Challenges Comparing Advanced Water Treatment Technologies
Produced water treatment systems are becoming more sophisticated and complex with the addition of tertiary treatment systems, sometimes referred to as polishing systems. Additionally, produced water chemistry, process design and discharge requirements are different from location to location. The advanced treatment systems can be needed when produced water is being reused or recycled and when produced water volume has increased beyond the treatment system’s capacity. The process variability makes the technologies used in these treatment systems diverse, such as reverse osmosis membrane systems, absorbent media and cleanable ceramic membranes.
With the variable process conditions, the specialized treatment systems’ performance parameters can differ greatly between products with similar technologies. Likewise, some common performance parameters lack universal definition, making comparisons between two systems difficult. For instance, there are many different methods to measure oil in water concentration, each with a different definition of what is considered oil. This paper will review the technology and performance parameters of three tertiary water treatment technologies and some of their typical applications.
Produced Water Discharge Requirements
There are no consistent global water conditions in oil fields due to the significant variability in reservoirs, production techniques and water discharge methods. Water can be up to 99% of the total liquids produced in some places, while other fields may not produce any water. Water salinities and mineral content can also vary from fresh to almost saturate brines.
Depending on the water salinity produced in an oil well and the destination of the water, the required discharge quality can vary significantly. Most treated produced water is injected into a formation for disposal. Some treated produced water is injected into the oil formation to increase oil production, while most is injected into a separate formation for permanent disposal. In some offshore operations, the produced water can be discharged to the surface.
In only a few locations, the highly treated produced water can be discharged to a fresh water source or recycled. In arid environments where the demand for water sources is high and with advanced water treatment, the produced water can be used as fracking water source, or irrigation. To meet the increased water quality requirements, water treatment companies have developed advanced water treatment systems. However, water quality requirements for reinjection are more lenient than for offshore discharge or recycling.
Water treatment systems can be efficiently designed for the process and discharge conditions expected during the initial phase of production and in many cases designed to accommodate expected production changes for the future. For instance, an oil well may have 10-20 per cent water during the first year of operation. However, the projected life of the well calls for adding injecting water into the oil formation to enhance the production after five years of operation. This is referred to as water flood where the water concentration can increase up to 75–90 per cent. With the expected change in produced water volumes, the original water treatment system can be designed to treat the higher water volumes and achieve discharge quality standards.
Water treatment systems are challenged when the production style change, the water quality needs change or there is insufficient room to add the system. In some offshore facilities, new oil production formations that add total liquid volume can be developed and fed into the same treatment system. The new field may not have been considered when designing the water treatment system.
Offshore facilities are limited by the space and weight constraints of the platform. They need small and lightweight solutions. Onshore facilities, on the other hand, can usually add more treatment tanks. If the production method is changed from the water flood to a different recovery method like steam, surfactant, or CO2 floods, the water treatment system may also need to change. The production technologies used in the secondary recovery method may not have been developed or considered when the original water treatment system was designed, and the system isn’t designed to treat the new production method.
Tertiary Treatment Systems/Polishing Filters
Storage tanks and pressurized vessels for oil, water and gas separation are considered the primary treatment systems. They usually have some internal structures and no moving parts. Most onshore facilities with plenty of space are well suited with this treatment. When liquid volumes grow, tanks can be added to improve treatment. When space is not available, or when higher water quality is needed, a secondary treatment system can be added. Secondary treatment systems can include enhanced gravity systems and gas flotation systems. Gas bubbles added to the water will remove oil as they float to the surface. Induced gas flotation and dissolved gas flotation are the common technologies used. When centrifugal forces in a hydrocyclone or centrifuge are added to the system to enhance the gravity separation, oil is removed much faster than under normal gravity systems.
When the primary or secondary treatment systems are no longer sufficient to meet the water quality needs, frequent oil slugs and discharge upsets can occur. To reduce these issues, a tertiary system can be added to the process. These systems can include absorbent media or coalescing media. Granular activated carbon is a common and successful treatment media, but the carbon has a high operating cost because it is consumed when removing oil from the water. To reduce operating costs, the ideal media can be regenerated in the field without needing to be replaced often or only used when upsets occur.
Due to the many different process conditions in oil fields and different water quality standards to be met, the many different treatment technologies and methods of measuring the performance must be connected to this discussion.
Organoclay
Xedia Process Solutions has developed an organoclay-based technology which can remove oil droplets and some dissolved oil from produced water processes and back washed in some cases. This technology can be retrofitted to some existing water treatment systems without adding new vessels. This technology is capable of reducing the free oil concentration from 100 ppm, typically seen after primary separation tanks, to less than 1 ppm and remove solids at the same time.
Oil and solid removal filters are added as the last stage of a water treatment system when the water is reused or discharged to the ocean. A traditional filter media can be consumed quickly when water volumes increase, polymers are added or high solids are present during upset conditions. However, with the higher loading capacity of the Xedia media, oil producers are able to achieve their needed water quality without adding new filter vessels. When the media is back washed, operating costs are reduced because the media does not need to be replaced as frequently.
