Initial Logging of Hole 735B
At 2000 hr on 24 October 97, we reentered Hole 735B; however, we encountered resistance almost immediately at a depth of 736 m (5 mbsf). We pulled clear of the seafloor, picked up the top drive, and using slow rotation reentered for the second time at 2130 hr. This time the end of the drill string was placed at a logging depth of 780 m (49 mbsf) without incident.
We initiated logging operations with the triple-combo consisting of the NGS, APS, HLDS, and the DLL probes. We also obtained temperature data using the Lamont-Doherty Earth Observatory (LDEO) temperature tool. The first logging run advanced to 492 mbsf or just 8 m short of the total depth of 500.0 mbsf. Our second logging run included the NGS, DSI, and the FMS. We had to abort this run because of data acquisition difficulties. We completed the first phase of logging operations by 1700 hr 25 October 1997.
Following a pipe trip to change over to a coring BHA, we again reentered Hole 735B and ran to bottom. The top drive was used at several points in the hole; however, minimal resistance was encountered on the trip because the last core recovered on Leg 118 was taken with the motor-driven core barrel (MDCB), we expected the lowermost 0.7 m of the hole to be under gauge for RCB operations. The bit tagged bottom at 504.8 mbsf, or 4.8 m below the depth that RCB coring ended on Leg 118, and 4.1 m below the depth of MDCB drilling. This discrepancy between the depth to the bottom of the hole recorded at the end of Leg 118 and the depth at which Leg 176 coring operations began remains unexplained.
Core Bit Number 1
We began continuous coring operations with Core 176-735B-89R at a total depth of 1235.8 m or 504.8 mbsf. The coring BHA consisted of a 9.875" x 2.313" C-7 tungsten carbide insert roller cone bit with 4 x 16 jets, an RCB outer core barrel assembly, eight each 8-1/4" drill collars, one set of mechanical drilling jars, two 8-1/4" drill collars, a tapered drill collar, and two stands of 5-1/2" drill pipe. We used a 20-ft drilling knobby and wear-knotted drill pipe at the top of the string through the upper/lower guide horn.
Our first cored interval advanced only 3.0 m to reach the proper place on the drilling kelly for a pipe connection. We followed this with two full length (9.6 m) cores. Partial cores (ranging in length from 3.7 to 8.0 m) were taken beginning with Core 176-735B-92R because of the extremely slow ROP. Core jammed inside the acetate butyrate core liners for three cores in a row, so beginning with Core 176-735B-96R, we deployed the core barrels without liners and continued this procedure for the remainder of the leg. We used Bentonite gel mud exclusively, circulating the mud in 20-30 bbl sweeps every core or every second core as required to enhance hole cleaning. This mud program was typical for the leg. No fill was noted when making connections, and no hole problems were identified. Upon recovering Core 176-735B-96R, we observed that the core catchers and cored material were missing. The threaded connection between the core catcher sub and the 11-1/8" inner barrel sub unscrewed, leaving the core barrel components and core material inside the pipe just above the bit. Either the connection had not been made up tightly enough or the 6.2 hr rotating time had vibrated the connection loose. We decided that attempting to fish the items out of the pipe would be futile, so the bit run was ended at a depth of 556.3 mbsf. We tripped the pipe back to the surface. The bit cleared the rig floor at 2145 hr and Core 176-735B-96R was recovered from the pipe. We found the core bit to be in good condition except for the inner (nose) row of carbide buttons. One of these was worn back nearly to the matrix material. The remaining cutting structure was in excellent condition as were the bearings. With the first core bit, we cored 51.5 m and recovered 34.32 m (66.6%) in 25.9 bit-rotating hours. The average ROP was 2.0 m/hr.
