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doi:10.2204/iodp.proc.301.106.2005

Hole completion

CORK observatory deployment operations during Expedition 301 are described in considerable detail in "Operations," and CORK designs and final configurations are described by Fisher et al. (this volume). In the remainder of this section, we provide an overview of CORK operations and configuration in Holes U1301A and U1301B.

Hole U1301A

Hole U1301A was drilled using a 14¾ inch tricone bit to a TD of 369.7 mbsf, 107.5 m into basement. This was originally intended to be the deeper of two basement holes drilled during Expedition 301, but after two unsuccessful attempts to install a long 10¾ inch casing string, we made Hole U1301A a shallower basement completion and installed the first CORK of the expedition (see "Operations"). We reconfigured the 10¾ inch casing string to extend to 277.1 mbsf, 14.9 m into basement. Once the casing was emplaced and cemented, we drilled out the casing shoe and checked the final depth of the 14¾ inch hole. Resistance was encountered at 296.2 mbsf (34.0 m into basement), so this became the maximum depth to which 4½ inch CORK casing could be run (Fig. F77).

The CORK system installed in Hole U1301A included a single monitoring interval. The large diameter of the borehole precluded setting a CORK packer in open hole, so the packer was set near the bottom of the 10¾ inch casing. Slotted 4½ inch casing was extended below the packer element to 291.8 mbsf so that OsmoSamplers and temperature sensors could be installed and protected for future recovery (Fisher et al., this volume).

We used surplus umbilical from Leg 196, comprising a single ½ inch packer inflation line, six ¼ inch pressure-monitoring and sampling lines, and one ⅛ inch sampling line. The ¼ inch and ⅛ inch lines were run through the packer and ended in small wire-wrapped screens. Four of the screens attached to ¼ inch lines were attached to the 4½ inch casing just below the packer element, and the remaining screens and lines were terminated roughly in the middle of the 4½ inch slotted casing (Fig. F77).

The pass-through across the 10¾ inch casing seal that was not plumbed to a formation sampling or monitoring line was connected to a three-way valve and manifold at the wellhead for future installation of pressure-monitoring instrumentation. This plumbing will allow monitoring of fluid pressure below the casing seal and above the packer element, to evaluate system integrity. All valves in the wellhead were left open during deployment to prevent air from being trapped in the sampling and monitoring lines. An OsmoSampler was attached to one of the fluid-sampling manifolds for short-term collection of fluids during the initial few weeks of borehole equilibration.

After we deployed the CORK system from the ship and reentered Hole U1301A, the CORK body was held a few meters above the cone so that we could deploy the instrument string. The Hole U1301A instrument string included four OsmoSampler packages. The uppermost OsmoSampler contains a copper coil for gas sampling. The next OsmoSampler contains microbiological incubation substrate. The third OsmoSampler has Teflon tubing for fluid sampling and rare earth tracer injection. The final OsmoSampler includes a module for acid injection into the sampling line to prevent precipitation of metal compounds. There is a single self-contained temperature logger in each of the OsmoSamplers (3.7 m apart), and two additional temperature instruments were installed 2.5 m and 7.5 m above the bottom plug (Fig. F77). Thus, temperature monitoring in Hole U1301A extends across ~24.2 m of upper basement. A 1 inch rod was welded across the opening in the bottom of the 4½ inch casing to prevent the OsmoSamplers from being lost through the plug if the line that holds these instruments breaks.

Instrument string deployment went smoothly, and the CORK was set in the cone and the packer was inflated in the casing. The submersible/ROV platform was assembled in the moonpool and lowered onto the CORK by wireline and released. We went back down with the camera to check the landing platform prior to releasing the CORK running tool. The platform appeared to be properly deployed, so we released from the CORK head, ending seafloor operations in Hole U1301A.

Hole U1301B

Hole U1301B was drilled to a total depth of 582.8 mbsf, 318 m into basement. The last 232 m were drilled and cored with a 9⅞ inch tricone RCB bit. Large sections of the upper 100 m of open hole were washed out, whereas the lower 130 m of open hole was generally to gauge (see "Wireline logging"). Several potential CORK packer seats were identified in the upper 100 m of open hole, and we planned to isolate the upper and lower parts of the open hole for monitoring. We did not consider setting one of the CORK packers in 10¾ inch casing because we were not confident that there was a good cement seal between this casing and the formation, potentially leaving the annulus between the 10¾ inch and 16 inch casing open all the way to the seafloor. In addition, this would have resulted in isolation of a basement zone that overlapped with that isolated below the CORK in Hole U1301A, just 36 m away.

After completing all other operations in Hole U1301B, we initially deployed a CORK system with three casing packers, with the intent of setting all three in open hole. However, the CORK casing system became stuck in the 10¾ inch casing, or in the throat of the reentry cone, and most of the casing, packers, and umbilical for this system were laid out onto the seafloor around the cone, with an unknown amount remaining in the 10¾ inch casing (see "Operations"). We believe that the CORK casing became stuck because it was too light in weight, particularly because the spring-centralizers (added to assist with passage through a break in the 10¾ inch casing) created excessive friction in the 10¾ inch casing.

Fortunately, we were able to fish all of the junked CORK casing and umbilical from the cone and surrounding seafloor, and we used components originally intended for the CORK in Hole 1027C to fabricate another CORK installation for Hole U1301B. We were ultimately successful in deploying this second CORK system.