Synthetic Walnut Shell
Walnut Shell Filters are a common treatment technology to remove free oil to low concentrations. Oil will adsorb (stick to the outside) to the nut shells, but not be absorbed (permanently stick to the internals). Because the nut shells are resilient, they can be back washed vigorously and returned to the treatment process with little damage or need to be refilled.
Siemens Water Technology has developed a new synthetic media, PerforMedia, to replace the nut shells without needing to change the design of an existing filter system. The synthetic media has better oil affinity and can remove oil at significantly higher concentrations than walnut shells. A typical specification calls for nut shells to remove 100 ppm and achieve less than 10 ppm in the discharge. However, the PerforMedia can meet the same discharge water quality when slugs are as high as 1000 ppm.
The size and shape of the media is designed to remove solids as well as free oil. The solid particles are trapped between the media particles and will be back washed with the oil. The PerforMedia will remove most oil droplets and solid particles as well. The success of the PerforMedia can be seen in the results of particle size distribution in the effluent. Almost all the particles remaining in the water are less than 10 microns.
When polymer floods are used to enhance oil recovery, a flotation treatment system efficiency can be reduced by up to 40%. The polymers increase the viscosity of the water thereby slowing particle movement. This translates into heavy loading of a downstream traditional nut shell filters. Because of the high possible loading and efficiency of PeforMedia, polymer flood produced water can be treated without frequent back washing and achieve oil concentrations less than 10ppm.
Adsorbant
Technologies are available for polishing produced water for the removal of dissolved hydrocarbons or very small droplets without regular media. The ProSep Osorb media is a silicon backbone based adsorbent media. A unique feature of this media is that under steam or methane the media can be regenerated and the hydrocarbons recovered.
Benzene is the most soluble hydrocarbon, with no droplets or gravity separation possible for concentrations below 300 ppm, depending on the process conditions. Osorb media is able to remove BTEX and highly soluble hydrocarbon to concentrations below 2 ppm.
Because Osorb media has a high affinity to hydrocarbon molecules, the process can handle upsets and fast changing concentrations in the inlet conditions. When used as a polishing filter, Osorb will need regular backwashing if the upstream process is not stable. However, even if frequent regeneration is required, the discharge water quality will be maintained.
Velia – MPPE
Similar to the Osorb media, Veolia MPPE (Macro Porous Polymer Extraction) technology will extract BTEX and other light / dissolved hydrocarbons to low concentrations. The MPPE media is a bead with a significant porous structure for oil adsorbance. The MPPE also can be regenerated with steam as needed.
The MPPE will improve the toxicity of the produced water by removing dissolved and dispersed hydrocarbons to below 1ppm. During high concentration upsets greater than 99% of the hydrocarbons will be removed. Because MPPE is highly effective at removing BTEX, NPD (Nathalenes, Phenanthrese, and Dibenzothiopenes), it is often used in natural gas field. An additional side benefit is the removal of mercury as well from the process, further improving the water toxicity.
Membranes
Some water generated in an oil field is different than the produced water from the formation. Flow back water from a well stimulation and slop water on a platform can have properties that vary widely. These waters can have high solids, strong acids, solid stabilized emulsions which can foul and challenge water treatment systems. Flow back and slop waters are usually treated in batches, with temporary systems. The designs, operating cost considerations and treatment methods are different for these waters.
Baleen Process Solutions has developed a bi-modal membrane system for removal of solids, oil and other dissolved components from flow back and slop waters. Dissolved components are considered below 0.2 microns. Some membrane technologies are used in produced water as a polishing step with a wash cycle. However, because the flow back water is often treated with a temporary installation, the Baleen technology can be treated once the system is returned to its facility.
The Baleen system is impressive when comparing inlet and outlet samples of the flow back water. The laboratory method used, EPA 1664A, has a minimum reporting limit of 5 mg/l. The Baleen system removed from 300 mg/l to non-detectable with the bi-modal membrane technology.
Dispersed, Dissolved and Water Soluble
Water treatment systems are often judged on their ability to remove oil. To understand the technology and performance of a system, it is good to know how oil is measured and what the data generated means. With no clear definition of oil, the method used to analyze a water sample will define oil for that process and method.
There are many terms used in the discussion of oil that can have different meanings. Oil is often described as dissolved, dispersed, emulsified, free, total, etc. In the oil and gas industry, oil is relatively easy to define in a generic sense. Oil is the mixed hydrocarbon produced in the wells. However, oil can range from extremely light natural gas condensates up to heavy crude oil, sometimes referred to as bitumen. Gravity separators, adsorbant media and membranes as well as analytical methods will behave differently when different oils are present.
In simple terms, there are two conditions for oil in water, dissolved and free. Dissolved oils are molecularly incorporated and uniformly mixed in the water. Free oil is entirely in droplet form throughout the water mixture. However, in practice, oil may be considered to be dissolved because the oil droplets are sufficiently small enough that separation does not occur without additional treatment.
