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Site C00021

H. Tobin, T. Hirose, D. Saffer, S. Toczko, L. Maeda, Y. Kubo, B. Boston, A. Broderick, K. Brown, A. Crespo-Blanc, E. Even, S. Fuchida, R. Fukuchi, S. Hammerschmidt, P. Henry, M. Josh, M.J. Jurado, H. Kitajima, M. Kitamura, A. Maia, M. Otsubo, J. Sample, A. Schleicher, H. Sone, C. Song, R. Valdez, Y. Yamamoto, K. Yang, Y. Sanada, Y. Kido, and Y. Hamada2


Transit from Shimizu, Japan (port)

Integrated Ocean Drilling Program (IODP) Expedition 348 began on 13 September 2013 when the D/V Chikyu left the port of Shimizu en route for Site C0002. Typhoon Man-yi’s approach on 14 September set the Chikyu into waiting on weather (WOW) status; the Chikyu remained in WOW until 16 September and then returned to Site C0002, arriving on 17 September. Upon return to Hole C0002F, the remotely operated vehicle (ROV) dove, and the seabed survey and transponder deployment was completed on 18 September. The ROV then checked the wellhead at Hole C0002G to observe the Long Term Borehole Monitoring System CORK.

Hole C0002M

The riser joints required pressure tests when the riser running equipment was rigged down to prepare for testing the small-diameter rotary coring barrel (SD-RCB) assembly, which continued until 19 September 2013. Although designated the “small-diameter” coring tool, the SD-RCB core is actually larger than standard RCB cores (7.3 versus 6.6 cm inner diameter [ID]); the bit itself has a smaller diameter (8½ inches versus the standard 10½ inch [or larger] drill bits) (see the “Methods” chapter [Tobin et al., 2015]). The guide horn was rigged up and the job completed by 20 September. The coring bottom-hole assembly (BHA) (Table T1) was made up on the rig floor and run into the hole (RIH) on 19 September. The Chikyu began drifting to the proposed well center, the center bit was dropped, and the BHA was washed down 30 m; Hole C0002M drilling began from 30.5 meters below seafloor (mbsf) (1996.5 meters below rig table [m BRT]). By 21 September, the BHA had drilled ahead to 475 mbsf, and coring began from that depth (Tables T2, T3), cutting 4 cores and ending at 512.5 mbsf (total depth [TD]). After coring operations were completed, 50 m3 of kill mud (1.30 sg) was spotted in the hole, and the BHA was pulled out of hole and recovered to the surface on 22 September.

Hole C0002F

The riserless guide horn assembly rig down began, and the rig up for riser running started; the ROV dove to deploy 3 transponders. This was completed, and the Chikyu moved to the well center. Pressure tests of riser joints continued until the risers and test plug were secured for another standby off Shionomisaki for WOW until 25 September. The Chikyu left Shionomisaki for the helicopter meeting point, for crew changes. The Chikyu then returned to the Site C0002 blow-out preventer (BOP) running point, 15 mi northwest of Site C0002, and prepared to run the BOP and riser. From 26 September 2013, the BOP was run down while low-pressure (300 psi) tests of the auxiliary line (kill and choke lines) began. The tests failed, so troubleshooting began, continuing with various connectors, riser joints, and other procedures. On 28 September, all lines passed low-pressure and high-pressure (5000 psi) tests. The BOP was landed on the BOP cart, and moonpool hoses were recovered for transit to Mikawa Bay on 29 September to pick up a repaired riser joint. The Chikyu reached the anchorage point on 30 September and, once loading was completed, returned to the Site C0002 BOP running point. On 1 October, however, the approach of Tropical Storm Fitow returned the Chikyu to WOW until the storm track and an unrelated weather front were observed. The logging-while-drilling (LWD) tools were loaded from the supply boat on 2 October during WOW.

Full operations resumed on 5 October. The BOP and riser were run, with subsea vibration data loggers for vortex-induced vibration (VIV) monitoring installed on slick joints. After running Joint 9 on 6 October, auxiliary line pressure tests were carried out. While running the BOP and riser, the ship drifted at 0.2 kt to Site C0002. The termination joint and last subsea vibration data loggers were installed on 9 October. BOP landing began after running the last sets of joints, on 10 October. The landing was successfully completed, followed by pressure tests of the 20 inch casing and wellhead connector.

On 11 October, a tropical depression developed into a typhoon and was predicted to approach the drill site. Therefore, the lower marine riser package (LMRP) was disconnected on 12 October, and the vessel moved to the stand-by site 17 mi north-northwest of Site C0002. WOW continued until Typhoon Wipha passed the site on 16 October, with the LMRP hung near the seafloor. Even after Typhoon Wipha passed, WOW continued because of the forecast of another strong typhoon reaching the area in a week. Therefore, riser pipe and LMRP recovery began on 16 October and continued to 19 October. The LMRP pick-up was suspended because of bad weather but was completed by 20 October. WOW continued until 25 October, when the Chikyu moved to the Site C0002 BOP running location. Typhoon Francesco’s course was similar to that of Wipha and approached the site on 25 October; the Chikyu resumed operations on 26 October.

