 | ||
| IODP publications Expeditions Apply to sail Sample requests Site survey data Search | ||
|
doi:10.2204/iodp.sp.316.2007 Drilling strategy, operations plan, and downhole measurementsTo achieve the scientific objectives, our primary operations at proposed Sites NT1-03B and NT2-01B will be to core and obtain downhole in situ formation measurements. Our operations plan and time estimate is shown in Table T1. Coring will progress from the hydraulic piston coring system (HPCS) to the extended shoe coring system (ESCS) to the rotary core barrel (RCB) to obtain the highest quality, most complete core samples. Formation temperature measurements (third-generation advanced piston corer temperature tool [APCT-3] and Davis-Villinger Temperature Probe [DVTP]) will be conducted during coring operations. Following coring, we may run downhole wireline logging measurements, with the specific package of logging tools to be determined based on the results of Expedition 314, the LWD component of the Stage 1 operation. Currently, all wireline logging operations for Expedition 316 are contingency only and will only be run if there are significant problems with LWD operations. Operations planThe operations plan and time estimate (see Table T1) are based on formations and depths inferred from seismic and regional geological interpretations without benefit of prior drilling in this immediate area. We have also based our plans on information from the landward ODP Leg 190 and 196 Sites 1175, 1176, and 1178, which are located >200 km west-southwest. Expedition 314 on the Chikyu will conduct LWD operations at proposed Sites NT1-03B and NT2-01B prior to Expedition 316. The LWD holes drilled at each site will be referred to as Hole A in all cases. We will use these operations and data to guide the operations described below. Our expedition plans to conduct operations in one hole (B) at Site NT1-03B (Fig. F4) (a second hole, C, is available as an alternate if the scientific target is not attained) and two holes (B and C) at Site NT2-01B (Fig. F6). An abbreviated list of primary sites, holes, drilling depths, and contingency plans can be found in "Site summaries." Proposed Site NT1-03BHole BHole B will be cored with the HPCS to refusal (~250 mbsf) and then with the ESCS to either (1) refusal, (2) significant problems with core quality, or (3) TD (~950 mbsf). We will conduct formation temperature measurements during HPCS coring (advanced piston corer temperature tool; APCT-3 if available) at a target spacing of every third core. Temperature probe measurements (DVTP or equivalent) will be taken during ESCS coring approximately every third core. If TD is reached in Hole B and LWD operations in Hole A were successful, the objectives of this site will have been reached, and Hole C (see below) operations will be cancelled. Hole CIf the target objective is not reached in Hole B because of ESCS refusal or poor core quality/recovery, Hole C will be drilled to reach the objective. Hole C will be drilled without coring to ~10 m above the TD of Hole B, where RCB coring will start and continue to a total target depth of 950 mbsf, assuming hole conditions permit. After the hole is conditioned, loaded with mud, and the bit is released in the hole, we may conduct wireline logging if LWD operations were unsuccessful. The specifications of the logging runs will depend on whether LWD operations in Hole A were successfully conducted during Expedition 314 at this site. If LWD operations were successful and the full planned suite of measurements were taken, there will no wireline runs. If LWD operations were unsuccessful because of poor borehole conditions, it is unlikely that hole conditions will allow successful wireline logging. However, if LWD operations did not reach this site because of time limitations or were not successful because of mechanical issues, there will be a logging run with density, porosity, natural gamma radiation, and electrical resistivity (triple combination [triple combo]) and a second wireline logging run with sonic velocity, resistivity imaging, and gamma radiation. In addition, a vertical seismic profile (VSP) will be conducted. Side wall formation pressure and fluid sampling tools are under consideration. To better understand the temperature regime seaward of the seismogenic zone and to observe active fluid circulation, open-hole mud temperature measurements during logging will be important. Proposed Site NT2-01BHole BHole B will be cored with the HPCS to refusal and then with the ESCS to refusal or 600 mbsf. Depending on core quality and recovery, we may switch to RCB in Hole B before refusal. We will conduct formation temperature measurements during HPCS coring (APCT-3 or equivalent) at a target spacing of every third core. Temperature probe measurements (DVTP or equivalent) will be taken during ESCS coring approximately every third core. TD for Hole B is 600 mbsf, and all operations in Hole B will cease once that depth is reached. Hole CPrior to drilling at Hole C, a jet-in test will be conducted to determine shallow subseafloor conditions. This will be followed by installation of a reentry cone and 20 inch surface casing to 60 mbsf. Hole C will be drilled without coring to ~10 m above the TD of Hole B, where RCB coring will start and continue to a total target depth of 1000 mbsf, assuming hole conditions permit. After the hole is conditioned, loaded with mud, and the bit is released in the hole, we may conduct wireline logging if LWD operations were unsuccessful. The specifications of the logging runs will depend on whether LWD operations were successfully conducted during Expedition 314 at this site. If LWD operations were successful and the full planned suite of measurements were taken, there will be no wireline runs. If LWD operations were unsuccessful because of poor borehole conditions, it is unlikely that hole conditions will allow successful wireline logging. If LWD operations did not reach this site because of time limitations or were not successful because of mechanical issues, there will be a logging run