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doi:10.2204/iodp.pr.337.2012

Operations

The Chikyu left Hachinohe Port at 1200 h on 26 July 2012 and arrived at Site C0020 after 7.5 h of transit. The corrosion cap of Hole C9001D, which was drilled to 647 m drilling depth below seafloor (DSF) and cased to 511 m DSF during the Chikyu shakedown cruise in 2006, was retrieved at the surface by 2400 h on 27 July. Ten transponders were installed on the seafloor by remotely operated vehicle on 28 July, and failure mode effect analysis for the Acoustic Position Reference System was completed on 29 July.

While conducting a surface pressure test of the blowout preventer (BOP), the science party was shuttled aboard by helicopter flights on 31 July. Technical problems were found during the BOP function test, and troubleshooting continued until 8 August. A successful BOP landing was confirmed on 11 August. Extensive BOP tests were carried out until completion of the pressure test on 14 August. Because of the delay in operations, planned spot coring in the 647–1220 mbsf interval was cancelled and a 17½ inch hole was started to drill out cement and reach 1220 mbsf.

On 16 August, we encountered total mud loss at 2375 m drilling depth below rig floor (DRF). After spotting lost circulation material twice, the mud loss rate decreased to 0.8 m³/h. A decision to continue drilling with seawater gel mud was made.

Drilling resumed on 18 August and reached 2471.5 m DRF (1263 m DSF); 13 inch casing was installed to this depth. Coring operations started on 25 August.

Riser drilling with spot cores continued to Core 337-C0020A-7R, which hit a low rate of penetration (ROP) interval at 2812.5 m DRF (1604.0 m DSF). The core included gravels of volcanic origin and showed different lithology from previous cores. The Co-Chief Scientists decided to take an industry-type large diameter core (LDC) at this depth.

LDC operations were stopped at 2834 m DRF before reaching the 27 m of drilling advance in the original plan, as core jamming was suspected when an increase in pump pressure and no penetration were observed. The LDC was recovered on deck on 30 August and 10.0 m of core was recovered from 21.5 m of advance. The core was cut into 1.0 m long sections at the middle pipe rack and transferred to the laboratories.

Rotary core barrel (RCB) coring resumed and Cores 337-C0020A-9R to 14R were taken before continuous coring through the coal-bearing horizons started at 1919 m DSF. After four consecutive RCB cores were recovered, the drill bit appeared to be worn out. Consequently, it was decided to change the drill bit.

After coring of Core 337-C0020A-18R, it was also decided that we continue RCB coring and cancel planned LDC operations. The new drill bit was installed and coring operations resumed on 4 September. Another seven consecutive RCB cores were taken from 3158.5 to 3212.0 m DRF (1950–2003.5 m DSF). Core recovery was high, and coal-bearing sequences were obtained. As we were able to sample various lithologies within and around the coal-bearing formation, the Co-Chief Scientists concluded that we fulfilled our operational mission in this interval. They decided to drill deeper to investigate the broader range of the hydrocarbon system and to explore the limits of life.

We deepened the hole while spot coring in 100 m intervals and reached the terminal depth of Hole C0020A at 2466 m DSF on 9 September. After pulling out of the hole, wireline logging operations started.

Logging Run 1 (Platform Express [PEX]) started on 10 September, followed by Runs 2 (Formation MicroImager [FMI]) and 3 (combinable magnetic resonance [CMR]). High-permeability layers were selected for the Modular Formation Dynamics Tester (MDT) based on the results from the first three runs. After a wiper trip, Run 4 (MDT-gamma ray [GR]) started on 12 September. Pretests for fluid mobility and formation pressure measurements were carried out at 31 horizons. Formation fluid samples were taken at 6 horizons of high mobility, and the 6 bottles were recovered on deck on 14 September. The sample bottles were delivered to the laboratory during the following Run 5 (vertical seismic profile [VSP]). The last run was completed on 15 September, ending scientific operations on the rig floor.

This expedition was originally scheduled for March–May 2011 but was postponed because of the Tohoku earthquake and the following tsunami hazard, which hit the eastern coast of Japan, including Hachinohe Port. The Chikyu suffered damage on the ship body and lost one of the thrusters during emergency evacuation from the port. Repair and reinstallation of the thruster were completed in June 2012, and this expedition was rescheduled for implementation in July 2012.

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