The LMRP was run into the water on 26 October, following the earlier procedure. A dummy landing was carried out when the vessel was 50 m west of Hole C0002F on 31 October. The vessel moved to the well center (1967.5 m BRT [1939 meters below sea level [mbsl]), and the LMRP was successfully landed on the BOP and locked by 31 October. After the BOP isolation test tool was run, Yellow BOP control pod (POD) pressure tests and Blue POD function tests of the BOP were conducted on 1 November.

The 17 inch LWD and GeoPilot BHA was made up and run in the hole on 2 November. Running the BHA in the hole while pumping and washing down continued until reaching the estimated top depth of the Number 2 shallow cement plug at 348.4 mbsf, but there was no indication of any decrease (45 kN) in weight on bit (WOB). The top of the Number 1 cement plug, however, was indicated by a clear decrease (15 kN) in WOB at 765.5 mbsf on 3 November. Drilling continued to 850.5 mbsf, and we conducted a pressure test of the 20 inch casing.

Hole C0002N

After circulation for hole cleaning, the sidetrack for Hole C0002N began at 860.3 mbsf on 4 November 2013. Sidetracking continued until the drilling inclination and azimuth reached 5° and 0°, respectively. On 5 November at 1191.5 mbsf, inclination was changed to 0°. Drilling continued to 1219 mbsf, followed by sweeping with 10 m3 of Hi-vis mud. During drilling in Hole C0002N, several episodes of overpull continued to occur. On 7 and 8 November during pulling out of hole (POOH) for maintenance, two overpull intervals were observed: (1) 240 kN at 1632.5 mbsf and (2) 200–250 kN between 1579.5 and 1577.5 mbsf (Table T4). On 10 November, when the LWD BHA was POOH for WOW stand-by, overpull, drag, and slack intervals were observed:

  • Drag (100–240 kN) between 2008.5 and 1404.5 mbsf,
  • Overpull (320 kN),
  • Slack (222 kN) at 1804.5 mbsf, and
  • Drag (100–250 kN) from 1545.2 mbsf.

After TD (2330 mbsf) was reached on 13 November, POOH during a wiper trip had two overpull intervals: (1) >300 kN between 2330 and 2023.5 mbsf and (2) ~300 kN between 2023.5 and 1807.5 mbsf. On 14 November after two sets of Hi-vis mud sweeping at 2327.5 mbsf, POOH began with two overpull events: (1) 2311.5 mbsf (400 kN) and (2) 2289.5 mbsf (500 kN). Pumping cleared these overpull intervals and no additional drag was observed.

Drilling paused on 9 November because the real-time measurement-while-drilling (MWD) signal was lost. After several attempts to recover the signal, the Operations Superintendent, Operation Liaison, and Expedition Project Manager agreed to continue drilling without real-time monitoring to 2008.5 mbsf. On 10 November, the LWD BHA was POOH to rig floor to remove the GeoPilot from the BHA, LWD memory data dumping, MWD pulser replacement, and mud-pump maintenance. The new 17 inch LWD BHA was run in the hole, and drilling resumed from 2008.5 mbsf on 11 November. Soon after, an active volume drop causing 48 m3/h of mud loss at 2036.5 mbsf was observed. Lost circulation material (LCM) and Hi-vis mud were spotted and mud-loss rates decreased to 1 m3/h by 12 November. Drilling resumed with another short interruption and spotting LCM due to reoccurring mud loss (27 m3/h) at 2117.5 mbsf. POOH continued until the BHA reached the 20 inch casing shoe. The LWD BHA was run back to the bottom to check and clean the hole; washing and reaming continued until 14 November.

On 15 November, the LWD BHA was POOH to 836 mbsf and resumed after flow check and BOP function test. The LWD BHA was laid down, and LWD memory data were successfully downloaded. Preparations to run and cement the 13⅜ inch casing began. After troubleshooting the flush-mount grip spider and power tong, running casing continued all day. On 18 November, the casing hanger joint was picked up and the 13⅜ inch casing was run with 5⅞ inch drill pipe from 341.5 to 501.5 mbsf, and the insert was changed to 6⅝ inch drill pipe after taper seating against the primary elevator insert was checked. Running 13⅜ inch casing on the 6⅝ inch landing string continued to 1993.5 mbsf without observing significant drag.

