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doi:10.2204/iodp.proc.327.107.2011

Status of CORKs installed before Expedition 327

CORKs installed prior to Expedition 327 were visited periodically following Leg 168 and Expedition 301 to download data, collect samples, and swap out instrumentation as needed to be prepared for Expedition 327. The last CORK servicing visit before Expedition 327 was in June–July 2010 during R/V Atlantis Expedition AT15-66, using the ROV Jason for seafloor operations. In this section we report on the status of CORKs installed before Expedition 327, as of the end of Expedition AT15-66. Operations involved in creating these CORKs, as well as details of hole completion, servicing operations, and CORK configuration through 2004 are described elsewhere (Becker and Davis, 2005; Davis and Becker, 2004; Expedition 301 Scientists, 2005a, 2005b; Fisher et al., 2005; Shipboard Scientific Party, 1997).

Hole U1301A

The Hole U1301A CORK was installed without a casing seal between the 16 and 10¾ inch casing strings, and cementing the 10¾ inch casing to the formation at depth was unsuccessful, so this CORK was not sealed at deployment. As a result, bottom seawater flowed through holes in the 10¾ inch casing hanger in the throat of the cone and down the annulus between the casing strings (Fisher et al., 2008). Flow down the hole spontaneously changed direction in fall 2007 after summer 2007 CORK servicing operations, and shimmering water was observed to be flowing from around the wellhead during a summer 2008 servicing expedition (Wheat et al., 2010). Although still not sealed, the flow of water from the formation and into the Hole U1301A CORK represents an improvement in conditions for this observatory system, as it provides an opportunity to sample formation fluids and assess the nature of in situ microbiological communities (Orcutt et al., 2011; Wheat et al., 2010). The spontaneous change in flow direction following several years of fluid flow down the hole and into the formation also places constraints on formation properties (Davis et al., 2010). However, the lack of a seafloor seal and the continued flow of formation fluid from Hole U1301A prevents assessment of basement pressure conditions around the hole and could make the interpretation of cross-hole response more complex during the long-term free-flow experiment. An attempt was made to seal this CORK with cement during Expedition 321T in summer 2009 (Fisher and Expedition 321T Scientific Party, 2010), but subsequent observations during dive operations in summer 2009 and 2010 indicate that the hole remained unsealed.

Although cement was observed to have completely filled the cone immediately after being injected during Expedition 321T, the current appearance of the hole indicates that much of the cement drained away, presumably through the same gaps through which warm formation fluid currently flows (Fig. F17). Although it is not sealed, the Hole U1301A CORK is instrumented at the wellhead and downhole. A pressure logging system with two gauges monitors pressures at two depths, immediately below the seafloor and below the packer near the base of 10¾ inch casing. Both of these gauges currently show hydrostatic conditions superimposed with noise associated with the turbulent flow of warm formation fluid up and around the wellhead.

The downhole instrument string in Hole U1301A was recovered and replaced using the submersible Alvin in summer 2009 (Tables T3, T4). The new string comprises (from top to bottom) a top plug, ⅜ inch Spectra cable, a middle sinker bar (35 lb), ⅜ inch Spectra cable, a bottom plug, three OsmoSampler systems (gas tight, MBIO, and acid addition), and a lower sinker bar (100 lb). Five autonomous temperature data loggers are deployed on the Spectra cable, and another three are deployed inside OsmoSampler systems (Table T3). The instrument string components hung below the lower plug are short enough to fit entirely within slotted (but uncoated) casing between the lower plug and the end of the bullnose at the base of 4½ inch CORK casing.

