Horizontal wells and directional drilling have many technical advantages over standard vertical wells for environmental remediation projects.
In the 1990s, the U.S. Environmental Protection Agency reported that approximately 75 percent of all soil and groundwater contamination was related to petroleum hydrocarbon spills and about 75 percent of that contamination occurs within 50 feet of the ground surface. Very often, contaminants released from leaking tanks or pipes or from surface spills migrate downward until they hit an impermeable layer in the soil or the ground water table. Because of this, we often find contamination spread out over large areas in relatively thin layers. The contaminants form a horizontal plane of contamination.
Environmental engineers have used vertical wells for many years as their primary access to the subsurface. When contaminants are planar in nature, the vertical wells intersect only a single point in that plane. Because a well can only influence a limited distance from the point it intersects the contamination, it takes many points in the plane to achieve effective coverage for a given plume. Many contaminated sites have literally hundreds of vertical wells installed to intersect the contaminant plume. For example, at a petrochemical plant in Louisiana, one owner installed 256 vertical wells at a cost of millions of dollars to capture a single plume.
Horizontal wells allow engineers to add a new dimension to their cleanup strategies. Instead of many vertical wells, a few horizontal wells can often suffice to form a line in the contaminant plane. Instead of a circular zone of influence around a vertical well, we form a horizontal zone of influence along the axis of the horizontal well. At the Louisiana site mentioned above, the 256 vertical wells were replaced with just seven horizontal wells.
Engineers have tried trenching as a means to install linear treatment systems. While effective as infiltration devices, disturbing the soil while putting in the trench often creates additional problems—soil vapor extraction systems or air sparge systems will often “short circuit” through the backfill in a trench, reducing the effectiveness of the system.
Vertical air sparge wells have been used extensively to increase oxygen levels in ground water. Often they are placed in a “picket fence” arrangement where they form a line downgradient from a contaminant plume. The theory is that the contaminated groundwater will flow past the fence of vertical wells and the contaminants will be degraded either through volatilization or biological activity. In practice, however, these installations have serious limitations. Most often, vertical air sparge wells affect only a very small zone around the wells. The disturbance of the soil column necessary to install the wells creates a short circuit to the surface. Most of the air sparged into the ground goes up the immediate vicinity of the borehole. In addition, the air that does escape into the soil forms a conical shape widening as the air moves vertically upward. The net result is a fence with many gaps at the bottom. A single horizontal well creates a continuous wall of oxygenated air across the horizontal contamination plane. There are no gaps and no short circuits.
In the past, contaminants under surface obstructions have been largely neglected in most cleanup efforts due to prohibitive costs and limited chances of success. This was particularly true under sites—airports and major highways are good examples—where operations at the surface precluded disruption or drilling equipment. Horizontal wells can be installed with little or no interference to surface operations. We can install them under buildings, ponds, landfills, roads, runways, wetlands, and pipelines. They can be very long (the longest wells are nearly 3,000 feet long) or very deep (the deepest horizontal wells have been installed over 250 feet deep). We can install horizontal monitoring wells under sites where vertical wells would potentially cross contaminate the subsurface.
Vertical wells and associated wellheads and conveyance lines must either be placed in vaults, or they protrude above ground, getting in the way and calling attention to the subsurface contamination. Trenching can tie up a site for weeks. Horizontal wells can be installed at an active site with little or no disruption of site activities.
A soil vapor extraction well eight feet deep by 400 feet long will generate 345 cubic yards of spoils, if trenched. The same well, if drilled horizontally using HDD, will generate about eight cubic yards of waste. At $50/ton for disposal, horizontal wells will save your project over $20,000.
A single horizontal extraction well can eliminate up to a dozen pumps from a treatment system, compared to a vertical well network. Or, better yet, a single horizontal biosparge well can eliminate the treatment system as well! At sites where risk-based analysis indicates that only minimal cleanup is warranted, horizontal biosparge wells provide an excellent, low cost means to treat residual contamination, hot spots, or contamination that is migrating off-site.
Why install multiple vertical wells, then spend even more to connect them using trenched conveyance lines, when you can install a single horizontal well with an integrated conveyance line?
Contact Directed Technologies Drilling, Inc. to learn more about how to best take advantages of horizontal wells for your environmental remediation project.