Deep borehole drilling extends beyond machining of individual components to the manufacturing of the borehole itself and the components inserted into it. Two applications at the extreme end of drilling technology — nuclear waste deep borehole disposal and enhanced geothermal systems (EGS) — require the integration of precision component manufacturing with deep drilling techniques that operate at depths exceeding 5,000 meters.
While these applications use drilling methods fundamentally different from the BTA and gun drilling covered elsewhere on this site (they use rotary drilling with tricone or PDC bits, not single-lip or multi-edge cutting tools), the components installed in these boreholes — waste canisters, casing strings, production tubing, and wellhead equipment — require conventional deep hole drilling and precision machining in their own manufacture.
This guide covers the manufacturing specifications for nuclear waste disposal canisters and geothermal well components that involve deep hole drilling operations.
Nuclear Waste Deep Borehole Disposal
The deep borehole disposal concept involves drilling a borehole 3–5 km deep into crystalline basement rock (typically granite), emplacing waste canisters in the lower 1–2 km, and sealing the upper portion with bentonite clay and cement. The canisters are manufactured using standard oil and gas industry tubular materials and manufacturing processes.
Canister Design Specifications
Two reference designs from published research establish the dimensional parameters.
MIT Reference Design (Hoag, 2006):
| Parameter | Specification | Notes |
|---|---|---|
| Inner diameter | 315.32 mm | Accommodates one PWR fuel assembly (214 mm wide) |
| Outer diameter | 339.7 mm | Standard oil well casing OD |
| Wall thickness | 12.19 mm | Carbon steel, API 5CT specification |
| Height | 5,000 mm | Single canister length |
| Material | Carbon steel (standard casing grade) | Per API 5CT, N-80 or L-80 grade |
| Optional liner OD | 406.4 mm | For retrievability |
| Optional liner wall | 9.52 mm | — |
| Drill bit for liner | 444.5 mm OD | Standard API bit size |
| Closure | Threaded connection | Standard API coupling |
The canister uses standard oil well casing sizes, making manufacturing compatible with existing seamless pipe production lines. The primary containment barrier is the geological formation and sealing plug, not the canister itself — the canister serves as a handling and emplacement container.
Norwegian Reference Design (BGE Technology, 2021):
| Parameter | Specification |
|---|---|
| Material | Corrosion-resistant austenitic stainless steel |
| Wall thickness | 80 mm |
| Closure method | Electron beam welding |
| Borehole depth | 3,500 m total |
| Disposal zone | Lowest 500 m |
| Outer diameter | Determined by borehole casing program |
The Norwegian design uses a thicker wall (80 mm vs 12 mm) because the canister is designed as the primary containment barrier, not the geological formation. The 80 mm wall provides corrosion allowance for the expected disposal period.
Canister Manufacturing Process
The manufacturing process for a deep borehole disposal canister involves:
- Seamless tube production — hot rolling or piercing of the steel billet to produce the tube shell
- Heat treatment — quenching and tempering to achieve the required mechanical properties
- End preparation — facing and threading of the pin and box ends (per API 5CT)
- NDT inspection — ultrasonic inspection of the full tube body, magnetic particle inspection of threaded areas
- Bottom closure — forging or welding of the bottom end cap
- Weld inspection — ultrasonic and radiographic weld examination
- Internal surface finishing — if required for inspection access or corrosion resistance
- Pressure testing — hydrostatic test per API 5CT requirements
- Surface coating — corrosion-resistant coating if specified
Borehole Casing for Waste Disposal
The borehole itself must be cased to maintain stability. The casing program uses standard API casing sizes:
| Interval | Hole Size | Casing OD | Notes |
|---|---|---|---|
| Conductor | 36–48 in | 30–40 in | Surface stability |
| Intermediate | 26 in | 20 in N-80 | Through overburden |
| Production | 17-1/2 to 12-1/4 in | 9-5/8 to 13-3/8 in | Through crystalline rock |
In crystalline rock, casing can often be omitted in the disposal zone below approximately 2,000 m depth if borehole stability analysis confirms the rock can remain open without support.
Enhanced Geothermal System (EGS) Well Components
EGS wells require larger diameters than conventional oil and gas wells at equivalent depths to accommodate the higher flow rates needed for economical heat extraction. A baseline EGS well targets 6,000 m depth with reservoir temperatures of 200°C.
Casing Program for a 6,000 m EGS Well
The casing design is driven primarily by collapse pressure from the geothermal reservoir. The telescoping design uses progressively smaller diameters with depth.
