Horizontal Wells Installed for Environmental Site Cleanup by Directional Technologies, Inc.

Environmental site cleanup can be expedited with a horizontal well system. You may be a bit skeptical if you have not yet used horizontal wells, but Directional Technologies, Inc. has successfully designed and installed horizontal wells for more than 24 years and based on that extensive experience we know they work for environmental site cleanup.

Below are some key elements that make horizontal well technology work and why horizontal wells are a viable remedial solution.

 

 

 

 

Environmental site cleanup has many complexities and vertical drilling has its limitations. The combination can lead to a cycle of problems that cause sleepless nights and headaches for many environmental project managers.

• Short-Circuiting
• A small Radius of Influence (ROI)
• Overuse of wells, ports, wellheads & piping, and way too much equipment onsite
• Site business disruptions
• Escalating Operation and Maintenance (O&M) costs
• Randomly distributed pockets of contamination that result in insufficient contact with contaminated soil
• Overexertion of limited screen intervals with excessive flow, pressure or vacuum, leading to
• Inefficient wells that are difficult to operate and maintain and need premature rehabilitation
• All of which extends system operation and delays site closure.

Environmental managers have been using horizontal wells to break out of this cycle of problems for over two decades, and the environmental industry’s collective experience applying directional drilling to provide effective solutions is growing every day.

Short-Circuiting

Short-circuiting is a common concern for vertical air sparge or soil vapor extraction wells, and significant effort is expended in grouting the wells to prevent short circuiting of air between the screen interval and the ground surface. By contrast, horizontal wells seem to be relatively immune to this problem. Even horizontal wells with little or no grout emplacement have shown no signs of short-circuiting during operation.

There are three reasons for this contrast:

1) Some vertical wells are completed with filter pack filling a measurable annular space, so the riser pipe is not in contact with the natural formation. The formation collapses around a horizontal well riser pipe soon after installation, and the formation in contact with the outer surface of the riser pipe is no more permeable than any unit volume of the formation.

2) The length of riser pipe along which short-circuiting air must travel is usually 5 to 50 times longer for a horizontal well than it would be for a vertical well at the same site. Target zones for SVE wells are commonly 5 to 20 feet deep, so air travels through a distance of 5 to 20 feet before it can short circuit to the atmosphere. Horizontal wells placed in that depth range require a step-back distance of at least 20 to 80 feet, and often this distance is 100 to 200 feet long to accommodate site restrictions.

3) Since many target zones are vertically anisotropic sedimentary systems with preferentially horizontal flow, flow distribution along a vertical screen exhibits more variability, and hence air will flow preferentially through discrete, short subsections of the screen, adjacent to stratigraphic zones with high pneumatic permeability. A horizontal screen in the same formation is normally placed largely within one stratigraphic zone and, therefore, the air will encounter the same hydraulic permeability along most of the screen. This reduces the likelihood that the well is over pressured or has excessive vacuum buildup, making horizontal wells less prone to short-circuiting.

Radius of Influence

Injection or extraction target zones for typical remediation systems are at least ten times longer than their thickness. Vertical injection well screens for soil vapor extraction, air sparging, bioamendment injection, or multiple phase extraction are usually between 2 and 20 feet long. By contrast, horizontal remediation well screens are hundreds of feet long. Vertical well tests commonly provide the basis for horizontal well screen design. The radius of influence measured in such tests does not provide an accurate estimate of a full zone of influence that horizontal well with its much longer screen will develop over time. Initially after startup, a horizontal well will have a zone of influence that is cigar shaped around the screen, with a capture zone about as wide as the zone of influence measured in a vertical well test in the same formation.

Unlike the vertical well, which will reach its maximum zone of influence relatively quickly, with little variation along its comparatively short screen, the horizontal well’s zone of influence will gradually develop into an elliptical zone, with the screen endpoints defining the focal points of the ellipse. The distance to the edge of the zone of influence will be greatest at the midpoint of the well. Only at the endpoints of the screen will the horizontal well’s zone of influence match the results of the vertical well pilot test. The zone of influence will exceed the vertical well test results along most of the horizontal screen.

This difference is due to two main factors. One is simply the length of the screen, which is too short in the vertical well for the elliptical shape to present much change in the distance to the edge of the zone of influence along the screen. The other is the stratified nature of typical target zones of sedimentary origin. Vertical anisotropy resulting from the stratification causes channeling and impedes fluid flow through the porous formation to the vertical screen in the most efficient pattern, producing the greatest rate of pore volume exchange. A horizontal screen placed largely within a single sedimentary unit will develop the most efficient flow pattern within that sedimentary unit.

Dead Zones and Pockets of Contamination

The large zone of influence, confined to the target zone without loss of fluid, pressure or vacuum to the atmosphere or irrelevant portions of the subsurface gives a horizontal well the ability to avoid dead zones that often develop between vertical wells. The long screen of a horizontal well can be smartly oriented to intersect natural heterogeneities in the target formation, including permeable lenses or fractures. Thanks to this advantage, horizontal well systems can produce more contaminant mass or inject more remediation agents directly into the most highly impacted pockets than an equivalent vertical well system.

Wellheads, Piping, and Site Logistics

The ability of a horizontal drilling crew to drill and install a well with minimal access to the ground surface above the well path makes it easy to install horizontal wells beneath almost any active business facility, including occupied buildings, manufacturing plants, retail stations, rail yards or any other grounds with active equipment, traffic and staff, without disrupting the business operations. Since the wellheads are laterally displaced from the screen locations, access to ports needed to operate the system is conveniently located along the periphery of the site, away from busy areas of the site. For this reason, as a general rule, a horizontal well system requires less conveyance piping, with fewer energy-consuming turns and elbows, than a comparable vertical well system. A much greater portion of the energy exerted in running a horizontal system is put to work directly in the target zone where it is needed, instead of being lost to complicated surface piping associated with a vertical well system.

Well Efficiency, Rehabilitation, and Lifespan

The efficient, direct access to the porous zones in need of remediation causes horizontal wells to work with lower flux (flow per unit screen area) across the screen than vertical wells. This limits the rate at which fine-grained particles enter the well. The specific capacity of a horizontal fluid recovery well in a typical formation targeted for remediation is much greater than that of a vertical well in the same formation. This means a horizontal well can produce fluid at low pressure compared to vertical wells. Consequently, horizontal wells can typically operate much longer before significant rehabilitation or, as is so often the case with vertical well systems, replacement of wells is needed. The total mass removed or remediation agent injected during the productive lifetime of a horizontal well is, as a rule, orders of magnitude greater than what a vertical well can produce or inject before it needs to be redeveloped or replaced.

Conclusion

It is not surprising, considering that horizontal wells deliver or recover fluids into or out of a large volume of soil, that many environmental consultants return to horizontal well technology again and again once they see how rapidly horizontal well systems can drive site closure.

Advantages include their length, orientation, and superior flow efficiency to overcome natural impediments to delivery or recovery found in most sedimentary, metamorphic or igneous target formations. Interestingly, the obvious site logistical advantages often trigger the initial horizontal well installation, but once they see how well the horizontal wells work, environmental consultants keep returning to the technology as an aggressive yet cost-effective remediation tool.

Some problems won’t be solved until you pick up the problem and turn it sideways. Contact us today to learn more about horizontal wells for environmental site cleanup and how they could be the solution for your next project.