Drilling proceeded to a depth of 48.7 mbsf where the center bit was recovered and an RCB core barrel was dropped. Permission was received to allow spot coring of this hole in 50-m increments to a depth of ~200 mbsf. RCB cores were recovered from depths of 48.7 to 57.8 mbsf (170 1042A-1R), 96.1 to 105.7 mbsf (170-1042A-2R), and 153.7 to 163.3 mbsf (170-1042A-3R). Core recovery was poor as is often the case during spot coring operations. An RCB center bit assembly was used for the drilled intervals between cores. DVTP temperature measurements were attempted after Cores 170-1042A-1R and 170-1042A-2R, but the data was bad.
Continuous RCB coring began with Core 170-1042A-4R at a depth of 201.8 mbsf and continued through Core 170-1042A-7R to a depth of 3845.1 (240.1 mbsf). Cores 170-1042A-6R and 7R were cut with 300-400 amps of torque and 300-400 psi, which was the beginning of hole problems that were never corrected. A total of 12 hr was spent conducting remedial work on the hole. Wiper trips, hole sweeps with Sepiolite and Quick Sweep material, and washing/reaming were all tried to no avail. Stalling torque, persistent back pressure, and a general inability to clean the hole eventually led to abandonment of further drilling/coring attempts at 2245 hr on 3 December 1996. A total depth of 240.1 mbsf was achieved, which was short of the desired 325 mbsf objective. The cause of the hole trouble was thought to be an inability to properly clean cuttings out of the hole (lost circulation), aggravated by formation overpressure, and a high angle fault zone at ~180 mbsf.
The hole was displaced with 70 bbls of 10.5 lb/gal mud and the drill string was pulled clear of the seafloor. Sepiolite mud sweeps of 30 bbls each were pumped at depths of 201.8 mbsf, 211.3 mbsf, and 220.9 mbsf. In addition, a 50 bbls Quick Sweep pill was pumped at a depth of 229.0 mbsf. Although the material was extremely viscous when pumped, it seemed to have little benefit over the standard Sepiolite sweeps. A pump pressure increase of 100-150 psi was all that was noted as the material came out of the bit and into the annulus of the hole.
Some higher order hydrocarbons were identified in this hole but not in large quantities. The numbers dropped off rapidly after drilling through the fault zone source. Hydrocarbon vacutainer analysis for Cores 170-1042A-3R and 4R above or within the fault identified 837,897 and 723,706 ppm methane, 829 and 792 ppm ethane, and 172 and 292 ppm propane, respectively. Higher hydrocarbons included IC4 = 40 and 79 ppm, NC4 = 5 and 4 ppm, and IC5 = 12 and 6 ppm. Only in Core 170-1042A-4R were NC5 (1 ppm) and IC6 (4 ppm) measured. Data from Cores 170-1042A-5R through 7R (no data on 170-1042A-6R), which are located below the fault zone, identified 238,342 and 70,307 ppm methane, 302 and 112 ppm ethane, and 122 and 28 propane respectively. Higher hydrocarbons included IC4 = 47 and 11 ppm, NC4 = 2 and 1 ppm, and IC5 = 3 and 7 ppm, and NC5 = 0.5 and 1 ppm.
Hole 1042B
Because we failed again to reach the high-priority target in Hole 1042A, a more radical solution was devised. We thought at least part of the drilling complications in the hole might be the result of inadequate annular clearance between the hole wall (nominal 9-7/8" diameter) and the drill collars/BHA (8-1/4" outside diameter). In fact, from a depth-achieved perspective one of the most successful holes of the leg was a hole drilled with the XCB drill bit, which drills a larger diameter (11-7/16") hole. We decided to make one last attempt to reach a minimum depth of 325 mbsf by drilling a 12-1/4" tricone drill bit hole before reentering for coring.
The ship was offset 50 m to the southeast while the BHA was made up. The assembly was tripped to the seafloor and Hole 1042B was spudded at 1615 hr 4 December 1996. The mudline was estimated at 3592.5 m, which was somewhat higher than at Hole 1042A. The 12-1/4" tricone bit rapidly drilled to a depth of 316.0 mbsf at an average ROP of 34.5 m/hr. There were no problems spudding with the drilling jars and correspondingly longer BHA (1 extra stand of drill collars). There were no hole problems experienced during the drilling. With the bit on bottom at 316.0 mbsf all parameters were normal. There was no evidence of overpressure and drilling torque was smooth and constant at 100 amps.
