Water Clarifiers
Water clarifier or flocculant is a family of products to help improve produced water quality by chemically flocculating and removing excess oil/grease content to meet discharge environmental specification or avoid sheen.
Before the produced water from oil gas production can be discharged into the environment, it needs to meet certain oil/grease content criteria, e.g. 29 mg/l average per month for Gulf of Mexico. In addition, if the water is to be reinjected, the solids may plug the pore throats in the near-well area of the injection wells. Produced water coming out of the oil/water/gas separators often contain residual oil and finely dispersed solids that are too high to be discharged or re-injected. Therefore, the produced water needs to be further processed to remove the excess residual oil and grease. This is often achieved by using a combination of equipment, e.g. hydro-cyclone, flotation cell, and chemicals. The chemical method of doing this is to add a flocculant, also called a “water clarifier,” “deoiler,” “oil-in water demulsifier,” “reverse emulsion breaker,” or “polyelectrolyte.”
Contents
What are dispersions and reverse emulsions?
Two immiscible liquids, such as oil and water, an emulsifying agent, and a source of agitation can form an emulsion. Most of the time, emulsions are water-in-oil. Dispersions and reverse emulsions are just the opposite: oil is the internal phase and water is the external phase.
More specifically, in a dispersion, the tiny droplets of oil will eventually break free and rise to the top of the water given enough time. A true reverse emulsion, however, will not separate regardless of the time involved.
Reverse emulsions are typically formed in the reservoir by natural processes. Additionally, they tend to form more readily with fresh water. This may be related to the specific gravities of these two liquids. Fresh water has a specific gravity close to 1.0, which is more similar to the lower specific gravity of oil than heavier brines with higher specific gravities.
How do dispersions and reverse emulsions affect production systems?
They can affect the quality of the water that’s produced with the oil and gas. This can have both geological and environmental implications for the producer.
Volumes of water are often produced as a by-product of the production process. Eventually, something has to be done with this water. On land, the water is typically injected back into the formation with injection, or disposal, wells. Water can be re-injected into the formation for water-flooding as well, which uses the water as means of stimulating production by pushing the reservoir fluids toward the wellbore. Off-shore, water is dumped overboard back into the seawater or injected in wells.
Land Operations
Geologically, water that has particles of oil or other small solids can damage a formation if forced into pore spaces not large enough to allow them to pass through. Additionally, in waterflood operations, these contaminants can clog pores and redirect the water away from the path designed to stimulate the well.
There also may be environmental restrictions which dictate what the quality of the water must be before it can be lawfully introduced back into the formation.
Offshore Operations
Water quality is an enormous issue in offshore operations. Strict environmental regulations require water to meet certain quality standards before it can be dumped back into the sea. Production operations that violate water quality requirements will be shut down.
In both scenarios, dispersions of oil in water and reverse emulsions must be broken to ensure continued production.
How do water clarifiers work?
Dispersions of oil and particles in water are stabilized by a number of factors, including charge repulsion. Polar molecules in crude oil, including resins and organic acids, as well as clays, scale, rust, and other polar products, will be present at the oil–water interface in oil-in-water emulsions. Flocculation of these particles must overcome the charge repulsion.
The effect of the water clarifier or flocculant is to counterbalance the charges on the dispersed oil and particles so that they will come together (flocculate). The flocculant may also bridge particles, causing them to come together. This is usually done with a large, charged molecule, such as a high molecular weight polymer or in situ–generated polymer. Once the polymer has bridged two or more particles, the charge on the polymer is more neutralized. This causes a shift in the conformation of the polymer from being open and linear to a more coiled or globular structure. Thus, the polymer has collapsed, enveloping flocculated particles as a “floc.”
Chemistry
Water clarifiers or flocculants used in the oil industry can be summarized as follows:
- Highly valent metal salts
- Cationic polymers
- Dithiocarbamates (DTCs; in situ–forming cationic “polymers”)
- Anionic polymers
- Nonionic polymers
- Amphoteric polymers
Ionic polymers (polyelectrolytes) make up the bulk of the primary flocculants used in the oil industry. If not properly applied, some chemistries can generate excessive solids that cause production issues.
Performance testing
The bottle test (similar to the standard bottle test for crude water-in-oil emulsions) is commonly
used to test flocculants. The flocculant is added to a sample of “oily” water and the capped bottle
shaken. After standing for a given time, the clarity of the water is examined. Flocculant concentrations determined by this test usually exceed that required in practice in the plant. It is normal practice to start testing at a dose of 10 ppm and then increase or decrease in the range 1–50 ppm. The
dose requirement is usually low compared with demulsifiers for resolving water-in-oil emulsions.
Many systems employ the action of gas bubbles to float both oil and solid contaminants out of
the water phase. These systems usually have high amounts of agitation and create foam or froth
above the water phase. Gas flotation testing is performed with a flotation test cell sometimes called
a “bench Wemco” to simulate plant practice.
How are water clarifiers applied?
Water clarifiers are typically continuously injected into the water leg of the production system, upstream of hydrocyclones and flotation cell. The product applied upstream of the hydrocyclone is typically considered reverse emulsion breaker to improve the hydrocyclone performance by increasing the dispersed oil droplet size as hydrocyclone is not very effective if the droplet size is below 10 µm.
