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Technical Report NTB 80-04

Feasibility study for large diameter boreholes for the deep drilling concept of a hight-level waste repository

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INTRODUCTION (Chapter 1)

The combustion of nuclear fuels generates radioactive wastes, some of which are very long lived and must be kept in confinement for very long periods, up to 100 000 years. One proposed method of confinement is to bury the waste containers in stable formations at depths of a few thousand metres, by means of holes drilled with existing oilwell equipment and techniques. This study endeavours to evaluate whether drilling these holes is possible with presently available technology.

DESCRIPTION OF THE CONCEPT (Chapter 2)

The formation chosen for the repository is granite, and the geological profile selected for a well depth of 2'000 m is:

0 to 1'000 m: sedimentary, overlaying

1'000 to 2'000 m: granite, massive.

Additional requirements calling for a minimum separation between two storage zones, and for a minimum impact on the environment of both the drilling and subsequent operations, lead to a concept where a certain number of deviated and orientated holes is drilled from a single location. One storage arrangement proposed has 9 wells, in line at the surface with a 6 m spacing, and diverging as the depth increases with a minimum angle to the vertical of 7°. By increasing the maximum allowable angle of a hole to 15°, one can design a storage arrangement with 19 wells drilled from the same rig location.

The characteristics of the geological formation govern the chemical and physical characteristics of the fluid used to drill a well, and also the architecture of such a well (particularly the depths at which intermediate technical casings must be used). Pending confirmation of the characteristics of the various overlaying formations (such confirmation will be given by exploratory holes, in which a number of parameters will be measured), the proposed architecture is:

  • 56" OD. conductor pipe set at 50 m
  • 36" OD. technical casing set at 800 m in a 48" diameter hole
  • 22" OD. final casing set at 2'000 m in a 30" diameter hole.

One important function of the drilling fluid is to evacuate the rock cuttings from the hole. This can be performed only if the fluid has a minimum ascending speed. With existing pumps, and "normal" circulation (fluid going down inside the drilling string and up in the annulus between borehole walls and drilling string), the maximum achievable diameter is 26" to 30". The drilling technique and sequence of operations will therefore be:

  • Setting 56" conductor pipe at 50 m.
  • Drilling 26" hole to 800 m, deviating and orientating such hole as per program.
  • Opening 26" hole to a diameter of 48", with air assisted reverse circulation technique.
  • Run and cement 36" casing.
  • Drill 30" hole to total depth, maintaining as much as possible the previous deviations and direction.
  • Run and cement 22" casing.

The exploratory wells will supply most of the information needed to adjust such a program and to estimate a realistic deviation for one well. One can assume that drilling such a hole will take 10 to 12 months.

TECHNOLOGICAL COMPONENTS (Chapter 3)

Conductor pipe

This can be selected from the available A.P.I. standardized line pipes, with a thickness of 1". Considering the short length of pipe to be set, the different sections will be welded together and the pipe can be either driven with heavy hammers or cemented conventionally into a predrilled hole.

Casing strings

Both 36" and 22" casing strings have larger diameters than the standard oilwell casings and must therefore be specially manufactured. Adopting design factors which are commonly used for oil wells, it is possible to select such a steel quality (K.55) and pipe thickness (1"), that any specialized steel plant will be able to manufacture these pipes without particular difficulty. Manufacturers of connectors can also extend the range of commercially available joints to the needed diameter of 36" and 22".

Cementing

The casing strings must be (at least partially) cemented to ensure separation between formations and to assist in supporting the weight of the casing. Conventional Portland cement, with different additives can be used for this purpose. Due to the very large volumes of slurry to be displaced and to the potential losses of slurry into weak limestones, the cementing operation will justify a careful preparation and planning and may require remedial actions, such as additional cementing into the annulus.

Piloting a well

Drilling a deviated and orientated well in order to reach a target at a certain depth is a very common operation in development drilling. The measuring equipment is available and the deviation control (or trajectory corrective) techniques are fully reliable. The storage arrangement as proposed allows some departure from the theoretical curves without jeopardizing a previously drilled well.

Drilling fluids

Mud is likely to be the most used fluid for the safe drilling of the holes. Due to the existence of a massive salt layer, it will not be possible to use the same mud for the whole well, and if mud treating section and pumping equipment are conventional, the storage of large volumes of drilling mud, some of it corrosive, will require original solutions.

Drilling equipment

The most powerful drilling machinery (pumping and lifting equipment particularly) available today is sufficient to drill and case the contemplated wells, and the down hole tools (drill pipes, drill collars and drilling bits), can be selected within the range of standard equipment.

Well Heads – Blowout Preventers

Well heads will have to be completely designed and developed for these unusual sizes, but they can be manufactured without any difficulty from welded steel plates. Blowout prevention will rely mostly on the knowledge of the formations and on specially made large diameter preventers, preferably ram type.

IMPACT ON THE ENVIRONMENT (Chapter 4)

Pollution and emissions can be kept to a minimum by specific design of the drilling machinery and of the area of recovery of cuttings, mud spills, etc.. In addition one must bear in mind that the drilling site will be used for a number of years, so that it is well worth designing and arranging it with particular care.

CONCLUSION

It is possible today to drill and case holes with such a diameter and at such depth as required for the fulfilment of the various requirements laid down in the "Deep Drilling Concept".

In addition, it is possible to design a particular architecture for individual holes, which will allow the drilling of a large number of wells from a single location, thereby reducing as much as possible the impact of drilling operations on the environment.

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