Horizontal Directional Drilling:

The technology for extracting groundwater from subsurface resources hasn?t advanced appreciably since drillers abandoned shovels and buckets for mechanized well drilling rigs. Shovels gave way to cable tool rigs in the early 1800s and mechanical improvements incrementally led to current well-drilling technology, typically rotary drive rigs with automated casing drivers. Despite mechanic?al advancements in advancing a borehole, however, the extraction technology has remained the same: drilling and casing a vertical bore that intercepts a water table. Even the venerable cable tool rig is still used for this purpose in some locales.

Meanwhile, a relatively new technology, shallow horizontal directional drilling (HDD) for utility and pipeline installation, has evolved rapidly over the past three decades. Unlike the deep directional bores used in oil exploration and production, which can extend thousands of feet deep, shallow HDD is performed from just a few feet to a few hundred feet beneath the ground surface. Shallow HDD bores typically are started from the surface at an acute angle, and are guided to a subsurface or remote surface target using sophisticated electronics packages that enable the driller to navigate and steer the drill along a desired path.

In the mid 1990s, this technology was adapted to install wells for the remediation of contaminated industrial sites, military bases, gas stations and other locations. The shallow depth capabilities, combined with the ability to steer a bore beneath obstacles along a predetermined path has greatly expanded the ability to access contaminated zones with appropriate treatment technologies.?

More recently, HDD has been applied to water resources development projects, with promising results. Vertical drilling is likely to remain a mainstay of the water industry, but HDD can be advantageous in some cases. This article explores some of the more challenging water development scenarios, and how HDD can provide a viable solution.

Common to all of these water resources solutions are their utilization of HDD?s key benefits. They require only limited surface disruption ? from a single, relatively small construction footprint, screens may be set tens of feet deep, while extending laterally for hundreds of feet in length, with no other effect on the surface environment. For wells that extend beneath fragile or protected ecosystems, this means that no roads must be constructed for installation or ongoing access for maintenance.

Another advantage of horizontal wells is their favorable screen to riser ratio. A horizontal well only needs to penetrate the overburden above the aquifer once (twice in the case of a double-ended well). This surface penetration is offset between four and five times the target depth of the well. Once at depth however, the length of the screened interval is only limited by the rig capacity and local geology.

A shallow, 50-foot deep well takes approximately 200 feet to achieve the target depth, but may be hundreds or even in excess of 1,500 feet in length. In contrast, a network of vertical wells drilled to intercept the same volume of aquifer would require multiple rig setups, multiple wellheads, and hundreds of feet of non-productive riser casing.

Thin, Shallow or Perched Aquifers

Many coastal or island communities, and some desert municipalities, must tap thin, shallow or vertically constrained perched aquifers for their water supply. In some locales, the best groundwater reserves are situated in thin, sinuous buried stream channels. Production of viable quantities of water from these formations can be challenging with vertical wells ? drawdown at even minimal pumping rates quickly exhausts the supply adjacent to the well, with the potential for damaging the formation. Multiple wells, spaced at intervals and pumped at lower rates may provide a more continuous water supply, but are costly to install and maintain. In coastal or island communities, saltwater intrusion is a classic problem ? overpumping of the ?bubble? of potable water floating above a brackish or saline layer can cause irreparable damage through saltwater intrusion.

In contrast, horizontal wells are ideally suited for these aquifers. A single horizontal well may be several hundred, up to a couple thousand feet long, intersecting the aquifer for most of its length and spreading the cone of depression along the full length of screen. The ratio of productive, screened well to unscreened riser casing is many times that of a single vertical well, and even more favorable compared to a network of vertical wells. Further, the horizontal well requires only one pump, and limited transfer piping to convey the water to distribution or treatment facilities. A vertical well network contains not only a high ratio of non-screened, non-productive casing, but each well requires its own pump, and the water must be conveyed to a central facility with multiple trenched pipelines.