While running the casing string, tight spots were encountered at 2014.5 and 2027.5 mbsf and a pressure buildup was observed; the pipe became stuck at 2024.5 mbsf. The casing was pulled up to 1993.5 mbsf after working the pipe slowly with overpull, but pack-off indication was observed. This continued while pulling back to 1802.5 mbsf, but on 19 November pressure and return flow rate returned to normal while pulling back to 1774.5 mbsf. At 1799.5 mbsf, the pump rate was increased to 50–100 strokes/min at pump pressure of 3.4–6.2 MPa to clean the annulus. Running casing resumed, although minor mud loss and tight spots were again encountered at 1954.5 mbsf. The tight spots were passed by increasing the flow rate; however, mud loss increased to 40 m3/h after the flow rate was again reduced. Working pipes, adjusting flow rate, and spotting LCM were repeated without success to push the casing beyond 2041.5 mbsf due to mud loss up to 50 m3/h. At the end, we decided to pull out the 13⅜ inch casing, shorten it, and run it again. POOH started on 20 November.

A new pressure-assisted drill pipe running tool assembly was made up and casing handling tools were rigged up on 21 November. Twenty-five joints of 13⅜ inch casing pipes were laid down, and running the shortened 13⅜ inch casing pipes began. No drag larger than 100 kN was observed while running to 1502 mbsf; however, minor mud loss (5–10 m3/h) was observed at 1994.5 mbsf. Working pipes and increasing flow rate allowed the casing to pass these tight spots. Landing the casing hanger onto the wellhead was completed on 22 November. The bottom depth of the 13⅜ inch casing was 2010 mbsf. Cementing the casing was followed by a 5500 psi pressure test. The BOP’s Blue POD function and pressure test was carried out successfully, ending on 24 November. Following in-line blowout protector and Hydrarack power swivel (HPS; National Oilwell Varco) rotary hose pressure tests, the 12¼ inch drill-out cement (DOC) BHA was made up and RIH. The cement top was tagged at 1897.7 mbsf on 25 November, although standby for WOW (cold-front passage) began. The drill string was pulled out to above the BOP, and mud in the riser was displaced with seawater. After WOW, operations resumed from displacing riser with 1.13 sg drilling mud. The top of cement was tagged again, circulated bottoms up, and rigged up for a casing pressure test. The pressure test was completed, and the test joint was rigged down to prepare for drilling out cement. On 27 November, drilling ahead confirmed the presence of cement at 2008.5 mbsf, and rig up for the shoe bond test began. The test was successful, pressurized to equivalent mud weight of 1.25 sg for 5 min. Drilling out cement recommenced, but when drilling reached 2028.4 mbsf, WOB and the HPS torque became unstable, fluctuating and suddenly increasing. It became clear that the pipe was stuck, and attempts to free the pipe by jarring began. Jarring was only paused for derrick inspection. The Schlumberger free point indicator tool (FPIT) was rigged up and run to survey for the drill string stuck point. The FPIT indicated that the stuck point was inside the 13⅜ inch casing, right above the Number 2 centralizer. The FPIT was recovered, and a dummy run of the back-off tool was run in hole on 29 November. The back-off tool planned to break the drill string above the Number 2 centralizer, between the Number 1 and 2 drill pipes. Wireline calibration with the casing collar locator (CCL) was used to correct the estimated colliding depth, as determined by wireline and drill pipe length. The back-off tool broke the drill pipe at 1886.35 mbsf, confirmed by torque changes (–37 to 0 kN/m) and a one-quarter rotation observed in the drill pipe at the rig floor. The colliding tool was recovered to the surface, and circulation with bottoms up was performed. The drill string was POOH to surface on 30 November, and the back-off point was confirmed and examined. No damage was observed on the bottom drill pipe pin connection, but the pipe joint was marked and removed from use. The fishing BHA was made up and run in hole, and by 1 December the fishing BHA reached and was screwed into the top of the fish. After pumping, checking hook load, and increasing torque, work continued on trying to work the stuck pipe. By 2 December, the fish had been jarred 724 times up and 173 times down. The Schlumberger FPIT was rigged up again and confirmed the free pipe measure to 1963 mbsf. A second FPIT run on 3 December confirmed a drill collar connection at 1969.88 mbsf. A dummy run with a 2.3 inch gauge also confirmed the drill collar connection. The back-off/colliding tool was prepared and RIH to 1960.58 mbsf, calibrated the firing depth with the CCL, and fired. The back-off tool was recovered to the surface, and free pipe was confirmed by monitoring hook load (4150 kN) after picking up 2 m. Circulation and bottoms up from 4 December was completed while the Schlumberger wireline tools were rigged down. The collided drill collar was recovered on deck on 5 December, and pin end examination of the drill collar found that the outer diameter had expanded by inch (8½ to 9 inches in size), while the pin thread was also expanded and cracked. A 12¼ inch BHA was made up and RIH to tag the top of the remaining fish; it tagged the top (slack off weight = 30 kN) at 1960.4 mbsf. The decision was made to kick off through the casing and begin a new sidetrack to continue drilling. The BHA was POOH to surface, after which a scraper BHA was made up and RIH.

