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doi:10.2204/iodp.proc.301.206.2009

CORK installations

Borehole observatories installed during Expedition 301 were designed to seal open holes so that thermal, pressure, and chemical conditions could equilibrate following the dissipation of the drilling disturbance; to facilitate collection of fluid and microbiological samples and temperature and pressure data using autonomous samplers and data logging systems; and to serve as long-term monitoring points for large-scale crustal testing. New observatory systems were designed to include a seafloor reentry cone and casing hanger(s); four concentric (nested) casing strings that penetrate through sediments and allow access to underlying basement; a series of seals (both between casing strings and between casing and the formation) that hydraulically isolate the open crustal interval at depth from the overlying ocean; downhole and seafloor instrumentation for collection of samples and data; and a seafloor wellhead that includes valves, fittings, electrical connections, and a landing platform so that the observatory can be serviced by submersible or ROV, allowing samples and data to be retrieved without recovery of the complete observatory assembly (Fisher et al.).

Hole 1026B was drilled to 295 mbsf, cased across the sediment/basement interface, and extended to 48 msb during Leg 168 (Shipboard Scientific Party, 1997). The original CORK installed in Hole 1026B included a data logger, pressure sensors, thermistors at multiple depths, and a fluid sampler, all of which were recovered in 1999 (except for the fluid sampler, which fell deeper into the hole). The original Hole 1026B CORK never completely sealed after being installed in 1996, and because crustal fluids are overpressured with respect to ambient hydrostatic conditions at Site 1026 (e.g., Davis et al., 1997; Davis and Becker, 2002; Fisher et al., 1997), this hole discharged fluid for years until the CORK was replaced during Expedition 301. As of the start of Expedition 301, warm (~64°C) altered basement fluid vented freely through the top of the wellhead.

Both of the Site U1301 boreholes contained four nested casing strings: 0.50 m (diameter) casing in the uppermost sediments, 0.41 m casing extending just across the sediment/basement interface, 0.27 m casing extending into basement, and 0.11 m inner CORK casing that houses instrument strings and plugs (Fig. F3). The two largest casing strings were sealed by collapse of unconsolidated sediments, and the 0.41 m string was also cemented across the sediment/basement interface. The annulus between 0.41 and 0.27 m casing strings at Site U1301 was supposed to contain a rubber mechanical casing seal near the seafloor, but this component was unfortunately not available for use during Expedition 301. An attempt was made to seal the 0.27 m casing strings at depth with cement, but rubbly basement prevented this cement from sealing between casing and the borehole wall. Operations were additionally complicated in Hole U1301B by the separation of the unwelded 0.27 m casing string into two sections, leaving a gap just above the sediment/basement interface (Fig. F3B).

The CORK installed in Hole U1301A included a casing packer (as part of the 0.11 m inner casing) that was set inside 0.27 m casing. In contrast, the CORK installed in Hole U1301B included two casing packers set in open hole, intended to hydraulically isolate sections of the upper crust (Fisher et al.; also see the "Site U1301" chapter).

Hole 1027C was drilled during Leg 168, 2200 m east of Hole 1026C, where sediment thickness is 575 m above a buried basement low (Figs. F1B, F3). Hole 1027C penetrated to 632 mbsf. The upper part of the hole was cased through sediments and uppermost basement, with 54 m of open hole. The open interval near the base of Hole 1027C comprises a diabase sill, intercalated sediments, and basalt breccia overlying 26 m of extrusive volcanic rocks (Shipboard Scientific Party, 1997). A CORK installed in Hole 1027C during Leg 168 included a data logger, pressure sensors, thermistors at multiple depths, and a fluid sampler. These instruments were retrieved in 1999 to allow recovery of a basement fluid sampler, and the pressure logging system was replaced. In contrast to Hole 1026B, Hole 1027C was underpressured with respect to ambient hydrostatic conditions (–26 kPa) (Davis and Becker, 2002). Researchers intended to replace the CORK system in Hole 1027C during Expedition 301 but ran out of time and supplies. Thus the CORK in Hole 1027C remained fully sealed and recorded formation pressure before, during, and after Expedition 301.

Expedition 301 CORKs and the preexisting CORK in Hole 1027C were visited with the ROV ROPOS soon after the drilling expedition in September 2004 and again in September 2005 with the submersible Alvin. Data recovered during these dives showed that the Hole 1026B observatory was sealed and operating as intended, although the pressure in Hole 1026B was recovering slowly from the thermal perturbation associated with 8 y of upflow of warm formation fluid from the unsealed hole. CORKs in Holes U1301A and U1301B were incompletely sealed, for reasons discussed earlier, allowing cold ocean-bottom water to flow into the formation following CORK installation. The flow of cold water into the crust at Site U1301 caused a measurable pressure perturbation at Site 1027, 2.4 km away, comprising an inadvertent crosshole test.

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