with density, porosity, natural gamma radiation, and electrical resistivity (triple combo); a second wireline logging run with sonic velocity, resistivity imaging, and gamma radiation; and a VSP will be conducted. Side wall formation pressure and fluid sampling tools are under consideration. To better understand the temperature regime seaward of the seismogenic zone and to observe active fluid circulation, open-hole mud temperature measurements during logging will be important. Following wireline logging, Hole C will be opened to a diameter of 17-1/2 inch and 13-3/8 inch casing will be run to the TD of the hole and cemented. If observations during the LWD operations (Expedition 314) or in Hole A indicate difficult hole conditions, a contingency plan is to install and cement 13-3/8 inch casing to a shallower depth determined by specific hole conditions and requirements. Drilling and coring will continue below the casing, and 9-5/8 inch casing will be installed and cemented into place after TD has been reached. Future observatory plansTentative plans for future observatory work at Site NT2-01B Hole C include return to the borehole to perforate the casing and cement at three intervals: (1) above, (2) within, and (3) below the splay fault for pore pressure monitoring. These intervals will be identified using information from Expeditions 314 (LWD transect) and 316. After conducting packer tests within each interval to determine formation permeability, a CORK-II type installation will be used to hydraulically isolate the intervals with packers inside the casing. A thermistor string or self-contained temperature loggers will provide in situ temperature and temperature changes through time at these three intervals and shallower portions of the borehole, and potentially long-term chemical sampling can be conducted (with OsmoSamplers or equivalent). Additional future stage work will add a paired borehole at this site for cross-hole hydrogeologic testing and strainmeters/seismometer. ContingencySubstantial risks to fully achieving our scientific objectives include (1) lack of data about hole conditions or formation properties, (2) seafloor conditions including slope (Fig. F9), and (3) potential problems with the Kuroshio Current (Fig. F10). There are minor risks associated with seafloor infrastructure (Fig. F11) and gas hydrate occurrence (Fig. F12); however, these factors have been well characterized and pose little or no risk. To mitigate any risks, we have devised a contingency strategy, as described below (Fig. F13). It is important to note that this contingency plan is based on our current state of knowledge at the time of writing this Scientific Prospectus and may be modified based on continuing NanTroSEIZE Project Management Team (NT-PMT) discussions, on continuing processing and interpretation of 3-D seismic data, and actual NanTroSEIZE Stage 1 operations. This contingency plan also reflects the linked objectives of multiple NanTroSEIZE Stage 1 expeditions by inclusion of "global" Stage 1 riserless coring objectives that would achieve overarching NanTroSEIZE and Stage 1 objectives. Proposed Site NT1-03Proposed Site NT1-03B is the primary choice of locations to achieve the frontal thrust objectives. This site has an estimated seafloor slope of 10°, and estimated depth to the frontal thrust is ~600 mbsf. Drilling conditions and data from Expedition 314 (LWD transect) will be used to decide the final drilling strategy at Site NT1-03. The primary plan is to drill with no reentry cone or casing if LWD indicates hole conditions are good. If Expedition 314 results indicate unstable hole conditions or if poor hole conditions are encountered at Hole B in this expedition, drilling in Site NT1-03B Hole C could use a reentry cone and case shallow unstable zones if time allows. Additional contingencies include moving to alternate Sites NT1-03A (second choice) or NT1-03C (third choice). Site NT1-03A has an estimated seafloor slope of 12°, which is at the limit for installation of a reentry cone and surface casing. If a reentry cone and casing are deemed necessary to achieve the objectives at this site, this alternative may not be acceptable unless camera or submersible survey identifies a suitably flat location. An additional uncertainty exists in the estimated depth of the frontal thrust resulting from potential inaccuracies in the velocity model. If the fault zone is shallower than expected, drilling may stop above the 950 mbsf nominal TD. If the fault zone is deeper than expected, additional drilling may be needed if time allows. Proposed Site NT2-01If LWD in Hole A (Expedition 314) or drilling in Hole B (Expedition 316) indicates unstable hole conditions, the contingency plan for Hole C, as discussed above, is to install 13-3/8 inch casing, drill and core, then install 9-5/8 inch casing. A strong Kuroshio Current could make casing operations difficult. In this case, a potential strategy would be to drill and RCB core as far as possible without a reentry cone and casing. An additional uncertainty exists in the estimated depth of the splay fault resulting from potential inaccuracies in the velocity model. If the splay fault is shallower than expected, drilling may stop above the 1000 mbsf nominal TD. If the fault zone is deeper than expected, additional drilling may be needed if time allows. If currents or hole conditions do not allow operations at Sites NT1-03 or NT2-01 or if we complete our objectives with sufficient time, we will proceed to coring at any remaining "global" Stage 1 riserless contingency sites. These global contingency sites were identified by the NT-PMT as important toward achieving overall scientific objectives of NanTroSEIZE Stage 1 drilling and will serve as common contingency sites for all of the Stage 1 expeditions. Coring at these contingency sites will follow in the order described as follows, beginning with the first site not already drilled during contingency operations during previous Stage 1 expeditions. Operations at all of these sites may require casing depending on hole conditions.
|
|