The scraper BHA tagged the fish at 1960.5 mbsf and started scraping the interior of the casing. The BHA was POOH and laid down to rig up for wireline runs with the Schlumberger cement bond log tool began. The cement bond log tool was run on 7 December and surveyed between 1958.5 and 1299.5 mbsf, confirming cement to 1352.5 mbsf. A junk basket run recovered ~600 g of cement, after which the bridge plug/CCL wireline run was rigged up and RIH; the bridge plug was set at 1950 mbsf. The wireline running tool was recovered, and a flat-bottom mill BHA was made up and RIH to confirm the bridge plug setting depth. The mill BHA tagged the bridge plug at 1948 mbsf on 8 December. Circulation and bottoms up completed with POOH for the BHA. The slight discrepancy (~2 m) in bridge plug depths necessitated another run with the CCL wireline assembly, which was rigged up and RIH. The CCL tagged the bridge plug and confirmed the top of plug depth at 1949.5 mbsf. Wireline recovery and rig down was completed early on 9 December.

The next operation was a BOP pressure test, running ~11 successful pressure tests. The weather began to pick up, and at 0230 h a planned emergency disconnect sequence was successfully carried out. The vessel moved 1.1 mi north of the well center by 0700 h. After this, the HPS was dismantled for a magnetic particle inspection of the main shaft of the main power unit; inspection passed. The vessel at this point was 1.2 mi northwest of the Hole C0002F well center, and we began rigging up the riser running tool. The Chikyu began drifting to well center as the LMRP running and landing tools were made up and RIH. On 11 December, the vessel had reached well center and, guided by the ROV, landed the LMRP on the BOP. Riser running tools were rigged down and began to displace the riser with KNPP (KCl-NaCl/polymer/PPG) mud, finishing on 12 December. The BOP was pressure tested again. There was a short 1 h break to service the HPS on 13 December, and running into the hole continued. The top of the bridge plug was tagged at 1948.5 mbsf, circulation and bottoms up was completed, and the simulation BHA was POOH to surface. The whipstock assembly was made up and attached to the BHA and RIH. The whipstock BHA was set at 1945.5 mbsf on 14 December, and milling out the window in the 13⅜ inch casing began.

Hole C0002N mud program

A primary difference between riser operations and riserless drilling is the use of weighted mud to prevent wellbore failure and to balance or control formation pore pressures (e.g., Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010; Strasser, Dugan, Kanagawa, Moore, Toczko, Maeda, and the Expedition 338 Scientists, 2014). Continuous monitoring of mud weight, annular pressure, mud losses, and other circulation data during riser drilling can provide useful constraints on formation pore fluid pressure and state of stress (e.g., Zoback, 2007). Problems related to mud weight or hole collapse may impact successful drilling or casing of the borehole itself, as well as the ability to conduct downhole measurements or to achieve postdrilling scientific objectives including observatory installations and active source seismic experiments. Because riser drilling remains relatively new in IODP, we follow on recent proceedings from IODP Expeditions 319 and 338 to describe key observations related to downhole (borehole) pressure, mud weight, and hole conditions while drilling Hole C0002N.

In general, mud weight is selected such that the pressure of the mud column in the borehole is sufficient to balance formation fluid pressure but remains below the fracture pressure (approximately equal to the least principal stress plus the tensile strength of the formation at a given depth). The initial mud weight for a section of hole is typically chosen on the basis of constraints on the fracture pressure from direct measurement by leak-off testing or extended leak-off testing at casing set points or from predrilling estimates based on models or seismic velocity information (e.g., Zoback, 2007). If mud weight is too low (an underbalanced condition), formation fluid may enter the borehole. This can be observed as “kicks” in mud gas or as increased annular pressure at times when pumping stops for pipe connections.

If mud weight is too high (overbalanced condition), the pressure of the mud in the borehole can cause fracturing of the formation and/or mud losses. The ensuing loss of circulation can reduce the effectiveness of cuttings circulation, potentially leading to pack-offs or hole collapse. If mud weight is too low, wellbore failure and cavings can lead to hole collapse and/or pack-offs; additionally, if the formation is overpressured, formation fluids can enter the borehole and lead to pressure or gas kicks.

In order to assess hole conditions, LWD annular-pressure-while-drilling (APWD) tools are commonly used, and the data are both recorded and pulsed to the surface in real time. The measured borehole pressures are typically reported both as absolute pressures (psig or equivalent) and as an equivalent circulating density (ECD; in g/cm3), computed as the total pressure divided by the product of depth below a reference datum (e.g., the rig floor) and gravitational acceleration.

Mud program operations

After drilling out cement at the 20 inch casing shoe at 2840 m BRT (872.5 mbsf) and sidetracking, we drilled to 4297.5 m BRT (2330 mbsf) in riser mode. We collected MWD data, including APWD) during these operations, providing a direct measurement of mud pressure at the bit (Fig. F1). During drilling, the mud weight was 1.12 g/cm3 in the upper part of the hole (to ~3640 m BRT) and was increased to 1.13 g/cm3 from that depth to TD. Mud was sampled and tested regularly by a service company to confirm the mud weight and chemical composition going in to the riser and exiting after circulation up the annulus.