Core Bit Number 2
Using the same BHA configuration and a new C-7 core bit, we function tested the mechanical drilling jars and ran in the hole (RIH). We deployed the subsea television camera and reentered the HRB in less than 15 min of ship maneuvering. These quick (15 min) reentries proved to be the rule for the remainder of the leg. In this case, the driller again could not induce the bit into the hole, and another reentry had to be made with the top drive picked up. On the second attempt, we reentered the HRB at 0300 hr 28 October 1997 and using slow top drive rotation we "walked" the bit into the off-center hole without incident. Coring began with Core 176-735B-97R from 556.3 mbsf. Coring on this bit run was impacted by large long-period swells, which translated into rig floor heaves of 2.0 to 3.0 m, making it difficult for the driller to maintain optimum weight-on-bit (WOB). The seas moderated on the third day, but by then it was time to pull the bit for replacement. Our original goal was to obtain a minimum of 40 rotating hours on this bit, but because of the large load fluctuations experienced, we decided to be conservative and terminate coring operations slightly earlier than planned. In addition, the pump pressure was elevated for the final three cores and we suspected that two of the bit jets may have become plugged. We suspended coring after cutting Core 176-735B-111R to a depth of 642.7 mbsf. The pipe was tripped back to the surface and the bit cleared the rig floor at 0930 hr on 30 October 1997. The core bit was again found to be in good condition except for the inner (nose) row of carbide buttons. These were worn back nearly to the matrix material. Two jets were plugged in the bit with a combination of cuttings and Teflon bit seal material. In addition, we identified a 5" crack extending longitudinally outward from the base of the pocket in the drilling jar body. With the second bit, we cored 86.4 m and recovered 64.05 m (74.1%) in 36.0 rotating hours. The average ROP was 2.4 m/hr.
Core Bit Number 3
We ran in the hole with a new C-7 core bit and a new set of mechanical drilling jars making another 15-min reentry at 1350 hr 30 October 1997. We ran to bottom and found 8 m of loose fill at total depth (TD). We initiated coring with Core 176-735B-112R at 642.7 mbsf. Our goal on this run was to replace the bit after completing Core 176-735B-124R; however, observations of core tapering and some suspicious markings in Core 176-735B-123R halted operations after only 3.4 m of advance on Core 176-735B-124R. With the hole depth at 752.1 mbsf, we tripped the pipe back to the surface, clearing the rig floor at 0530 hr on 2 November 1997. Surprisingly, we again found the bit to be in good condition with effective bearing seals and no sign of imminent catastrophic failure. The internal cutter wear was more advanced than seen on the two earlier bits. The inner (nose) row of carbide buttons was completely worn back into the matrix material, and one of the two opposite cutters with three carbide inserts also was worn back to the matrix material. In addition, some wear was evident at the tip of the core guide, indicating that ROP would have begun to suffer soon. With the third bit, we cored 109.4 m and recovered 96.43 m (88.1%) in 43.8 bit rotating hours. The average ROP was 2.5 m/hr.
Core Bit Number 4
As a precaution, because of the earlier cracking incident, we again laid out the drilling jars for nondestructive testing (NDT) inspection and pressure testing prior to running them back in the hole with the fourth core bit. During the NDT inspection, we identified small crack indications initiating from the two corners at the base of each pocket in the jar body. We set these jars aside and because the remaining set of drilling jars had not yet passed pressure testing, this BHA was made up without jars. At 0845 hr 2 November 1997, we reentered the hole and ran to 742.0 mbsf or ~10 m off bottom. Using the Tensor electronic multishot tool we conducted a wireline drift survey from 742.0 mbsf to 500.0 mbsf. Six measurements were made at 50-m increments, and a hole deviation of 4.5°+ 0.3° was measured. Upon completing the survey, we began coring Core 176-735B-125R from 752.1 mbsf. While cutting Core 176-735B-136R, we noted a drop in circulating pressure of 100 psi and assumed that the RCB bit seal had failed. Although the bit continued to recover gauge core and advanced at a respectable penetration rate, we decided to again be conservative and pull the bit after 46.8 hr. We halted coring after recovering Core 176-735B 140R to 889.3 mbsf. The bit arrived at the rig floor by 0500 hr 5 November 1997. Just as on earlier bit runs, we found the bit to be in good condition with no sign of eminent catastrophic failure. As expected the internal cutter wear was slightly more advanced than on previous bits, with the inner (nose) row of carbide buttons completely worn back into the matrix material and both of the two opposite cutters with three carbide inserts each also found worn back to the matrix material. Slight wear was evident at the tip of the core guide. Bearing seals were considered effective and bearing condition was actually slightly better (tighter) than that of the previous bit. With the fourth bit, we cored 137.2 m and recovered 106.15 m (77.4%) in 46.8 bit rotating hours. The average ROP was 2.9 m/hr.
