Case 1 Texas Gulf Coast Industrial Landfill Site

Before the advent of horizontal wellbore technology, vertical wells were used to recover contaminated groundwater under landfills. In aquifers with high hydraulic conductivity, vertical wells placed along the periphery of a landfill can influence the aquifer below the landfill. If the size of the plume is small compared to the size of the landfill, migration of contaminants from the core of the plume below the landfill to vertical extraction wells along the periphery spreads the contamination. Low hydraulic conductivity requires placement of vertical wells within the landfill, penetrating the landfill liner. Long-term sealing of the liner around the vertical well casing cannot be guaranteed. Horizontal wells avoid these problems by placing a production screen directly in the plume without penetrating the landfill.

In 1991, a horizontal well was installed beneath an RCRA facility on the Gulf Coast of Texas to recover contaminated groundwater. The landfill occupies an area approximately 900 ft by 900 ft. Its depth is 30 ft, and the slope along its sides is 27° from horizontal (Figure 3). Numerical modeling prior to installation predicted that a single horizontal well would be hy-

GRAY-TAN SILTY CLAY

RED-BROWN CLAY

RED-BROWN CLAY

GRAY CLAY

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FINE SAND

GRAY CLAY

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Figure 3 Texas Gulf Coast landfill—cross section with horizontal well path.

GRAY-TAN SILTY CLAY

RED-BROWN CLAY

RED-BROWN CLAY

FINE SAND

draulically more efficient than a vertical well system [18]. The single horizontal well replaces five vertical recovery wells. Fewer pumps and a lower volume of recovered water save operation and maintenance costs of the remediation system.

1. Hydrogeology

Interbedded, 20-30 ft-thick reddish brown clay and gray and tan silty or sandy clay characterize the subsurface from 0 to 98 ft depth, corresponding to 26 ft above mean sea level (MSL) to 72 ft below MSL (Figure 3). The uppermost silty clay layer and the top of the underlying reddish-brown clay layer have been excavated to accommodate the landfill. Tan fine sand occurs at 100-128 ft depth from the surface, or 70-95 ft below the base of the landfill. Blue-gray and black-gray clay underlies the sand. The silty or sandy clay layers and the sand layer are saturated. The reddish-brown clays are aquicludes or aquitards. The tan sand layer is the target of the remediation effort. The hydraulic conductivity, K, of the sand is 30 ft/day, and stora-tivity, S, is 1 X 10-5, indicative of a confined aquifer. There is no significant potentiometric gradient in the absence of pumping.

2. Well Specifications

Sixty feet of 6%-in. slotted stainless steel screen was installed beneath the landfill at a total vertical depth of 114 ft. This depth is 4.7 ft below mid-height in the aquifer, giving the well an eccentricity of 4.7 ft. The wellhead protrudes at a 41.5° angle, 20 ft from the edge of the landfill. The horizontal displacement from the wellhead to the beginning of the screen is 248 ft, reflecting a radius of curvature of 275 ft. The curved section of the well was cased with 10 -in. of high-density polyethylene liner to isolate soil zones during drilling and to provide structural support for the inner stainless steel casing.

3. Drilling Procedure

Drilling commenced with augering of a 16-in. wellbore to a measured depth of 37 ft, which corresponds to the kickoff point for the curved section of the wellbore. A steel conductor pipe, 14 in. in nominal diameter, was cemented in the 16-in. wellbore. The cement was allowed to set for 12 hr. The curve drilling assembly then drilled and simultaneously cased a 13-in. borehole with an effective drilling radius of 275 ft to a measured depth of 272 ft. The curved section was drilled using a 13-in. wing bit. High-density polyethylene casing 10 in. thick was then cemented into the curved section of the wellbore. After the cement had set, the horizontal drilling assembly drilled an 83/4-in. wellbore to a measured depth of 352 ft, with a final inclination of 90.5° from vertical. The horizontal section was drilled using an 83/4-in. rock bit. Schedule 40, 316 SS ERW stainless screen casing, with 65/s-in. outer diameter, was installed along the entire length of the wellbore. Only the 60-ft section of stainless steel casing installed within the target zone was slotted. Slot size was 0.02 in. TWenty-foot sections of stainless steel casing were welded together, increasing installation time. Six 10-ft sections of prepacked screen were installed inside the stainless steel screen in the target zone. The prepacked screen was attached directly to an electric submersible pump oriented horizontally within the unscreened stainless steel casing. Well development required 18 hr, after which clear water was recovered. Residual drilling fluids were flushed out of the well with a submersible water jetting tool.

Drilling, installation, and well development required approximately 10 days. During drilling, continuous updates of inclination and tool face orientation were provided to ensure proper wellbore placement. TruTracker surveys were taken as needed, at least once every 20 ft, to provide actual bit locations with respect to the proposed well path direction. The horizontal wellbore remained within a 4-ft vertical target range, thus exceeding the required accuracy of ± 5 ft (Figure 3). The well remained within a 20-ft-wide horizontal target zone (Figure 4). Approximately 18,000 gal of cuttings, drilling fluids, and cement returns were generated dur-

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Figure 4 Texas Gulf Coast landfill—plan view of horizontal well path.

ing the installation of the horizontal well. Nearly half of that volume was generated during installation of the curved section of the well.

4. Well Performance

The well produces up to 7 gpm of water, creating drawdown of 1-2 ft at distances of up to 300 ft radially from the midpoint of the well. The drawdown created by the horizontal well is consistent with results of analytical modeling shown in Figure 5 (distances are given in feet, and the contour interval is 0.1 ft). The model assumes steady-state flow in a homogeneous, isotropic aquifer. Less than 100 ft from the well, the potentiometric surface defines an elliptical

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Figure 5 Texas Gulf Coast landfill—horizontal well drawdown contour map.

trough of depression. Beyond 100 ft, potentiometric surface contours are circular. The model indicates that drawdown along the well is 2.7 ft at a pumping rate of 7 gpm.

The specific capacity of the horizontal well is only 1.16 times that of a comparable vertical well with fully penetrating screen. Therefore, the primary advantage of this horizontal well is not hydraulic, but logistical. The risks involved in penetrating an existing landfill liner, which would be required for vertical well installation in this scenario, outweigh the cost difference.

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