. Preferred oxidizer is sodium hypochlorite (NaClO)