Technical Note 25


Leg 147 was the first of several legs planned to explore the concept of offset drilling on high angle, hard rock sites exposing lower crust and mantle rocks. The drilling sites were expected to be on bare rock exposures with up to 25° slopes and shallow sediment covers. An intensive operations and engineering effort at ODP led to the development of new or modified tools to accommodate the new operating requirements. The three-legged Hard Rock Base (HRB), capable of handling up to 25° slopes, was mated with a new Dril-Quip dual-casing hanger system. A Cam Actuated Drill Ahead (CADA) feature was added to the running tool to permit spudding the hole after the HRB was set without tripping the pipe. New Rotary Core Barrel (RCB) core bits were developed for very hard rock, and close catch core catchers were developed to try to improve recovery in highly fractured rock.

Hole 894A (Site HD-3)

On arrival at Hess Deep, a TV survey on the summit of the intra-rift ridge found gabbro outcrops with 0.5 to 6.5 m of soft sediment, scattered patches of small cobbles 5 to 20 cm in diameter, and isolated massive outcrops and large blocks up to 2 m diameter. Hole 894A was spudded-in on the ridge summit. Core 1R (0-6.0 mbsf) was interpreted as regolith; therefore, coring was terminated.

Hole 894B (Site HD-3)

Hole 894B was located above a steep 15-m-high scarp of massive gabbros on a 35-m-wide sedimented bench with a slope of up to 20° in 3031.0 m water depth. Core 1R (0 to 7.0 mbsf) consisted of two fragments of metagabbro. Coring was terminated, but surface conditions appeared to be adequate for an HRB site.

Hole 894C (Site HD-3)

Hole 894C was positioned on the crest of the ridge, and the HRB was deployed near Hole B. The running tool with the CADA drill-ahead feature was used for the first time. It was not possible to see around the HRB with the TV camera, and the running tool (with the CADA feature engaged) did not permit unlatching for surveys and then moving the base; however, the HRB location was believed to be correct based on the angle, water depth and beacon coordinates. A brief stability test was performed.

The CADA tool was unjayed, and Hole 894C was spudded in 3044 m water depth. The 14-3/4 in. hole was drilled to 31 m. During a wiper trip, the BHA parted. The HRB had been set by mistake on a steep sediment slope about 60 m east and downslope from the intended hard rock ledge. The most probable cause of the positioning error was that the HRB swung out of position in the process of moving the vessel and resetting the legs three times. The sediment slope was destabilized by circulation while drilling, causing the sediment to slump and undermine the HRB leg on the downslope side. The HRB tilted further, and the zip lift groove on the second drill collar was cut by the CADA tool neck. When the bit was pulled to the seafloor, the HRB tilted, bowed, and broke the BHA, allowing the HRB to topple over downslope.

Hole 894D (Site HD-3)

Hole 894D was spudded on a flat sedimented area on the west side of the ridge summit in 3024.0 m water depth. Core 1R to 2R (0-19.5 mbsf) recovered 1.52 m of granular sediment and basalt fragments. The hole was terminated because of the very unstable upper hole, which ruled out the possibility of using a reentry cone.

Hole 894E (Site HD-3)

Hole 894E was spudded on flat sedimented terrain at the ridge summit in 3024.6 m water depth. Cores 1R to 3R (0-28.7 mbsf) recovered 3.03 m of foram ooze, gabbroic sand, gabbro, and basalt. The hole was terminated because of the very unstable upper hole, which ruled out the possibility of using a reentry cone.

Hole 894F (Site HD-3)

An area was found southwest of the ridge summit with flat-lying blocky outcrops covered by 1 m of sediment. Hole 894F was spudded just north and upslope of Hole B. Cores 1R to 3R (0-25.7 mbsf) recovered 1.80 m of gabbro and basalt. Hole and surface conditions were good for HRB deployment; therefore, the precise spot was marked with a beacon released by the VIT beacon (an ODP first).