Core Bit Number 5-First Deployment
During the previous bit run, we rebuilt the final remaining set of drilling jars and added packing to the seal chamber. The jars passed a pressure test of 1500 psi for 10 min and were judged acceptable for use. A new C-7 core bit was made up and after 15 min we made yet another routine reentry at 0800 hr 5 November 1997. We were tripping the bit to bottom when it encountered a hard bridge at a depth of 123 mbsf. After picking up the top drive, we again encountered an obstruction, this time at 117.0 mbsf. We spent close to 1-1/2 hr attempting to clear the bridge with the C-7 core bit to no avail. The top drive repeatedly stalled out with little or no WOB each time it contacted the obstruction. After deliberation, we decided that we would be more successful using a more massive tricone drill bit, so the drill string was recovered back to the rig floor by 1545 hr that same day.
Tricone Drill Bit Deployment Number 1
Leaving the BHA configuration unchanged, we exchanged the Rock Bit Industries (RBI) C-7 style core bit for a Smith F57 tricone drill bit. We reentered the hole at 1915 hr 5 November 1997 (15 min reentry), and the pipe was RIH tagging the obstruction at the same depth as before (117.0 mbsf). Top drive stalling continued to be a problem during our attempts to clear the bridge. This was aggravated by significant heave resulting from long period swells generated by a significant low pressure cell some 80 km to the south of the drilling location. Ultimately, by using high revolutions per minute (RPM; 100-130) coupled with low WOB and some skillful drilling by the ODL Drilling Superintendent, a foothold was attained in the bridge. All total, in less than an hour, we had cleared the hole once again. Top drive stalling torques of 750 amps and overpulls of up to 60K lb were experienced during the episode. We theorized that an angular piece of material must have sloughed off of the side of the hole leaving a high-angle bridge that caused both core and drill bits to wedge immediately upon contact. We subsequently washed/reamed the hole to bottom to ensure that any other bridges or hard fill on bottom would be broken up by the more rugged tricone drill bit before resuming coring operations. No other bridges were identified, however, and only 2.0 m of soft fill was encountered on bottom. We circulated this out in short order and pumped a 50 bbl Bentonite gel mud sweep. We then tripped the pipe back to return the BHA to a coring configuration. The tricone drill bit cleared the rig floor at 0630 hr 6 November 1997.
Core Bit Number 5-Second Deployment
The same C-7 core bit was made up that was run initially when the hole obstruction was first encountered. The bit was undamaged and showed no wear from its first deployment. The drilling jars failed a routine pressure test, and because they were the only remaining set of jars, rig time was taken to add packing and retest. We then made up the jars with the remaining BHA and tripped the drill string to bottom, reentering the hole at 1230 hr on 6 November 1997. We encountered no problems passing the previously bridged area, and the pipe went straight to bottom without incident. No fill was detected at TD, and we began coring Core 176-735B-141R from 889.3 mbsf. While we were cutting the second core of this bit run, the drilling torque escalated to 600 amps and we experienced 60K lb overpull picking up off bottom. High torque, top drive stalling, and overpull of 40-60K lb continued through Core 176-735B-145R. Although we encountered no problems after Core 176-735B-146R, we pumped three 30 bbl bentonite gel sweeps after Cores 176-735B-147R, 148R, and 150R. Beginning with Core 176-735B-150R, we restored the original mud program of 30 bbl gel mud sweeps every other core. We stopped coring while cutting Core 176-735B-151R at a depth of 987.5 mbsf when drilling torque escalated to more than 50 amps above normal and ROP dropped off to 1.6 m/hr. The bit cleared the seafloor at 0250 hr and was at the rig floor by 0415 hr 9 November 1997. As before, the bit was in good condition with no sign of imminent catastrophic failure. The internal cutters were severely worn with the inner (nose) row of carbide buttons completely worn back into the matrix material. Both of the two opposite cutters with three carbide inserts were also worn back to the matrix material. Bearing seals were effective and bearing condition was good. With the fifth bit, we cored a total of 98.2 m and recovered 91.28 m (93.0%) in 47.3 bit rotating hours. The average ROP was 2.1 m/hr.
