Horizontal directional drilling (HDD) has been successfully used in drilling oil and gas wells, and more recently in installing water and sewer lines beneath busy city streets and under environmentally protected areas such as palustrine wetlands, estuaries, lagoons, natural levees and tarns. The practice of using HDD for installing remediation wells began in the 1990s to counter the escalating costs of traditional remediation methods such as soil excavation and pumping and treating ground water.
When a gas or oil well has completed its useful service life, it is usually decommissioned and shut down to prevent the leakage of fluids and gasses and to isolate it from aquifers. Sometimes, contaminants such as spilled oil or gasoline are left undiluted beneath the surface. This can be effectively removed using HDD remediation techniques such as air sparging, in-situ bioremediation and soil vapor extraction.
In 1993, Directional Technologies Inc. successfully used a mid-sized directional rig to install three horizontal soil vapor extraction wells at John F. Kennedy International Airport. As a result, today there are over 1,000 horizontal remediation wells worldwide. Over the years, horizontal remediation wells have proven to be a solution to many problems that are faced during cleanup at remediation sites in addition to being environmentally safe. (For more on wells, check out The Essentials to Drilling and Boring Wells.)
Unlike vertical wells, horizontal wells that are dug horizontally beneath the earth’s surface are able to make contact and draw out contaminants from a greater area. A single horizontal well can replace a number of vertical wells that will be required for the same area, making it a very economical and time saving option. Other benefits include less interruption to surface activities, multi-dimensional screen exposure, faster site closures and low operational and maintenance costs. Some well remediation methods are:
Soil Vapor Extraction
This is a technique that uses vacuum to extract the contaminants. The horizontal bore has two sections: one that extracts the vapors, and the other section to carry the vapors safely to the surface.
Soil Venting
This technique works by injecting air into the groundwater or soil at low psi and volume. This helps in greatly increasing the biological activity of bacteria present in soil, consequently helping in removing contaminants.
Air Sparging
Air sparging is carried out by distributing air through the area of contamination. A horizontal air sparging system is installed to induce better soil vapor extraction. As air passes through the water, it separates and mobilizes the lighter contaminants while steam mobilizes the heavier contaminants for collection and treatment.
HDD for Water Wells
Water wells or borewells have traditionally been dug using vertical downward methods. However, there is another way down that has recently picked up some steam, and that is digging water wells using HDD. The ability of HDD to navigate below the surface, beneath and around obstacles and cause minimum surface disruption was considered for use in water resource development projects. The bore is started at an acute angle from a point on the surface and guided using electronic guiding technology to a water source which may be subsurface or at a remote location on the surface.
The path for drilling is predetermined using geotechnical and soil investigations. Though traditional vertical well digging may never stop, using horizontal wells has its many benefits, of which is a favorable screen-to-riser ratio and access to a greater volume of aquifer. (For more on geotechnical reports, see Getting Technical: Information Required in a Geotechnical Investigation Report.)
HDD is very advantageous in typical situations such as availability of surface water bodies such as lakes and reservoirs, fractured bedrock, perched, thin and shallow aquifers and aquifer recharge. Some of these situations are discussed below.
Surface Water Bodies
Some communities are situated right next to large water bodies such as rivers, lakes and reservoirs. Horizontal wells can be dug from the surface to penetrate the shallow sediments or alluvial deposits beneath them, giving the additional advantage of natural filtration through sand and gravel while preventing scouring or damage and disturbance to aquatic life. For additional capacity, gravity drained radial horizontal wells can be installed that drain into a central vertical well.
Fractured Bedrock
It is a practical difficulty and operational hazard to install vertical wells and safely reach an available water source in mountainous regions. Some bedrock aquifers have vertically running water veins and some water sources are in unstable landslide masses, in which cases HDD can be a great alternative to traditional well digging. Difficult drilling conditions such as those encountered in fractured bedrock and landslide masses can be accomplished using steerable air hammers. HDD can also be used to drill gravity systems that require no pumps for operating them.
Perched Aquifers
Communities around coasts, islands and deserts often have to depend on aquifers that are thin, shallow or perched or located in buried stream channels. This can prove to be a problem for vertical wells, as the water available for usage will be limited unless multiple wells are dug at the location, resulting in cost escalation. HDD can be used to dig a single horizontal well that may be several feet long while intersecting it for almost the entirety of its length. Compared to a vertical well, and even multiple vertical wells, the screen-to-riser ratio is very favorable.
Digging vertical wells is a standard, effective way for reaching a known groundwater source and will always be around as a proven method for obtaining water. However, some situations, such as those mentioned above, may not be the ideal condition to install vertical wells, and that’s where HDD can prove to be an economical and safe way to reach and procure groundwater for human use.