Hole 894G (Site HD-3)

The only remaining HRB was set 10 m north of Hole 894F as a retrievable installation (i.e., without cementing the casing) because of the persistent hole problems, which may have exacerbated subsequent hole cleaning problems. The running tool was released (CADA was locked out), and a TV survey verified the location (with the option to move the base) before the decision to drill was made. The legs had a uniform 1 m of penetration into the soft sediment cover, and the tilt beacon read 11° by 20°.

Hole 849G was spudded and a 14-3/4 in. hole was drilled to 18.6 mbsf. The hole condition appeared to be good, and 17.68 m of 13-3/8 in. casing (with flush joint connections) was run. The casing stopped on the lip of the hole, and offsetting the ship to change the ship-to-hole alignment was not successful; therefore, the casing was pulled. A 14-3/4 in bit would not reenter the old hole. The top hole drilled very slowly with high torque and overpull, indicating that the holes were misaligned. Below 4 mbsf, the bit intercepted the old hole and was run to 18.6 mbsf TD. A 16.67 m section of 13-3/8 in. casing stopped 1.25 m below the surface. Subsequent information from logging, core examination and marks on the equipment confirmed that there was a combination of problems:

The hole was cleaned out to TD, and Cores 1R to 3R (18.6-39.6 mbsf) recovered 3.85 m of olivine gabbro and basalt. Except for slab boulders on the surface, conditions were good in gabbro to 20 mbsf; however, the fractured gabbro started falling into the hole (Core 3R contained a piece of the hole wall). A 14-3/4 in. hole had to be drilled out twice to TD. A 31.0 m section of 13-3/8 in casing was run with a 12-1/4 in. pilot bit, but it could not be worked below 4.6 mbsf and was pulled. A 5.88 m section of 13-3/8 in. casing was set to attempt to pin the HRB, and the hole was deepened from 39.6 to 45.0 mbsf. A 41.56 m section of 10-3/4 in. casing with a 12-1/4 in. pilot bit could not be run; however, it was shortened to 35.56 m and set at 33.0 mbsf.

RCB Cores 4R to 14R (45.0-118.8 mbsf) recovered 32.32 m of gabbronorite. The hole could not be cleaned out, and the pipe stuck briefly despite repeated reaming and two wiper trips. At 118.8 mbsf the bit was pulled as a precaution. The stabilizer pads were worn away, and the body was worn down 1/4 in.

The hole was cleaned out with a 9-7/8 in. RCB core bit to 118.8 mbsf. After Core 15R (118.8-122.8 mbsf), the hole could not be reamed back to TD because of high torque, and the bit was pulled. The wear pads and body were worn down 1/2 in. A 9-7/8 in. drill bit was run to wipe out ledges and doglegs in the hole and drilled 3.0 m to 125.8 m. The bit sub and stabilizer blades had heavy wear from abrasion, but the bit showed no wear, indicating that ledges were the problem.

A 9-7/8 in. CC-9 RCB bit with integral spiral stabilizer blades and close catch design was then run in an effort to clean out the hole to TD and core ahead with lower torque. The hole was reamed to 125.8 m TD, and RCB Cores 17R to 19R (125.8-145.6 mbsf) recovered 5.75 m of gabbronorite and basalt. The hole was packing off and could not be reamed back to bottom. The bit had vertical abrasion marks on top of the stabilizer blades. Ledges, deviation and inability to clean the hole continued to frustrate attempts to core; therefore, a BHA with two stabilizers was run in an attempt to straighten the hole and wipe out the ledges. The hole was reamed to 145.6 m TD with high stabilizer torque. Core 20R (145.6-154.5 mbsf) recovered 1.37 m of gabbronorite and basalt. The drill pipe subsequently plugged and was cleared again four times.