Groundwater in Connection to Surface Water

For communities adjacent to large bodies of surface water, such as lakes, reservoirs or rivers, water may be obtained directly from the body, or from multiple shallow wells installed along the shoreline. More sophisticated solutions include the Ranney collector type of well, with radial, horizontal laterals connecting to a central caisson.

Fisheries agencies are becoming more demanding in the construction and ongoing maintenance of intake weirs that directly pull water from rivers and lakes, as protection of fish habitat is becoming an increasingly contentious issue. These structures are also prone to damage from storms and flooding. Vertical wells along shorelines have similar issues to the wells in thin aquifers noted above, in that their productive zones are limited and it usually requires many wells to achieve useable volumes. The Ranney collector system, first developed in the 1930s, is an effective technology to capture groundwater in connection with surface water, but the systems themselves are generally expensive to construct, with costs often exceeding $1 million for a relatively modest system.

Horizontal directional drilling provides an elegant solution in these scenarios. One or more horizontal wells can be drilled from the surface, extending into the alluvial deposits or shallow sediments beneath surface water bodies. These wells take full advantage of the natural filtering capacity of the in situ sand and gravel, and protect the intake from flood damage or scouring. If additional capacity is required, multiple radial horizontal wells that gravity-drain to a central, vertical wet well can be installed. A Ranney collector system requires manned jack-and-bore operations at the bottom of the caisson, with attendant requirements for dewatering and accommodation for worker safety. In contrast, since all of the work for a radial HDD collector is performed from the surface, the wet well can be of considerably smaller diameter than a Ranney collector caisson and none of the risks and costs of manned operations at depth are incurred.

Fractured Bedrock in Mountainous Regions

Mountainous terrain can pose both construction and operational challenges for water suppliers. On steep, sometimes unstable slopes it can be difficult to safely place a vertical drill rig in a position to effectively intercept water-bearing formations. In some fractured bedrock aquifers, the water-bearing joints run vertically, exacerbating the problem. In other areas, water production is from landslide masses that are relatively unstable. In more remote locations, getting power to the site to operate pumps is problematic.

Horizontal directional drilling can provide a solution to many of these challenges. New advances in steerable air hammers has enabled the installation of wells in extremely difficult drilling conditions in fractured bedrock, landslide materials and buried talus. The ability to design a bore path that intersects known fracture systems can enhance water recovery. Further, horizontal directional wells can be drilled as gravity systems, requiring no pumps for operation. For smaller water systems that rely on natural springs as a source, HDD can be an excellent option to enhance recovery and maximize utilization of water rights.

Aquifer Recharge and Brine Infiltration

Point source discharges of potable water from water treatment systems are highly regulated and usually difficult to site. These include outfalls from municipal sewage plants, or discharges from facilities for treating industrial or remediation site wastewater. Permitting and design of such discharges must consider site ecology and wildlife impacts, erosion control, protection of the discharge structure from flooding or severe weather damage, and a host of other factors.

Subsurface discharge of treated water can simplify permitting and design considerations. Horizontal wells can extend for hundreds of feet within a receptive aquifer, and can be designed to disperse high water volumes with slight impact to surface activities or ecosystems.

At Sand City, Calif., a 750-foot horizontal infiltration well installed parallel to the beach and 50 feet below the low tide mark handles the entire waste brine stream from the city?s 300 acre-foot brackish water desalination plant. The well is drilled beneath protected Monterey Bay coastal dunes, which serve as habitat for endangered bird and reptile species. The horizontal well enables the waste brine (at nearly natural salinity as discharged from the plant) to mix with natural seawater beneath the sea floor before entering the bay, eliminating ecological issues related to potential salinity imbalances, and requiring no outfall structures, which were prohibited in the area.

Conclusion

Although vertical well drilling is a proven method to tap groundwater resources, and will continue to be a standard technology for water supply, HDD has made significant advances for water supply in unusual or difficult scenarios. If conventional drilling isn?t the answer to your water supply problem, lateral thinking may lead to better results.

Michael Lubrecht, L.G., is a Senior Geologist and Dan Ombalski, PG, is President of Directed Technologies Drilling.

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