BBC: Drilling Techniques
 
Preliminary Boreholes : Phase II
Test Boreholes in Phase III
Drilling Parameter Recorder
 

Preliminary Boreholes : Phase II

During Phase II of the Year I research, four preliminary boreholes will be drilled with a direct rotary drill rig. Three of the preliminary boreholes will be within the contaminant plume (i.e., TCE, DCE and VC predominant zones of the plume). Past studies indicate that there are zones of varying contaminant concentrations within the bedrock (Weston, 1993).

Numerous bedrock wells have been installed at Pease Site 32. Weston (1993) describes the telescoping casing used at Site 32 for installing shallow bedrock wells. The telescoped casing is designed to isolate the surficial lithology above the marine clay sediment layer from the lithology below. (See Appendix D for discussion of the lithology at Site 32.) The boreholes were started using a 14-inch casing, which was telescoped to 8-inch down to bedrock.

The BBC's drilling will also be done using telescoping casing to isolate the overburden and the shallow weathered bedrock zones from the competent bedrock below. Drilling through the overburden will commence using a 10-cm (4 in.) inside diameter (ID) hollow stem auger. It is anticipated that the USGS will perform the overburden drilling and sampling (See Appendix A – USGS letter). Continuous sampling of the overburden will be performed using a standard split spoon sampler inside the casing. Typical depths of the overburden at Site 32 are from 3 to 12 meters (Weston, 1993). The samplers will be driven ahead of the 10 cm augers. Lexan liners will be used in the split spoon, which will be sealed with caps and stored on site for future reference. Samples will be screened for contaminants using either a photoionization detector (HNU meter), or an organic vapor analyzer (OVA). Those samples that emit organic vapors detected by the meters will be handled as hazardous waste. Drilling will be halted once the weathered bedrock zone has been encountered. The auger hole will be grouted up to the surface with a cement and bentonite grout as the augers are withdrawn from the borehole. Sampling boreholes will be drilled and grouted at all four preliminary well sites.

A second subcontractor (to be selected based on bids) will be brought in to drill down through the overburden with a 30.5-cm (12-inch) casing. The borehole will extend at least 30 cm into the weathered bedrock. The 30.5-cm casing will then be pressure grouted in place, such that grout will be forced up the annular space between the casing and the outside of the borehole. This casing completion (grouting) will essentially seal off the overburden, and the grout will displace or stabilize contaminants that may have been in the borehole fluids. Thirty-centimeter casings will be set within 3 meters of the exploratory boreholes at each of the four preliminary well locations.

The remainder of the drilling will be done by the USGS (See Appendix A – USGS letter). The grout in the 30.5-cm casing will be drilled out with a 25.4-cm bit, and a 20-cm casing will be extended down to the weathered bedrock using a rotary wash technique. Direct rotary wash will be used as opposed to air rotary techniques to prevent the introduction of air into the subsurface, especially into bedrock fractures. The drilling fluid used for the rotary wash drilling will be clean formation water from on-site wells with similar water chemistry to the water from the bedrock being drilled.

The weathered bedrock will be cored with an HQ (or smaller) wire-line triple tube core barrel and a diamond coring bit. The HQ core barrel will retrieve a 6 cm diameter core. Continuous cores will be taken down to competent bedrock. It is anticipated, based on lithologic studies at Site 32, that the weathered bedrock zone is less than five meters. Clean formation water, obtained from an uncontaminated well near Site 32, will be used to cool the coring bit. Once it has been established that competent bedrock has been encountered, the hole will be reamed with a 25.4-cm diameter bit using direct rotary wash techniques.

The water will be collected from a clean-on site well in a sterile polyethylene truck-mounted tank the day before it is needed, and brought to the drill site. The headspace in the tank will be filled with an inert gas (argon) to minimize exposure to air and argon will be bubbled through the water to remove oxygen if the borehole is anoxic or suboxic. This procedure is specified because we are anticipating low yields from appropriate on-site wells. Should an appropriate supply well be found relatively close to the drill site which produces sufficient water to match the drill rig demand, a direct connection will be made to the drill rig, thus eliminating the need for the tank. Clean pipe and hose will be used for this connection. The chemical constituents in the drilling water will be monitored prior to its use to be sure that it is similar to groundwater in the vicinity of Site 32 (except for the lack of organic contaminants). A sample of this water will also be taken and analyzed each drilling day.

The 20-cm casing will be advanced behind the drill bit used to ream the hole through the weathered bedrock zone and at least 30 cm into competent bedrock. This casing will be grouted into place to isolate the competent bedrock zone from the shallow weathered bedrock. The casing will be drilled out down to the competent bedrock once the grout has set. Coring will continue in the competent bedrock to the design depth without additional casing (typically 50 meters below ground surface (bgs)).

