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doi:10.2204/iodp.proc.301.103.2005

CORK configurations and deployments during Expedition 301

In this section, we summarize CORK mechanical and hydraulic configurations and describe the instrument strings and CORK head samplers deployed in each borehole. The next section describes initial operations and preliminary results for the first ROV visit to the Expedition 301 CORKs during September 2004. Table T5 summarizes key characteristics of each CORK system, and schematics illustrating CORK plumbing and the subseafloor layout of each observatory are presented hole by hole. Table T6 lists varieties and depths of temperature loggers deployed in each CORK system.

Hole 1026B

The mechanical and plumbing configurations of the CORK deployed in Hole 1026B are summarized in Figures F11 and F12. Hole 1026B was drilled during Leg 168, originally to a total depth (TD) of 295.2 meters below seafloor (mbsf), 48.1 m subbasement (msb) (Shipboard Scientific Party, 1997). However, upper basement proved to be unstable and a piece of drill pipe was installed as a "liner" to keep the hole open. This liner extends up into the 10¾ inch casing and prevents direct exposure of open hole in basement. There is clearly a good hydraulic connection between basement and the cased hole because this hole has produced copious amounts of formation fluid during the last 8 y (e.g., Cowen et al., 2003; Fisher et al., 1997; Wheat et al., 2004), but these fluids must pass between the 10¾ inch casing and the liner before rising up the hole.

As a result of having the liner blocking the bottom of the hole, the Expedition 301 CORK in Hole 1026B was set with the packer in 10¾ inch casing, ~100 m above hole TD. The instrument string was positioned almost entirely beyond the end of the 4½ inch CORK casing, within the 10¾ inch cased hole, because there was no danger that poor hole conditions would "hang up" and prevent recovery of the instrument string (Fig. F11). The Hole 1026B CORK was deployed with a single CORK head OsmoSampler, which was subsequently recovered during September 2004 ROV operations (Fig. F13).

The instrument string deployed in this CORK contained three OsmoSampler systems, one each for gas sampling, acid injection and microbiological growth, and tracer injection. Each of the last two OsmoSampler systems contained a single temperature logger. No additional temperature loggers were deployed in Hole 1026B because the instrument string did not extend into basement and upper basement thermal conditions have been determined at this site during earlier drillship and CORK experiments.

A single OsmoSampling system was attached to the CORK head during deployment to monitor the open hole interval below the casing packer and was subsequently recovered during September 2004 ROV operations. All valves on the CORK head were left open on deployment and subsequently closed by the ROV.

Hole U1301A

The mechanical and plumbing configurations of the CORK deployed in Hole U1301A are shown in Figures F14 and F15. Hole U1301A was drilled to TD of 369.7 mbsf, 107.5 m into basement. We originally attempted to set a long string of 10¾ inch casing with the shoe close to TD, in the hope of making Hole U1301A the deeper basement installation. Unfortunately, we were unable to land the long casing string and elected to deploy a shorter casing string, making Hole U1301A a shallower basement installation. Casing was eventually set at 277.1 mbsf (14.9 msb), leaving as much as 92.6 m of open 14¾ inch hole below (Fig. F14). However, when we reentered Hole U1301A to check hole depth before making up the CORK, resistance was encountered at 296.2 mbsf, <20 m beyond the 10¾ inch casing shoe. Because hole conditions were unstable and the amount of open hole was limited, we decided to deploy a 4½ inch CORK casing that extended to 291.2 mbsf (29.0 msb).

The OsmoSampler systems we deployed comprised four sections (one of each kind of sampler) plus a sinker bar, requiring 17.4 m below the lower CORK plug, so we cut vertical slots into the lowermost 18 m of 4½ inch casing. The OsmoSampler systems and sinker bar were placed entirely within this slotted casing, allowing reasonably good communication with the surrounding open hole and assuring that we will be able to recover these instruments several years later. We placed six autonomous temperature loggers on the instrument string, one within each of the four OsmoSampler systems and two on the Spectra line above the lower CORK seal plug (Table T6; Fig. F14). We welded a steel rod across the opening in the "bull nose" at the end of the 4½ inch casing to make sure that the instrument string will not fall out of the casing, even if the line above the instruments were to part.

Because we isolated a single interval in the open hole but we used surplus umbilical tubing from Leg 196 (with six ⅜ inch and one ⅛ inch lines), we had redundant sampling lines available to depth. We placed four sample screens immediately below the casing packer inside the 10¾ inch casing, where they are extremely well protected, and placed the other three screens in the middle of the slotted 4½ inch casing, surrounded by open hole (Figs. F14, F15). Sampling through the various tubings and comparing results to samples collected with downhole OsmoSamplers will help with assessing how well mixed the borehole is and whether details of tube and screen placement influence sample fluid chemistry.