| Interval | Hole Size | Casing OD | Grade | Depth |
|---|---|---|---|---|
| Conductor | 48 in | 40 in (line pipe) | X-56/X-66 | 0–15 m |
| Surface | 36 in | 30 in, 310 ppf | X-56/X-66 | 0–150 m |
| Intermediate 1 | 26 in | 20 in, 169 ppf | N-80, BTC | 0–1,500 m |
| Production 1 (liner) | 17-1/2 in | 13-3/8 in, 72 ppf | N-80, Vam Top | 1,450–3,000 m |
| Production 2 (liner) | 12-1/4 in | 9-5/8 in, 53.5 ppf | P-110, BTC | 1,500–5,200 m |
| Production 3 (liner) | 8-1/2 in | 7 in, 32 ppf | P-110, BTC | 5,100–6,100 m |
Casing Material Grades for Geothermal Service
| Grade | Minimum Yield (MPa) | Application | Max Temperature |
|---|---|---|---|
| N-80 | 552 | Intermediate casing | 230°C |
| L-80 | 552 | Sour service intermediate | 230°C |
| P-110 | 758 | Production liner | 230°C |
| T-95 | 655 | Premium production | 260°C |
| Q-125 | 862 | High-strength production | 260°C |
| V-140 | 965 | Ultra-high strength | 260°C |
| 13Cr (L-80) | 552 | CO₂ corrosion resistance | 230°C |
| Duplex 2205 | 450 | High chloride resistance | 250°C |
| Super duplex 2507 | 550 | Extreme chloride resistance | 250°C |
| Ti Grade 29 | 480 | Highest corrosion resistance | 350°C+ |
For temperatures above 230°C, standard API casing grades may experience strength degradation. Proprietary grades with enhanced high-temperature performance are available for superhot geothermal wells (>300°C).
Manufacturing of Large-Diameter Casing
For casing sizes above 20 inches OD, standard API seamless pipe may not be available. These sizes are manufactured by:
- Plate rolling and welding — steel plate is roll-formed into a cylinder and seam-welded longitudinally
- Girth welding — individual joints (typically 40 ft length) are welded together when run into the hole
- Riser-type connections — breech block connectors can be welded to casing sections for faster makeup
Each welded connection requires approximately 2.5 hours of rig and crew welding time. Suppliers accustomed to manufacturing drive piles and riser pipe for offshore oil and gas can fabricate these large-diameter welded pipes.
Production Tubing and Downhole Components
Geothermal production wells require downhole components that must survive extended exposure to high-temperature, corrosive brine.
Production Tubing:
| Parameter | Specification |
|---|---|
| Typical OD | 4-1/2 to 7 in |
| Material | L-80, 13Cr, or duplex stainless |
| Connection | Premium threaded (Vam Top, TenarisHydril) |
| Surface finish (bore) | Ra 0.8–1.6 µm (for scale resistance) |
Electrical Submersible Pump (ESP) Housing:
| Parameter | Specification |
|---|---|
| Minimum casing ID for ESP clearance | ~16 in (406 mm) |
| Material | Corrosion-resistant alloy or coated carbon steel |
| Operating limit | 200°C (standard); 350°C (specialized) |
Wellhead Equipment:
- Machined from forged alloy steel (4140, 4340, or stainless)
- Pressure rating: 2,000–5,000 PSI (EGS stimulation pressure)
- Temperature rating: 200°C+ continuous
- Bore diameter: matches production casing ID
- BTA drilling for through-bore and side outlets
Material Selection for Geothermal Service
Material selection for geothermal well components depends on the downhole environment chemistry.
| Environment Severity | Chloride (ppm) | Temperature | Recommended Material |
|---|---|---|---|
| Low | < 10,000 | < 150°C | Carbon steel N-80 / L-80 |
| Moderate | 10,000–100,000 | 150–230°C | 13Cr, duplex 2205 |
| High | > 100,000 | 230–300°C | Super duplex 2507, Ti Grade 29 |
| Extreme | > 100,000 | > 300°C | Ni-Cr-Mo alloys (SM2245, SM2550), advanced Ti alloys |
The key corrosion mechanisms in geothermal wells include:
- Sulfide stress cracking (SSC) — from H₂S in the reservoir fluid
- Stress corrosion cracking (SCC) — from chlorides at high temperature
- Pitting and crevice corrosion — from localized chloride concentration
- Erosion-corrosion — from high-velocity brine containing suspended solids
- Scaling — silica and carbonate scale deposition on tubing walls
Drilling and Completion Components
The drilling assembly for geothermal wells includes components manufactured with precision bores:
Drill collars:
| Parameter | Specification |
|---|---|
| OD | 6-1/2 to 11 in |
| ID (bore) | 2-13/16 to 4-1/4 in |
| Material | AISI 4145H modified |
| Surface finish (bore) | Ra 1.6–3.2 µm |
| Connection | API 4-1/2 IF to 7-5/8 REG |
Borehole casing accessories:
- Centralizers — bow-spring or rigid type, welded to casing OD
- Float collars and shoes — machined from ductile iron or steel, threaded per API casing specifications
- Stage cementing tools — with precision-bored ports, sliding sleeves, and sealing surfaces
Quality Assurance
Both waste disposal and geothermal components require strict quality assurance:
| Inspection | Method | Standard |
|---|---|---|
| Full-body ultrasonic | UT | API 5CT / API 5DP |
| Thread inspection | MPI or DPI | API Spec 7-2 |
| Wall thickness | UT gauging | API 5CT |
| Dimensional | Gauges and calipers | Per API specifications |
| Hydrostatic pressure | Water or nitrogen | API 5CT |
| Tensile and hardness | Destructive (witness) | Per ASTM |
For geothermal production casing, additional testing may be required:
- SSC testing per NACE TM0177 (for sour service)
- Sulfide stress corrosion cracking per NACE MR0175 / ISO 15156
- High-temperature tensile at reservoir temperature
For related reading, see the Energy and Power Generation Deep Hole Drilling Guide, the Deep Hole Drilling for Oil & Gas, and the Deep Hole Drilling Quality Standards Guide.