The ledges in the hole could not be removed with a stabilized BHA despite repeated reaming, and the hole could not be cleaned out by circulating mud sweeps. High torque stalled the top drive, and circulating pressures increased. The pipe stuck while reaming to bottom with the bit at 146.6 m, but the pipe was freed and pulled out. The top stabilizer had severe blade wear, indicating it had been reaming on ledges. Another CC-9 bit was unsuccessful in a final attempt to clean out the hole. The pipe stuck briefly, and coring was terminated. A TV survey found a new washout in the sediment downslope from the HRB. The HLDT (one arm caliper) indicated alternating cavities to 18 in. diameter and ledges to 10.2 in. diameter. A multishot survey instrument read a 5° angle at 56 mbsf.

HD-3 Coring Problems

Hole 894G was probably started on an angle because of the hole-to-ship misalignment in strong surface currents, which compromised subsequent drilling/coring/casing operations. The hole was spudded without a centering bushing; therefore, it may have been non-concentric with the reentry cone throat. Many hard basalt dikes intruded into the softer fractured gabbro, leaving ledges and cavities that contributed to hole cleaning, deviation and rotating problems. Highly fractured gabbros were loosely cemented with soft alteration products and became unstable after a few days. Hole conditions were too unstable to risk leaving the pipe hanging for a survey, and attempts to straighten the hole with stabilization were unsuccessful. The caliper log showed a steady decrease in hole diameter through successive ledges culminating in a 10.2 in. diameter hole through the ledge at 74 mbsf. Heavy reaming with stabilizers was required below 74 mbsf, which indicates that the hole angle probably continued to build by about 1°/10 m to 12° to 15° at 154.5 mbsf TD. Recovery was poorest (with high rop) in the fractured gabbros and best in the harder basalt dikes (with low rop).

Hole 895A (Site HD-4)

Hole 895A was spudded in 3832 m water depth in the southeast quadrant of the ridge. Core 1R to 2R (0-17.2 mbsf), recovered 2.38 m of serpentinized breccia, harzburgite, clay, and basalt. Loose surface rubble resulted in heavy fill and high torque. A 3-m heave in long period swells reduced heave compensator control of weight-on-bit. The DC pin parted at the top of the second drill collar 5.5 m below the mud line.

Hole 895B (Site HD-4)

Hole 895B was spudded in 3832.0 m water depth. Core 1R (0-10.3 mbsf) recovered 1.02 m of serpentinized harzburgite and dunite. The hole was unstable with high erratic torque and heavy fill and could not be cleaned out.

Hole 895C (Site HD-4)

Hole 895C was spudded in 3831.0 m water depth. Core 1R to 4R (0-37.6 mbsf), recovered 5.79 m. Hole conditions were good (the best of the leg). While cutting Core 5R, the BHA parted 0.55 m down on the fourth drill collar at the top of the zip lift groove. An unsuccessful attempt was made to fish the BHA in open hole. Three cracked pins were found in the BHA.

Hole 895D (Site HD-4)

Hole 895D was spudded near Hole C in 3832.0 m water depth. Cores 1R to 9R (0-93.7 mbsf) recovered 19.99 m of serpentinized harzburgite and dunite, basalt, and troctolite despite unstable hole conditions (some 0.5 in. rock fragments were recovered). The bit was pulled because high torque was stalling the rotary near bottom suggesting a possible undergauge bit problem. Another CC-9 bit was run, and the hole was reamed from 23 to 90 mbsf; however, the crossover between the DC's parted in the box threads. An unsuccessful attempt was made to recover the BHA.

Hole 895E, (Site HD-4)

Hole 895E was spudded in 3764 mbsf water depth about 270 m north of Hole D. Cores 1R to 8R (0-87.6 mbsf) recovered 32.93 m of spinel bearing olivine gabbro, serpentinized dunite and harzburgite, and troctolite (37.6% recovery). The harzburgites and dunites were impregnated with gabbroic melt, which strongly suggests that this section is the "crust/mantle transition" or "Moho". Hole conditions started deteriorating after Core 5R, and torque and overpull increased from unstable rock falling into the hole. The pipe stuck and could not be freed; therefore, it was severed.