Three seafloor OsmoSampler systems were deployed on the Hole U1301A wellhead in summer 2010 (Fig. F17C, F17D), and all are connected to the open basement interval at depth: one with two ~300 m long PTFE sampling coils, one with two ~300 m long PTFE coils and five FLOCS chambers, and one with two ~300 m long copper-tubing coils. The PTFE-coil OsmoSampler was installed in the left side of the bay on the left sample port and is controlled by Valve 4. The FLOCS OsmoSampler was installed on the right side of the bay on the right sample port and is controlled by Valve 3. The copper OsmoSampler was installed in the center of the bay and is supplied by the right sample port, controlled by Valve 2. At the end of summer 2010 operations, Valves 2, 3, and 4 were open and Valves 1, 5, and 6 were closed (Fig. F17D). The end of the T-handle on Valve 6 broke off during recovery of an older OsmoSampler that had been installed on the right sample port, but fortunately this occurred after the valve had been closed.

A geomicrobiology sensor and fluid sampling system (“GeoMICROBE” instrument sled, Wheat et al.) was deployed with the sample inlet connected to the lower of two Aeroquip connectors in the microbiology sampling bay (Fig. F17E). This system is programmed to pull 550 mL individual fluid samples and filter samples collected from volumes of 8 L from the borehole at ~3 week intervals until the system’s recovery in summer 2011. These samples will be useful in assessing the return of borehole conditions to a predrilling state, the nature of formation fluids at depth, and the timing and extent of tracer arrival following injection into Hole U1362B during Expedition 327 (Fisher, Cowen, et al.). The GeoMICROBE instrument sled is also equipped with a suite of in-line fluid sensors (e.g., temperature, flow, oxygen, pH, and electrode potential) and an electrochemical (voltametry) analyzer that operate simultaneously with fluid collection and filtering. A weight stack that had previously been deployed on the top plug of Hole U1301B was moved to Hole U1301A in summer 2010, clearing the former wellhead for instrument string recovery and providing additional downward force on the top plug (Fig. F17F).

Hole U1301B

The Hole U1301B CORK was also deployed during Expedition 301 without casing seals and with incomplete cementing at depth, leaving the borehole observatory unsealed. This resulted in long-term flow down Hole U1301B that led to a pressure perturbation in the surrounding crust that was resolved by formation pressure monitoring below the Hole 1027C CORK, 2.4 km to the east (Davis et al., 2010; Fisher et al., 2008). Unlike Hole U1301A, downward flow into Hole U1301B did not reverse spontaneously, most likely because this hole is deeper and has a greater open basement interval, requiring a larger excess formation pressure to overcome the hydrothermal siphon created by the imposition of cold bottom water in the hole during drilling and other operations. Two attempts were made to cement the cone around the CORK wellhead in Hole U1301B using Alvin and a seafloor cement delivery system in summer 2006 and 2007, but neither of these efforts was successful, although there was an observable pressure response as a result of cement delivery to the cone in summer 2007. The cone around the CORK wellhead was filled with cement during Expedition 321T, and on the basis of seafloor observations made during servicing in summer 2009 and 2010, much of the cement remained in the cone.

Pressure data downloaded 3 weeks after cementing in summer 2009 and 2010, prior to Expedition 327, and thermal data collected from Hole U1301B during Expedition 327 (see the “Site U1301” chapter) suggest that Expedition 321T cementing was successful in sealing this borehole observatory. However, pressure records from the deepest two intervals in basement indicate that the pressure monitoring lines were blocked soon after cementing operations. Anomalously high pressures detected within these gauges (400–1200 kPa in excess of hydrostatic pressures measured in summer 2010) appear to result from the heating of sealed parts of the pressure monitoring system rather than from actual formation conditions. In contrast, pressure data from the shallowest monitoring interval in Hole U1301B shows evidence of a continuing return to superhydrostatic conditions, following a 5 y thermal and pressure perturbation associated with rapid downhole flow.

