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doi:10.2204/iodp.pr.341S.2013

Hole 858G CORK replacement

Background

As part of Expedition 341S, we planned to reinstrument Hole 858G (Table T1) in the sediment-filled Middle Valley axial rift of the northernmost Juan de Fuca Ridge for long-term hydrologic monitoring. This hole and its companion Hole 857D (Fig. F10) were equipped with the first CORK hydrologic observatories in 1991 during Ocean Drilling Program (ODP) Leg 139 (Davis et al., 1992) to determine the thermal and hydrologic state within the buried permeable igneous crust of the valley and the driving forces for fluid flow through the seafloor in the vent field where Hole 858G is situated. For reasons not fully understood at the time, the CORK seals of Hole 858G failed after roughly 1.4 y, after which the formation sensor recorded only hydrostatic (seafloor) pressure (Fig. F11). This CORK and the one in Hole 857D were replaced during a second phase of drilling in the area, ODP Leg 169. The latter remains operational and the 16 y continuous record of seafloor and formation pressure has provided information well beyond the scope originally anticipated for the experiment. Unfortunately, the Hole 858G CORK failed again, this time in roughly 1 y. Inspection of the failed seals and mineral deposits within the original CORK recovered during Leg 169 suggested that the seals had suffered from exposure to high-temperature formation fluids, although this factor alone cannot have been responsible. Natural formation temperatures at this location do not reach the ~270°C temperature of the Middle Valley hydrothermal system until ~80 mbsf. Temperature at the level of the CORK seals situated at ~1 mbsf should have been close to the bottom water temperature. Failure of some component within the CORK body may have caused initial leakage, leading to increased temperatures, chemical imbrittlement, and ultimate failure of the main CORK seals (see discussion in Fouquet, Zierenberg, Miller, et al., 1998).

Despite these problems, the data collected prior to seal failure provided a valuable complement to those from the CORK in Hole 857D. They provided an approximate lower limit on the formation pressure available to drive flow from the igneous crustal hydrothermal reservoir vertically through seafloor vents, as well as an estimate for the differential pressure available to drive flow within the sediment-sealed permeable reservoir (e.g., Davis and Becker, 1994).

Motivation for revitalizing the Hole 858G CORK

A number of factors have accumulated since the time that the monitoring experiments were originally undertaken, leading us to the Expedition 341S reinstallation effort:

1. From a scientific perspective, long-term monitoring experiments at a number of sites in tectonically active settings (Juan de Fuca Ridge axis and flank, Mariana forearc, Costa Rica prism, and Nankai accretionary prism) have revealed that formation fluid pressure variations provide a sensitive proxy for volumetric strain. Transient events related to coseismic, postseismic, and aseismic deformation have been seen at all of these locations, and the observations are leading to new understanding about the episodic nature of deformation, seismic energy efficiency, and regional interseismic strain accumulation. Some of the best examples come from Middle Valley Hole 857D, which has provided an unprecedented look at seismotectonic processes at ridge axes by virtue of its location, the local sensitivity of pressure to strain, and the very long continuous record of pressure (Fig. F12). One example contained in this record shows very clearly how formation pressure responds to coseismic elastic strain produced by three earthquakes, postseismic slip, and hydrologic readjustment (Fig. F13). Pressure transients like this are numerous throughout the history of recording in Hole 857D. They reflect strain generated by slip on faults within Middle Valley and in the surrounding region, specifically the neighboring West Valley Rift, the Endeavour axial segment to the south, and the Nootka Fault to the northeast. Future events captured in Middle Valley and at other operational CORK sites in the region will provide new insights into the complex mixture of hydrologic and viscoelastic response to fault rupture and allow them to be separated. Data from one hole by itself makes this separation equivocal, so a combination of data from Holes 857D and 858G would be invaluable.

2. From a technical perspective, a number of things fortified the justification for revitalizing this hole. High reliability of CORK instrumentation has been demonstrated through successful long-term operations at many sites. Instruments deployed during Leg 196 (Nankai) have been operating continuously since their deployment in 2001, those in the Middle America subduction zone off Costa Rica since 2002, and the one in Hole 857D deployed during Leg 169 since 1996. Improvements in power consumption, memory capacity, and resolution now permit detection of much more subtle signals, and connections to the NEPTUNE observatory cable infrastructure will open up great opportunities. Much higher sampling frequency can be achieved, allowing observations to reach into the seismic frequency band, and the observations can be placed in context of co-located seismic and hydrologic records that will be collected at all NEPTUNE nodes with broad-band seismometers and a variety of seafloor monitoring instruments.