The final Hole U1301B CORK system included two casing packers set in open hole at depths of 428.8 and 472.0 mbsf (163.6 and 206.8 m subbasement [msb], respectively) (Fig. F78). The lowermost packer element isolates the deepest ~120 m of Hole U1301B, whereas this and the shallowest packer isolate a 42 m thick interval within the transition between shallower and deeper parts of the hole. Based on caliper and other wireline log responses, it appears that the upper 100 m of the hole has formation properties associated with the most brecciated and rubbly parts of basement. In contrast, the lower 120 m of the hole appears to be more massive, although it still appears to have relatively high bulk permeability (see "Packer experiments"). The Hole U1301B CORK system also includes an upper monitored interval, comprising uppermost basement and the entire 10¾ inch and 16 inch casing strings below the cone. It should be possible to assess the quality of the hydrogeologic seal at the seafloor using the pressure monitoring line into this uppermost interval. The upper zone includes no fluid sampling, but this interval is sampled in nearby Hole U1301A.

The Hole U1301B CORK system included an umbilical containing nine separate lines: a single ½ inch packer inflation line, four ¼ inch pressure-monitoring and sampling lines, and four ¼ inch sampling lines. A separate ½ inch Tefzel (Teflon variant) microbiological sampling line was run to the deepest monitored zone. Four small intake screens were deployed below each of the packer elements.

The bottom of the CORK installation was considerably different from that originally planned. We had hoped to minimize the amount of metal below the deepest packer to reduce contamination and its influence on fluid and microbiological sampling and incubation. However, we learned from the first (attempted) CORK deployment in Hole U1301B that it is essential to put enough weight at the bottom of the CORK casing string to pull the system into the hole, particularly when the long casing string is completely unsupported in the water column above the reentry cone. For this reason, we deployed 35 m of drill collars and crossovers below the lower packer, including ~10,000 lb of metal. We put plastic heat shrink tubing on the end of the lower CORK packer casing joint, above the first drill collar, to help isolate the titanium intake screen from the contaminating influence of the underlying stainless steel.

As with other CORK deployments during Expedition 301, all sampling and monitoring valves were left open on deployment to make sure that no air was trapped in the lines. These valves will be closed during a ROV visit to the CORK 3 weeks after Expedition 301. Three OsmoSamplers were attached at the wellhead for short-term collection of fluids in the isolated basement zones during the initial few weeks of CORK equilibration. Two of these were recovered during the ROV visit, but the third remains in place to continue sampling fluids during the first year of borehole recovery. This sampler will be replaced when other samplers are deployed by submersible in summer 2005.

Once the CORK system was deployed most of the way into Hole U1301B, we deployed the internal string of temperature loggers, OsmoSamplers, and microbiological substrate. Because we knew that we would not be deploying a new CORK in Hole 1027C during Expedition 301, we deployed extra instruments in Hole U1301B. The downhole sensor and sampling string included 14 autonomous temperature loggers and six OsmoSamplers packages, including two microbiological growth packages (Fig. F78). Temperature monitoring extends from ~1 m below top of basement to ~263 m into basement, with typical sensor spacing of ~20–25 m. All of the downhole OsmoSamplers and microbiology growth systems have their intake lines extending beyond the bottom of the 4½ inch CORK casing system, in open hole.

The uppermost OsmoSampler contains a copper coil for gas sampling. The next OsmoSampler contains microbiological substrate in flow cells. The third OsmoSampler has Teflon tubing for fluid sampling and additional microbiological substrate. The fourth OsmoSampler includes a module for acid injection into the sampling line, to prevent precipitation of metal compounds, and microbiological substrate. The fifth OsmoSampler is injecting rare earth element tracers, and the final OsmoSampler module is configured for acid addition. There is a stainless steel sinker bar at the bottom of the sensor and sampling string. Three of the OsmoSamplers contain autonomous temperature loggers (Fig. F78).

Preparation and deployment of the instrument string went smoothly, although we had to replace the shear pin in the plug-setting tool when the first one failed to release the top plug. After release of the instruments, the CORK was set in the cone and the packers were inflated. The submersible/ROV platform was assembled in the moonpool and lowered onto the CORK by wireline and then released. We watched the platform being deployed and confirmed that it was properly seated on the cone.

The submersible/ROV platform had been modified to allow us to cement the Hole U1301B CORK into place in an effort to seal the annulus between 10¾ inch and 16 inch casing strings. We tripped the pipe back to the ship and made up a cementing BHA, and then returned to the seafloor and "reentered" the cone through the hole in the landing platform. We pumped a plug of bentonite mud followed immediately by the last cement left on the ship.

Final operations around Hole U1301B included a fishing trip to remove a piece of CORK casing from the initial deployment that was sticking vertically from the seafloor and might have posed a hazard to submersible and ROV operations. This piece of casing was pulled from the seafloor and deposited 300 m west of Hole U1301B, in the same pile where the rest of the CORK casing had been deposited. We conducted a camera survey of a 100 m × 100 m box around Holes U1301A and U1301B and two 200 m long swaths to the east and north of the holes that could be used as approach corridors during submersible or ROV operations. We found no other items on the seafloor that might pose a hazard for future operations.

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