For sites with high permeability geologies that have high groundwater flux rates, effective heating can be a challenge. For these sites, Steam Enhanced Extraction (SEE) may be the best technology fit, either by itself or in conjunction with other heating technologies such as Thermal Conduction Heating (TCH) or Electrical Resistance Heating (ERH). If you are considering thermal remediation at a site with high groundwater flux, then read on and we’ll provide an overview of what SEE is, how it works, what it can treat, and how it is applied.
What to learn more? Register for next week’s webinar, What is SEE and how is it Applied where my co-worker Nikole Huard and I will discuss the basics of SEE and provide you with examples of SEE projects we have done.
What is Steam Enhanced Extraction?
SEE is the injection of steam into the subsurface combined with robust liquid and vapor extraction. The steam is injected into a series of steam injection wells (SIWs) distributed throughout the treatment zone. Typically, a ring of SIWs is also located around the perimeter of the treatment zone to create a steam barrier to prevent COCs from being pushed to the outside and assist with hydraulic control. The liquid and vapor are extracted via a network of multiphase extraction (MPE) wells located within the treatment zone. The MPEs provide both hydraulic and pneumatic control and remove the COCs from the targeted volume.
Interesting fact: the amount of water that must be extracted by the MPEs to ensure hydraulic control ranges between 1.5 and 2 times the amount of water injected as steam, depending on the groundwater velocity at the site. For example, if 1,000 lbs. per hour of steam is injected, which is equal to 2 gpm of water, then 3 to 4 gpm must be extracted to maintain hydraulic control.
How does Steam Enhanced Extraction work?
The SIWs consist of small diameter, easy-to-install, riser pipe and short sections of screen. Typical pipe and screen diameters are 1 or 1.5 inches, with the screen lengths typically ranging between 1 and 5 ft. The short screen intervals are necessary to ensure the effective horizontal spread of the steam radially before it rises due to buoyancy. If short steam injection intervals are used, the spacing between the SIWs can range from 20 to 40 feet for typical treatment depths and hydraulic properties, namely the vertical and horizontal hydraulic conductivity and the ratio between the two, or anisotropy of the formation.
Initially, the steam injected into the cold treatment zone condenses at the SIWs. Gradually, as more and more energy and heat are injected and transferred to the formation through the steam condensing, the formation around the SIWs heats up and eventually reaches steam temperatures (variable depending on steam injection pressure and hydrostatic pressures). As this progressively occurs, a steam front moves out radially around the wells, pushing a condensate front consisting of condensed steam and COCs, and contaminant vapors toward the MPE wells located in between the SIWs. This is why the MPE well system must be designed to extract 1.5 to 2 times the amount of water injected as steam in order to maintain effective hydraulic control. Not only does the injected water as steam need to be captured, but also the water that is displaced by the steam zone along with any water migrating into the treatment area under natural gradients. Vapor extraction at the MPEs safely removes any volatilized COCs. Eventually, the condensate and steam front reach the MPE wells, and steam breakthrough occurs.
Unlike at TCH and ERH sites, at typical SEE sites, MPE and treatment system inlet concentrations and mass removal rates are relatively low during heat up until the condensate front arrives at the MPEs and steam breakthrough occurs. Then, COC concentrations and mass removal rates are very high.
Following steam breakthrough and after mass removal rates begin to subside, a series of pressure cycles are performed to ensure contaminant removal from low permeability zones. Rapidly decreasing the steam injection pressure results in a sudden decrease in the pressures within the formation. This creates a thermodynamically instability and water present in the low permeable zones rapidly boils and steam strips any COCs present, pushing them into the permeable portions of the formation for extraction.
What COCs can be treated with Steam Enhanced Extraction?
Permeable sites can be heated effectively with SEE to temperatures at or above 100°C, depending on hydrostatic pressures. Thus, SEE can be used to treat VOCs, CVOCs, BTEX, naphthalene, chlorobenzene, trimethylbenzene, and other low boiling point SVOCs. SEE can also be used to remove and recover LNAPLs such as petroleum hydrocarbons and coal tars and creosote. Significant reductions in the viscosity of NAPLs can be achieved by heating them to ~100°C, making them much more recoverable by pumping than at ambient temperatures.
What sites are suitable for Steam Enhanced Extraction?
For SEE to be effective, the hydraulic conductivity of the formation must be high enough to allow efficient injection and migration of the steam. Typically, geologic formations with hydraulic permeabilities greater than 10-3 cm/sec or 2.8 ft/day are suitable for SEE. Note, sites with a combination of low and high-permeable strata (e.g., silts and clays interbedded with sand and gravel units), can be treated using a combination of thermal remediation technologies. For example, TCH or ERH can be used for the low permeable strata while SEE can be used for the high permeable strata. These kinds of combined technology remedies can be a very cost-effective way to treat sites with complex geologies.
Steam, contaminant vapors, air, and liquids removed by the MPEs are directed to a treatment system where cooling, condensing, and phase separation are performed to create separate vapor and liquid streams for effective treatment and removal of the COCs. Treated air and water are then discharged to the atmosphere and POTW, respectively.
Hopefully, you now have a better understanding of what SEE is, what it can treat, and how it is used to treat sites with high groundwater flux zones. If you would like to learn more about SEE and how it’s applied, register for next week’s webinar: What is SEE and how is it applied?
We would also love to hear from you, so please don’t hesitate to contact us with your questions about SEE or ISTR in general!
Steffen Griepke is the Vice President of Technology at TerraTherm. In this role, he helps clients determine if a thermal remedy would work on their project site and, if so, which thermal technology or combination of technologies would be the most cost-effective. He then leads the design of the remedy and guides its implementation, operation,…