The competent bedrock will be continuously cored using a wire-line HQ (or smaller) triple-tube core barrel with a diamond core bit, similar to the weathered bedrock zone. The HQ core bit will leave a 10 cm hole. Again, formation water from an on-site well will be used as the drilling water as described previously. Samples of the drilling water will be collected prior to drilling to evaluate the potential for microbiological contamination of the formation and the cores. The cores will be taken in 1.5 meter runs.

The first two cores from the first preliminary hole will be drilled using conventional equipment and techniques, without special measures to limit/prevent microbial contamination. The first day of coring will use tracer-labeled drilling water. The first cores will be withdrawn from the hole, and the core barrel transported directly to the on-site anaerobic glove chamber, where the core will be extruded under anaerobic conditions, as needed. The cores will be processed for the presence of tracers (See Section VI.C). Heat sensitive tape will be applied to the core barrel end bit to measure the maximum temperatures achieved during drilling (Colwell et al., 1992).

Modified drilling procedures will also be tested in the first borehole to evaluate their effectiveness. To prevent water from within the core barrel from contaminating the core while it is still in the barrel, a wax plug (seal) will be poured in the bottom of the core barrel. As the core barrel is advanced, the wax seal will be pushed along the inside of the core barrel, keeping the drilling fluids from seeping down the core barrel or into the core. To prevent water pressure from displacing the wax seal, formation water will be used to fill the drill stem (from the top) thus maintaining an equivalent water pressure inside and outside of the core barrel. The wax plug will isolate the core from water in the core barrel and drill stem during the coring and removal of the core.

When the core barrel is removed, the drilling water will be pumped into the borehole to bring the water level to the top of the casing. This will minimize the water pressure differential between the inside and the outside of the core barrel. It will also facilitate sealing the barrel from the atmosphere prior to removal from the well. The core barrel will be withdrawn so that the lower end of it remains submerged to prevent the introduction of air. When the barrel end is just below the top of the casing, a plug of sterile modeling clay will be packed into it to keep air from entering as it is withdrawn from the well. The barrel will be removed and immediately placed in the anaerobic glove box, where the air will be flushed out of the glove box with inert gas before the ends of the barrel are opened.

Once the coring has been completed to the design depth, the hole will be reamed to 15.4 cm in diameter using direct rotary wash techniques with the same clean formation water used during other drilling steps. The larger diameter will facilitate geophysical and acoustic televiewer logging of the preliminary boreholes.

The well surface completions will be similar to other existing monitoring wells at Pease. The 30.5 and 20-cm casings will be cut off with the latter casing slightly below the outer casing. The 30.5-cm pipe will serve as the protective casing, and will have a locking cap installed. In most instances, the well will be nearly flush-mounted at the ground surface, with a relief above existing grades of no more than 5 cm. In wooded or wetland locations, wells will have a minimum of a 60 cm stick-up in order to readily find them as well as prevent temporary flooding from making them inaccessible.

Tracer experiments will be conducted in the first preliminary borehole, located in the control (uncontaminated) region, to evaluate the potential for vertical migration of the contaminants during drilling, in particular microbial contamination (See Section VI.C). The first test will entail the placement of a bacterial tracer (Chromobacterium) on top of the weathered bedrock before the first core of it is collected. The tracer will be placed ahead of the casing and will act as a surrogate in determining whether microorganisms found in the bedrock are native or transported from above during drilling.

An additional tracer test will be performed where both a bacterial tracer (INA Pseudomonas) and a chemical tracer are added to the drilling water. The chemical tracer will be lithium bromide, in concentrations of at least 50 ppm above the background concentrations. The addition of tracers will occur only on the first day of competent bedrock coring in the first preliminary borehole. Consecutive cores of the weathered bedrock will be tested for the presence of the chemical and bacterial tracers to evaluate vertical migration of the tracer and /or drilling fluid contamination. The first two cores drilled with the tracer-labelled drilling fluid will be taken without any special drilling procedures (i.e., no wax plug or modelling clay plug used). The cores will be analyzed for the presence of the tracers in the fractures. The subsequent competent bedrock cores on the first day of drilling will be retrieved using the special handling and drilling techniques (e.g., wax plug and modeling clay plug). These cores will be drilled using the same tracer-labeled drilling water. Analyses of the fractures for the presence of the tracer will provide an evaluation of the effectiveness of the special coring procedures in isolating the core from the drilling fluids. Drilling fluid containing only the chemical tracer will be used for the remainder of the drilling in the first preliminary hole.

