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doi:10.2204/iodp.sp.327.2010

Risks and contingency

There are a number of risks to achieving the primary and secondary objectives of this program.

Poor hole conditions in basement

Past experience drilling basement in this area shows that hole conditions in upper basement can be poor, especially the uppermost 100 m at the locations proposed for Holes SR-2A and SR-2B, which are located along the same buried basement ridge as Sites 1026 and U1301. This is the primary reason for drilling through the uppermost 100 m of basement with a tricone bit in the basement holes, without coring, and installing 10¾ inch casing as soon as possible. Poor hole conditions and rubbly basement may also prevent sealing the base of the 10¾ inch casing with cement, which is why we have developed a mechanical casing seal system for use between the 16 and 10¾ inch casing. We still plan to pump cement with LCM at the base of the 10¾ inch casing, but this will be done after the casing is installed and (presumably) sealed against the 16 inch casing. The risk of poor hole conditions will also be mitigated by using a BHA that puts drill collars in the open hole across the entire length of exposed rock. This will prevent rubble from falling into the hole on top of the drill collars, a strategy that worked extremely well during Expedition 301. We are less concerned about poor hole conditions in the uppermost 100 m of basement at Site 1027. Drilling and coring at this site during Leg 168 was highly successful and showed that basement at this location is more altered and cemented than it is at nearby Sites 1026 and U1301. For this reason, we plan to core the interval between 19 and 59 msb in Hole 1027C.

Poor hole conditions could also make deployment of new CORKs challenging if instruments deployed within these CORKs extend into open hole. For this reason, we plan to deploy all CORK instrumentation so that it resides inside perforated casing rather than in open hole. We note that even in Hole U1301B, where hole conditions were generally good at depth during Expedition 301, there was apparently some collapse of basement rocks over the years between instrument string deployment in 2004 and the attempt to recover this string by submersible in 2009.

Poor weather/sea state

Many of the borehole operations, including installation of casing and cementing, packer experiments, and deployment of CORKs and instrument strings, are sensitive to ambient weather and sea state. Operations using the drill string in close proximity to installed CORK systems, including cementing planned for Hole U1301A and string recovery in Hole U1301B, may be especially difficult to complete if weather or sea conditions are unfavorable. We will pay careful attention to existing and forecasted weather and sea conditions and adjust operations accordingly so that we have the best opportunity to complete delicate operations when conditions permit. Even when weather and sea conditions are favorable, care must be taken to avoid damaging the CORK wellheads as part of planned operations. There will be particular risks when the BHA is positioned close to a wellhead, either immediately before or immediately after reentry. Particularly when reentering adjacent to the CORK in Hole U1301A, the ship may need to be offset slightly away from the wellhead just before removing the BHA from the cone so that it does not swing into the wellhead when it is free of the cone. It may be difficult to see the platform and wellhead clearly during these operations.

Difficulty sealing CORK observatories

We have learned from past experience that it can be challenging to seal CORK observatories, but the new approach planned for Expedition 327 (using both a casing seal and cementing with LCM at depth) should prove more effective than approaches taken during earlier expeditions. In addition, we have designed CORK instrument strings to be heavier to help hold the CORK plugs in place despite elevated (natural) formation fluid pressures in basement. Finally, we will take a more aggressive approach to use of LCM while completing remedial cementing activities in Hole U1301A than was taken during Expedition 321T.

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