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doi:10.2204/iodp.proc.336.109.2012

Expedition 336 CORKs: detailed summaries for each CORK installed

Hole 395A

The configuration of the CORK as it was initially deployed in Hole 395A is summarized in Figure F19 and is described in more detail in the “Site 395” chapter (Expedition 336 Scientists, 2012b). The Hole 395A CORK was designed to monitor four zones within igneous basement: a shallow interval just below the casing in uppermost basement, two intermediate intervals of basement, and a deep interval within the lowermost cased basement. These intervals were determined based on previous logging data, which defined an upper rubbly, high-flow zone, an intermediate high-flow zone, and a lower low-flow warmer basement regime at the bottom of Hole 395A. The upper crustal portion was intended to be sealed with a combination packer (inflatable and swellable) in the 11¾ inch casing, and the borehole was to be sealed with an inflation packer against the wall of the basaltic formation at the lower portion of the interval. In this interval, slotted casing was deployed from 114 to 140 mbsf. The middle zone ranges from 147 to 462 mbsf and has two sampling horizons from 211 to 282 mbsf and 409 to 436 mbsf. The deeper section of the borehole was to be sealed with an inflation packer set at 460 mbsf. Umbilicals with internal stainless steel or Tefzel tubing were strapped to the outside of the casing and connected to miniscreens located at ~120, 430, and 506 mbsf. At each depth, three stainless steel miniscreens were attached to stainless steel tubes (one with ⅛ inch diameter and two with ¼ inch diameter). A fourth miniscreen of titanium was attached to a Tefzel tube (½ inch diameter). An additional ½ inch stainless steel tube was used to inflate the packers, which have a check valve that opens at 25 psi.

Downhole OsmoSamplers, microbiological experiments, and temperature and oxygen sensors were all deployed in the Hole 395A CORK. Four sets of downhole OsmoSampler packages were deployed to target four sampling horizons. Sensors and experimental packages were deployed within slotted fiberglass casing within the top three intervals and within perforated, coated casing and drill collars in the lowermost interval.

The Hole 395A CORK instrument string contains eight FLOCS units (Fig. F18): four in the enrichment OsmoSampler packages and four in the MBIO packages. The enrichment OsmoSampler FLOCS units pull formation fluids mixed with enrichment solution through two serially connected chambers containing cassettes of different substrates (basalts, olivine, siderite, sphalerite, chalcopyrite, pyrrhotite, hematite, pyrite, and glass wool and beads). Separate osmotic pumps irrigate each chamber of the FLOCS unit in the MBIO OsmoSampler packages. One chamber contains basalts, olivine, siderite, sphalerite, chalcopyrite, pyrrhotite, and hematite; the other chamber contains larger volumes of glassy basalt and pyrite, plus glass wool and beads. All of the FLOCS have eight panels of rock chips (2–4 chips/panel, ~3 mm × 3 mm) mounted on one side of the FLOCS body to allow passive colonization on polished rock chips (as opposed to the slow advective pumped colonization in the chambers). The FLOCS used in the enrichment OsmoSampler package contains grids of barite, Hole 395A basalts collected during Leg 45, sphalerite, pyrite, goethite, and hematite. The FLOCS used in the MBIO OsmoSampler package contains grids of rhyolite, glassy basalt, Hole 395A basalts, olivine, chalcopyrite, pyrrhotite, and magnetite.

The assembly and installation of the CORK in Hole 395A proceeded well until the CORK head broke off during the final step of releasing the CORK running tool, as described in more detail in the “Site 395” chapter (Expedition 336 Scientists, 2012b). The CORK head experienced forces that bent the wellhead and severed its 5 inch pipe ~4 m below the top of the reentry cone (Fig. F20), and knocked the landing seat from its original centralized position (Fig. F21). The Spectra rope and umbilicals were also severed, leaving the downhole tool string in place. Based on the portion of the CORK wellhead recovered, the upper end of the remaining 5 inch diameter cup packer subassembly near the seafloor (5 inch pipe mandrel) is not completely rounded but may be open enough to allow recovery of the internal downhole samplers (using fishing tools), sensors, and experiments in the future (Fig. F20). Several stainless steel tubes likely extend above the cup packers and the top of the 5 inch casing. Damage to stabilizing fins above the cup packers suggests that they may have been too large in diameter to enter the throat of the reentry cone (DSDP documentation indicated a 24 inch ID, but this is now thought to be less), which may have been the root cause of the installation failure. Similar damage was observed on the Leg 174B CORK that was recovered (it, too, did not fully land). The CORK pressure logging system was recovered along with the broken-off wellhead. The recorded data do not definitively resolve whether or not the downhole CORK packers inflated. A plan is being formulated to recover the downhole instrument string in the future with the aid of an ROV.

