Management of Commercially Generated Radioactive Waste - U.S. ...

6.13
For oil field rotary drilling, standard oil field drill equipment would be used. In this
method, a drill bit attached to a drill pipe is rotated from the surface, and drilling mud is
circulated through the drill pipe to carry cuttings to the surface. The drilling mud also
assists in providing borehole stability, provides lubrication and cooling, and minimizes pipe
sticking. Substantial rotary drilling experience exists; however, most of the drilling has
been in sedimentary formations.
At least the upper portions of deep rotary drilled holes would be cased; and, in fact,
the entire hole may need to be cased for borehole stability, as in the reference concept (LBL
1979). As described there, cement grouts would be pumped from the bottom of the hole up
around the steel casing to seal the casing against the drilled borehole. If the entire
borehole were cased, then the hole could be bailed dry (depending on the depth of the hole),
and could be left standing open for extended periods. If the bottom portion of the hole were
not cased, it is unlikely that the borehole would stay open if the hole were bailed dry.
Some fluid, probably with a density somewhat higher than that of fresh water, would therefore
be required in the open hole at all times.
There is little experience at drilling in hard, crystalline rocks, although such rocks
may pose no more, or fewer, problems than.drilling ultra-deep wells in sedimentary rocks. A
limited number of oil field rigs are capable of drilling to 8,000-m (25,000 ft) depths and
beyond, and there are presently four rigs in the U.S. capable of drilling to a depth of 9,000
m. The bottom portions of such holes have been drilled with a 16.5 cm (6-1/2 in.) diameter
bit, and the holes were cased to the bottom. There is some experience in drilling geothermal
wells where formation temperatures are 30-0 C (approximately 600 F) as anticipated in VDH
drilling; however, these wells have not been drilled much below 3,000 m (10,000 ft).
It is believed that deeper and larger diameter holes could be drilled. A maximum well
depth of about 11,000 m (36,000 ft) in rocks where borehole stability is not a problem is be-
lieved possible, using a 20-cm (7-7/8 in.)-diameter bit for the bottom hole. Depths of 9,000
m could be achieved with 31-cm (12-1/4 in.)-diameter bits in crystalline rocks where no gas
pressure exists. For very strong rocks, the bottom part of the hole might be left open. In
fact, for the 31-cm-diameter hole, the bottom part of the hole may have to remain open be-
cause current rigs (with current casing) would not be able to set casing to the bottom of a
9,000 m hole. A drill rig with a 15,000-m (50,000-ft)-depth capability has been designed but
not operated which would utilize the largest available components. It would provide a 22-cm
(8-1/2-in.)-diameter hole at total depth (Drilling DCW 1979). Salt has been drilled success-
fully to about 4,600 m (15,000 ft); below this, borehole closure prohibits further drilling.
Emplacement. The technology for emplacing waste canisters is not fully developed at
present. Some technology for emplacing items to depths less than 10,000 m exists. For exam-
ple, the Deep Sea Drilling Project has a hydraulically operated down-hole device that discon-
nects the boring bits.