An open borehole provides an avenue where vertical migration of contaminants may occur from contaminated fracture zones to clean fractures. To prevent this potential migration and cross-contamination, and still maintain the preliminary borehole for future groundwater quality sampling or other testing, a plug will be installed in the borehole. The plug will consist of a diffusion multi-level sampler (DMLS), or a 10 cm PVC pipe (with flexible solvent-resistant diaphragm-type seals) that isolates each fracture zone in a borehole. The DMLS is a passive sampling device, containing vials with a semi- or permeable membrane. Each vial contains sterile distilled (or deionized) and deoxygenated water. The vials are isolated from vertical groundwater flow in the borehole by flexible neoprene diaphragms, which seal against the borehole wall. The modular construction with blank pipe between devices, will allow sampling for groundwater quality from discrete fracture zones. Over time, the membrane allows the sterile, deoxygenated water to come into equilibrium with the water flowing in the formation. The DMLS is removed after a discrete time, and the water in the vial is analyzed for various chemical constituents. The residence time needed for the vial's contents to come to equilibrium with the formation water will be determined as outlined in Section VI.D.

 

Test Boreholes in Phase III

The test boreholes will be drilled during Phase III specifically to recover cores containing fractures, which can be used for evaluating in situ microbial metabolic processes. The primary objective of the drilling will be to obtain a core that has not been significantly contaminated by microorganisms from other vertical locations in the well. Consequently, the drilling procedure will be formulated to prevent the introduction of microbes in the drilling fluid or the vertical migration of microbes from the overburden or weathered rock to the competent bedrock. At the same time, the core will be kept under conditions similar to those that existed in situ. In most cases, this will preclude significant exposure to air. The procedures used in this phase of drilling will be those that were developed and tested during the drilling of the preliminary boreholes in Phase II.

The test holes will be sited based on all available data gathered on existing wells and cores, and on the data from the preliminary boreholes. These data include the fracture density, fracture locations, and fracture attitude. These properties will be used to site each test borehole (presently envisioned to be within 10 meters upgradient of the preliminary well) and also to predict the specific depths of the fractures in each test well location (See Section VI.B). These depths will be the target zones for coring. The test wells will be drilled in the same zones as the preliminary wells (one control and one in each zone of the contaminant plume: VC, DCE, and TCE, respectively).

The drilling procedure will be similar to that used for the preliminary boreholes, with the exception that the auger rig with continuous sampling in the overburden will not be used. In addition, continuous coring in the weathered bedrock will not occur. Telescoped casing will still be used. A subcontractor, using direct rotary wash techniques, will drill a borehole down to weathered bedrock to set and grout a 30.5-cm casing. Clean formation water will be used as the drilling fluid. The 30.5-cm casing will be pressure grouted into place. Once the grout has set, the 30.5-cm casing will be drilled with an 25.4 -cm bit, and a 20 cm casing will be advanced behind the drill bit through the weathered bedrock and at least 60 cm into competent bedrock. The depths will be chosen based on data collected while drilling the nearby preliminary borehole. The 20-cm casing will be pressure-grouted into place. A 15 cm borehole will be drilled down to the first coring zone using a roller-cone drill bit and direct rotary wash techniques. The rotary wash will use clean formation water from an existing well as described in Section VI.A.b.

When the desired coring depth is reached, where a fracture intersecting the preliminary borehole is thought to intersect the test borehole, a core will be taken using a 10-cm diameter triple-tube core barrel with a diamond bit. The design coring depth will be established as the depth two meters above the predicted depth of the target fracture. The bit will leave a 14-cm diameter hole. Cores will be taken in 1.5 meter runs. It is anticipated that this coring will use the wax plug and modeling clay plug described previously. Each run will be processed separately. However, this procedure may be modified based on the results of the tracer tests performed during the drilling of the preliminary wells.. If the procedures described here are not as effective as required, appropriate modifications will be made to the procedures and tested prior to the second phase of drilling.

Each core will be extruded in the first section of the anaerobic glove chamber, and samples will be taken for the microbiological analyses (See Section VI.C). The atmosphere in the glove chamber (consisting of 2 glove bags in series) will be sampled with an HNU or OVA meter to evaluate whether organic contaminants are present. When all microbial extraction procedures are complete, the core will be sealed and prepared for transport to the UNH geology laboratories.

If a suitable fracture is not contained in the recovered core, an additional run will be made, using a second sterile core barrel. This process will continue until a satisfactory core is retrieved. Because cores will be taken from discontinuous levels, the boring will be reamed to a 15 cm diameter to the depth of the borehole, and advanced to the design depth of the next core with the same drill bit. The coring procedures will be repeated at the new level.

Test wells will be completed in a manner similar to the preliminary wells to maintain security of the well. In-well isolation devices (flexible diaphragm seals or DMLS devices) will be installed to prevent vertical migration of groundwater within the well, while still maintaining the borehole for future characterization and testing.

 

Drilling Parameter Recorder: Real Time Continuous Recording
(above: typical profile)

  • Depth (meters)/Penetration Rate (meters/hour)
  • Rotary Speed (rpm)
  • Thrust Pressure (bars)
  • Hold Back Pressure (bars)
  • Torque (bars)
  • Drilling Water Pressure (bars)
  • Drilling Water Flow Rate (liters/minute)
  • Driller’s Control Button


 

 
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