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doi:10.2204/iodp.sp.335.2010

Operations plan

Background

Following ODP Leg 206, IODP Expeditions 309 and 312 made significant progress in recovering an intact section of the upper oceanic crust. Average rates of recovery and penetration are summarized in Figure F13 and Table T1. The inverse relationship between spreading rate and depth to axial melt lenses has been confirmed, supporting the strategy of drilling in crust formed at a superfast spreading rate to achieve the first upper crustal penetration. Expedition 335 should deepen Hole 1256D sufficiently into plutonic rocks to obtain definitive answers to longstanding questions about the structure and composition of the oceanic crust and about mechanisms of crustal accretion.

The IODP Science Planning Committee requested in 2006 (SPC Consensus 0603-19) that the United States Implementing Organization (USIO) identify the operational requirements for further drilling in Hole 1256D. Texas A&M University (TAMU) Engineers from the USIO presented their operational plan to an audience of scientists and independent drilling engineers at the Mission Moho Workshop (Portland, Oregon, 7–9 September 2006) for technical review. There was consensus support for the plan proposed, and the USIO report is available in the full report of the Mission Moho workshop (page 39; available online at www.iodp.org/mission-moho-workshop/).

Four deepening scenarios were considered:

  1. Resume rotary core barrel (RCB) coring in Hole 1256D using large-volume (100–150 bbl) high-viscosity mud sweeps combined with frequent bit trips.

  2. Enlarge the hole to 18½ inches to isolate the out-of-gauge section using 13⅜ inch casing.

  3. Forego 13⅜ inch casing and enlarge the hole to 14¾ inches to isolate the out-of-gauge section using 10¾ inch casing.

  4. Offset and start a new hole following the casing strategy employed during Leg 206.

Several key points were noted by USIO-TAMU and independent engineers at the Mission Moho Workshop. Hole 1256D is in excellent condition, and remedial engineering operations such as reaming and casing are premature. It is important to recognize that neither the offshore industry nor scientific ocean drilling operators have ever attempted to open up an existing deep basement hole to any significant depth in basalt and insert casing. Regardless of the casing strategy proposed (2 or 3), attempting to open any portion of Hole 1256D to accommodate casing would require significant new hardware and numerous pipe trips and would extend over several expeditions. Such operations would require unproven technology and would be extremely challenging with substantial risk of irreparable damage to Hole 1256D without any further coring or recovery. Approaches 2 and 3 are not viable options and are not considered further. Further deepening of Hole 1256D using large mud sweeps is the only strategy that begins with coring, is the most likely to return samples from deeper in the hole, and has the largest possibility of building on the tremendous success of previous deepening operations on Expedition 309 and 312 that employed proven drilling techniques.

Operations plan for Expedition 335

The operations plan for Expedition 335 is to resume RCB coring with frequent large-volume, high-viscosity mud sweeps. During Expeditions 309 and 312, large-volume mud sweeps were effective at clearing cuttings from the hole, but at the end of Phase 1 operations, mud stock depletion precluded implementation of this approach throughout the whole of Expedition 312. Following the recommendations of the IODP Expedition 309/312 Operations Review Task Force, the JOIDES Resolution will be stocked with at least 60 T of sepiolite and attapulgite for use during Expedition 335. Assuming reasonable hole conditions and an average rate of penetration comparable to previous coring in this hole, TAMU engineers estimate that this approach should deepen Hole 1256D ~350 m within 39 days (Table T2; Fig. F14).

Before conditioning the hole for further deepening, an attempt will be made to acquire an equilibrium temperature profile and recover a water sample before the thermal structure of the crust is perturbed by cleaning and drilling operations (see next section). We will reenter the hole with a tricone drilling bit on a bit release and slowly descend past the zone around 900 mbsf that caused an obstruction during Expedition 312. If the hole is clear, we will withdraw the pipe, drop the bit on the seafloor, reenter to below the rat hole, and log the hole with the triple combination (triple combo) tool string, including the Modular Temperature Tool (MTT). If the wireline conditions are suitably benign, we will attempt to take a borehole water sample from near the bottom of the hole, where the temperature is anticipated to be close to the currently accepted limit of life, using the Water Sampling and Temperature Tool (WSTP).

Logging/downhole measurements strategy

Downhole logging will be an important complement to coring operations during Expedition 335 and will assist in the attainment of the scientific objectives by providing continuous in situ geophysical measurements of the drilled basalts, dikes, and gabbros that are commonly incompletely recovered. The logging program will establish the igneous stratigraphy, magmatic morphology, and variations in seawater-basaltic alteration as a function of depth, as well as allow direct correlation of wireline measurements with discrete laboratory measurements on the recovered core. In addition, core-log integration will be enhanced through the use of the DMT Digital Color 360° CoreScan system or a similar tool to digitally record the outer surface of all cores, as employed during Leg 206 and Expeditions 309 and 312. Wireline data will be used in conjunction with core images and structural and magnetic data to reorient veins, fractures, and other features back into the geographic reference frame.