Hole U1382A

The configuration of the primary parts of the subseafloor CORKed observatory installed in Hole U1382A are depicted in Figure F22. The Hole U1382A CORK isolates one upper-basement interval for experiments and instruments, which are all deployed within coated perforated casing within the borehole. On the basis of drilling rate and core recovery, the screens and downhole instrument string targeted a single zone centered at 161 mbsf. Because of the need to keep the CORK in tension, 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 188.7 mbsf), six perforated 6 inch OD drill collars, a crossover, a 15 m long section of perforated 5½ inch OD casing, a crossover to fiberglass, five nonslotted fiberglass (4½ inch; OD = 4.57 inches, ID = 3.89 inches, connection OD = 5¾ inches) casings, a crossover (5 inch OD, 4.05 inch ID), a landing seat (3.375 inch), and a combination packer (inflatable and swellable) that was set in casing with its base at 101.4 mbsf.

Downhole OsmoSamplers, microbiological experiments, and temperature and oxygen sensors were all deployed in the Hole U1382A CORK, identically to the systems described above for Hole 395A. One set of downhole OsmoSampler packages, an oxygen probe, and temperature probes were deployed to target the single sampling horizon. Sensors and experimental packages were deployed within slotted fiberglass and perforated steel casing (Fig. F16). The Hole U1382A CORK instrument string contains a total of four FLOCS units (Fig. F18): two in the enrichment OsmoSampler package and two in the MBIO packages. See above for further details on these downhole experimental packages (Hole 395A).

In addition to the downhole instrumentation packages, wellhead experimental packages were deployed in Hole U1382A as well. As described above, these packages consisted of a standard OsmoSampler package and an MBIO OsmoSampler package (each with a 12-membrane osmotic pump and a 300 m Teflon coil) connected to the sampling line of a fast-flow osmotic pump. The MBIO OsmoSampler package at the Hole U1382A wellhead consisted of FLOCS units containing siderite, rhyolite, chalcopyrite, pyrite, pyrrhotite, sphaelerite, basalts, olivine, goethite, hematite, and glass wool and beads.

Hole U1383B

The configuration of the cone, casing, and potential CORK profile for Hole U1383B is depicted in Figure F23. A landing platform was also installed by free fall, which was slightly off center within the cone in Hole U1383B. Approximately 18 m of open hole is associated with U1383B, which was intended for future CORK installation (see below).

Hole U1383C

The configuration of the primary parts of the subseafloor CORK observatory installed in Hole U1383C are depicted in Figure F24. Three observatory intervals were isolated at shallow, middle, and deep depths. Sensors and experimental packages were deployed within slotted fiberglass casing within the top two intervals and within perforated, coated steel casing and drill collars in the lowermost interval (Fig. F24). The CORK screens and downhole instrument string were selected on the basis of drilling rate, core recovery, and caliper logging data. The upper zone is isolated by a combination packer and landing seat in the casing (58.4 mbsf) 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.

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 OD drill collars, a crossover, a shortened 3.12 m long section of perforated 5½ inch OD casing to maximize the amount of open borehole below, a crossover to fiberglass, a landing seat (2⅞ inch), and an inflatable packer.

Downhole OsmoSamplers, microbiological experiments, and temperature and oxygen sensors were all deployed in the Hole U1383C CORK, identical to the systems described above for Hole 395A. The Hole U1383C CORK instrument string contains a total of six FLOCS units (Fig. F24): three in the enrichment OsmoSampler packages and three in the MBIO packages. See above (Hole 395A) for further details on these downhole experimental packages.

In addition to the downhole instrumentation packages, wellhead experimental packages were deployed in Hole U1383C, coupled to the fast-flow OsmoSampler systems described above and identical to the wellhead systems deployed in Hole U1382A.

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