A wiper trip identified tight hole from 3830.0 m (237.5 mbsf) to 3797.0 m (204.5 mbsf). A drilling torque of 500 amps and overpulls of 30 kips were initially seen but were reduced to normal after working the pipe through the affected area. Sepiolite mud sweeps of 30 bbls each were circulated at depths of 90.6, 119.5, 148.2, 177.1, 205.9, and 316.0 mbsf while drilling the 12 1/4" hole. After the wiper trip, the hole was displaced with 180 bbls of Sepiolite drilling fluid and the pipe was tripped back to within 100 m of the surface. The hole was again tight through 204.5 to 237.5 mbsf with the same 500 amps of torque and 30 kips of overpull required.
A free fall funnel (FFF) was made up and deployed, and it was followed by the subsea TV camera (VIT frame). The bit was observed with the TV as it was pulled clear of the FFF and seafloor at 1530 hr 5 December. The drill string was tripped back to the surface while recovering the subsea TV. Then the drill string was tripped back to the seafloor with an RCB BHA and the drilling line was slipped and cut before attempting reentry. The search for the FFF and subsequent vessel maneuvering took 2 hr. The pipe was RIH without picking up the top drive until encountering an obstruction at 151.5 mbsf. Drag was 10-15 kips so the pipe was pulled back to a depth of 128.0 mbsf and the top drive was picked up at that point. The subsequent trip required heavy reaming all the way to bottom. Initially there were two circulating pumps online at 75 strokes per minute (spm) each. Eventually this was cut back to a single pump at 75 spm to reduce the back pressure and hydraulic pump effect resulting from the very tight formation. The rate of progress increased, torque was reduced, and hole problems declined with the use of a single circulating pump.
Once on bottom, RCB coring commenced and continued to a depth of 90.7 mbsf through alternating green mudstone and brecciated sandstone cemented with carbonates. A second wireline run was required to recover Core 170-1042B-4R when the overshot shear pin failed. Core 170 1042B-7R was recovered with an empty liner, so the chisel type bit deplugger was deployed. After recovering the deplugger, a 30 bbls Sepiolite mud sweep was pumped. Pump pressures increased by 400 psi when the pill came out of the bit. This was accompanied by elevated torque as well. Hole conditions remained acceptable for the cutting of Core 170-1042B-8R. However, the consensus of opinion was to stop coring at that point before hole conditions deteriorated. Core recovery was a poor 11.9% for the eight cores recovered, because coring parameters had to be set to optimize hole stability.
The hole was displaced with 85 bbls of 10.5 lb/gal mud, the top drive was set back, and the pipe was recovered back to the surface. The bit cleared the seafloor at 1845 hr 7 December 1996 and the ship began DP offsetting to Site 1043 while the pipe trip continued. No water samples or formation temperature measurements were taken in this hole and no significant hydrocarbons were detected.
Operations at Site 1043 (CR-6)
Before completing operations at Site 1042 with LWD, Site 1043 was cored and logged (see Site 1043 section). The vessel then returned to Site 1042.
Hole 1042C
Logging-while-drilling at Hole 1042C was the last hole operation of Leg 170. As before, the jars were pressure tested (low = 500 psi and high = 3,000 psi) prior to making up the LWD BHA and tricone drill bit. This time the Compensated Density Neutron tool was left out of the BHA. Based on the previously cored holes, we thought that the potential for penetrating below the target reflector at this site was higher if we did not use the stabilizer associated with the CDN tool. The CDR tool was made up as part of a standard BHA including the McCullough mechanical drilling jars. As before, it was considered safe to run the jars because the firm bottom was likely to be drillable in a short period of time using the tricone bit. In addition, the sea state remained calm. The drill string was tripped to the seafloor filling the pipe every 30 stands.
Hole 1042C was spudded at 1345 hr 13 December 1996. Drilling continued at a target rate of 35 m/hr until 1515 hr 13 December 1996 when the driller was advised by Anadrill/LDEO that they wished to reduce the ROP back to 25 m/hr. At a depth of 35.0 mbsf the target ROP was decreased accordingly. Drilling continued without incident to a depth of 257 mbsf where pump pressure was elevated by 150 psi and some drill string torque was noted. The hole was swept with 30 bbls of Sepiolite mud and the drilling parameters returned to normal. Drilling continued to a total depth of 297.6 mbsf. A steady increase in drilling torque coupled with persistent high pump pressure was noted. Another 30 bbl sweep with Sepiolite mud did not improve the hole condition. Since drilling was within 2.5 hr of being halted because of time constraints, we decided to halt further LWD operations at that point. The hole was displaced with 48 bbls of 10.5 lb/gal mud. The drill string was then pulled clear of the mudline and the drilling line was slipped and cut for the last time. The bit reached the rig floor at 2030 hr 14 December 1996. The rig floor was secured, thrusters raised, and the vessel got underway for Balboa, Panama at 2200 hr 14 December 1996, ending Hole 1042C.