Case Study: Escambia Superfund Site in Pensacola, Fla.

(Editor’s Note: This is the second of a two-part series on HDD and environmental remediation wells. In September, an overview of this market was presented.)

In the summer 2009, Directed Technologies Drilling Inc. (DTD) installed one of its longest horizontal remediation wells to date at the Escambia Superfund site in Pensacola, Fla.

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The 1,450-ft long, multi-screen well intersects a naphthalene plume in groundwater at a depth of 100 ft, down gradient from a source area on the site. The Escambia site encompasses an old wood treatment plant that used creosote and pentachlorophenol for wood preservation for four decades and was placed on the National Priorities List (Superfund sites) in 1994. The site has been the focus of ongoing cleanup efforts since then to remove or reduce soil and groundwater contamination. Black & Veatch, Alpharetta, Ga., is currently managing the site characterization and cleanup activities.

For this pilot project, DTD designed and then installed three, 2-in. diameter well casings and screens in a single borehole. The well screens were staggered across the width of the plume and used three different screening materials to assess their effectiveness for future cleanup efforts. Oxygen is injected into the screened sections to treat the contaminated groundwater as it moves past the well.

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The design of an oxygen injection well is calculated the same as an air sparge or other injection well. However, since the oxygen can only be produced at a fixed rate set by the oxygen generators, it is not feasible to tweak flow rates once in production by simply turning up the blowers. This means that the design effort must be carefully undertaken to assure the desired performance as designed, without post-installation adjustments.

“The design of the well screens was critical to optimize the oxygen injection rate,” noted DTD president Dan Ombalski, who also did the well design. “This pilot project involved three separate well screen sections, each of a different material type and screen design, to test the effectiveness of very low injection rates.”  

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The installation also included two tremie pipes that were used to inject grout to isolate the separated screened sections. DTD installed the multiple pipes in a single pull and used special techniques in order to minimize the potential for damage to the well screen.

DTD mobilized its American Augers DD210 drilling rig for the project. The rig can produce 210,000 lbs of thrust or pullback and 25,000 ft-lbs of torque. This big-rig capability proved useful in drilling and pullback through the  fine sand formations at the project site. To support the drilling operations, DTD also mobilized a Mud Technology International (MTI) mud system, which included a mixing hopper, storage capacity, a scalping screen, desanding shakers and desilting hydrocyclones. The mud system also included a separate skid-mounted, high-pressure triplex mud pump to supply drilling fluid to the drill rig.

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Navigation was accomplished with a Tru-Tracker system, which induces an artificial magnetic field by energizing a wire loop that is constructed on the ground surface. Navigation for the bore was complicated by the site topography. The screened section of the well was 100 ft below ground elevation, but prior to reaching that zone, the borehole traversed beneath a 50-ft tall stockpile of recycled asphalt stacked above the entry section, then crossed beneath an active railroad switchyard before entering the project site.

Observed senior drilling supervisor James Ditto: “The mountain of asphalt was a particular challenge. Ordinarily, we would set up a project like this with one or two coils. Due to the steepness of the asphalt pile, we had to break our navigation system into several coils and energize them as we progressed. It added a couple of days to our project setup.”

Due to the length of the well, crossing of the rail switchyard and steep terrain on the asphalt pile, three separate loops were constructed and activated individually as the boring progressed. A short section of the entry was drilled “blind” without coil coverage. The boring progressed through this section without incident and emerged on target when it re-encountered the final tracking coil.

After preparing the well casing for installation and setup of the drilling equipment, the pilot bore was drilled in three days and the pullback was accomplished in one day. The unexpectedly fine sand proved to be challenging, since the mud system was sized to remove larger sand grains. Increased sand content in the drilling fluid reduces the carrying capacity of the fluid and increases wear on pump seals and other mud system components. The mud system required constant monitoring and adjustments were made continuously to keep the sand content as low as possible. Pullback proceeded smoothly, but it was necessary to pre-ream the bore to a larger diameter to reduce skin friction on the casing during pullback.

“The flowing sands in the portions of the bore did increase our pullback pressures, but we were able to continue without difficulty,” noted Ditto. “The combination of selecting the right rig for the job and using a steel carrier casing enabled us to pull harder than we could have if we were pulling back unprotected HDPE well materials.”

After the well materials were pulled back, additional work was required to remove protective material from the well screens and casing. Both ends of the well were then grouted to the surface, using the tremie pipes that had been pulled into the bore with the well screen and casing.

In addition to the American Augers rig, DTD also mobilized its Vermeer 24×40 drill rig in order to prepare well materials for installation. Additional support equipment included two rough terrain, extended reach forklifts; two excavators; a backhoe; a bulk water storage tank for make-up water; and two rolloff containers for collection of drill spoils and excess drilling mud.

Initial and continuing tests of the wells show promising results for site cleanup, and Black & Veatch is evaluating the relative performance of each well type for potential use in additional wells at the site. Several monitoring wells have been installed along a line parallel to the horizontal well in order to monitor its performance. To date, these have shown excellent diffusion of injected oxygen throughout the groundwater column.

The Escambia project fits the model of many environmental HDD projects. It had a lead time of several years from initial discussions with the client to actual field work. Logistics at the site were also challenging, requiring duplication of several pieces of heavy equipment due to the length of the bore and the many obstructions between the entry and exit. The project overall was a challenging one, but dedication to quality at every step of the process resulted in a successful installation.

Michael Lubrecht, LG, is with Directed Technologies Drilling, based in Port Orchard, Wash.