Previous   |    Next

doi:10.2204/iodp.proc.336.104.2012

CORK observatory

Hole U1382A was cased through the sediment section (~90 mbsf) with 10¾ inch casing to 102 mbsf (see “Operations,” Fig. F1). The casing was cemented in place with cement that included Cello-Flake lost-circulation material. The cement extended 14 m uphole from the casing shoe inside the casing. Below the casing (1 m), the cement reached 103 mbsf. The hole was then RCB cored to 210 mbsf. Hole instabilities caused fill in the deepest ~1.5 m of the hole, based on a depth check with the logging bit after the aborted packer operations (see “Packer experiments”).

On the basis of drilling rate and core recovery, the screens and downhole instrument string targeted a zone centered at 161 mbsf (Fig. F1). Only one zone was targeted in this CORK. 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 a depth of 188.7 mbsf), six perforated 6 inch diameter drill collars, a crossover, a 15 m long section of perforated 5½ inch diameter casing, a crossover to fiberglass, five nonslotted fiberglass casings, a crossover, a landing seat (3⅜ inch), and a combination packer (inflatable and swellable) that was set in casing with its base at 101.4 mbsf (see Figs. F1, F2; 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 drill collars were connected, painted with an epoxy, and hung in the derrick several days prior to use. As these sections were lowered through the moonpool, they were wiped with 10% ethanol and painted with a fast-drying epoxy paint (Alocit 28) that also dries in water. The perforated 5½ inch casing sections were connected one at time, cleaned, and painted as they were lowered past the moonpool. Above the combination packer is more steel casing, which was 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 ~159–161.6 mbsf (see Fig. F1; Table T3). Nine miniscreens were deployed: two were attached to ½ inch diameter Tefzel tubing, one was attached to ¼ inch stainless steel tubing for pressure, and six were attached to stainless steel tubing bundles for geochemistry (three with ⅛ inch diameter and three with ¼ inch diameter). An additional ½ inch diameter stainless steel tube was used to inflate the packer, which has a check valve that opens at 25 psi (172 kPa).

The wellhead is a standard lateral CORK (L-CORK); however, the 4 inch diameter ball valve was removed and inspected. No visible cracks were evident, but it was replaced by a steel cap.

Downhole samplers and experiments

The downhole tool string consists of six different 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). 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 ½ inch (1.27 cm) diameter 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 diameter 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 an outer diameter of 2⅜ inches (7.30 cm) in order to fit within the confines of the borehole, which was constrained by the inner diameter of the landing seat that isolates the monitoring interval inside the 4½ 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 that are connected in series and open on one end, with an eight-membrane pump attached to the other end. Similarly, the gas sampler has three copper sample coils and a single eight-membrane pump. 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 T-connector, with the other T-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 T-connector that leads to the standard package. Fluids flow through two FLOCS columns before reaching the first Teflon sample coil. Attached to the FLOCS are 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 borehole 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 this expedition is described in Edwards et al. (2012). The Hole U1382A CORK instrument string contains two FLOCS units: one in the enrichment OsmoSampler package and one in the MBIO package. The enrichment OsmoSampler FLOCS unit pulls 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; for details see Edwards et al., 2012). Separate osmotic pumps irrigate each chamber of the FLOCS unit in the MBIO package. 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 glass 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 395A 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 T4). One novel dissolved-oxygen probe (Aanderaa Optode 4330, thermal couplers, and digital recorders) was deployed. This recorder was programmed to measure dissolved oxygen and temperature once per day, with enough memory and battery life to record data for ~5 y. Excluding the temperature sensor in the dissolved-oxygen probe, two 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 Fisher et al., 2005).

The remaining elements of the downhole tool string include the sinker bar, sealing plugs, and Spectra rope. A sinker bar (150 lb in water) was placed at the bottom of the string to help pull the string elements into the hole. A middle sinker bar (100 lb in water) was placed above the plug to help pull in the rope attached to the top plug. Below the top plug (1 m) is a smaller sinker bar (10 lb in water), with the joining Spectra rope covered by heavy-duty hydraulic hose. The sinker bars and sealing plug are made of stainless steel and coated with Xylan. 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 (see Fig. F1). In Table T4 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 the length between seals. Additional Spectra rope was desired to ensure that the plugs would reach the sealing seats with 1–2 m of excess rope.

After the downhole OsmoSampler string was deployed, we deployed an OsmoSampler system attached to the CORK head, tapping into the targeted zone ⅛ inch diameter stainless steel umbilical line. This system consists of standard and MBIO OsmoSampler packages and a new fast-flow osmotic pump (for additional details see Edwards et al., 2012). The fast-flow pump was attached in parallel to two 17 L reservoirs, with the intakes joined by a T-connector and directly attached to a modified handle system. The handle was connected to the valve for the ⅛ inch stainless steel umbilical line that samples the targeted zone. The narrow-diameter sample line was chosen to minimize the dead volume of the sample line and therefore the residence time of the fluid in the line. The fast-flow pump was modified to reduce the pumping rate to 150 mL/day (from 300 mL/day) by placing a Mylar sheet with five 1 inch diameter circular holes between the membrane and membrane support grid. This rate was chosen so that the 34 L freshwater reservoir would last 7.5 months, longer than the 6 month deployment time. This pump was placed in seawater and kept at 4°C for 4 days prior to deployment to verify the pumping rate and condition the membrane. The standard and MBIO packages, each with a 12-membrane pump and 1 Teflon sample coil, were attached (via T-connectors) to the intake tubing between the handle and the pump reservoir in order to subsample the formation fluids before entering the pump reservoir. The system was attached to the wellhead via a network of zip ties and rubber bands so that the package would remain on the CORK during deployment but allow recovery with an ROV.

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. The top plug was deployed last but initially did not latch properly; it released with too little pull based on a gauge reading of several hundred pounds per square inch. After some modification it was decided that the top plug latch may be working. With the instrument string in place, the CORK running tool was secured to the CORK, and a new hydraulic hose was made to connect to the packer inflation line. The CORK was lowered ~5 m below the ship with all the valves open for 10 min. The CORK was then brought back to the ship, where the valves (including the pressure purge valves) were closed, the cap to the underwater mateable connector was removed, and the fast-flow OsmoSampler package was installed. During the deployment of the CORK, water surged up the casing because of the swell and the lowering of the casing, which suggests that the latch on the top plug was not latching as designed, allowing water to flow up the casing when the ship was descending in the trough. However, the plugs sealed the borehole when the ship ascended with the next wave. The CORK and ROV platform were landed, and the CORK running tool unlatched as planned.

Top of page   |    Previous   |    Next