The objectives of oil and gas produced water treatment include meeting discharge regulations, reusing treated produced water in oil and gas operations, developing agricultural water uses, aquifer restoration, cattle and animal drinking water, and even water for human consumption. Current produced water technologies and their successful applications have advantages and disadvantages and the suitability can be assessed on the basis of those factors.Water produced during oil and gas extraction operations (FRAC) constitutes the industry’s most important waste stream on the basis of volume. The oil and gas industry produces approximately 14 billion bbls of water annually by some estimates. The water varies greatly in quality and quantity and in some cases the water can be a useful by-product or even a saleable commodity. Produced water is most often considered a waste, but the industry is beginning to consider this material as a potential profit stream. Whether waste or commodity, produced water has management costs that need to be kept in-line with each specific production project and region or it could adversely affect the life of the well, thereby leaving substantial recoverable reserves in the ground. Produced water handling practices must also be environmentally protective. Produced water handling methodology depends on the composition of produced water, location, quantity and the availability of resources. The thing with produced water is the volume increases as the well ages & the water component may go up to 98% also. The strict regulations on discharging the produced water are forcing the Oil producers to go for better treatment schemes.Some of the options available to the oil and gas operator for managing produced water might include the following:– Avoid production of water onto the surface – Using polymer gels that block water contributing fissures or fractures or Downhole Water Separators which separate water from oil or gas streams downhole and reinject it into suitable formations. This option eliminates waste water and is one of the more elegant solutions, but is not always possible.
– Inject produced water – Inject the produced water into the same formation or another suitable formation; involves transportation of produced water from the producing to the injection site. Treatment of the injectate to reduce fouling and scaling agents and bacteria might be necessary. While waste water is generated in this option, the waste is emplaced back underground.
– Discharge produced water – Treat the produced water to meet onshore or offshore discharge regulations. In some cases the treatment of produced water might not be necessary.
– Reuse in oil and gas operations – Treat the produced water to meet the quality required to use it for drilling, stimulation, and work over operations. In EOR (Extended Oil Recovery) operations, this options is very beneficial as treated produced water can be used as BFW for once through Boilers used for Huff & Puff or continuous steam injection or the purified water is also used to prepare the Polymer solutions to inject into oil wells.
– Consume in beneficial use – In some cases, significant treatment of produced water is required to meet the quality required for beneficial uses such as irrigation, drinking water for private use or in public water systems.The general objectives for operators when they plan produced water treatment are:1. De-oiling – Removal of free and dispersed oil and grease (Hydrocarbons) present in produced water.
– Soluble organics removal – Removal of dissolved organics.
– Disinfection – Removal of bacteria, microorganisms, algae, etc.
– Suspended solids removal – Removal of suspended particles, sand, turbidity, etc.
– Dissolved gas removal – Removal of light hydrocarbon gases, carbon dioxide, hydrogen sulphide, etc.
– Desalination or demineralization – Removal of dissolved salts, sulphates, nitrates, contaminants, scaling agents, etc.
– Softening – Removal of excess water hardness.
– Sodium Adsorption Ratio (SAR) adjustment – Addition of calcium or magnesium ions into the produced water to adjust sodicity levels prior to irrigation.
– Miscellaneous – Naturally occurring radioactive materials (NORM) removal.Oil and Grease Removal (De-oiling):Oil and grease in produced water includes free oil, dispersed oil (small oil droplets), and emulsified oil. Oil and grease discharge, along with produced water, involves coOil and grease removal methods depend on the end usage of treated water and composition of oil in the produced water.Table below shows typical performance for oil removal treatment as expressed by oil droplet size.
|Oil Removal Technology||Minimum size of particles removed (microns)|
|API gravity separator||150|
|Corrugated plate separator||40|
|Induced gas floatation (no flocculants)||25|
|Induced gas floatation (with flocculants)||3 – 5|
|Hydroclone||10 – 15|
The all above options can be offered by AES based on case to case basis. Soluble Organics Removal:Extraction of dissolved hydrocarbons with on-site liquid condensate (such as available from gas compression units) has been attempted by some oil and gas producers. Extraction enables enhanced removal of dispersed oil by coalescing small oil droplets during the process. This process utilizes the principle of higher solubility of heavier hydrocarbons in lighter hydrocarbons.Adsorption is a widely accepted technology for the removal of soluble hydrocarbons from the produced water. Adsorption columns are packed with porous solid material known as adsorbent. The hydrocarbons present in the produced water adhere onto the surface of adsorbent and are eventually retained within the porous structure. The effluent from the adsorption column contains little or no hydrocarbons. Highly porous adsorbents with higher surface area offer better performance. Activated carbon, nutshell media (Wall Nut, Pecan shell), modified organoclay, etc. are some of the adsorbents used for produced water treatment. The major concern of adsorption operation is the requirement of retention time which limits the throughput capacity.
Oxidation of soluble organics and contaminants (bacteria nitrate etc.) is an effective produced water treatment. Ozone and hydrogen peroxide are strong oxidizers. Oxidation process produce dissolved carbon dioxide which can be removed by air stripping. Ultraviolet (UV) light has also been used for oxidation, which de-ionizes water into hydrogen and hydroxyl ions. Hydroxyl ions are also strong oxidizers. UV light also kills bacteria and other microorganisms to disinfect the water. Oxidation is able to reduce chemical oxygen demand (COD), biological oxygen demand (BOD), organics, iron, nitrite, manganese, cyanides, hydrogen sulphide and aromatic hydrocarbons. Biological decomposition (combined oxidation and reduction) of produced water in a constructed wetland is also useful for treating raw produced water and treating post-treatment water after it has been through other treatment processes. Plants and microorganisms are utilized to naturally decompose the contaminants from the produced water.
Removal of dissolved solid, salts or impurities is often the most important part of water treatment systems. TDS in produced water ranges from <2000 ppm to >150,000 ppm. Average TDS content in seawater is approximately 35,000 ppm. The choice of desalination method depends on TDS content and compatibility of the treatment system to work under the presence of extra contaminants present in the produced water. Various technologies like evaporation, distillation, membrane filtration, electric separation and chemical treatments (Lime Softening, ion exchange etc.) to remove TDS from the produced water. The choice of technology depends upon the produced water TDS, end application and several other factors.
Removal of bacteria, viruses, microorganisms, algae, etc. from the produced water is necessary to prevent scaling and water contamination. Microorganisms occur naturally in the produced water or may be added during de-oiling treatments. Advanced filtration techniques are one of the effective technologies used to remove microorganisms. UV light treatment, chorine or iodine disinfection, ozone treatment are other treatments available to disinfect produced water.
Produced water softening, SAR adjustments and removal of trace contaminants, pollutants, naturally occurring radioactive materials (NORM), etc. are the part of produced water treatment in some regions depending on the composition of produced water.