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doi:10.2204/iodp.proc.327.103.2011

Borehole observatories

CORK deployment operations during Expedition 327 are described in considerable detail in “Operations,” and CORK designs and configurations are discussed by Fisher, Wheat, et al. In the remainder of this section, we provide an overview of observatory operations and configurations in Holes U1362A and U1362B.

Hole U1362A

Hole U1362A was drilled to a total depth of 528.0 mbsf, equivalent to 292.0 msb (Fig. F48; Table T1). The lowermost 182.0 m of the hole was drilled with a 9⅞ inch bit, whereas the uppermost 37.5 m of open basement hole (below the 10¾ inch casing shoe) was drilled with a 14¾ inch bit. The end of the CORK installation was placed at 469.7 mbsf (233.7 msb), 58.3 m above the total depth of the hole, but a drill string depth check immediately before packer testing indicated fill at 519.0 mbsf (283.0 msb), placing 49.3 m of open hole below the base of the CORK. The base of the CORK assembly comprises (from the bottom up) a bullnose, three perforated drill collars, a section of perforated 5½ inch casing, and the deepest set of CORK casing packers (one inflatable and two swellable elements). All of the parts from the base of the inflatable packer downward are coated to reduce reactivity. The deepest set of packers separates the lower and upper monitored basement intervals (Fig. F48).

The CORK deployed in Hole U1362A monitors two basement intervals: a shallow interval extending from the base of the 10¾ inch casing to the top of the deepest set of swellable packers (307.5–417.5 mbsf) and a deeper interval extending from the base of the deepest inflatable packer to the bottom of the hole (429.2–528.0 mbsf). Pressure in both intervals is monitored through ¼ inch stainless steel tubing connected to miniscreens installed just below the inflatable packers at the top of the isolated intervals. A third pressure gauge is set to monitor seafloor pressure but can be switched to monitor the cased interval above the upper set of swellable packers. Pressure lines were de-aired on deployment and left open so that an installed pressure logger would record borehole and seafloor conditions immediately.

Eleven autonomous temperature probes were deployed, including two installed in OsmoSamplers (Wheat et al.) suspended inside perforated and coated drill collars at depth in the hole (Fig. F48; Table T11). The downhole OsmoSampler string comprises six separate instruments and a 200 lb (in water) sinker bar (Table T12). Three ½ inch stainless steel fluid sampling lines are configured at two depths (two sampling below packers in the upper interval and one sampling below packers in the lower interval). No wellhead OsmoSamplers were installed when the Hole U1362A CORK was deployed so that the system can remain closed during the subsequent year of pressure recovery (until a planned servicing expedition in summer 2011). One ½ inch microbiology sampling line (PTFE inner liner within a stainless steel sheath and outer plastic protective layer) ends in a titanium miniscreen that rests on perforated and coated 5½ inch casing, 7 m below the base of the deepest inflatable packer, just above the perforated collars (Table T13).

The CORK was deployed from the drillship and lowered to the seafloor, and the OsmoSampler and temperature logger string were installed before reentering Hole U1362A. The tapered top plug seals into the CORK wellhead with an O-ring and is held in place with a latch system. The instrument string was assembled as it was lowered into the hole, and then the top plug was attached and lowered into the wellhead. The top plug was lowered into place in the wellhead using the coring line, and latching was verified by pulling up with several hundred pounds once the plug was in position. A weakened overshot shear pin allowed release by jarring off with a single stroke.

Inspection with the VIT camera showed that at least the sinker bar and one OsmoSampler (and perhaps more than one) extended beyond the end of the CORK bullnose. A check on planned cable and CORK lengths did not explain why the instrument string was apparently too long, but this situation required recovery of the string before we could reenter Hole U1362A. We did not have a latch-release tool available on the JOIDES Resolution, so we had to return the CORK to the ship to remove and shorten the string. We were able to recover the CORK body and secure it above the moonpool so that the top of the wellhead extended through the rotary table. The CORK running tool was removed, and the top plug was extracted by turning release bolts on either side of the wellhead. We picked up the top plug just high enough to secure it and put sufficient slack in the cable to remove ~10 m of length. The top plug was again latched into place and the CORK was returned to the seafloor. We reentered Hole U1362A and slowly lowered the CORK into place, encountering no resistance or other difficulties as the CORK was landed in the cone.

A subsequent test of a ~25 m section of new Spectra cable using the traveling block on the drilling rig indicated that cable stretch on the order of 2% should be expected for the new Spectra cable deployed in the Expedition 327 CORKs, somewhat more than the ~1% stretch previously calculated on the basis of manufacturer specifications. The total stretch appeared to occur mostly in an initial set of ~1.5% when the Spectra cable was first loaded, with a small amount of additional stretch proportional to loads in the 500–1000 lb range. The larger overall stretch factor of 2% was used for preparation of the Hole U1362B instrument string.