Three OsmoSamplers were recovered from the wellhead in Hole U1301B in summer 2010 prior to Expedition 327, and three new OsmoSamplers were deployed (Fig. F18). The system deployed in the center of the bay uses two ~300 m long copper coils and is attached to Valve 5, which samples formation fluids from the deepest basement monitoring interval. The other two OsmoSamplers each consist of an osmotic pump and two ~300 m long PTFE coils attached to the right sample port, as well as four FLOCS chambers attached to the left sample port. The left-side OsmoSampler was plumbed to the deep formation interval using Valves 1 and 4, and the right-side OsmoSampler was plumbed to the intermediate depth interval using Valves 3 and 6. At the end of the summer 2010 dive series, Valves 1, 3, 4, 5, and 6 were open and Valve 2 was closed (Fig. F18D).

Part of the original instrument string deployed in Hole U1301B was recovered and replaced during Expedition 327 (see the “Site U1301” chapter). The new string comprises a 51 m Spectra cable on which three autonomous temperature probes and a 200 lb sinker bar are mounted (Table T3). These instruments were installed to monitor the continuing return of Hole U1301B to predrilling conditions following a long period of disturbance associated with rapid downhole flow of cold bottom water.

Hole 1026B

The instrument string deployed in Hole 1026B during Expedition 301 was recovered in summer 2008 and replaced with several OsmoSampler packages suspended at the base of a thermistor cable. The thermistor cable has sensors with millikelvin temperature resolution at 14 m spacing and provides data in real time through the NEPTUNE Canada regional network (www.neptunecanada.ca/). The Hole 1026B pressure data logger, deployed using the submersible Alvin in 2007, is also transmitting data through the NEPTUNE Canada network (Fig. F19). Both systems are fully operational; seafloor and borehole pressure and seafloor temperature are measured at a frequency of 1 Hz, and borehole temperatures at 16 depths are measured at a frequency of once per 0.5 h (programmable to once per 10 s). Formation pressure is sampled at a single depth interval in uppermost basement, and the thermistor cable monitors conditions within the sedimentary section down to 200 mbsf because a drill pipe liner installed during Leg 168 extends upward from basement and into the 10¾ inch casing, preventing instrumentation inside the CORK from extending across the sediment/basement interface. Bacterial growth and mineral precipitation on the Hole 1026B CORK wellhead and variations and trends in downhole pressures and temperatures following installation of the new thermistor cable in 2008 suggest that Hole 1026B is incompletely sealed, once again.

Three new and refurbished OsmoSampler systems were deployed on the Hole 1026B CORK wellhead during dive operations in summer 2010. A pump with two ~300 m long copper coils was placed on the left side of the sampling bay, a pump with two ~300 m long PTFE coils and three FLOCS chambers in series was placed in the center of the bay, and a pump with two ~300 m long PTFE coils was placed on the right side of the bay. These systems are plumbed through Valves 4, 5, and 6, respectively, which were left open as of the end of dive operations. Valves 1, 2, and 3 remained closed (Fig. F19F).

Hole 1027C

Unfortunately, it was not possible to recover the old CORK, deepen the hole, and install a new CORK system during Expedition 327, as was originally planned (see the “Site 1027” chapter), and the old CORK remains installed in this hole. Pressure in this CORK was monitored with an earlier generation data logger and gauge system, which also served as a top plug for the wellhead. This system had been connected to a battery pack and communications interface (using the same underwater mateable connector as that used by more recent CORK systems) placed on the landing platform in summer 2007. The battery pack and communications interface were disconnected from the data logger and recovered in summer 2009, but the handle to the brass electrical connector attached to the top of the old data logger broke off when removal was attempted. It appears that the connector is corroded in place on top of the wellhead. This was confirmed with a subsequent visit with the ROPOS in October 2010, when an attempt was made to rotate and pry the connector free. After 20 min of vigorous lifting, hammering, and rotation, the brass connector remained firmly fixed to the steel logger body, but the logger appeared to rotate within the CORK body. The old data logger may still be collecting pressure data, but accessing these data will require removal of the brass connector. Researchers are planning during summer 2011 ROV operations to remove the connector and old logger from the wellhead and install an adapter and pass-through fitting that allows use of a modern pressure logging system. There are no fluid or microbiology sampling systems installed on or below this CORK wellhead.

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