IODP

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

Hole 858G CORK replacement

Background

As part of Expedition 341S, we plan to re-instrument Hole 858G 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. F12) were equipped with the first CORK hydrologic observatories in 1991 during 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 only hydrostatic (seafloor) pressure was recorded by the formation sensor (Fig. F13). 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 a depth of roughly 80 mbsf. Temperature at the level of the CORK seals situated only ~1 mbsf should have been close to the bottom water temperature. Failure of some component within the CORK body may have caused the initial leakage, leading to increased temperatures, chemical imbrittlement, and ultimate failure of the main CORK seals (see discussion in Fouquet 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, and they provided 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

Since the time that the monitoring experiments were originally undertaken, several factors have accumulated to lead us to the current project:

  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 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. F14). 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. F15). Pressure transients like this are numerous through the history of recording at Hole 857D. They reflect strain generated by slip on faults within Middle Valley (as in Fig. F15) 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. We anticipate that future events captured at both Holes 857D and 858G 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 the combination of data from Holes 857D and 858G will be invaluable.

  2. From a technical perspective, a number of things fortify 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 (Mikada, Becker, Moore, Klaus, et al., 2002; Davis et al., 2009, submitted), those in the Middle America subduction zone off Costa Rica since 2002 (Morris, Villinger, and Klaus [Eds.], 2006; Davis and Villinger, 2006; Davis et al., 2011), and the one in Hole 857D deployed during Leg 169 since 1996 (Fouquet, Zierenberg, Miller, et al., 1998; Davis et al., 2006; Hooft et al., 2010; Inderbitzen, 2013). Improvements in power consumption, memory capacity, and resolution now permit detection of much more subtle signals. And in this instance, connection 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 this NEPTUNE node with a broadband seismometer and a variety of seafloor vent monitoring instruments.

  3. From a financial perspective, setting up existing holes like 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 ROV.

Hole 858G CORK replacement operations plan/drilling strategy

A scheme is proposed by which pressure monitoring can be re-established and later access can be gained to the interior of the borehole. The operations plan is presented in Table T2 and consists of two main operational steps:

  1. Removing the existing CORK body and 370 m long thermistor string. This would be done with the existing ODP CORK retrieval 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 cannot be reused and so would also be available for sampling for microbiology. Expedition 336 recovery operations are bound to provide an incomplete analogy for the 858G CORK recovery, however. Recovery of the original CORK 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 have passed since the second CORK was installed, similar challenges and risks of failure can certainly be expected.

  2. Installing the new seal stack and instrument. This step will 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. F16) will comprise, from bottom up, (a) three joints of drill collars, adding up to a weight (~10,600 lb in water) that will 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 would support the instrument package at a convenient position for submersible operations. The top of the stack would mate with one of the standard IODP CORK running tools and would include a removable axial plug for possible future downhole access. Valved plumbing and ports will 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, will contain a current-generation high-precision pressure recorder capable of resolving pressure to roughly 10 parts per billion at 1 Hz sampling frequency and a wellhead temperature sensor that has a resolution of the order of ~0.1 mK. The instrument package will be equipped with batteries to run in autonomous low-sampling-rate mode (15–60 s sampling interval) for a total of roughly 15 y, to facilitate initial autonomous (pre-NEPTUNE connection) monitoring and back-up operations during cable-power downtime. When cable-connected, the instrument will automatically switch into continuous 1 Hz sampling mode, with time-stamped pressure and temperature data passed to the cable via an RS422 serial port. Like others deployed during CORK drilling legs since 2001, the package is designed to be removed and replaced easily by submersible or ROV, augmented with an external battery supply, or connected to a communications cable. Changing position of the three-way valve allows the instrument to be removed while the formation remains sealed, as well as periodic calibration of the formation pressure sensor.

Hole 858G CORK replacement risks and contingency

As mentioned above, there are some inherent risks to the removal of the existing Hole 858G CORK and installation of a new CORK. These are as follows:

  1. Engagement of the CORK pulling tool may prove difficult or impossible depending upon sea state at the time of the attempted recovery.

    • Mitigation: Stand-by until weather/sea state improves.

  2. Once the existing CORK has been engaged with the CORK pulling tool, the CORK latching mechanism may not release properly. History has shown that over time the functionality of this type of latch can become compromised by corrosion and wasting of the metal latch rods.

    • Mitigation: Use brute force of drill string overpull to remove existing CORK.

  3. When recovering the existing CORK, parts of it might be left in the hole. This could prevent installation of the replacement CORK.

    • Mitigation: Use drill string to fish out, push down, or move aside whatever might be blocking the throat of the reentry cone/casing.

  4. After removing the existing CORK, the seal surface may be corroded or built up with mineral deposits or sea life. If so, the replacement CORK may not properly seal inside the casing.

    • Mitigation 1: See description of “seal stack” above.

      Mitigation 2: Investigate purchase and/or fabrication of a clean-out sub for the bottom of the drill string during the depth check pipe trip.

      Mitigation 3: Investigate possible fabrication of a “water-jet” sub to be run above clean-out sub that would use circulation water/pump pressure to “blast” clean the internal diameter of the casing at the level of the seals.

  5. Weather could deteriorate after removal of existing CORK and prevent installation of the replacement CORK. The hole may have to be left open.

    • Mitigation 1: Stand by until sea state improves or time runs out.

      Mitigation 2: If hole must be left sealed, the only option would be to seal with cement.

  6. Retrieving the existing CORK may take all the time available. No additional time is available. If so, the hole may have to be left open, or if time available and scientifically sensible, the hole could be sealed with cement.