Over the interval from 2800 to ~3200 m BRT, the ECD computed from APWD measurements gradually increased from 1.144 to 1.162 g/cm3 (Fig. F1B). Between ~3200 and ~3950 m BRT, ECD remained relatively constant and ranged between 1.156 and 1.164 g/cm3, with a few peaks as high as 1.168 g/cm3. At ~3950 m BRT, ECD increased sharply and remained between 1.17 and 1.18 g/cm3 to TD. The increase in ECD at ~3950 m BRT corresponds to an increase in rate of penetration (ROP) from ~20 to 35 m/h for most of the section above and to ~45–50 m/h below. The pumping rate during drilling remained constant at ~950 gal/min.

The measured pressures correspond to values of ECD up to 0.05 greater than the nominal mud weight mixed at the surface, likely due to the combination of cuttings load in the annulus and dynamic pressures generated during drilling and pumping. Throughout drilling, annular pressure was consistently smaller than the lithostatic stress, but close to or slightly in excess of the leak-off pressure of ~1.15 g/cm3 ECD determined from a leak-off test (LOT) conducted at 2841.5 m BRT during Expedition 338 (Strasser, Dugan, Kanagawa, Moore, Toczko, Maeda, and the Expedition 338 Scientists, 2014) (Fig. F1).

Large mud losses occurred on 11–12 November while drilling the lowermost ~300 m of the hole, including losses of up to 48 m3/h when the bit depth was 4004 m BRT and up to ~15 m3/h when bit depth was 4083 m BRT (Fig. F2). The first major mud loss was noted at ~2355 on 11 November, with an initial loss rate of 48 m3/h during pumping (i.e., dynamic mud loss), which decreased to 15 m3/h over ~30 min of circulation and drilling (Fig. F2A, F2B). Following this, mud losses were monitored after shutting the pumps off for ~20 min (static mud loss) and then during pumping at 400 gal/min. Both the static mud loss rate and the dynamic rate measured at 400 gal/min were 6 m3/h. The ECD at the time of these mud losses ranged from 1.16 to 1.172 g/cm3. After spotting LCM, the mud losses were significantly reduced to 1–5 m3/h during drilling to 4083 m BRT. During this leg of the drilling, ECD ranged from 1.158 to 1.165 g/cm3 with pumps off (for pipe connections) to 1.164–1.172 g/cm3 during circulation and drilling ahead (Fig. F2C, F2D). In comparison, prior to the onset of mud loss, ECD was 1.15–1.16 g/cm3 with pumps off and ~1.16–1.164 g/cm3 while circulating. After additional mud losses were noted when the bit depth was 4083 m BRT, LCM was again added to the mud, mud loss stopped (<1 m3/h), and drilling resumed. During drilling of the lowermost hole from 4083 to 4297 m BRT, ECD ranged from 1.167 g/cm3 with pumps off to >1.17 g/cm3 while circulating.

After drilling and cleaning the hole, we attempted to run and land a 13⅜ inch casing string to 4279.5 m BRT. However, while running the casing past ~3990 m BRT, large mud losses (45–50 m3/h) occurred while circulating. Ultimately, the mud losses limited our ability to clear cuttings and cavings from the hole while lowering the casing by either increasing pumping rate or mud weight, and we pulled out of the hole and shortened the casing string.

Clear responses to mud losses in the riser hole were recorded in a long-term borehole observatory installed at Hole C0002G, located ~110 m west-southwest of Hole C0002N (Fig. F3) (Kopf, Araki, Toczko, and the Expedition 332 Scientists, 2011). These signals include changes in pore fluid pressure, tilt, and strain (Fig. F4) and provide additional constraints on the location and timing of the mud loss in the riser hole.


The entire depth of the riser hole (2840–4297 m BRT) was drilled using a nominal mud weight (1.12–1.13 g/cm3) less than, but near, the leak-off pressure at the 2840 m BRT 20 inch casing shoe (ECD of ~1.15 g/cm3). The APWD data show that the pressure during circulation ranged from ~1.14 g/cm3 to values as high as ~1.18 g/cm3. For most of the hole, the difference in downhole pressure between drilling ahead (pumps on) and at pipe connections (pumps off) is consistently equivalent to a difference in ECD of ~0.01 g/cm3 (Fig. F2). The remainder of the difference between the nominal mud weight and measured ECD can therefore be attributed to cuttings load. This component of the downhole pressure ranges from 0.014 g/cm3 at the top of the riser hole (~2840–2950 m BRT) to ~0.04 g/cm3 in the lower portion of the hole (Fig. F1).