3. From a financial perspective, setting up existing holes like Hole 858G for long-term monitoring in a time-efficient and economical way makes good sense, as it takes advantage of existing infrastructure like NEPTUNE, which will minimize the need for costly and time consuming site visits using a ship and submersible or remotely operated vehicle (ROV).

Operations plan/Drilling strategy

A new scheme was proposed in the Ancillary Project Letter (APL) that led to this project through which pressure monitoring can be reestablished and later access can be gained to the interior of an existing borehole. For Hole 858G, the operations plan consisted of two main operational steps:

1. Removing the existing CORK body and 370 m long thermistor string. This was to be done with the existing ODP CORK pulling tool and would require a single pipe trip. A comparable operation was recently carried out in Hole 395A during IODP Expedition 336, with the data logger and thermistor cable removed and sampled for microbiology when the CORK body reached the rig floor. The thermistor string in Hole 858G was also to be made available for microbiological sampling. Even though the CORK in Hole 395A had been in place since 1997, we knew that the recovery efforts at Hole 858G would be more difficult because of the subseafloor environmental setting. Recovery of the original CORK in Hole 858G during Leg 169, 5 y after its installation, was made challenging by the accumulation of hydrothermal mineral deposits within the CORK body and by corrosion of some of the CORK structural components, including those providing a connection to the latch/unlatch mechanism. Given that >16 y had passed since the second CORK was installed, even greater challenges and risks of failure were expected.

2. Installing the new seal stack and instrument. This step was to follow a pipe trip to clean the hole of any hydraulically resistive mineral deposits precipitated inside the 11¾ inch casing from water ascending the hole over the history of leakage. The seal stack (Fig. F14) comprised, from bottom up, (a) three joints of drill collars, their combined weight sufficient to overcome the piston force imposed by the formation overpressure on the seals, roughly 2700 lb; (b) multiple cup seals to land inside the top of the 11¾ inch casing below the casing hanger; (c) landing webs to support the stack in the reentry cone/casing hanger; and (d) a section of reinforced 7 inch casing that positions the instrument package at a convenient position for submersible operations. The top of the stack mates with one of the standard IODP CORK running tools and includes a removable axial plug for possible future downhole access. Valved plumbing and ports provide for pressure monitoring and fluid sampling. An instrument package, connected to the hydraulic access line via a pressure-balanced connector and a three-way valve, includes a current-generation high-precision pressure recorder capable of resolving pressure to roughly 10 ppb at 1 Hz sampling frequency, along with a wellhead temperature sensor that has a resolution of the order of ~0.1 mK. It is equipped with batteries to run in autonomous low–sampling rate mode (15–60 s sampling interval) for roughly 15 y, to facilitate initial autonomous (pre-NEPTUNE connection) monitoring and backup operations during cable-power downtime. When cable connected, such instruments automatically switch into continuous 1 Hz sampling mode, with time-stamped pressure and temperature data passed to the cable via an RS422 serial port.

Plans to recover the old CORK from Hole 858G were thwarted, despite concerted efforts (see “Operations”). Repeated pulling on the release sleeve, increasing to a peak of >150,000 lb, resulted only in the release sleeve breaking not far below the latch dogs. Corrosion or mineralization apparently kept the sleeve from shifting as designed and the latching ring from disengaging. Use of the running tool during a subsequent pipe trip was also unsuccessful; rough weather and poor VIT camera visibility combined with the very small target offered by the modified running tool precluded mating of the running tool with the upper CORK head.

Fortunately, the vigorous recovery operations, which subjected the wellhead to high pulling forces, large side-loading, and heavy impacts by the bottom-hole assembly drill collars, appear to have resulted in no substantial damage to the CORK body, and there were no signs of leakage from any parts of the CORK at the end of recovery operations. Thus, one contingency remains: to deploy the pressure recording instrument on the wellhead platform and connect it to the valved fluid sampling port on the original CORK wellhead body. This may be attempted as early as June 2013. The unused new CORK body will be stored at IODP/Texas A&M University (TAMU; USA) and be available for use in any other hole where single-interval pressure monitoring or fluid sampling is justified. Alterations may be necessary to ensure compatibility in holes with different casing hangers and internal diameters. Fabrication drawings will be archived at IODP, and the deployment procedure is documented in “Appendix B.”

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