Technology Overview For Horizontal Remediation Wells
Industry analysts indicate that hundreds of horizontal remediation wells are installed each year. Applications included air sparging, biosparging, bioventing, soil vapor extraction, landfill leachate collection, landfill gas collection, free product recovery, and sampling for site assessment.
Horizontal remediation wells are screened bores installed along or across subsurface contaminant plumes. They interact with the surrounding formation and are essential components of remediation systems. HRWs can be used to extract liquids and vapors or to introduce air, reactive chemicals, or biological stimulants.
Horizontal wells differ from horizontal borings in that borings are not screened and provide only a conduit for water, gas, power, or fiber cables. Borings are passive and the placement of borings is less critical than HRWs. Borings do not interact with the formation; horizontal remediation wells do.
A typical drill bit that is used on a horizontal remediation well project.
Horizontal remediation wells can be used in the remediation field to lower the cost of a project; access areas that cannot be accessed using vertical wells or trenches; and reduce the number of pieces of aboveground remedial equipment such as pumps, blowers, and carbon canisters.
The drilling fluids, drill methods, development requirements, and other facets of horizontal wells and horizontal well installation are vastly different from those associated with horizontal bores even though the same drill rig may be used for both. The Directional Technologies team is skilled and experienced at installing horizontal remediation wells properly and understands the differences between boring and HRWs.
While there is no universally applicable optimum design, a typical design employs 4"-diameter HDPE and 400' of screened interval. Longer screened intervals generally require larger diameters. Shorter screened intervals can sometimes be installed with smaller diameter well materials.
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Benefits
Some of the benefits of directionally drilled horizontal remediation wells include:
- Maximum exposure to the contaminated zone, either along or across the plume
- A zone of influence greater than several vertical wells
- Construction that does not disrupt ongoing commercial activity
- Minimum site restoration
This drill rig was set up behind the gas staion pumps. The horizontal wells were installed without disturbing the business.
Performance
Directionally drilled horizontal remediation wells may dramatically reduce the time of clean up, increase the sphere of influence, and decrease the disruption to a site. For example, horizontal air sparge wells have shown zones of influence 70 feet perpendicular to the well and soil vapor extraction wells have shown 120-foot perpendicular zones of influences. These may reduce remediation time while reducing maintenance, equipment and operation costs.
Limitations
The price of the project is determined by several factors, such as the length and size of the well, the expected ground conditions, and the contaminant. If the expected ground conditions are bedrock, or other hard to drill conditions, horizontal remediation wells may be cost prohibitive.
Design Rules of Thumb
Every site is unique and every remediation technology should be selected to draw on the experience of the engineer and contractor as well as meet the needs of the client and address the characteristics of the site. With this as a caveat, the following Rules of Thumb have been described by HRW designers and constructors:
- Soil Vapor Extraction (SVE) wells are generally constructed to withdraw 2 cfm per foot of screen. Blowers and compressors must be sized accordingly
- Air Sparge wells are usually constructed to inject 1 cfm per foot of screen
- Bio-sparge systems are usually constructed to inject 0.5 cfm per foot of screen.
- A single horizontal well will often cover the same area and provide the same treatment as 10-50 vertical wells, depending upon site conditions.
- For air sparge and biosparge applications, a horizontal well will often have a zone of influence approximately three times the radius of influence (as measured at the water table) of a vertical well with the same screen depth.
- A horizontal soil vapor extraction well will have a much greater distance of influence than an air sparge or biosparge well placed in the same stratigraphy.
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