Using the heat flow calculated based on temperature measurement in Hole 1256C (109 mW/m2), the temperature at the sediment/basement boundary (35°C) and appropriate thermal conductivities for lavas, dikes, and gabbroic rocks, we predict that the ambient temperature at 1700 mbsf in Hole 1256D should be ~100°–140°C, significantly cooler than was encountered in Hole 504B (~170°C) at these depths (Fig. F15). Our operations schedule includes time for preliminary logging of Hole 1256D, which would require a dedicated round trip of the drill string, so that an equilibrium temperature profile and water sample can be recovered before the thermal structure of the crust is perturbed by drilling operations. We also expect to evaluate the diameter of the borehole with the triple combo tool string. In the unexpected case of significant fill and/or large changes in hole diameter, additional passes with the Formation MicroScanner (FMS) or the Ultrasonic Borehole Imager (UBI) may be made to evaluate the need for casing. A full suite of wireline logging tools will be deployed after the completion of drilling operations, following an order of operations used at the end of Expedition 312 and including a final temperature profile of the hole. The hole will be flushed with freshwater in order to improve the quality of resistivity measurements.

Precoring logging

  1. Triple combo, including the MTT. The tool will be run into the hole after lowering the drill string to ~1000 mbsf to confirm that the hole is still open.

  2. If conditions permit, water sampling with the WSTP.

Postcoring logging

The hole will be flushed with freshwater after completion of coring to reduce the electrical resistivity contrast between the borehole fluid and gabbroic wall rocks and enhance the quality of the images. This follows good practice recommended following wireline logging of gabbros during Leg 176 (Dick, Natland, Miller, et al., 1999) and undertaken during logging operations in IODP Hole U1309B (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, et al., 2006).

  1. Triple-combo, including the MTT and the High-Resolution Laterolog Array (HRLA) resistivity tool.

  2. Vertical seismic profile with the Versatile Seismic Imager (VSI).

  3. UBI.

  4. FMS-sonic.

  5. MTT, HRLA, and Hostile Environment Gamma Ray Sonde (HNGS) for a final temperature measurement.

The triple combo will be the first tool run to establish the hole conditions. The order of the other tool strings will be adjusted to ensure that the vertical seismic profile is acquired during daylight operations for marine mammal watch.

The characteristics of the tools are briefly described below.

Triple combo tool string

The triple combo consists of five probes:

  1. The Accelerator Porosity Sonde (APS) uses an electronic neutron source to measure the porosity of the formation.

  2. The Hostile Environment Litho-Density Sonde (HLDS) measures bulk density. It includes a caliper that will provide an assessment of the hole quality.

  3. The HNGS measures the natural radioactivity of the formation and provides indications of Th, U, and K concentrations. These measurements can then be integrated with analytical studies to determine downhole geochemical variations in alteration.

  4. The HRLA tool measures rock resistivity at two invasion depths.

  5. The Lamont-Doherty Earth Observatory (LDEO) MTT tool will be attached at the bottom of the tool string to measure borehole temperature.

Formation MicroScanner/Dipole Sonic Imager tool string

The FMS-sonic tool string has two main components:

  1. The Dipole Sonic Imager (DSI) records a full set of acoustic waveforms to measure the compressional and shear velocity of the formation (VP and VS). VP can be combined with the density log to generate synthetic seismograms and provide high-resolution seismic/well integration.

  2. The FMS consists of four orthogonal pads with 16 electrodes on each pad to record high-resolution microresistivity images of the borehole wall, which are useful for the identification of volcanic units and tectonic features (e.g., the presence of fractures and faults and their orientations). It includes a General Purpose Inclinometry Tool (GPIT) to orient the images and adjust them for tool motion. The HNGS is included in this tool string to allow correlation with other logging runs for establishing consistent depth estimates.

Ultrasonic Borehole Imager

The UBI measures the amplitude and transit time of an acoustic wave propagated into the formation. It provides high-resolution images with 100% borehole wall coverage, allowing the detection of small-scale fractures. The GPIT is deployed with the UBI and enables borehole images, fractures, and other structural features to be oriented. This can provide important information on the local stress field and borehole geometry even within the casing. The HNGS is included in this tool string to allow correlation with other logging runs for establishing consistent depth estimates.

Verstile Seismic Imager

The VSI will be used to acquire a vertical seismic profile over the deeper section of the hole. It will be lowered in the borehole and anchored at 10–25 m depth intervals against the borehole wall to record the waves emitted by two 250 in3 Sercel G guns (Table T3) in parallel clusters suspended 2–7 m below sea surface. The result will be a complete depth-traveltime relationship to position the hole in the seismic stratigraphy as we target the Layer 2/3 boundary.

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