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doi:10.2204/iodp.sp.334.2010 Drilling and coring strategyDrilling strategyThe overall operations plan and time estimates are summarized in Table T1. An alternate operations plan substituting logging while drilling (LWD) for wireline logging has been prepared and is summarized in Table T2. Alternate sites have been selected and are presented in Table T3. Time estimates are based on formation lithologies and depths inferred from seismic and regional geological interpretations, including prior drilling in this area (Legs 84, 170, and 205). After departing from Balboa, Panama, we will transit for ~1.6 days to the Costa Rica sites and prepare for drilling operations. The proposed drilling strategy is to begin by drilling at proposed Site CRIS-3B following by proposed Site CRIS-4A. Two holes will be cored at the each site. At both sites, the first hole will be cored with the advanced piston coring (APC) system to refusal. The coring system will then be changed over to the extended core barrel (XCB) system and will be cored to refusal, which has been estimated at 500 meters below seafloor (mbsf). The second hole will be drilled to just above the final depth of Hole A and will then be cored with the rotary core barrel (RCB) coring system to ~950–1000 mbsf. Temperature and pressure measurements will be taken at both sites in order to try to establish temperature and pressure gradients for both sites (see "Downhole tools and logging strategy"). As per policy, all holes in continental margins into consolidated sediments will be plugged and abandoned with cement plugs. Drilling operations in shallow water require special operational precautions to ensure safety for crew and equipment. Weather conditions (particularly sea state and resulting heave behavior of the vessel) are critical (see "Risks and contingency"). Coring strategyThe first hole at each site (Hole A) will be cored with APC/XCB to refusal depth (estimated to be ~500 mbsf). The second hole (Hole B) at each site will be drilled without coring by installing a center bit to a depth slightly above the refusal depth of Hole A (e.g., ~490 mbsf), and RCB coring will extend from this depth to the target depth. While drilling/coring, a number of advanced piston coring temperature tool (APCT-3) and sediment temperature (SET)/sediment temperature pressure (SET-P) probe measurements will be made, as formation conditions permit, to complement existing data. Core orientation with the FlexIt tool will also be measured during the APC-cored sections at each site. After completing coring in Holes A and B at each site, the hole will be conditioned, displaced with logging mud, and the hole will be logged as per the logging plan (see "Downhole tools and logging strategy"). |
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