The submersible/ROV platform was assembled in the moonpool and lowered onto the CORK by wireline and released. Unfortunately, the platform release system failed initially to disengage at one of three latch points, but it was eventually worked loose. However, the platform was observed to be cocked and hung up on the top of the CORK wellhead on one side. We recovered the VIT camera frame, which was attached to the platform running tool, and then returned with the camera to assess the position of the platform. We were able to land the platform by nudging it along one side with the VIT camera frame and then recovered the camera system and secured it for departure. Upon its return, the platform release system was found to have been hung up by a bolt installed on the top of the CORK wellhead that had too high a profile. A lower profile bolt was used on the Hole U1362B CORK.

Hole U1362B

Observatory installation operations in Hole U1362B were similar in some ways to those in Hole U1362A, but there were important differences. Hole U1362B was drilled through 242 m of sediment and 117 m of basement to a total depth of 359.0 mbsf (Fig. F49; Table T2). Thus, Hole U1362B had 175 m less open basement than did Hole U1362A. In addition, the basement into which Hole U1362B was drilled was restricted entirely to the rubbly and unstable part of the upper crust, meaning it was not possible to set CORK casing packers in the open hole. Thus, the Hole U1326B CORK was designed to monitor only a single interval of upper basement, with packers set in the 10¾ inch casing shoe. Hole U1362B also served as the perturbation point for the 24 h pumping and tracer injection experiment (see “Hydrologic experiments” and Fisher, Cowen, et al.), which was completed just prior to CORK installation.

The open-basement interval that was configured for monitoring with the Hole U1362B CORK extends from the base of the 10¾ inch casing shoe at 272.0 mbsf to the total depth of 359.0 mbsf. This interval includes 11.0 m of 14¾ inch hole and an additional 70.0 m of 9⅞ inch hole. The bullnose at the end of the CORK assembly was placed at 311.0 mbsf, 47.5 m above total depth and 36.5 m above the depth of fill encountered with the drill bit during a final depth check of the hole made just before assembling and running the CORK. At the base of the CORK assembly are (from the bottom up) a bullnose, three perforated drill collars, a section of perforated 5½ inch casing, and a single set of CORK casing packers (one inflatable and two swellable). All parts from the base of the inflatable packer downward are coated to reduce reactivity. The packers separate the monitored basement interval from the annular gap between the 10¾ and 16 inch casing strings (Fig. F49).

Pressure in the basement interval is monitored through ¼ inch stainless steel tubing connected to a miniscreen installed just below the inflatable packer near the bottom of the 10¾ inch casing shoe. A second pressure gauge in the wellhead monitors the cased annular interval that extends from above the cement around the 10¾ inch casing shoe to the 10¾ inch swellable casing packer set against the 16 inch casing near the seafloor. A third dedicated gauge in the wellhead monitors seafloor pressure. Pressure lines extending to depth were de-aired on deployment, and the valves were oriented so that the wellhead pressure logger would immediately record borehole, cased interval, and seafloor conditions.

Eight autonomous temperature probes were deployed across a range of depths, including two installed in OsmoSamplers suspended inside the perforated and coated drill collars at depth (Fig. F49; Table T14). The downhole OsmoSampler string comprises six separate instruments and a 200 lb sinker bar (Table T15). Three ½ inch stainless steel fluid sampling lines terminate in miniscreens installed 4 m from the base of the perforated 5½ inch casing, providing sampling redundancy. As for the Hole U1362A CORK, no wellhead OsmoSamplers were installed on deployment so that the system could remain closed during the subsequent year of pressure recovery. A single ½ inch PTFE microbiology sampling line ends in a titanium miniscreen that rests at the base of the perforated and coated 5½ inch casing, 7 m below the base of the deepest inflatable packer, just above the perforated collars (Table T16).

The CORK was deployed from the drillship and lowered to the seafloor, and the OsmoSampler and temperature logger string were installed before reentry into Hole U1362B. The instrument string was assembled as it was lowered into the CORK, and then the top plug and latch system were lowered into place in the wellhead using the coring line. We confirmed that the string was not too long by monitoring the end of the CORK bullnose with the VIT camera during string deployment and verifying that the sinker bar did not emerge. The top plug was landed, and latching was verified by pulling up with 500–800 lb once the plug was in position; then the sinker bar was released. However, while making a pipe connection after reentering Hole U1362B but before the CORK was landed, operations personnel noted water flowing from the top of the pipe at the rig floor, suggesting that the instrument plug might not be latched into place. A sinker bar was run down the pipe on the coring line and then latched into the top plug. At that time, we could not verify that the plug was latched, and it is possible that the positive latch indication from a few hours earlier was incorrectly interpreted because of ship heave (which causes coring line tension to vary). Unfortunately, we were then unable to unlatch from the top plug, so it was brought to the surface for inspection. No problems were noted, but a different top plug was configured for deployment and the instrument string was lowered for a second time into the CORK. Once again, we were not able to verify latching, but we were able to shear off the CORK after six or seven attempts. As long as the top plug is in position, the O-ring should seal the CORK whether or not the plug is latched.

Once the CORK was landed in the reentry cone, the casing packer was inflated using the rig pumps. The inflation lines held pressure when the drill string was shut in from the ship, suggesting that the top plug was fully landed. Once the packer was inflated, we deployed the ROV platform and detached from the CORK and verified the positioning of the top plug with the VIT camera.

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