Hole 895F, (Site HD-4)

Hole 895F was spudded in 3704.0 mbrf water depth on a sediment slope with small boulders and cobbles 200 m north of Hole E. The bit encountered 4 m of very soft sediment. Cores 1R to 2R (0-26.2 mbsf) recovered 1.98 m of serpentinized harzburgite and dunite. Hole conditions were very unstable, and coring was terminated.

Hole 894G, (Site HD-3) Logging & HRB Recovery

The HRB at Hole 894G was reentered for logging. The FMS tool could not be worked past a ledge at 71.6 m. The tool indicated a fairly constant hole angle of 4° at 240° azimuth, but it stuck while being pulled into the pipe. 10-3/4 in. casing and hanger were pulled out with the FMS tool, which was jammed in the bit by a broken sensor pad. The HRB was recovered.

HD-4 Coring Problems

A total of 272.9 m (23.7% recovery) of serpentinized peridotites impregnated or crosscut by gabbros was cored at Site HD-4. Each cored section had to be rereamed, apparently because the rock was relieving internal stresses by closing-in the well bore, and there was heavy pad wear on bits and stabilizers. Unstable hole conditions were encountered with constant high torque stalling the top drive, high overpull and the annulus packing-off at circulation rates above 150 gpm. Large rock chips accumulated above the bit. Viscous gel sweeps and/or increasing the pump rate caused the debris to pack-off above the bit; therefore, it was not possible to clean most holes. Tandem 20 bbl mud sweeps were fairly effective in cleaning the more stable holes such as 895B/C. Many core sections exhibit a pronounced curvature, and broke-off in roughly 0.2 m disks after contacting the inner core barrel wall.

Future Offset Drilling

Future offset drilling programs in unstable formations might be more productive if precise drill sites were marked with beacons or floats on dives. A bare rock pilot hole should first be cored as deep as possible to confirm the lithology, determine rock stability, and obtain logs. A reentry cone with a short surface casing should be drilled-in (using an underreamer) and cemented to anchor the base. Drilling should be done with stabilized BHAs to wipe out the ledges, and deviation should be controlled. Multiple casing strings would be run as required (probably about every 70 to 100 m) for deep penetrations in unstable formations.

Leg 147 - Operational Problems and Suggested Solutions

1. Site survey data were not adequate:

2. More time is needed to core multiple Pilot Holes to evaluate sediment cover and rock consolidation near the surface (but more risk to drill string), in order to identify Deep Hole location.

3. TV/Sonar/Drill String difficult to control accurately.

4. BHA failures when spudding on hard, sloping, bare rock.

5. The HRB is unstable on sedimented slopes and falls over as circulation washes sediment from under the legs.

6. Hole 894G started at 4°-5° angle because ship was offset to counter strong, shifting surface-currents. This caused alignment problems when attempting to reenter with bits and casing.

7. Non-concentric hole was drilled because HRB tilted, and drill string fell to low side of HRB casing throat.

8. Cam-Actuated-Dril-Ahead (CADA) running tool did not permit disengaging from base to verify position (without tripping ) before spudding.

9. Alternating cavities and hard ledges caused problems with hole angle, torque, drag, hole cleaning, clearance, and running casing. Also prevented effective concentric hole enlargement with pilot bit and stabilizers and caused stuck pipe problems.

10. Rubble continued to fall in the hole and fluid circulation occurred around the casing (making hole cleaning impossible) because the surface casing was not cemented (to allow remaining HRB to be moved if required).

11. Hole enlargement from unstable formations led to hole cleaning problems.

12. Difficulty running casing in unstable holes with Cavity/ Ledge / Alignment problems.

To Operation Superintendent's Perception of Leg 153

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