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doi:10.2204/iodp.pr.319.2009

Operational strategy

Our drilling plan for Expedition 319 included one riser borehole in the Kumano Basin at proposed Site NT2-11B (drilled Site C0009; Fig. F4A) and one riserless hole at proposed Site NT2-01J (drilled Site C0010; Fig. F4B).

To meet the scientific and engineering objectives, primary operations at both sites were to drill and case to TD (Araki et al., 2009). Because of anticipated challenges and uncertainty in operations schedules with both riser and riserless operations, we also developed a detailed suite of contingency operations for the expedition. Our primary contingency operations included installing casing and a smart plug at IODP forearc basin Site C0002 (Araki et al., 2009) and drilling and coring sediment inputs Site NT1-01 proposed for Expedition 322 (Saito et al., 2009) (Fig. F3).

Planned operations at Site C0009 began with running and jetting-in a 36 inch conductor casing to 55 mbsf. Following this, the wellhead and blowout preventer (BOP) were to be set at the seafloor, and a 26 inch riserless hole drilled to 700 mbsf with MWD, including annular pressure while drilling (APWD). After installing and cementing 20 inch casing in the upper 700 m, the planned operations were to lower the BOP, connect it at the wellhead, drill a 12¼ inch riser hole from 700 to 1510 mbsf with MWD including recovery and analysis of cuttings, and then rotary core barrel (RCB) core from 1510 to 1600 mbsf. After coring and opening the cored hole to 12¼ inches, three wireline logging runs were planned from ~700 to ~1600 mbsf: (1) the Schlumberger Platform Express (PEX), including the Highly Integrated Gamma Ray Neutron Sonde, Three-Detector Lithology Density tool, and High-Resolution Laterolog Array (HRLA); (2) Formation MicroImager (FMI) and Sonic Scanner; and (3) MDT. After wireline logging, the hole was to be opened to 17 inches and cased to TD with 13 inch casing. Drilling mud gas collection was planned during all riser drilling operations. After cementing the casing, a cement-bond log (CBL) was planned inside the casing, followed by a zero-offset VSP and a two-ship walkaway VSP experiment coordinated with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) R/V Kairei. The VSP experiment was planned for 2 or more days, but the duration depended on operational progress and the drilling schedule. Following the VSP, we planned to suspend the riser hole by setting a corrosion cap, with a contingency plan to hang a short (~100 m) thermistor string into the casing for later retrieval if time permitted.

We completed essentially all of the planned scientific and engineering objectives at Site C0009, with riser operations ending on 31 July (as scheduled, including contingency days). Because of time constraints at the time of casing and cementing, the CBL was eliminated from operations. In addition, because of the combination of time constraints on the drilling schedule and limitations on the availability of the Kairei (shooting vessel), the planned two-ship VSP experiment was shortened slightly. A total of 19 contingency days were included in the original schedule and were fully used, primarily because of recovery of a lost bottom-hole assembly (BHA)/drilling ahead tool (DAT) during riserless drilling, problems with the dynamic positioning system (DPS), mechanical problems related to riser system installation (riser tensioners and tensioner load ring), and weather. In other cases, operations were considerably faster than scheduled, including the retrieval of the riser system after completion of riser drilling operations and installation of the 20 inch casing string.

Planned operations at proposed Site NT2-01J (drilled Site C0010) included jetting-in a 20 inch casing string to 35 mbsf, followed by hole reentry and drilling of a 12¼ inch hole with MWD and a limited suite of LWD (GVR, including GR and azimuthal resistivity-at-the-bit [RAB] measurement) to TD at 560 mbsf. After drilling, 9 inch casing with two screened casing joints (~11 m long each) spanning the megasplay fault would be installed to ~550 mbsf, leaving an ~10 m open "rathole." Based on seismic data, we estimated the depth of the megasplay fault at 410 mbsf; we anticipated adjusting the exact depth for the casing screens shortly before running the casing, using the newly acquired LWD/MWD data. After cementing the casing, we planned an observatory sensor dummy run to measure shock and acceleration experienced by highly sensitive strainmeter and seismometer instruments to assess operations for future reentry and installation, followed by a wireline temperature log inside the casing to identify the top of cement in the annulus. The hole was to be suspended by installing a retrievable casing packer (modified to include the smart plug instrument package below) at ~390 m and setting a corrosion cap. The smart plug is designed to thread to a crossover joint at the bottom of the bridge plug and includes a self-contained temperature sensor and data logger, as well as a pressure gauge and data logger package. These instruments will be in hydrologic communication with the fault zone at the screened interval and will monitor formation pore pressure and temperature from the time the bridge plug is set until it is retrieved at the beginning of future permanent riserless observatory installation operations.

We completed all of the planned operations at Site C0010, with the exception of the wireline temperature log, which we eliminated to preserve time for the anticipated challenges of running casing in the strong Kuroshio Current and after discussion with the operations group resolved that it would be of limited use to define the top of cement. In addition, drilling with LWD/MWD was suspended after reaching 482 m LWD depth below seafloor (LSF) (Table T1) in order to evacuate for a typhoon and resumed after ~2 days. Based on poor data quality related to ship heave and stick-slip, we reamed and relogged a critical interval near the megasplay fault zone from 348 to 418 m LSF before running down to the previous TD and drilling to a final TD of 555 m LSF.

In practice, our contingency options evolved during the expedition because of time constraints and the limited availability of personnel for coring operations. In response to these constraints, we developed additional contingency plans during the expedition, which would generate data of high value to the overall NanTroSEIZE drilling project. These operations included LWD/MWD at proposed Sites NT1-07 and NT1-01, which were the primary and contingency sites, respectively, for Expedition 322 (Saito et al., 2009). In the final 5 days of Expedition 319, we moved to Site C0011 (proposed Site NT1-07), dropped 6 transponders, and drilled with LWD/MWD to a TD of 952 m LSF. Data from Site C0011 are not described here but will be included in the Expedition 322 Preliminary Report along with results of planned coring, downhole measurements, and wireline logging.

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