The clear increase in ECD at ~3900 m BRT coincides with an increase in ROP and cuttings load in the annulus as described above. This is consistent with the general observation that increased ROP is associated with higher ECD, because for a constant pumping (circulation) rate of mud in the riser, a higher ROP will lead to a larger concentration of cuttings (Fig. F5). Thus, the increased ECD in this zone could simply be caused by a higher ROP associated with drilling through a weaker formation. It is also possible that formation overpressure could have contributed to both the increased ROP by weakening the rock and to the ECD by flow of fluid from the formation into the borehole. However, throughout drilling there is no evidence of ballooning or of fluid influx or gas kicks when pumps were shut down during pipe connections (Fig. F2). This suggests that formation overpressures exceeding the mud weight are unlikely.

Both static and dynamic mud losses incurred while drilling the interval from 4004 to 4083 m BRT indicate that the leak-off pressure was likely equivalent to an ECD of ~1.162 g/cm3 (Fig. F2). Above this value, mud losses were observed, and when pumps were off, both mud and pressure losses were noted (Fig. F2A, F2B). At the time of maximum mud loss, pressure in the annulus decreased even while pumping (Fig. F2A). At circulating pressures below an ECD of ~1.155–1.16 g/cm3, no losses were observed (Fig. F2E). After the addition of LCM, drilling at slightly higher pressures (corresponding to an ECD of 1.167–1.172 g/cm3) was possible without substantial mud loss but was likely near or slightly above the leak-off pressure (Fig. F2C, F2D).

Pore-pressure monitoring in the Kumano Basin section and the uppermost accretionary wedge at the observatory Hole C0002G document a clear and rapid response to mud loss in the riser hole (Fig. F3). These responses include a gradual rise in pore pressure within both intervals following mud loss while drilling at 4004 and 4083 m BRT and a rapid rise in pressure following mud loss while running the 13⅜ inch casing. Increases in pressure following the mud losses while drilling are clear in both the Kumano Basin and upper accretionary prism intervals. The pressure history in the Kumano Basin monitoring interval is well fit by a transient pump-test model, consistent with small but continuous mud losses while drilling the interval from 4004 to 4083 m BRT. The pressure record in the upper accretionary prism follows a similar trend but is not as well fit by a simple pump-test model. A large pressure increase in response to mud loss during casing operations is observed only in the Kumano Basin monitoring interval. Following a sharp increase, the pressure decays and is mirrored by a gradual pressure rise in the accretionary prism monitoring interval.

Based on the rapid responses to mud loss in the riser hole observed in monitoring zones at 757–780 (in the Kumano Basin strata) and 937–980 mbsf (upper accretionary prism), we interpret that mud loss in the riser hole was localized at or near the casing shoe at 2840 m BRT. This is also consistent with the observation that pressure increased only in the Kumano Basin monitoring interval following mud loss during casing operations and with the expectation that the lowest leak-off point (LOP) pressure should be at shallow depth in the basin fill, where the horizontal principal stresses are less than the overburden. Together with the observed ECD at the time of mud loss, these observations suggest that mud loss was triggered by exceeding the LOP very slightly at the 2840 m BRT 20 inch casing shoe. After addition of LCM, drilling could continue at pressures slightly above the LOP without large losses.

Hole C0002P

Hole C0002P began at the top of the window, at 1936.5 mbsf. Milling out the casing window continued to 1954.5 mbsf, when dressing the window and reaming began. The window was checked to ensure that the drill string would pass through without hanging up. The drill string was then pulled up into the 13⅜ inch casing in preparation for a LOT. Two LOTs were set up and run to test the strength of the formation (see “Downhole measurements”). The tests were completed, and circulation began to increase mud weight to 1.18 sg. The milling BHA was POOH and laid down. The kick-off BHA was made up and run into the hole on 16 December 2013. Once at 1952.5 mbsf, the kick-off BHA began rotating and drilling out the 12¼ inch sidetrack hole. Mud weight was raised to 1.23 sg while the BHA was alternately rotated and slid forward to increase inclination and therefore offset from Hole C0002N. Drilling the kick-off continued, with one short stuck pipe incident at 1045 h on 17 December. Jarring broke the hold the formation had on the drill pipe, and it was freed. Reaming up to the casing window was completed, and then the drill string was pulled into the casing so that the drilling mud could be conditioned and weight increased to 1.28 sg. Drilling resumed, reaching 2067.7 mbsf on 18 December. A short 45 min period of stuck pipe occurred again, but the pipe was freed after another short session of jarring. The kick-off BHA was POOH to above the BOP because of rough weather; WOW continued until 19 December. The kick-off BHA was run back to the bottom on 20 December, and drilling resumed. The kick-off inclination reached up to 3.9° by 2107.5 mbsf (16.35 m above the bit) and then started to drop by 2162.5 mbsf. After sweeping out the hole with 10 m3 of Hi-vis mud, the kick-off BHA was POOH to the rig floor on 21 December. The 10⅝ inch RCB BHA (Table T1) was run in the hole on 22 December. RCB coring ran from 22 to 24 December; the coring interval was 2163.0–2218.5 mbsf (6 cores; Tables T2, T3). The RCB BHA was POOH to the rig floor and laid down on 25 December. Routine BOP testing and HPS maintenance (25.5 h) were completed on 26 December before LWD operations began. The 12¼ inch LWD BHA was made up, RIH, and continued until 27 December. The hole was washed down from 2149.5 to 2152.5 mbsf, 10 m above the top of the coring interval, and then opened from 10⅝ to 12¼ inches over the coring interval with the MWD/LWD BHA. Drilling ahead with MWD/LWD continued on 28 December; however, when the bit depth reached 2263 mbsf, MWD data telemetry stopped. All attempts to resolve the problem failed; therefore, the OSI, Co-Chief Scientists, and EPM agreed to drill ahead without real-time monitoring from 0215 h on 28 December. Drilling resumed and proceeded to 2601.5 mbsf with relatively stable hole conditions by 29 December. A wiper trip back to 2148 mbsf found no excess drag. Drilling resumed and proceeded to 3058.5 mbsf by 31 December. The hole was swept out with 10 m3 of Hi-vis mud, and a wiper trip to 2582.5 mbsf was conducted.

