TRANSIT DARWIN TO SITE 1108 (ACE-8A)
At 0715 hr on 11 June the last line was released and we departed Darwin 2 days earlier than planned for our first drill location, Site 1108 (ACE-8A). The transit was uneventful and we made excellent time (averaging 11.5 nmi/hr ) in calm seas. Because we would be operating in Papua New Guinea (PNG) waters, we were required to clear PNG customs and immigration before drilling operations could begin. A helicopter rendezvous with the ship was scheduled to minimize the time necessary to complete the requirement. We had to arrive at the rendezvous location early on the morning of 16 June, but our transit speeds were better than expected; therefore, on 15 June we reduced speed for several hours. We used this time to pick up and space out drill collarsan operation that would have had to take place once we arrived at our first site.
The first helicopter landed on the ship at 16 June 1998 south of Alotau at 10°55.0´S, 150°20.0´E. Passengers on this flight included a PNG customs agent, a PNG quarantine official, a representative from ODPs agent in PNG, and the ODP logistics coordinator. After the helicopter landed we immediately resumed full speed for the second rendezvous location ~20 nmi to the east. The helicopter departed from the ship at 0655 hr for Alotau to pick up the second PNG observer who was due to arrive about 0800 hr. Following the formalities of clearing the ship into PNG waters, the PNG officials were given a tour of the ships laboratory facilities. The helicopter returned for the second rendezvous (11°10.0´S, 150°50.0´E) at ~1000 hr that same morning. This time the pilot did not shut down his engines. After the second PNG observer (Paul Kia, Geological Survey of PNG) disembarked, the PNG customs/quarantine officials and ODPs PNG agent boarded the helicopter and departed for PNG. As soon as the helicopter lifted off, we resumed full speed for Site 1108 (ACE-8A).
We left the Coral Sea and turned north entering the Solomon Sea via the Jomard Passage, picking up a favorable current along the way, which boosted our speed, temporarily at least, to nearly 14 nmi/hr. Overall, our transit speeds averaged 11.5 nmi/hr in calm seas. After arriving at the Global Positioning System (GPS) positions for Site 1108, we lowered the thrusters/hydrophones and switched into dynamic positioning (DP) mode. After stabilizing on the GPS position, we deployed a Datasonic commandable positioning beacon at 0115 hr.
Hole 1108A was devoted strictly to obtaining information on the seafloor sediments for a reentry cone deployment planned for later in the leg. We fit together a bottom-hole assembly (BHA) with an 18 1/2-in tricone drill bit attached to the bottom stand of 8 1/4-in drill collars and lowered it to the seafloor. The subsea TV camera was deployed during the pipe trip, and while observing the seafloor, we spudded Hole 1108A at 0815 hr on 17 June.
The 3.5-kHz depth recorder indicated a seafloor depth of 3157.4 mbsl. We observed the drill bit tag the seafloor at 3162.7 mbsl. For the next hour and 40 min, we conducted a jet-in test into the moderately hard seafloor, eventually reaching a depth of 16.3 mbsf. The test was suspended at that point because little progress was being made at flow rates of up to 110 strokes per minute (spm). The drilling assembly was pulled clear of the seafloor at 1000 hr, the subsea television camera was recovered, and the hole officially ended at 1530 hr when the bit cleared the rig floor.
We had originally planned Hole 1108B as an advanced hydraulic piston corer/extended core barrel (APC/XCB) hole to ~600 mbsf. Since the jet-in test results indicated that APC coring was not possible and that XCB coring was unlikely to progress very far, we decided to start RCB coring. After putting together an rotary core barrel (RCB) drilling assembly (including a single stand of drill collars above a set of mechanical drilling jars), we lowered the drill string to the seafloor. During the pipe trip the ship was offset 50 m to the southeast on a bearing of 120°.
Upon reaching the 3162.7 mbsl seafloor depth of the previous hole, the driller had not recognized any indication of bottom (reduction of total drill string weight). Even after advancing another single joint of pipe to a depth of 3172.0 mbsl, no weight loss was observed. We checked the drillers pipe tally that was verified as correct and recovered the core barrel, which was empty. After adding another joint of drill pipe, the seafloor indication was tagged at a depth of 3177.3 m or 14.6 m, deeper than at the Hole 1108A jet-in test location.
Hole 1108B was officially spudded at 0100 hr on 18 June 1998 and continuous RCB coring began using very little weight on bit (WOB) and slow revolutions per minute (RPM). We were forced to proceed cautiously because of the lack of stabilization for the bottom-hole assembly (BHA) (it was still above the seafloor) and the hard surface sediments. The upper 50 m of the borehole was characterized by erratic torque, overpull and drag, poor recovery, and low rates of penetration (ROP). Typically 12 m of sediment filled the bottom of the borehole between connections. Liberal amounts of drilling mud (sepiolite) were used, and the hole condition gradually improved with depth. Because of the relatively unstable hole conditions and layers of semi-indurated sediments, we were unable to make temperature measurements in the formation.
