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Victoria University Antarctic Research Expedition Science and Logistics Reports 1998-99: VUWAE 43

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Drilling CRP-1 and CRP-2 has now proven that the good CIROS- 1 drilling experience was anomalous even taking into consideration that at CIROS- 1 we had the advantage of 3 rotary/coring drill strings. CRP drill holes thus far are probably more representative of the drilling conditions over much of the Antarctic shallow continental shelf. We now have to expect:
  • Sea floor surface sediments consisting of unconsolidated young [Quaternary?] glacial diamicts, possible enriched in clasts, and even clast supported. Unlithified muddy/sandy matrix. Unknown thickness but likely to be between 20 and 50 metres. Embedding the sea riser in such sediments to a depth of around 20 m bsf is a slow and difficult process and likely to take around 10 days of the 45 day drilling window. This time includes cementing the sea riser, though experience indicates this is worth doing only when it has been fully embedded (i.e. at least 12 m bsf). Experience also shows that these sediments can be cored some tens of metres ahead of the sea riser once it is supported a few metres into the sea floor.
  • CRP-1 and the upper part of CRP-2 were younger than expected (16-20 instead of 28-30 Ma). Some strata were poorly lithified, and included soft running sands. These sediments are normally difficult to drill, recover quality core and maintain a stable hole. The problem is compounded when these strata are windowed to the sea floor up-dip [shown by down-hole logging in CRP-2], allowing almost complete loss of drilling fluid and also the incursion of sea water at depth causing uncompacted sands to fluidise. The zones of most concern in the older strata were at ~ 80, 120, 160, 270 and 460 mbsf. Two of the zones lay below the cemented H casing, which had been cemented at 200 m bsf. Tests indicated that the casing was not set deep enough to provide adequate control for over-pressured fluids. CRP-2 had minor inflammable gas shows detected by the gas monitoring system on two occasions which only lasted a few minutes each. No effervescence was observed in the core on these occasions or at other times and the gas shows were considered minor and transitory Of more concern, however, is that the well returned significant volumes of drilling fluid when the inner tube was pulled to recover the core. On only one occasion did the returning fluid appear to be cut with other down-hole fluid. A pressure test showed that at the initiation of flow the locked in fluid pressure was in order of 160 psi and minor flow was still evident after 36 hours of down-hole logging. It was not clear at the time if this effect was a result of gas pockets or due to the formation relaxing after expansion caused from drill fluid pressures during the coring process. We are seeking advice from other experts familiar with slim-hole coring in gas-bearing sediments, but for the moment we need to consider the possibility of closed gas pockets that may exist up-dip in the formation.