Case 2 Industrial Chemical Plant in Southern Louisiana

Contaminated clayey soils beneath industrial plants are commonly excavated to prevent migration of contaminants into underlying aquifers. Low hydraulic conductivities of such soils require close spacing of vertical wells, but dense wellfields are difficult or impossible to install because of logistical obstructions such as underground utility lines and overhead steel structures. Furthermore, drilling vertical wells in the midst of active operating facilities raises concerns about worker health and safety as well as plant productivity losses. Recently, several horizontal wells have been installed as an alternative to excavation in sandy clay soils beneath petrochemical complexes along the Mississippi River in the industrial corridor between Baton Rouge and New Orleans, Louisiana. Two such wells were installed at a petrochemical complex located south of Baton Rouge. The wells were installed to recover dissolved ethylene dichloride and monochlorobenzene from shallow silty clay soils.

1. Hydrogeology

Clay and silt dominate the subsurface to a depth of 60 ft at the site. Eight soil strata have been identified. The top 8-10 ft consists of dark brown to gray clay and orange, green, and brown to gray silty clay. Below that is a 10-ft-thick orange and gray to brown clayey silt, which is underlain by alternating clay and silt. The 10 ft of silt is the target of the remediation effort. Its hydraulic conductivity is 1.7 ft/day in the vicinity of the contaminant plume. Vertical permeability in the clayey soils above and below the silt is approximately 3 X 10 ~6 ft/day. The clay sediments behave like aquitards. The potentiometric gradient in the 10 ft of silt is 0.002 east-northeast.

2. Well Placement

The shallower of the two wells (Well A) was installed at a total vertical depth of 12 ft beneath an existing superstructure. The overhead structures and the concrete foundation below ground level contained active unit process equipment. The horizontal section of the well is 356 ft long, and it was installed along the midline of a 20-ft-wide corridor bounded by 60-ft-deep concrete and wood pilings. A vertical well was located only 5 ft away from the wellpath, demanding high placement accuracy. The deeper well (Well B) was installed at a total vertical depth of 14 ft beneath an existing concrete road, along a pipe rack and railroad loading dock. The horizontal screen of the well is 400 ft long, and it was installed between pilings supporting the pipe rack on one side and a freshwater drainage ditch on the other side.

3. Well Specifications

The curved section of Well A (Figure 6) is constructed of 103/4-in. -high-density polyethylene casing cemented in a curved 12'/4-in. wellbore. The curve ends at a measured depth (measured along the wellbore) of 80 ft. The horizontal section is completed with 6%-in. high-density polyethylene slotted liner (slot size is 0.02 in.) installed within an 85/s-in. wellbore. The horizontal section ends at a measured depth of 436 ft. Blank (unslotted) high-density polyethylene

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Figure 6 Louisiana chemical plant—horizontal well path, well A.

casing is installed inside the 103/4-in. curved casing, rising to the wellhead. An electric submersible pump, rated at 10 gpm, was placed inside the 65/a-in. casing at the bottom of the curved section of the well. Stainless steel wire-wrapped prepacked screen was installed inside the horizontal high-density polyethylene casing and attached to the pump to filter silt-sized formation material. Horizontal displacement between the wellhead and the beginning of the screen is approximately 70 ft. The horizontal section remains within a 3-ft tolerance envelope, ranging in total vertical depth from 11.8 to 14.9 ft. Horizontal accuracy is within 2 ft of the planned termination point. Well B is constructed similarly to Well A (Figure 7), with total vertical depth ranging from 12.4 to 17 ft.

Four undisturbed horizontal core samples were extracted during drilling—three from the shallower well and one from the deeper well. Each core is 5 ft long and 2 in. in diameter. Coring is accomplished by setting a hydraulic coring tool (Figure 8) into the soil at the end of the

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Figure 7 Louisiana chemical plant—horizontal well path, well B.

Figure 8 Hydraulic coring tool. (Source: Eastman Christensen Environmental Systems.)

Figure 8 Hydraulic coring tool. (Source: Eastman Christensen Environmental Systems.)

wellbore with a moderate amount of pushdown force to isolate the inner barrel from the drilling fluid and contaminated soil and groundwater. Pressure is hydraulically applied to create a constant load on the outer tube of the coring device. An accumulator located on the drilling rig is plumbed in parallel with the drill string circulation path to maintain a constant punch force. Pressure is raised to the calculated punch release force, which breaks shear pins and accelerates the inner tube into the formation. Pressure is maintained in the system to hold the outer tube against the formation to prevent the drilling medium from coming into contact with the sample as it is pulled back into the outer barrel. The core barrel is then retrieved to the surface for recovery of the undisturbed sample. The sample is contained within a disposable plastic liner. The core barrel requires a radius of curvature of at least 100 ft to pass through a 5!/2-in. borehole.

The total groundwater recovery of the two wells is 23 gpm, which is comparable to over 50 vertical wells in the same water-bearing zone. Vertical wells as far as 70 ft away from the horizontal wells exhibit drawdown caused by pumping from the horizontal wells.

4. Analytical Modeling

Results of analytical modeling of capture zones that develop in response to groundwater recovery from the two horizontal wells are shown in Figure 9. Recovery of the contaminant plume in the vicinity of the tank car unloading areas, process sump, and glyoxal unit, shown in Figure 9, can be achieved with two 400-ft-long horizontal wells placed within the waterbearing silt zone. The southern well would be placed 30 ft north of the glyoxal unit, as shown

Figure 9 Louisiana chemical plant—groundwater capture zones.

in Figure 9. Shaded areas indicate 4-year and 8-year capture zones of the two wells. The capture zones are calculated using a groundwater code called PAT [19], which assumes a homogeneous, isotropic aquifer. The assumed aquifer parameters, input data, and the results are listed in Table 1.

Results of analytical modeling of aquifer response to groundwater recovery from the two horizontal wells are presented in Figure 10 (distances are in feet and the contour interval is 1 ft). The shaded region represents the chlorobenzene (MCB) plume. Groundwater recovery rates are calculated using HWELL, an analytical model [20],

Aquifer response to the two proposed horizontal recovery wells is estimated by simulating each horizontal well by a series of closely spaced vertical wells with total pumping rate equal to that calculated by HWELL. This method provides a good estimate of drawdown curves generated by more complex, numerical methods, and hence it is a good tool for predicting actual drawdown contours. The model used for this simulation is a modified version of THWELLS [21], which solves the Theis equation for multiple wells and accounts for unconfined aquifer conditions. The model assumes non-steady-state conditions in a homogeneous, isotropic aquifer with infinite areal extent and with a hydraulic gradient of 0.002. Time elapsed since the onset of pumping is 100 days. The analytical models suggest that the two horizontal wells installed at the plant could capture the plume during an 8- 10-year period if the total pumping rate were only 5 gpm. Actual recovery rates are over four times that amount, so closure levels of contaminant concentrations may be achieved sooner.

Horizontal wells offer significant advantages over vertical wells in environmental remediation and protection in many hydrogeological scenarios. These include thin and/or low-permeability aquifers, where many closely spaced vertical wells are replaced by a single horizontal well. Current high installation cost of a horizontal well compared to a vertical well is offset by operation and maintenance cost savings. New developments in horizontal drilling technology will further reduce the cost of installing horizontal wells, and subsurface pollution control with horizontal wells should become as common as vertical wells.

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