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doi:10.2204/iodp.sp.314.2007 Drilling strategyTo achieve the overall scientific objectives of the NanTroSEIZE project, drilling is planned at eight sites. As many as three sites target the incoming plate section, one will sample the frontal thrust of the accretionary wedge, two sites target the megasplay fault system at different depths, one site will sample the megasplay uplift history recorded in the forearc basin sediments, and one ultradeep site targets the plate interface in the seismogenic zone. NanTroSEIZE Stage 1 calls for drilling and sampling in riserless mode at six of the sites, including the following:
Primary drill sitesDuring Expedition 314, the order of drilling at the six sites is determined by scientific priority and operational difficulty. The full set of LWD/MWD tools will first be deployed at proposed Site NT1-01A at the far southeast of the Kumano Basin in the Nankai Trough. An LWD/MWD hole at proposed Site NT1-01A will be drilled to the top of basement at an estimated depth of 600 mbsf. Next, proposed Site NT2-03B is to be drilled into the upper thrust sheet and through a possible fault zone at ~800 mbsf to a total depth of 1000 mbsf or greater. The third site, proposed Site NT3-01B, is located in the Kumano forearc basin. This site is a pilot for the deep riser drilling in later stages. To facilitate achievement of the 1400 m deep objective, installation of casing is planned for the upper 1000 m before drilling deeper. This will require that LWD logging be conducted in two or more separate runs. Proposed Site NT2-01B is located at the shallower part of the megasplay OOST thrust system at the outer arc high and is planned to reach a maximum depth of 1000 mbsf, passing through the splay fault reflector at ~800 mbsf and continuing ~200 m into the footwall silts and clays of the accretionary prism. Next, we plan to drill the frontal thrust site, proposed Site NT1-03A, to a depth of as much as 1000 mbsf, though only 950 m of drilling is currently scheduled. Finally, we plan to turn to proposed Site NT1-07A in the incoming sediments of the Shikoku Basin, where the plan is to drill ~1200 m to reach the top of the oceanic crustal rocks of the basement. In the event that drilling conditions are difficult at proposed Sites NT2-01B and NT2-03B, casing will be available as a contingency for the upper 600 m. For planning purposes, this casing plan is included in the operation time estimate at proposed Site NT2-03 (Table T1), but not at proposed Site NT2-01; however, this contingency covers drilling at both sites. Contingency/alternate sitesA total of 15 contingency days are factored into the expedition for time lost as a result of weather, equipment, and drilling. If the LWD operations at the primary sites are completed ahead of schedule or if operations at one of the primary sites cannot be completed because of currents or other difficulties, then drilling may be conducted at (1) an additional site, proposed Site NT2-04 (Fig. F13), and (2) proposed Site NT1-03 to greater depth in order to reach farther into the deformed footwall of the frontal thrust, since EPSP has approved penetration at this site to as deep as 1800 mbsf. If the LWD operations are behind schedule, drilling may stop at shallower depths than the target depth at proposed Site NT2-03 and all other sites, as needed. In addition, most of the primary sites are accompanied by one or more approved backup sites (e.g., proposed Site NT2-03C; see "Site summaries" for details). In the event that the primary site is deemed unsuitable, either before or during operations at that site, then drilling at the respective backup sites will become the first alternative strategy. Logging strategyDifficulties in attempted wireline logging during previous legs at Nankai and other accretionary prisms and, conversely, the successful experience of Leg 196 (Mikada, Becker, Moore, Klaus, et al., 2002) at Nankai clearly indicate that LWD is required to acquire in situ physical properties data in the Nankai accretionary prism. LWD surveys have been very successful in obtaining logs from active prisms during the following ODP legs and IODP expeditions:
During Expedition 314, we will employ both LWD and MWD technologies. LWD and MWD tools measure different parameters. LWD tools are formation evaluation tools while drilling; they measure formation properties with instruments located in the drill collars above the drill bit. MWD tools are also located in the drill collars and measure downhole engineering drilling parameters (e.g., weight on bit, torque, etc.). Another function of the MWD tool is transmitting real-time MWD data and a limited set of LWD data through drilling fluid inside the drill pipe by means of a modulated pressure wave, or mud pulsing, which is then monitored in real time at the rig floor. Most LWD data, at least those of highest resolution and thus larger size, are recorded in memory at the downhole tool and retrieved when the tools reach the surface. For safety monitoring purposes, the MWD tool will be coupled with APWD for all holes drilled during this expedition. MWD tools enable both APWD and MWD data to be transmitted uphole when the tools are used in conjunction, along with a subsampling of the LWD data. The term "LWD" is often used generically for LWD and MWD-APWD measurements. Logging data complement core-based information because they are continuous and not subject to core recovery and because additional parameters or scales of investigation can be measured. At the time of this writing, we are planning to deploy the following Schlumberger MWD and LWD instruments (www.slb.com/content/services/drilling/imaging/index.asp):
To ensure good resistivity image resolution and gamma ray data quality, ~20–30 m/h rate of penetration is required. |
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