Hole C0002P also had hole condition problems: overpulls of 200 kN at 3000.5 mbsf and 250 kN at 2993.5 mbsf; HPS stall, partial pack-off, and bleed-off stand pipe pressure at 2944.5 mbsf; and two tight spots at 2755.5–2753.5 and 2716.5–2710.5 mbsf. Mud weight was increased from 1.28 to 1.30 sg to improve hole conditions, and a large amount of cavings was recovered on the shale shaker. While running the LWD BHA back to the bottom from 2582.5 mbsf, hole conditions worsened; the BHA stuck twice, once at 2973.5 and again at 3041.5 mbsf. Each time, the pipe was freed after dropping pumping pressure and working the drill pipe. Reaming downward continued, reaching 3058.5 mbsf by 1 January 2014; a heavy load of cavings continued to be seen on the shale shakers. The HPS stalled three times while working pipe to make connections, so mud weight was increased to 1.32 sg. While racking back stands during POOH, two overpull events were observed: 450 kN at 2980.5 mbsf and 200 kN at 1945.5 mbsf. After working the drill string, there was no more overpull past the same interval.

On 2 January, the LWD BHA was recovered on deck, and downloading memory data began. Gamma radiation, pressure while drilling, and resistivity were downloaded by 0545 h and XBAT data were recovered by 0615 h, but the AFR data recovery port was damaged, preventing data download, therefore requiring shipping to the Halliburton base in Thailand for data recovery.

Reaming out the open hole was the next phase in operations. The 14½ inch Anderreamer underreamer BHA, with no logging tools, was made up and RIH, passing the BOP and casing window with no indications of drag. The underreamer was activated and, after confirming activation, reaming the 12¼ inch hole to 14½ inches from 1963.5 mbsf began. Reaming down operations were occasionally paused for mud-pump maintenance. Hole opening continued, with no excess drag observed over the next several days of reaming. Traces of gas and cavings were encountered throughout this period of drilling. From 8 January, several episodes of HPS stalling at 2867.5 mbsf from heave increase were noted. The approach of a cold front weather system required another POOH for WOW to above the BOP for standby. The cold front passed, and WOW continued until 9 January. Between 0930 and 2045 h on 10 January, drillers reamed up and down 9 times between 2877.2 and 2907.2 mbsf, with frequent HPS stalls, tight spots, and one hole pack-off at 2902.5 mbsf. The pack-off initially had no mud return, but continuous working the pipe and reduction in pumping pressure first resulted in a gradually strengthening return; HPS speed was increased and the pipe was confirmed freed by 1500 h. Backreaming from 2879.5 to 2849.5 mbsf and then again reaming down through 2879.5–2906.5 mbsf while circulating mud resulted in a decrease in cuttings volume and also a stabilization in HPS torque. Hole opening resumed from 2045 h with a 2 h pause from 2330 h to perform more maintenance on the Number 1 mud pump. Hole opening resumed from 0130 h on 11 January, opening the hole from 2912.5 to 2960.5 mbsf. The hole packed off at 0645 h while reaming up to 2934.5 mbsf, but the drill string was freed by 0715 h. Another pack-off occurred at 1345 h when back down at 2960.5 mbsf and was freed by 1530 h. Sweeping out the hole with 40 m3 of Hi-vis mud was carried out in three sessions between 1530 and 2115 h. At that point, troubleshooting the HPS torque wrench was required after the torque wrench die slipped while trying to break a connection between the saver sub and the drill pipe. Once fixed, the drill string was POOH to 2728.5 mbsf. Drag was observed while pulling out to 2936.5 mbsf, but no stuck pipe incidents occurred. The Anderreamer underreamer BHA was recovered on deck on 12 January; examination of the reamer cutters found excessive wear and damage. The BHA was laid down, and a new underreamer BHA was rigged up and RIH. After a short pause to cut and slip the drilling line, the BHA was RIH on 13 January. Once reaching 1957.5 mbsf, the Anderreamer was activated. A series of issues with the mud pumps started impacting drilling, with frequent pauses in drilling to swab out and replace cylinders of the mud pumps. Reaming down resumed and by 14 January had reached 2960.5 mbsf. Hole opening had progressed 4 m when the hole packed off, leaving the drill pipe stuck in the hole. Operations to free the pipe began immediately, pumping Hi-vis mud downhole more than 8 times. Attempts to fire the jars met with no success, leading the drillers to speculate that the stuck point was located above the jars in the hole. A 1 h pause to swab and change the Number 1 and 2 cylinder of the Number 2 mud pump was followed by more attempts to free the drill pipe. Spotting 6 m3 of clean lube at 1800 h was again followed by working the pipe near maximum overpull. It was noticed that the overpull began to decrease with each attempt, and after setting the tension to neutral and applying 30 kNm of torque, the pipe was freed. Sweeping the hole began and circulation and bottoms up was finished by 15 January. The BHA was pulled back to 1928.5 mbsf for a series of wiper trips; no excess drag was observed while passing through the 13⅜ inch casing window. From 0830 to 1915 h, reaming advanced without much trouble, until taking 160 kN WOB at 2925.5 mbsf. Reaming to 2964.5 mbsf encountered numerous tight spots before reaching 2964.5 mbsf by 2130 h. Circulation and bottoms up to clean the hole began and continued until 16 January. Hi-vis mud (30 m3) was spotted, and the underreamer was POOH to surface. The mechanical jar, Anderreamer underreamer, float sub, and bit were laid down by 1800 h.

