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CORK observatory

Hole U1383C was cased through the 38.3 m thick sediment section with 10¾ inch casing to 60.4 mbsf and a 14¾ inch rathole to 69.5 mbsf. The casing was cemented in place with cement that included Cello-Flake LCM. When it was drilled out, the cement extended 16.6 m uphole from the casing shoe inside the casing. The hole was then RCB cored to 331.5 mbsf. After cleaning, a lack of fill was verified during logging.

The CORK screens and downhole instrument string targeted three zones selected on the basis of drilling rate, core recovery, and caliper log. The upper zone is defined by a combination packer and landing seat (58.4 mbsf) in the casing and the first open-hole packer and landing seat at 145.7 mbsf. Within this section the miniscreens are centered at 100 mbsf, with the slotted portion of the casing extending from 76 to 129 mbsf. The middle zone is isolated by the two open-hole packers and landing seats, with the bottom landing seat at 199.9 mbsf. Within this section the screens are centered at 162 mbsf, with the slotted casing extending from 146 to 181 mbsf. The deep zone is defined as the bottom of the deepest landing seat (199.9 mbsf) to the bottom of the hole (331.5 mbsf), with miniscreens beginning at 203 mbsf.

Because of the need to keep the CORK in tension and potential issues regarding a ledge within the borehole, additional heavy, perforated, resin-coated drill collars were used at the bottom of the CORK assembly. The monitoring section of the CORK comprises (from the bottom up) a bullnose that is not restricted (terminates at 247.6 mbsf, leaving 83.9 m of open borehole for future open-hole logging), 10 perforated 6¼ inch diameter drill collars, a crossover, a shortened 3.12 m long section of perforated 5½ inch diameter casing to maximize the amount of open borehole below, a crossover to fiberglass, a landing seat (2⅞ inch), and an inflatable packer (see Figs. F2, F3; Table T3).

All steel portions are coated with either Xylan, TK-34XT, or Amerlock to reduce reactivity (Edwards et al., 2012; Orcutt et al., 2012). However, due to handling operations, some steel was exposed. The bullnose and the first drill collars were connected, painted with an epoxy, and hung in the derrick several days prior to use. The additional four drill collars were added to this string and painted before being lowered to the moonpool area, where all of the collars were inspected. Grease (pipe dope) was wiped with 10% ethanol, and scratches in the epoxy were painted with a fast-drying epoxy paint (Alocit 28) that also dries in water. The perforated 5½ inch casing section was connected, cleaned, and painted as it was lowered past the moonpool. Both ends of the inflatable packers were painted with fast-drying epoxy paint (Alocit 28), as were any scratches in the steel landing seats and crossovers. Some grease was wiped from the fiberglass casing. Above the combination packer is more steel casing, which is untreated because none of this section is exposed to the formation of interest.

Umbilicals with internal stainless steel or Tefzel tubing were strapped to the outside of the casing and connected to miniscreens located at ~100, 162, and 203 mbsf (see Fig. F2; Table T3). Four miniscreens were deployed at 203 mbsf. Three of these screens were attached to ½ inch diameter Tefzel tubing, ⅛ inch diameter stainless steel tubing, and ¼ inch diameter tubing, ending at the base of the 5½ inch casing. The ¼ inch stainless steel tubing and miniscreen for pressure monitoring were attached directly to the packer, ~3 m shallower than the other miniscreens. Miniscreen locations for the other two depths (100 and 162 mbsf) were staggered. The ½ inch diameter Tefzel tubing was positioned at the prescribed depth, which is ~1 m deeper than the ⅛ inch diameter stainless steel tubing and ¼ inch diameter tubing for geochemistry. These miniscreens were ~1 m deeper than the ¼ inch stainless steel tubing for pressure monitoring. A single ½ inch diameter stainless steel tube was used to inflate all three packers in series, and each has a check valve that opens at 25 psi (172 kPa).

The wellhead was a standard lateral CORK (L-CORK) with a 4 inch diameter ball valve.

Downhole samplers and experiments

The downhole tool string consists of six OsmoSampler packages, a dissolved-oxygen sensor and recorder, two miniature temperature recorders, sinker bars, sealing plugs, and interspersed sections of ⅜ inch (0.95 cm) Spectra rope (Table T4). In addition, the wellhead was populated with a pressure logger capable of monitoring each of the three horizons and bottom seawater and a fast-flow OsmoSampler that included standard and MBIO OsmoSampler packages. Complete details of this deployment are provided in Edwards et al. (2012) and are summarized in the “CORK observatory” sections of the other site chapters in this volume. More information on the various OsmoSampler packages is provided in detail in Wheat et al. (2011).

Scientific and operational implications

The CORK plumbing was finished before the CORK head was landed on the moonpool doors. The CORK running tool was removed, exposing the top of the wellhead at the rig floor, and the bushings were installed to stabilize the CORK. The instrument string was deployed as designed. Lastly, the top plug was deployed and latched properly. A new hydraulic hose was made to connect to the packer inflation line, and the CORK was lowered ~5 m below the ship with all the valves open for 10 min. The wellhead was brought back to the moonpool, where the valves (including the pressure purge valves) were closed, the cap to the underwater mateable connector was removed, the ball valve was closed with the cap in place, and the fast-flow OsmoSampler package was installed. During this 10 min dunking of the wellhead, seawater was at the level of the open ball valves (the lowest open port of the wellhead plumbing), which indicates that the shallowest borehole plug sealed as planned. The CORK and ROV platform were landed, and the CORK running tool unlatched as planned.