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doi:10.2204/iodp.proc.336.103.2012

CORK observatory

Hole 395A was drilled during DSDP Leg 45 (Melson, Rabinowitz, et al., 1979). The hole was cased through the sediment section with 11.75 inch casing to 112 mbsf. The hole was then drilled to 664 mbsf. Hole instabilities caused fill in the deepest ~60 m of the hole, but the remaining ~600 m was found to be open on revisits during DSDP Leg 78B and ODP Legs 109 and 174B. During Expedition 336 we lowered the DEBI-t and drill bit to 600 mbsf but did not encounter the bottom of the hole. Because of long-standing hole stability, we were confident in fabricating a multilevel CORK design that penetrated much of the open hole. The number of levels and placement of packers were determined on the basis of previous downhole logs and hydrologic (packer) tests, the desire to compartmentalize the upper crust and lower crust, and the diameter of instruments and seals.

On the basis of temperature, caliper, and spontaneous potential data, three distinct hydrologic zones were identified for compartmentalization (Becker, Malone, et al., 1998; Bartetzko et al., 2001) (Fig. F11). The upper zone (112–147 mbsf) represents an opportunity to focus on the extent of sediment–basalt exchange of microbes and ions. This upper crustal portion was sealed with a combination packer (inflatable and swellable) in the 11.75 inch casing, and the borehole was 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. The deeper section of the borehole was sealed with an inflation packer. In this depth range the spontaneous potential data are consistent with multiple zones of fluid influx. To capture some of this flow, two slotted casing sections were deployed from 211 to 282 mbsf and from 409 to 436 mbsf. Depths greater than 462 mbsf represent the lower crustal zone, which is less permeable and warmer than the two zones above. Here a combination of slotted and perforated casing and drill collars was used from 473 to 532 mbsf.

Because of the need to keep the CORK in tension, heavy perforated drill collars were used at the bottom of the CORK assembly. The deepest section of the CORK comprises (from the bottom up) a bullnose that is not restricted (terminates at 532.8 mbsf), six perforated 6 inch drill collars, a crossover, a section of perforated 5.5 inch casing, a crossover to fiberglass, three 4.5 inch slotted fiberglass casings, a single nonslotted fiberglass casing, a crossover, a landing seat (2.875 inch), and an inflatable packer. All steel portions are coated with either Xylan, TK-34XT, or Amerlock to reduce reactivity (Edwards et al., 2012; Orcutt et al., 2012). However, steel that was exposed as a result of handling operations was painted with a fast-drying epoxy paint (Alocit 28) that could dry in the water while the CORK was being lowered below the rig floor. The casing exposed to the middle and upper sections is fiberglass, some of which was slotted (Edwards et al., 2012), except for the packer that separates the upper and middle sections. Like the exposed steel below, the exposed steel on the packer was painted with the Alocit epoxy. Above the combination packer is steel casing, which was untreated because this section is within the Hole 395A 11.75 inch casing and is not exposed to the formation. 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, four miniscreens were deployed and attached to stainless steel tubes (one with ⅛ inch diameter and two with ¼ inch diameter) and a Tefzel tube (0.5 inch diameter). An additional 0.5 inch stainless steel tube was used to inflate the packers, which have a check valve that opens at 25 psi.

Downhole samplers and experiments

The downhole tool string consists of four sets of six different OsmoSampler packages, dissolved oxygen sensors and recorders, miniature temperature recorders, sinker bars, sealing plugs, and interspersed sections of ⅜ inch (0.95 cm) Spectra rope (Table T3). Complete details of this deployment are provided in Edwards et al. (2012), but a summary is provided here for completeness. The general features of OsmoSamplers are summarized in detail in Wheat et al. (2011).

In brief, OsmoSampler packages consist of a series of small-bore tubing, osmotic pumps, and in some cases microbial substrate materials (Flow-through Osmo Colonization System [FLOCS]; Orcutt et al., 2010, 2011). Each package includes a 0.5 inch (1.27 cm) stainless steel strength member and stainless steel couplers to attach to other packages, line, sinker bars, or sealing plugs. All of the pump parts, excluding the membranes and O-rings, are made from polyvinyl chloride (PVC). Membranes were purchased from Alzet (2ML1), and O-rings are silicon based. Sample tubing is  inch copper or Teflon with an inner diameter of ~1.19 mm and a length of 1000 ft (304 m). This tubing is spooled onto rods such that the rod and tubing fit within the inner diameter of a protective tube of clear PVC. The OsmoSampler packages were made with two different outer diameters (2.875 inch [7.30 cm] and 2.5 inch [6.35 cm]) in order to fit within the confines of the borehole, which was constrained by the inner diameter of the three landing seats that isolate the three monitoring intervals inside the 4.5 inch casing (tapered gravity seals).

Six types of OsmoSampler packages were deployed: standard, gas, acid addition, BioOsmoSampling System (BOSS), enrichment, and microbiology (MBIO) (Wheat et al., 2011). The standard package consists of three Teflon sample coils and a pump with either eight or five membranes, with more membranes for the larger diameter packages. Similarly, the gas package has three copper sample coils, with the number of membranes depending on the outer diameter of the tubing. The acid-addition package is a standard package with two additional Teflon coils and an additional one-membrane pump at the intake. This pump draws borehole fluids into one coil while expelling saturated salt from the pump into the other coil filled with dilute subboiled HCl (20 mL 6N HCl in 500 mL of deionized distilled [18.2 MΩ] water). This acid-filled coil is attached to a tee, with the other tee positions connected to a short inlet to sample borehole fluids and to the intake of the equivalent of a standard OsmoSampler package. The BOSS package is identical to the acid-addition package, except that a solution of 2 mL of saturated HgCl₂ in 75% RNAlater (Ambion) replaces the dilute acid solution. The enrichment package also has the same configuration, but a solution of 1.2 mM nitrate in sterile seawater replaces the dilute acid solution, and a FLOCS microbial colonization experiment (Orcutt et al., 2011, see below) is connected to the tee that leads to the standard package. Fluids flow through two FLOCS columns before reaching the first Teflon sample coil. Attached to the FLOCS is a series of mineral chips that are exposed directly to the formation. The MBIO package consists of two standard packages, each with a FLOCS experiment attached to the intake. Only one FLOCS column is attached to each standard package, and mineral chips are included and exposed to the formation.

