Horizontal Remediation Wells
Drilling and Well Construction
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Types of Horizontal Wells |
Steering the Drill Bit |
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Installation of Casing and Screen |
Well Development
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Prior to drilling vertical or horizontal remediation wells (HRWs), the site must be adequately characterized to determine suitable well placement with respect to contamination, groundwater flow, geologic features, receptors, etc. However, design and construction of HRWs is typically more complex. HRW design is highly variable due to factors such as design flow rate, length of the screen, total depth, total well length, soil type(s), and the type of remedy. In addition, the borehole path must be mapped out based on contaminant plume location, treatment objective, site geology, accessibility for drilling equipment, and the allowable radius of curvature of the drill pipe and well materials.
The well materials for HRWs are like those of vertical wells (e.g., stainless steel, carbon steel, and polyvinyl chloride [PVC]). Both slotted and wire-wrapped screens are available. The slot size and design of the well screen will depend on the grain size of the formation, other subsurface conditions, and well performance parameters, but are selected to maximize groundwater or soil vapor flow while restricting entry of particles. Design also will depend on the well application, such as injection or extraction of water or materials, their density, and the required injection or extraction rates. Note that various slot sizes may be selected for use along the length of the well to ensure uniform recovery or placement of chemicals or amendments.
Advancing Drill Rods at an Angle.
(Courtesy of Ellingson-DTD)
In horizontal directional drilling (HDD), a drill rig advances the drill string in the ground while the bore path is controlled using one of several locating techniques (discussed below). Borings are advanced to the desired depth at a shallow angle, typically 10 to 20 degrees from horizontal. Depending on target depth, the entry and exit bores might have to be located a significant distance away from the treatment area. The boring's radius of curvature defines how tightly the boring curves from the point of entry to the desired depth of installation. A larger build radius in the well design puts less stress on the drilling equipment and well materials but requires a longer boring and larger setback distance. The rule of thumb is a minimum of 5 feet of setback for every foot of depth. This may restrict the usefulness of HDD where contamination is deep and access is limited; however, HRWs can be drilled to 200 feet or deeper.
Continuous and Blind Wells. (EPA, 1997)
An HRW can be installed as a continuous well using both an entry hole and an exit hole, or as a blind well by using an entry hole only. To install a continuous well, the drill penetrates the ground surface at the entry angle, descends to the desired depth, traverses the desired distance, rises to the surface, and exits the ground. The drill rig is then used to pull the well casing into place through the exit hole. For a blind well, the drill enters the ground, descends to the desired depth, traverses the desired distance, and is withdrawn from the borehole. The drill rig is then used to push the well casing into place through the entry hole. A knock-off installation method has been developed in which a 5-inch drill pipe and drill bit assembly are advanced with 3- or 4-inch well materials. The well materials lock into the bit, and the drill pipe is withdrawn from the hole, leaving the bit behind.
Duckbill Bits. (Courtesy of Ellingson-DTD)
Schematic of a Walkover Locator to
Track a Horizontal Drill Bit. (EPA, 2017
)
The drillhead, which is positioned at the leading end of the drill string, includes the drill bit – the most common being the duckbill bit – and electronic locating equipment. Using the electronic locating equipment as an aid, the operator steers the borehole in the direction of the bit's orientation. When the bit is rotating, the borehole advances in a straight path. Halting rotation of the assembly steers the borehole to the left or right based on the bit's orientation.
The ability to know precisely where the drillhead is located underground is vital to the successful installation of a HRW. Examples of locating technologies used in HDD:
- Walkover locators are most often used in HRW installation for installations less than 80-100 feet deep. A battery-operated transmitter called a sonde transmits wireless radio signals from the drillhead behind the bit to a handheld receiver carried by a crew member on the surface. These signals inform the driller about the drill bit's location, depth, and orientation, allowing the driller to adjust the drill bit to follow the desired borepath.
- Remote magnetic systems have a sensor in the sonde that senses its location and orientation relative to a magnetic field, either the Earth's natural magnetic field or an induced magnetic field.
