2 The 4 Technology Solutions
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An On-Line Version of a Column First Published in:
Environmental Technology Sept./Oct. 1997 Vol. 7 No. 5 Pages 25-26
by: David B. Vance
The Groundwater Column in the last issue examined the use of reactive barriers. This column will review one of the newer technologies available for the installation of reactive barriers, horizontal wells. In addition to chemical and biological reactive barriers, horizontal borings can be used for groundwater extraction and control, air sparging, bioremediation, groundwater injection, for vadose zone soil vapor extraction or bioventing systems, and free product recovery. Although, in areas where there is a great deal of fluctuation in the groundwater table, use for free product recovery can be problematic.
Horizontal well technology was first utilized in the late 1920s by the petroleum industry. In the 1970s the technology was applied by utility companies to cross rivers and other natural or manmade barriers. The use of horizontal drilling technology for environmental applications began in the late 1980s and has escalated in the 90s.
Horizontal wells offer distinct advantages over trenching or vertical wells. Trenching produces massive quantities of excavation spoils, and is economically limited in depth of application. In addition, horizontal wells can be installed beneath structures and other surface obstructions that would be impossible to access using trenches or vertical wells.
Horizontal wells have a unique advantage that originates in the geometry of the typical contaminated groundwater system. Horizontal permeability is on the order of ten times greater than vertical, due to the stratigraphic layering of near surface soil horizons. Causing contaminant plumes to spread horizontally. Horizontal wells can be installed through contaminated zones, along their leading edges, or along a property line. All active groundwater remediation systems rely on the mass transport of water, air, or other chemicals. Mass transport is induced through the interface offered by the screened sections of a well that are exposed to the contaminated strata. Compared to vertical wells, horizontal wells can increase well screen interface by an order of magnitude or more.
Various drilling heads and cutting removal technologies have been applied to horizontal drilling. Fluid-cutting systems, mechanical cutting, augers, percussion drilling, and sonic methods are all available. The selection of which is dependent upon the geology and competency of the subsurface (and the selected technology vendor). With appropriate techniques, bore holes may be extended through difficult conditions such as gravel formations, coral reefs, even boulders or bedrock.
The removal of cuttings is usually based on mud slurry systems for installation beneath the groundwater table and air systems for installation in the vadose zone. The formulation and engineering of a mud slurry system takes great care, historically this has been one of the most common points for the failure of an installation. The mixture must be capable of holding the hole open, removing the cuttings without erosion of the walls of the well bore, and capable of decomposing with time to restore the well bore to permeable operation.
Inherent with the horizontal drilling process is the ability to accurately direct the placement of the horizontal well bore. There are two dominant methods of achieving directional control of the drill head: magnetometer/accelerometer and radio beacon. A magnetometer/accelerometer array is expensive and the long configuration of an array makes them prohibitive for short radius bends. They are also subject to magnetic interference which can be a serious issue around tanks or other surface or near surface structures (i.e. buildings, piers and piles, pipelines, or utility runs). Radio beacons previously have been limited to a depth of 25 feet or less, although there have been instances where down hole wirelines have been used to increase accessible depth.
Well casings for horizontal wells must be more highly engineered than those used for vertical wells. They must have great tensile strength to withstand the significant forces associated with installation. Compressive strength is required to resist the overburden load as the horizontal well bore collapses with time. The screen pack is also a critical issue with regards to installation and subsequent operational efficiency. Conventional sand packs can be installed, but they require great care. Many vendors now use prepacked screens. These are nested screens, an exterior screen (typically HDPE), a packing system (sand, filter cloth, or other filter media or mixtures of media), and an internal screen typically constructed of stainless steel. These prepacked screens are stiffer and subject to larger turn radii, but obviate the difficulties associated with installing a conventional sand pack in a horizontal boring.
Horizontal drilling is a relatively costly process to utilize. It is use is driven by necessity, or the economic advantages of scale. Necessity drivers are generally the need for the installation of a remediation system underneath a structure or facility that can not be disturbed. Economics of scale come in to play with regards to large contaminant plumes. A single horizontal boring can replace 10 to 30 vertical wells in a plume of large aerial extent. The break even point for the choice between horizontal or vertical wells occurs around the need for 4 to 5 vertical wells. Installations requiring more than 5 vertical wells can often be more cost effectively addressed using horizontal wells.
There are two basic methods of horizontal well installation. The simplest involves two ends, boring downward to near the desired depth at one end, curving the boring to the horizontal, traversing the required path and distance horizontally, and then recurving to the surface at the other end. The casing is then pulled back through the boring from the distal end. A more sophisticated approach is to use a single end, through which the boring is advanced, followed by the casing being pushed through the boring, a more difficult, risky, and time consuming process. Horizontal drilling using the two-end approach can cost as little as $30 to $40 per linear foot, the cost for singled ended systems start at $80 to $90 per linear foot.
As with many other technologies applied to the environmental industry, developers over the last decade have focused on the creation of a "just good enough" technology. Placement accuracy is sufficient, boring sizes are minimal, and construction materials are designed for the limited life of the project. Successful installation of horizontal wells depend upon: the selection of a vendor who is flexible enough to utilize an approach that is appropriate for the subsurface condition of your site; who is experienced in the use of the specialized drilling equipment and screens; who can engineer the application as well as the installation procedures; and who is experienced enough to be quick. Aside from failure due to inadequate mud engineering, taking too long is another key source of failure, the well bores simply cave in before installation is complete.
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