The Hole U1301A CORK was designed to allow continuous monitoring of two subseafloor intervals, one that included the open hole and one that included the annulus between the 10¾ inch casing and 4½ inch casing, to allow assessment of the integrity quality of the main CORK seal (Fig. F15). A single OsmoSampling system was attached to the CORK head during deployment, to monitor the open hole interval below the casing packer, and was subsequently recovered during September 2004 ROV operations. All CORK head valves were left open on deployment and subsequently closed by the ROV.

Hole U1301B

The mechanical and hydraulic configurations of the Hole U1301B CORK are shown in Figures F17 and F18. Hole U1301B was drilled to 582.8 mbsf, 317.6 m into basement. Core and wireline logs indicate that we penetrated the boundary between to two lithologically distinct intervals in upper basement: an extremely rubbly and unstable upper interval, extending from the sediment/basement interface to ~470 mbsf (207.5 msb), and a more massive and layered interval below this depth. Packer experiments demonstrate that the formation surrounding the borehole is extremely permeable throughout the drilled interval, but suggest that the lower 100–120 m of the hole may be somewhat less permeable than the upper part. We selected seats for CORK packers with this information in mind, and with the intention of hydraulically separating and monitoring these two distinct intervals of upper oceanic crust.

We installed a long 10¾ inch casing string in Hole U1301B, isolating and stabilizing the uppermost 80 m of rubbly basement. As discussed elsewhere, we were unsuccessful in deploying our original Hole U1301B CORK system intended for use in the deeper basement hole (see the "Expedition 301 summary" and "Site U1301" chapters), which included three 4½ inch casing packers, but we successfully deployed a two-packer CORK system that allowed isolation of three crustal zones. A depth check immediately prior to CORK installation showed that the hole remained open almost to TD, giving us confidence that we could set packers in open hole and deploy OsmoSampler packages beyond the end of the 4½ inch CORK casing.

The Hole U1301B CORK head required modification prior to deployment. Because the Hole U1301B reentry cone settled below the mudline and a large ring of cuttings collected around the cone, we were concerned that the CORK head might sit too low below the seafloor for OsmoSamplers and valves to be accessed by an ROV or submersible during servicing visits. For this reason, the Hole U1301B CORK head was extended by 2.0 m by adding a short section of 4½ inch casing below the original CORK seal, attaching a new seal below the extension, and welding gussets around the new casing and the old seal for strength and stability (Fig. F19A).

CORK casing packers were set in open basement at 403.3 mbsf (138.1 msb) and 473.5 mbsf (208.3 msb), allowing isolation of three distinct basement intervals. The deeper two isolated intervals in Hole U1301B are >33.6 m deeper than the TD of nearby Hole U1301A, whereas the upper interval in Hole U1301B overlaps with the monitored zone in Hole U1301A (Fig. F10 in the "Expedition 301 summary" chapter). The upper monitored interval in Hole U1301B also includes the annulus between the 4½ inch and 10¾ inch casing and the annulus between the 10¾ inch and 16 inch casing. The former was sealed with the main CORK seal (Fig. F3), and we attempted to seal the latter with cement that was pumped into the reentry cone around the CORK head near the end of operations in Hole U1301B.

The instrument string deployed below the CORK in Hole U1301B included six OsmoSampler systems: gas sampling, tracer injection, acid addition, and three for microbiological growth experiments (one of these also having acid addition) (Fig. F17). Three of these OsmoSampler systems contained autonomous temperature loggers, and ten additional temperature loggers were placed on the Spectra line above the bottom seal plug and below the top of basement (Fig. F17; Table T6). The instrument string was positioned so that all of the OsmoSamplers and the sinker bar extended beyond the end of the 4½ inch CORK casing, in open basement. Fluid sampling screens at the end of tubing that extended to the CORK head were positioned immediately below the casing packers in open hole (Fig. F18).

The Hole U1301B CORK was deployed with three OsmoSampler systems on the CORK head, two monitoring the middle interval, and one monitoring the deepest interval, and all valves were positioned open (Fig. F19). One of the CORK head OsmoSamplers was recovered during September 2004 ROV operations, leaving two systems to collect fluids from the middle interval, and the remaining valves on the CORK head were closed.

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