To investigate the activity and diversity of microorganisms in deep basement, microbial colonization experiments with defined mineral substrates are placed in the formation to encourage in situ growth seeded by planktonic microorganisms in the borehole fluids. The concept, design, and demonstration of the FLOCS is discussed in detail elsewhere (Orcutt et al., 2010, 2011; Wheat et al., 2011) and is only briefly outlined here. The design of the FLOCS units used during Expedition 336 is described in Edwards et al. (2012). The Hole 395A CORK instrument string contains a total of eight FLOCS units: 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 (Table T3) containing cassettes of different substrates (basalts, olivine, siderite, sphalerite, chalcopyrite, pyrrhotite, hematite, pyrite, and glass wool and beads; for details see Edwards et al., 2012). Separate osmo 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; see Edwards et al., 2012, for details). The enrichment OsmoSampler FLOCS contains grids of barite, Hole 395 basalts, sphalerite, pyrite, goethite, and hematite. The MBIO OsmoSampler FLOCS contains grids of rhyolite, glassy basalt, Hole 395A basalt, olivine, chalcopyrite, pyrrhotite, and magnetite.

Attempts were made to minimize potential contamination of the FLOCS experiments. All rock substrates were autoclaved prior to use, and gloves were worn during assembly of the units. Prior to connection to the OsmoSampler package, the chambers of all FLOCS units were flushed with ethanol, distilled water, and filter-sterilized (0.2 µm mesh) Site 395 surface seawater, and the exterior of the FLOCS body with the mounted rock-chip panels was washed in a sterile Whirl-Pak bag with ethanol and kept closed in baked (450°C) aluminum foil wrappers. Assembled OsmoSampler packages sat for 1–2 days prior to deployment, with the intake for the FLOCS chambers connected to a syringe of sterile filtered seawater. The rock-chip panels were exposed to minimally circulating air during this time (i.e., circulation was restricted to the few small openings in the OsmoSampler package sleeves).

Autonomous temperature and oxygen measurement probes were deployed at different locations along the instrument string (Table T3). Two novel dissolved-oxygen probes (Aanderaa Optode 4330, thermal couplers, and digital recorders) were deployed in the uppermost section and the lower middle sections of the CORK. Because of their diameter, these instruments could not pass below the deepest landing seat. These recorders were programmed to measure dissolved oxygen and temperature once per day, with enough memory and battery life to record data for ~5 y. Excluding the two temperature sensors in the dissolved-oxygen probes, seven other temperature probes were deployed. These probes were purchased from Antares and Onset, and they fit within holes drilled into the plastic couplers in the OsmoSampler packages. Both probes provide a dynamic range that covers the expected temperatures (2°–30°C), but the Antares probes have greater sensitivity (see Edwards et al., 2012).

The remaining elements of the downhole tool string include the sinker bar, sealing plugs, and Spectra rope. A sinker bar was placed at the bottom of the string to help pull the string elements into the hole. Both the sinker bars and the sealing plugs are made of stainless steel and coated with Xylan. Three different sealing plugs were fabricated with sealing surfaces with diameters of 3.375, 3.125, and 2.875 inches. The Spectra rope (⅜ inch) allowed us to space out the OsmoSampler packages, sinker bars, and plugs to achieve the scientific goal of isolating and sampling specific horizons. In Table T3 the exact lengths of Spectra rope are provided, as well as the expected length when taking into account a stretch of 1.5%. Note that some of the Spectra depths in the table exceed that of the hole. This additional Spectra rope was desired to ensure that the plugs would reach the sealing seats.

Scientific and operational implications

The CORK was deployed from the drillship and lowered about 100 m below the ship. The downhole instrument string was deployed, but the seafloor (top) plug did not latch after several attempts. The CORK was retrieved, and the top of the wellhead was brought to the rig floor. Several attempts with two different top plugs had the same result. Given that this hole is slightly underpressured, the top plug was deployed as a nonlatching gravity seal. Although the wellhead was exposed, the ball valve on the wellhead was replaced by a plug because the ball valve was cracked during the initial lowering. During repairs, the inner CORK tubing was filled with water to the level of the ball valve port. This water remained, indicating the upper gravity plug (3.375 inch) was sealed.

The CORK was then deployed and apparently landed in the reentry cone. During subsequent operations the wellhead underwent compressional forces that bent the wellhead and severed it above the cup packers, ~4 m below the reentry cone. This act also parted the Spectra rope and the umbilicals, leaving the downhole tool string in place. On the basis of recovered pieces of the wellhead, the CORK tubing near the seafloor is likely not completely rounded, but it may not be closed enough to restrict recovery of the downhole samplers, sensors, and experiments. Several stainless steel tubes likely extend above the cup packers and the top of the 4.5 inch casing. These tubes may impede recovery of the downhole instrument string. A plan is being formulated to recover the downhole instrument string in 4 y with an ROV.

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 actually inflated. The seafloor gauge and three formation gauges show very similar slight increases in pressures after the time of attempted inflation, but the similarity to the seafloor gauge could be consistent with a tidal influence without inflation of the downhole packers.

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