Produced water treatment
Produced water comes up with the oil and gas production. Produced water is a term for reservoir water that is produced along with the oil and gas phase. Alternatively, the water is discharged to the sea or sewerage. It involves a set of conditions and strict regulations, aimed at preventing well contamination and water pollution. In recent years this kind of water management has turned difficult.
Technologies and strategies applied to produced water comprise Minimization, Recycle/ Re-use and Disposal.
Onshore produced water can be re-injected to provide additional oil and gas recovery, treated and used for civil usage or disposed on the reservoir according to regulations and permits. Technologies for onshore locations include Coalescing plate separation (CPI/WOSEP), Induced gas flotation (IGF) and Multimedia filtration.
Offshore produced water can be re-injected into reservoir, disposed as discharge to the sea after suitable treatment or transported onshore for suitable treatments and subsequent disposal. Technologies employed in offshore locations are Cyclonic de-oiling and de-sanding, Compact flotation (CFU) and Sand cleaning.
Nowadays, it has become essential to recover and use this water again due to the more and more stringent environmental limits (the components of produced water must exhibit persistent, toxic or bio accumulative properties) and the always increasing water demand.
One of the most challenging aspects of produced and flowback waters is that the composition of these waters is often highly variable, especially when they are reused for additional production operations.
Primary treatment of produced water treatment:
In this process removal of suspended hydrocarbons components and solids are taking place. The produced water, leaving the three-phase separator which separates oil, gas and water coming from the wellhead, is sent to the gravity separator (API separator). API Seperator then removes oils and other light fractions with a lower density than water. These fractions, removed by an oil skimmer, are then sent to the oil recovery stage. The settled particles are conveyed on the bottom of the separator and transferred to the oil sludge processing. The water separated from oil reaches the free surface on the side opposite to the power supply and it is sent to a mixing tank. Appropriate amounts of coagulants and flocculants are added here to facilitate the particles coalescence by sedimentation.
Next, water enters to Dissolved Gas floatation Stage. Here, nitrogen or natural gas (to avoid explosions upon contact with hydrocarbons) is blown to separate oil, suspended solids and other macromolecules.
The produced water coming from the DGF stage is further deoiled and then sent in another mixing tank, where chemical reagents are added (polyelectrolytes, caustic soda and aluminum chloride); then, the stream is treated in a sedimentation tank to remove metals. Before the secondary treatment stages, produced water passes through sand filters to ensure a further reduction of pollutants.
This method is based on adsorption, biodegradation, stripping, membrane separation. It eliminates the dissolved aromatic hydrocarbons such as BTEX and NPD along with organic compounds, suspended solids and oil. It was possible to establish that the treatment with the vibrating membrane system VSEP (Vibratory Shear Enhances Process). Advantages of this method are best process with single operation, high purity without adding another chemical and amount of waste will be very less.
Salt removal is done in the tertiary treatment phase. Water coming from secondary processes is entering to this phase. Reverse osmosis principle is utilized in tertiary treatment, mainly reducing the levels of nitrates and phosphates. Antiscalant and chemicals are added to water in the upstream of reverse osmosis stage to prevent precipitation of low soluble salts on the membrane surface. The permeate is then pressurized and sent to the reverse osmosis stage. The process scheme includes also an energy recovery device (R) and a booster pump.
**The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.