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Next Section | Table of Contents doi:10.2204/iodp.sp.309312.2005
The operational strategy for Expeditions 309 and 312 is to core as deep as possible in Hole 1256D, including complete borehole coverage with conventional wireline logging tools (Table T1). Operational time for these expeditions includes 38 days on site for Expedition 309 and 35 days on site for Expedition 312. Using pipe-trip times appropriate for the water and hole depths and assuming 50 h of rotation on each of 1215 CC-9 rotary core barrel (RCB) coring bits (determined to be the best available for coring at Site 1256 during Leg 206), we expect to achieve a minimum depth of at least 1450 m into basement (1700 meters below seafloor [mbsf]). A penetration rate of 1.5 m/h has been assumed in this estimate, which was the average rate of penetration toward the end of drilling during Leg 206 and is similar to rates achieved during operations in sheeted dikes in Hole 504B during Legs 140 and 148 (Dick, Erzinger, Stokking, et al., 1992; Alt, Kinoshita, Stokking, et al., 1993). For a spreading rate of >200 mm/y, the low-velocity zone interpreted as a melt lens is predicted to occur at a depth between 725 and 1000 m at the ridge axis (Fig. F4). The steep magnetic inclinations of the uppermost lavas sampled in Holes 1256C and 1256D (Wilson, Teagle, Acton, et al., 2003) as well as the ~100 m thickness of the ponded lava flow (Units 1256C-18 and 1256D-1) suggest that the magma flowed or was erupted a considerable distance off axis (~5 km). Assuming that there is at least 100200 m of additional lavas that flowed relatively short distances from the axis (12 km), we predict that gabbros representing the frozen axial low-velocity zone should occur at ~9251300 msb (11751550 mbsf), providing 150525 m of gabbro penetration if the proposed drilling scenario is achieved (Fig. F5). Using the lowermost heat flow measurement taken 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 that encountered in Hole 504B (~170°C) at these depths. Our operations schedule includes time for preliminary logging of Hole 1256D, which will 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 and eccentricity of the borehole with the triple combination (triple combo) tool string. In the unexpected case of significant fill and/or large changes in hole diameter, additional passes with Formation MicroScanner (FMS) or 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 similar to that used during Leg 206 (see "Triple Combination Tool String," "Formation MicroScannerDipole Sonic Imager Tool String," "Ultrasonic Borehole Imager," and "Three-Component Well Seismic Tool" in "Logging Strategy"). We also expect to run a three-component borehole magnetometer. We have requested funding to deploy the DMT Digital Color 360° CoreScan system or similar to digitally record the outer surface of all cores. Core-logging integration will be enhanced through the use of this system, as used during Leg 206. Contingency Plans Should Drilling Difficulties Arise in Hole 1256D Although hole developments and drilling in Hole 1256D during Leg 206 were exhaustive and prepared the site for the best possible chance of successful deep drilling down to the gabbros, drilling upper oceanic basement will always be technically challenging and risk of hole collapse or drill string failure will always exist. As such, it is prudent that a range of contingency options be considered and approved precruise so that the best decisions can be made should the need arise. The underlying philosophy for this experiment to drill a complete section of in situ upper oceanic crust was to construct a borehole engineered specifically for deep drilling. In Hole 1256D, the borehole infrastructure includes a large reentry cone supported by 95 m of 20 inch casing and 270 m of 16 inch casing that penetrates completely through the sedimentary overburden and is cemented ~20 m into the basement. This arrangement allows two further casing strings (133/8 inch and then 103/4 inch) to be deployed in the hole should the need arise to isolate and armor a section of basement. Our drilling strategy and contingency plans must protect the integrity of Hole 1256D, in which there has already been significant investment, as well as utilize the engineering opportunities allowed by the ocean bottom structures already in place. Potential failings that may occur during Expeditions 309 and 312 fall into two broad categories: (1) failure of the drill string, requiring fishing operations, and (2) collapse and instability of the borehole itself. Drill string failures may be directly linked to borehole wall collapses. To mitigate against drill string failures we will adopt a conservative drilling strategy, with consistent core recovery taking priority over rapid penetration. Where necessary, half-cores (4.5 m) will be taken if core recovery becomes very low, so at least some rocks will be recovered from all intervals. The drilling operations team will ensure that Expeditions 309 and 312 will sail with the best possible arsenal of fishing tools (fishing jars, intensifiers), so should drill string or other equipment be lost in Hole 1256D we will have available all potentially useful tools to recover the lost equipment or mill it out. Borehole failures are potentially more difficult to tackle. Casing a section of borehole a significant distance into Hole 1256D will require a very large expenditure of time and equipment. Deployment of the next-sized casing string (133/8 inch) will require the hole to be reamed to an internal diameter of 18.5 inches before casing can be inserted. Table T2 shows an estimation of the time required to open Hole 1256D to case a blockage at ~500 msb. Such an operation would take ~34 days, similar to the number of operational days on site for Expedition 309. Although installation of a smaller-diameter casing string (103/4 inch) would require less milling and would be less technically difficult, such an approach would not allow further casing strings to be inserted into Hole 1256D and would make redundant the difficult engineering operations required to cement 16 inch casing into basement during Leg 206. It should be noted that a deep basement casing operation has never been attempted in scientific ocean drilling, and the best reaming strategies and tools remain unknown. As such, we will adopt one of the following strategies should a borehole breach occur that requires a significant casing string to be installed in Hole 1256D.
1. To prepare for opening and casing Hole 1256D we will test hole opening strategies and tools including the deployment of 14.5 x 18.5 inch bicenter reamers coupled to 97/8 inch wobble bits and other potential tools to test their effectiveness and determine progress rates so that the best reaming and casing strategy can be developed ashore for future operations at the site. |