ETL 1110-1-178
10 Dec 96
b. Horizontal wells are particularly suited to recovering contaminants distributed as broad,
flat layers. Contaminant plumes usually are horizontally extensive and relatively thin vertically.
Such a distribution may occur when light non-aqueous phase liquid (LNAPL) floats on a water
table or when dense non-aqueous phase liquid (DNAPL) accumulates on a low-permeability bed.
With the use of directional drilling, a screen can be placed parallel to the contaminated layer,
whereas the screen of a vertical well may only intersect a very small portion of the contaminated
layer. Therefore, horizontal wells better mimic this plume geometry and place more screen in
contact with the target zone on a per well basis.
c. In tight formations, where vertical fractures provide the primary flow paths, horizontal
wells can intersect many vertical fractures. This application facilitates access to the preferred flow
paths, increases well discharge, and controls fluid flow in the formation.
d. Some advantages of horizontal wells are unrelated to hydrologic performance. In many
locations, such as beneath landfills, tanks, buildings, roads, lagoons, or bodies of water, access
limitations prohibit entry by a drill rig, prevent penetration by a vertical hole, or restrict the
aboveground facilities necessary for recovery operations. As a result, recovery, sampling, or
monitoring with conventional drilling technology is difficult beneath many structures that may be
sources of contaminants. Horizontal or inclined wells overcome those difficulties by allowing the
drill rig to be adjacent to the obstructing structure, and the well boring to be created beneath it.
In addition, horizontal or inclined wells help keep the above-well system components out of the
way of such things as landfill equipment, service station traffic, and the local wildlife habitat.
6. Actions Required.
a. Because horizontal wells have only recently been applied to solve environmental problems,
there has been relatively little information published concerning the benefits and application of
horizontal drilling technology. One of the most comprehensive pieces of information on
horizontal wells can be found in EPA/625/R-94/003. The manual covers horizontal well
construction, design considerations, applications, and case histories.
b. This guidance should be carefully considered by all staff involved in groundwater and soil
remediation. Horizontal directional drilling has applications for ground-water remediation,
leachate collection, in-situ air stripping (sparging), in-situ bioremediation, soil vapor extraction,
thermally enhanced vapor extraction (steam injection), and grouting. Additional references are
listed in Appendix A. Appendix B is a list of some Internet web sites relating to horizontal
drilling.
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