At 91.3 mbsf (after Core 10R) we decided to make a wiper trip. The drilling in the last two cores had indicated that the formation was getting more lithified and appeared to be more stable. Drilling parameters became more consistent and core recovery continued to improve. The partial wiper trip was strictly precautionary because the drilling jars and last stand of drill collars were about to go below the mudline. We raised the drill string to a depth of 42.7 mbsf. At 47.7 mbsf the driller noted 70,000 lb overpull and was unable to rotate the drill string. After working the pipe for a few minutes, it became unstuck and a sepiolite pill was circulated. This portion of the hole was reamed several times until there was no longer any indication of drag or overpull. We then proceeded with running the pipe to bottom, where we encountered 3 m of fill. Another sepiolite pill was circulated while drilling out the fill. All drilling conditions were normal and the hole appeared to be in excellent condition at that point. When the bit was back on the bottom, a portion of the hole again apparently collapsed, trapping the pipe. The pipe was worked free and another sepiolite pill was circulated. Coring resumed some 2.75 hr later once the wiper trip was completed and the hole appeared to be stabilized.
The RCB coring continued through Core 41R to a depth of 389.0 mbsf. Higher than normal core barrel pump pressures (250300 psi vs. normal values of 150 psi) and lower recovery led us to suspect that we may have lost some core in the pipe or that something was partially plugging the bit throat. A core barrel was pumped downhole and then recovered without advancing the drill bit. Upon recovery we found a small rock lodged in the lower core catcher. A bit deplugger with painted latch and landing shoulders (to document that it latched fully down and extended through the bit throat) was deployed. Once recovered, the deplugger indicated proper latching, and pump pressures returned to normal. The RCB coring progressed to 485.2 mbsf; however, core recovery and ROP deteriorated with depth possibly because of an increase in the degree of fractures in the formation. Toward the bottom of the hole, hydrocarbon headspace C1/C2 were decreasing and higher hydrocarbons such as n-C5> (pentane) started to appear. We wondered what impact this trend might have regarding our plan to advance a reentry hole down to a projected depth of 1150 mbsf.
At 1630 hr on 22 June, the hole collapsed just after starting to cut Core 52R. The top drive continually stalled out and the pipe was pulled free with 30,000 lb of overpull. At that point, the annulus plugged off, preventing circulation. We pulled the pipe free again. This time with 110,000 lb overpull. Attempts to actuate the mechanical drilling jars were not successful, leading us to believe that the material holding the pipe had fallen in from above. The drilling assembly was literally dragged up the hole to a 253.7 mbsf using 20,000 to 60,000 lb overpull. At this depth we were able to resume circulation and torque/pump pressure returned to normal. We continued pulling the pipe to 114.7 mbsf without overpull. After recovering the core barrel (which had not been advanced before the pipe became stuck), we washed and drilled back to the bottom of the hole but could not pass 436.7 mbsf, which was 48.5 m above the bottom of the hole. At that point the hole packed off again, cutting off circulation and preventing top-drive rotation. The pipe was pulled back to 416.7 mbsf before we regained the ability to pump fluids and rotate the drill string. For the next several hours, the pipe was cycled up and down in an unsuccessful attempt to get back to the original depth of the hole. Eventually, we decided that further coring in this hole was not possible. The pipe was placed at 379.0 mbsf and an Adara heat-flow shoe was deployed on a core barrel through the pipe to obtain a temperature gradient in the borehole. We knew that the data would not reflect accurate formation temperature but considered the data to be insurance in case the temperature-logging tool (TLT), which was to be run during the first wireline logging run, might not be able to be deployed very far into the hole. Temperatures were measured for 10 min each at 0, 388, 300, 200, and 100 mbsf with the circulating pumps shut off. Results of the Adara temperature profile, inside the drill pipe, indicated a linear temperature gradient of ~35.4°C/km. We later determined this to be significantly lower than the actual gradient.
Once the Adara temperature measurements were completed we made two wireline runs to activate the mechanical bit release (MBR) to drop the bit in the hole in preparation for logging. The hole was displaced with 150 bbl of sepiolite mud, and we pulled the drill string to a depth of 99.7 mbsf. During the pipe trip, the driller encountered overpulls of 20,00040,000 lb to a depth of 225.7 mbsf.
After rigging up for logging, the triple combo was assembled and run in the hole. Unfortunately, the logging tools were only able to reach 62 m below the end of the pipe into the open hole. This section of hole was logged twice, and the remaining logging program for the hole was abandoned. Analysis of the temperature data, gathered with the TLT on the bottom of the triple combo and the Adara tool, yielded a temperature gradient of ~100°C/km. To displace the hole with weighted mud before abandoning this site, we attempted to lower the pipe further into the hole, but it could not be lowered more than 0.5 m before encountering an obstruction. After the hole was filled with 31 bbl of 10.5 pounds per gallon (ppg) bentonite gel mud, the pipe was pulled clear of the seafloor at 0415 hr on 23 June. Hole 1108B ended when the bit cleared the rig floor at 0900 hr. The acoustic positioning beacon was left on the seafloor in an active mode because we were planning to return to this site after drilling Site 1109.
A total of 51 cores were recovered from 485.2 m of section with a recovery of 148.6 m (30%). The average ROP was 12.3 m/hr.
To 180 Site 1109
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