- Gyroscopic steering tools, a highly accurate type of HDD locating technology, can work at depths greater than 100 feet and do not require any wireless communication or surface access. Optical gyroscopes and accelerometers track changes in the drill bit's position and orientation. Note that because it is more expensive than a walkover locator or remote magnetic systems, it is a more significant loss to lose a gyroscopic steering tool in the drilling process.
Depth, borepath, cost, and potential for electromagnetic interference (e.g., due to buried drums, pipes, or reinforced concrete) will determine which strategy is selected.
HDD rigs pump drilling fluid down the borehole while drilling to maintain borehole stability, remove cuttings, cool the drillhead and bit, and lubricate the well materials to be installed. Bentonite clay-based drilling fluid is often used in general HDD applications, but several manufacturers have developed drilling fluids, such as biopolymers, specifically for HRWs. There is a higher potential for borehole collapse in HRWs than vertical wells. Drillers with extensive HDD experience will be able to most effectively select and use drilling fluids based on site-specific conditions and needs. For instance, a soil vapor extraction well might require a drilling fluid that degrades quickly, such as a biopolymer, to expedite testing and use.
Installation of Casing and Screen
Examples of HDPE Well Screens Showing Vertical and Horizontal Slot. (Courtesy of Directional Technologies, Inc.)
Prior to installing a well, the casing and screen materials need to be inspected to ensure the correct size was delivered and the materials were not damaged during shipping. The well screen and casing are pushed or pulled into the borehole, so the well material must be able to withstand the compression or tension as well as the curvature of the borepath. Care must be taken to avoid damage to the well screen through abrasion or breakage.
EniroFlex Well Screen. (Courtesy of Directional Technologies, Inc.)
Installers use software (often proprietary) to model fluid flow to optimize well screen selection for site-specific conditions and applications. Effective distribution of fluid across the screened length of a HRW used for injection or extraction is necessary for optimal operation. Distribution can be achieved by varying the length and/or apertures of screen slots. These slots may be oriented vertically or horizontally, depending on the well design and tensile strength of the material. Another approach is to install a segmented well, where multiple small-diameter wells are nested together in one borehole and sealed at the target intervals.
Construction of a vertical well typically includes the installation of a filter pack to further prevent formation materials from entering the well and from clogging the screen; the approach often involves pouring fine-grained sand through a tremie pipe into the annular space between the screen and borehole wall. With HRW installations, however, gravity causes formation materials to collapse and fill the small annular space around the screen, which can make it even more challenging to keep fine-grained particles out of the well. Specialized screens are available that have added filtration capabilities. Specialized screens are available with additional filtration capabilities. One example is constructed of inner- and outer-slotted HDPE pipes around a geocomposite layer (EnviroFlex, Ellingson-DTD, Bellfonte, Pennsylvania).
Schematic of a nested well used for HRW
(Vertebrae™, EN Rx, Inc, Decatur, Texas)
Due to limitations of the borehole size, nested wells are not common with HDD, but applications do exist. For example, three 2-inch-diameter oxygen injection wells were installed within a horizontal boring at the Escambia Wood–Pensacola Superfund site. The screened intervals were isolated by installing Portland cement/bentonite seals via a tremie pipe during pullback of the casing. In addition, the Vertebrae™ nested well system allows for multiple small-diameter (approximately ½ inch) screens to be used in HRWs for injection or sparging. Each screen is separated by a bentonite and Portland cement seal, which is injected in the intervals between the screens.
Due to the length and curvature of HRWs as well as the frequent absence of a filter pack, development is critical to remove drilling fluids and any soil particles that plugged the well screen as it was advanced through the soil. Development is typically accomplished by over-pressuring or jetting the well.
Overpressuring involves capping one end of the well and pumping water under high pressure into the other so the pressure dislodges clogged particles. Over-pressuring typically is not used for screens longer than 200 feet (EPA, 2017). Jetting involves pushing or pulling a hydraulic jet, which is a pipe with holes attached to the drill rods, through which water is pumped. The jet is moved along the screened length to dislodge particles. The well may be pumped to remove the jetted water. Jetting may take longer, but it is typically more effective than overpressuring.