Emulsions by definition are any mixture of oil and water. Often oil which cannot easily be removed by the water treatment technology is referred to as an emulsion. In primary separation vessels, an emulsion can be water with 1–5 per cent oil present. However, after gravity based produced water system, high oil concentration upsets in the 50–150 ppm range are often referred to as emulsions because they cannot be separated.
The makeup of the oil can affect the emulsions. Natural gas condensates tend to have high BTEX concentrations, which have high solubility. With their low gravity, gas condensates separate quickly in gravity separators. Heavy bitumen doesn’t have gravity separate well, but has low solubility. The tertiary water treatment system must be selected to remove the type of emulsions or upsets which occur in the process.
Additionally, Organic Acids (water solube oils) are polar molecules present in most produced water that are highly soluble in water. WSO concentrations are usually stable after gravity separators because they are not removed.
Oil in Water Measurement
The common practice for analyzing a water sample is to add a specific volume of a solvent (10:1 ratio) and a strong acid. Next, the sample bottle is vigorously shaken for 2 minutes and then the solvent is analyzed for oil concentration after it has settled. The acid addition causes the polar hydrocarbons (including water soluble organics) to be readily extracted by the solvent. Some procedures then pass the extracted solvent through a silica gel so that only the non-polar hydrocarbons remain in the solvent.
In other methods, a specific sized filter is used to remove all large particles and droplets to understand the particle size distribution of the oil and solids.
The choice of solvent is important to the results. Solvents will have different extraction efficiencies for different hydrocarbons. The choice of solvent can have a significant effect on the presence of waxes and asphaltenes in the final analysis.
Many of the polishing filter systems describe the ability to reduce oil concentration to below 1 ppm, and remove BTEX and other light hydrocarbons. A common analysis method is the EPA 1664A method. This method is used for reporting oil concentrations in discharged waters to the local government environmental agencies in many parts of the world.
In simple terms, the EPA 1664A method uses hexane to extract a water sample, separates the hexane and allows the hexane to evaporate. The results are the weight of the oil residue remaining after evaporation. This method has a minimum reporting limit of 5 ppm. Because of the evaporation step light hydrocarbons are under reports by this method. This analytical method cannot be used for light hydrocarbon processes or processes that aim to achieve low concentrations. The figure included shows how the EPA method does not have any response to changing concentrations of a process, while an InfraRed technology does.
The procedures for preparing a water sample for analysis can be manipulated to best demonstrate the performance of a water treatment system. For instance, if a treatment system is designed to remove 99% of the oil and solid droplets that are 10 microns and larger, the sample can be prepared to differentiate particles larger than 10 microns. The method might first test a sample of water using a solvent and acid, then test another sample which is first passed through a 10 micro filter, and then the acid and solvent used to prepare the sample. When the results of the second test are subtracted from the first, the results will be the concentration of oil that was in droplets greater than 10 microns.
InfraRed analytical methods are often used for water analysis. The solvent used must be considered for the extraction efficiency, operating cost and solvent disposal. If the extraction solvent is a hydrocarbon, it must be evaporated before analysis of the remaining oil film can be performed. Similar to the EPA 1664A method, the IR analysis of light hydrocarbons will not have much resolution at low concentrations. Non-hydrocarbon solvents can be used without evaporation, allowing for better sensitivity to light hydrocarbons. For these solvents the analysis is made in the solvent for a quick result.
UV Fluorescence is also common for water sample analysis. All common solvents (hydrocarbons and nonhydrocarbons) can be used with UV Fluorescence. For instance, when analyzing heavy, high asphaltenic crude oils, toluene can be used as an extraction solvent. For highest accuracy, UV Fluorescence analyzers should be calibrated to the target oil in the process. However, when used with a generic calibration, the results can indicate the removal efficiency of a treatment system by comparing samples before and after treatment.
UV Fluorescence can operate with different light wavelengths. When analysis of light hydrocarbons or low concentrations is needed, a deep UV light source, with a short wavelength should be chosen. Most crude oils and light gas condensates can be measured below 1 ppm. However, when a heavy oil is to be measured and toluene is to be used as the extraction solvent, a light source with a longer wavelength should be chosen. Toluene can be measured by short wavelength light sources, but is optically clear to longer wavelengths and therefore not measured.
Conclusions
Advances in water treatment technologies are allowing discharge water quality to improve significantly. With these advances, the water can be re-used or recycled in ways not previously available. With many different technologies available to choose from, the operators must have a good description of their process, discharge requirements and operating budget. The analytical methods used for confirming the process are also improving. The oil in water measurement method can be modified to suit the treatment system to fairly depict the separation performance. To best monitor the process, the water treatment companies will need to define the performance of their system based on the ability to analyze the water samples.
Acknowledgements
Thank you to Xedia, Siemens, Prosep, Veolia and Baleen for supplying their polishing system performance and product photos