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Scientific objectives and operational strategy

Long-term monitoring within a network of boreholes along the Kii drilling transect is one of the central objectives of NanTroSEIZE and is critical in order to shed light on potential changes in mechanical and hydrologic properties through the seismic cycle, define the upper transition from aseismic to seismic deformation and distribution of interseismic strain accumulation, and assess the role of the megasplay in accommodating slip. Therefore, Expedition 332 followed up on the riserless observatory installations begun during Expedition 319 in summer 2009. Our activities focused on two sites:

  • Site C0010, drilled and cased across the shallow megasplay fault and into its footwall, has been identified as a high-priority target for permanent long-term monitoring. However, because no LTBMS was available for Expedition 319, a simple temporary pore pressure and temperature observatory (SmartPlug) was installed in 2009 (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). The objectives for Expedition 332 included recovering the SmartPlug and replacing it with the upgraded temporary observatory that also includes geochemical sampling capability and in situ microbiological culturing media, termed a “GeniusPlug.”

  • Site C0002 is also identified as a high-priority target for permanent long-term monitoring. Ultimately, observatory instruments at this location will be installed in two boreholes: one ~1 km deep riserless LTBMS and one deep riser completion at ~5–7 km below seafloor. The main goal of Expedition 332 was to drill and case a borehole at Site C0002 and install an integrated LTBMS that combines thermal, hydrologic, and geodynamic sensors to monitor pore fluid pressure, temperature, strain, tilt, and seismicity. The installation contains four major components: bottom-hole instruments and hydraulic screens, tubing to provide mechanical support for downhole cables and hydraulic lines, a swellable packer to provide hydraulic isolation between the formation below and the overlying ocean, and a wellhead similar to those used for previous circulation obviation retrofit kit (CORK) installations during ODP and IODP (Becker and Davis, 2005). The wellhead is designed to land at the casing hanger and suspend the tubing, packer, and instruments below. The instruments include a broadband seismometer, a tiltmeter combo, a thermistor string, and a strainmeter (see “Site C0002 riserless observatory”).

The operations summary for Expedition 332 is shown in Table T1. After departure from Shingu, Japan, port on 27 October 2010, the D/V Chikyu evacuated from Typhoon Chaba, carried out additional loading of equipment at sea, and started drilling operations at Site C0010 on 2 November. At Site C0010, operations began with an ROV dive to place transponders on the seafloor and recalibrate the dynamic positioning system. The Chikyu then moved to a low-current area on 3 November to start lowering the casing packer retrieving assembly and running tool. While lowering the string through and in the Kuroshio Current, two accelerometer packages to monitor vortex-induced vibration (VIV) were attached to the drill string at two depth levels. In parallel, ropes were attached the upper 500 m of the drill string, where current velocities are highest. Once the bottom-hole assembly (BHA) neared the seafloor, the ROV was launched to assist reentry into Hole C0010A, which took place around noon on 5 November. The assembly latched onto the retrievable bridge plug 5 h later, and the bridge plug and SmartPlug were pulled out of the hole in the early morning of 6 November. At 1400 h on 7 November, the bridge plug and SmartPlug were recovered on the rig floor. We detached the SmartPlug and downloaded and processed the stored data. Hole C0010A was covered with the corrosion cap and transponders were recovered before moving to Site C0002.

At Site C0002, transponders were already in place from drilling during IODP Expedition 326 (NanTroSEIZE Stage 3: plate boundary deep riser: top hole engineering) in summer 2010 (Expedition 326 Scientists, 2011). The Chikyu moved upstream of Site C0002 on 13 November and prepared to run the drilling BHA into the water. Anti-VIV ropes were attached to the string and the vessel started to drift toward the site. Jetting in began on 15 November. After completing additional loading via supply boat, reaming of the hole and drilling using the LWD/MWD BHA commenced on 17 November. First, a 12¼ inch hole was drilled to 892 mbsf and logging data were collected with a limited suite of LWD/MWD tools (MWD PowerPulse/TeleScope, annular pressure while drilling [APWD], and arcVISION array resistivity compensated tool with natural gamma radiation detection and attenuation resistivity). After pulling the drill string, a 10⅝ inch BHA was run in to deepen the hole to a TD of 980 mbsf. This was mostly done to maintain a small hole diameter and minimize disturbance in the anticipated target depth for the cemented instruments. LWD data were collected during drilling of the 10⅝ inch hole from 892 to 980 mbsf, with a section of the hole relogged because of failure in the live streaming of LWD data. After a wiper trip and some reaming, the hole was found to be in good condition and casing (including screened casing joints set for 757–780 mbsf) was lowered on 22 November. During casing operations, the accelerometer packages as well as anti-VIV ropes were run again. Casing operations were completed early on 25 November, and the BHA was pulled out of the hole in order to have a short port call at Shimizu, Japan, on 27 November.

Once back at the site of Hole C0002G, drilling out the cement started on 29 November before the hole was swept and spotted with high-density brine and a high-viscosity temporary gel for later reentry and LTBMS hole completion. Early on 30 November the CORK running tool was picked up, running in the hole began, and the flatpack umbilical, three cables for CORK instruments, and the thermistor string were terminated and connected. From 1 December, joints of 3½ inch tubing were slowly run down with four centralizers per joint, with additional tie wraps and metal bands attached every ~1 m to secure the cables, thermistor, and flatpack. From 2 December onward, the number of centralizers per joint was reduced from four to two, with a corresponding increase in speed and efficiency. The swellable packer was slit and the cables were fed through its body seal a day later before lowering tubing commenced. Both prior and after packer operations, the CORK instruments were tested via an interface and extension leads to a container unit in the moonpool area. From late on 2 December into the morning of 3 December, the CORK head was picked up on the rig floor, latched to the CORK running tool, and lowered into the moonpool area. The electrical cables and flatpack umbilical were measured, cut, and terminated thereafter and tested prior to and after mounting them to the CORK head assembly. All tests were successful so the ROV platform could be attached, and the CORK head assembly finally entered the water in the evening of 5 December. Three more CORK component tests were carried out: before drifting to the site (CORK head at 350 meters below sea level), after drifting (above the hole), and after landing of the CORK head in the well. The last of these tests was successfully completed late on 8 December. Cementing through a cement port at 937 mbsf took place on the morning of 9 December, and a final check of the hydrogeologic unit at the CORK head took place by ROV 15 h later before the transponders were recovered. Expedition 332 ended at sea on 11 December, and scientists were exchanged via helicopter.