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A “continuous” well is drilled from a surface entry point to the required vertical depth, continued through the horizontal section then up to the surface at a defined exit point. The well is constructed at the exit site then pulled back through the borehole to the entry point. A “blind” well is drilled to the required vertical depth of the horizontal section then terminated at the end of this section. The well is built at the entry point and pushed into the borehole. Both well types are drilled to the same accuracy and the well screens are placed in the horizontal sections.
Continuous wells are normally completed in areas where sufficient room and access are available at the exit location. The wells are installed where the entire well is located on one property and access agreements are not required for the exit location. Additionally, continuous wells are completed in geology such as washed gravel or cobble formations where blind wells are very difficult to install.
Blind wells are completed under properties where access agreements are difficult or impossible to obtain. Also, in areas where exit locations are not available, such as chemical plants, rail yards and airports. Back to Menu
The well length and depth is a function of several parameters, including well materials and subsurface conditions. To date, the longest continuous horizontal environmental well is 2850 feet in length. The longest blind well is 1185 feet long. The deepest well was installed to 235 feet below ground surface. In general, formations with large gravel, cobbles and boulders limit the length and depth of horizontal well completions. Steel well materials, because of high tensile and compressive strengths, can be placed in longer/deeper wells than high-density polyethylene. Back to Menu
Drilling fluid selection should be based on the well use and geologic conditions. For wells installed below the water table (pump and treat, groundwater containment, free product recovery and air/biosparging) bentonite-based drilling fluids are suitable. Bentonite fluids provide excellent viscosity (required to remove cuttings from the hole) and form a thin, removable wall cake. The wall cake stops the bentonite fluid from infiltrating deep into the formation and prevents contaminants from the formation from entering the borehole during drilling operations. However, a detailed well development plan should be provided to insure that the drilling fluids are removed from the formation during development. Both physical (jetting, surging and swabbing) and chemical (acids and polyphosphates) development techniques should be used. For wells installed above the water table, biodegradable drilling fluids should be used. These drilling fluids develop viscosity to remove cuttings from the borehole. Our experience has shown that the biopolymer drilling fluids form poor wall cakes and fluid loss to the formation can occur. However, the biodegradable drilling fluids can be broken down using chlorine or enzymes. Back to Menu
Well development consists of both physical and chemical treatments. Physical treatments include high-pressure jetting of either water or a chemical solution through a jetting tool. The tool is run into the hole on drill pipe and rotated through the screen section. Other physical treatments include swabbing the well or overpumping the well, allowing the formation pressure to aid in well development.
Chemical treatments can include acids (citric or aceatic) and polyphosphates for wells drilled with bentonite fluids, to chlorine or enzyme treatments for wells drilled with biopolymer drilling fluids. The chemicals can be introduced into the wells through a jetting tool or through a packer assembly. Back to Menu
Concurrent Technologies Corporation (CTC) has documented 1,050 horizontal environmental well installations from the late 1980s to the end of 1999. The National Ground Water Association has a bibliography of over 100 articles detailing horizontal well usage. Additionally, horizontal wells have been utilized in the oil and gas industry since the 1940s. Longbore is by far the leading installer of long horizontal wells (over 600 feet). Back to Menu
Depending on the geology of each particular site, one horizontal well can replace up to 50 vertical wells. At a site in Louisiana, four horizontal wells replaced 224 vertical wells on a pump and treat project. Because one horizontal well can replace many vertical wells, the overall remediation cost may be considerably less with a horizontal well system. For example, the cost of running electricity, control systems and instrumentation to many vertical wells is much more expensive that one horizontal well. Additionally, site restrictions, manifold piping and long-term maintenance of a multiple-well system is much more expensive than a one-well system. Back to Menu
Screen and casing for horizontal wells has consisted of PVC, HDPE, steel, stainless steel and fiberglass. The two main materials are HDPE and steel/stainless steel for continuous wells. Back to Menu
- The well can be installed in areas inaccessible to traditional vertical drilling (rail yards, chemical plants, refineries, airports).
- The screen can be placed throughout the length of the contaminant plume.
- Fewer wells are required than with traditional vertical systems, reducing cost by:
- Less surface disruption at the site.
- Less manifold piping, instrumentation and controls wiring and electrical connections.
- Less operational and maintenance costs for the system.
- Less security concerns.
- Less restriction on site use.
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Installation time can vary based on the length and depth of the well and the complexity of the geology. For example, Longbore recently completed a 1434-foot continuous well (HDPE screen and casing) completed in a gravel and cobble formation. The installation required eight days, with an additional two days of well development. Wells of 200 feet or less have been completed in less than three days. Blind wells normally require slightly more time for installation. Longbore completed a blind well, total length of 1100 feet with 685 feet of screen, in fifteen days including rig up/down and well development. Back to Menu
Horizontal wells have been used for removal of both free-phase LNAPL and DNAPL as well as dissolved-phase contaminants. Back to Menu
Costs can vary over a wide range being dependent on a number of factors including the following:
- Length and depth of the well
- Well materials composition
- Ultimate use of the well
- Drilling fluid
- Development criteria
- Geologic conditions
- Type of site activities
- Site location and topography
- Blind or continuous well
A major cost variable is the type of well screen used. The least expensive is HDPE and the most expensive is sintered stainless steel, which can be 20 times more expensive. Back to Menu
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