It was decided to end reaming and drilling and move ahead with running and cementing the 11¾ inch liner casing. Rig up continued until 18 January, when the liner was run to 966.5 m MSL while setting the guide shoe joint, float collar joint, landing collar joint, and the 3.5 inch ball in the float collar joint. The 11¾ inch liner casing hanger and packer assembly were picked up and made to the casing joints. No drag was observed while running casing to 2922.5 mbsf. Slack was checked and weight picked up before circulating after breaking pipe to drop the 1¾ inch ball. Setting the liner hanger began when the 2¼ inch ball was dropped and chased; the first three attempts to set the hanger failed. After picking up 1 m and applying pressure, confirmation that the setting tool had released the hanger was made. Attempts to circulate failed, with no mud return; the liner-casing packer had accidentally set during the attempts to set the hanger. Pressure tests confirmed that the annulus pressure was holding (up to 7.8 MPa), and there was no communication with the formation below. Two LOTs were performed; the second ended in an injection test. The liner packer was pressure tested at 5000 psi for 5 min and then at 4000 psi for 15 min; both tests passed. After the LOTs, the liner hanger running tool was POOH to surface, laying out the cement stand on 19 January. A cement diverter assembly was made up and RIH and passed through the top of the 11¾ inch liner without taking any drag. The first of 2 cement “squeezes” was completed early on 20 January, pumping 120.5 bbl of cement. The second began from 0500 h and pumped a total of 94 bbl of cement. Once completed, the diverter was circulated clean, and then POOH to 1632.5 mbsf when 7.9 m3 of Hi-vis mud was spotted. Once the circulation was complete, the diverter assembly was POOH to surface. The cement plug was pressure tested on 21 January.

Preparations to recover the risers began, and the Blue POD on the BOP was unlatched and the BOP disconnected from the wellhead on 22 January, while the ROV dove to set the corrosion cap and recover transponders. Riser joint recovery operation began once the vessel was 50 m offset from Hole C0002F well center. By 23 January, the gooseneck and termination joints had been recovered and riser joints proper began to be recovered as the vessel continued to drift north-northwest. The BOP was recovered and set on the BOP cart over the moonpool by 25 January. The ROV dove to complete transponder recovery. Once complete, the ROV was recovered to the surface, while riser-handling tools were rigged down. The riser tensioners were tested on 27–28 January while general rig equipment services were performed. The Chikyu left the site on 29 January for facilities in Irago, Aichi Prefecture, Japan.

1 Tobin, H., Hirose, T., Saffer, D., Toczko, S., Maeda, L., Kubo, Y., Boston, B., Broderick, A., Brown, K., Crespo-Blanc, A., Even, E., Fuchida, S., Fukuchi, R., Hammerschmidt, S., Henry, P., Josh, M., Jurado, M.J., Kitajima, H., Kitamura, M., Maia, A., Otsubo, M., Sample, J., Schleicher, A., Sone, H., Song, C., Valdez, R., Yamamoto, Y., Yang, K., Sanada, Y., Kido, Y., and Hamada, Y., 2015. Site C0002. In Tobin, H., Hirose, T., Saffer, D., Toczko, S., Maeda, L., Kubo, Y., and the Expedition 348 Scientists, Proc. IODP, 348: College Station, TX (Integrated Ocean Drilling Program). doi:10.2204/iodp.proc.348.103.2015

2Expedition 348 Scientists’ addresses.

Publication: 29 January 2015
MS 348-103