Core Bit Number 6
We made up yet another new C-7 core bit with the BHA; however, drilling jars were not included this time because during the NDT inspection the final remaining set was also identified as having crack indications. The last set of jars was set aside and the remaining BHA was run in the hole. Reentry was made at 0700 hr 9 November 1997, and the drill string was run to the bottom without incident. Only 0.5 m of fill was present on bottom. We initiated coring with Core 176-735B-152R at a depth of 987.5 mbsf and continued coring through Core 176-735B-163R to a depth of 1099.4 mbsf. The customary 20-30 bbl gel mud sweeps were increased at this point to every core rather than every other core. We terminated coring early while cutting Core 176-735B-163R, because the drilling torque abruptly increased more than 50 amps and the ROP dropped to 2.5 m/hr from earlier, higher rates. The bit reached the rig floor by 2330 hr 11 November 1997 and proved to be in good condition with no sign of imminent catastrophic failure. The internal cutter wear was severe, however, and the inner (nose) row of carbide buttons was completely worn back into the matrix material as were both of the two opposite cutters with three carbide inserts. One insert had fallen out of the core gauge row and another had broken off. Bearing seals were effective and bearing condition was as good or better than any of the previous bits. The sixth bit cored 111.9 m and recovered 106.35 m (95.0%) in 45.9 bit rotating hours. The average ROP was 2.4 m/hr.
Core Bit Number 7
We made up the seventh new C-7 core bit and weld repaired a crack in the latch sleeve prior to running in the hole. The BHA was run as before without drilling jars. We reentered at 0245 hr 12 November 1997 and ran the drill string to bottom where we tagged ~2.0 m of fill. This was easily circulated out. We initiated coring with Core 176-735B-164R at a depth of 1099.4 mbsf and continued to a depth of 1191.1 mbsf. As before, we pumped 20 bbl Bentonite mud sweeps after every core and again ran without liners. No fill was identified after any of the connections; however, while cutting Core 176-735B-174R, the drilling torque increased dramatically from the normal 360-400 amps to 500-600 amps. The torque dropped back to normal when the bit was picked up off bottom. We had only 33.3 rotating hours on the bit, but it was impossible to rule out the possibility of a potential bit problem. We therefore decided to err on the side of conservatism, and the final core was recovered after advancing only 3.0 m. Upon clearing the rig floor at 0430 hrs 14 November 1997, we found the bit to be in excellent condition, similar to the second core bit run of 36.0 hr. We noticed a high degree of wear and burnishing on the crossover sub between the 5" and 5-1/2" drill pipe during the pipe trip. This may have been the source of the excessive drilling torque. A check of the pipe tally placed the crossover at or near seafloor in the vicinity of the HRB. The seventh bit cored a total of 91.7 m and recovered 86.81 m (94.7%) in 33.3 bit rotating hours. The average ROP was 2.8 m/hr.
Core Bit Number 8
We ran the eighth new C-7 core bit in the hole and replaced the 5-1/2" to 5" crossover sub. In addition, we added 10 more stands of 5" drill pipe to the string to remove the crossover from the area of the HRB. We made another routine 15-min reentry (0700 hr 14 November 1997), and the bit was run to bottom. There was no indication of fill on bottom, and drilling torque was normal. We began coring Core 176-735B-175R at a depth of 1191.1 mbsf and continued to a depth of 1360.6 mbsf. We terminated coring after advancing 5.6 m on Core 176-735B-193R when the drilling torque increased by 100 amps. The bit had 53.8 rotating hours at that time so we took the conservative approach and pulled the bit. The bit reached the rig floor at 1530 hr 17 November 1997, and we found it to be in good condition. One cover plate from the grease reservoir was missing, however, and this may have contributed to the increased torque downhole. The eighth bit cored 169.5 m and recovered 140.80 m (83.1%) in 53.8 bit rotating hours. The average ROP was 3.1 m/hr. The increased ROP and decreased recovery rate was indicative of the more fractured nature of the formation through much of this interval.
Core Bit Number 9
We ran the ninth new C-7 core bit in the hole with an additional five stands of 5" drill pipe to keep the 5" x 5-1/2" crossover sub well above the HRB. After slipping and cutting the drill line for the first time during this leg, we reentered the hole at 2045 hr 17 November 1997. We ran to a depth of 1295.0 mbsf, where the bit met an obstruction and was unable to pass. We picked up the top drive and were able to ream the remaining 65.6 m without difficulty. We found no indication of any fill on bottom, and the drilling torque was normal. We initiated coring with Core 176-735B-194R at a depth of 1360.6 mbsf and continued to a depth of 1386.4 mbsf. Coring was terminated at 3.1 m on Core 176-735B-197R because of deterioration in the weather. Aside from the heave compensator beginning to exceed operating limits, we experienced several 3% (5 ea) and 5% (2 ea) positioning alarms. We stopped coring after only 8.8 rotating hours on the bit and began to pull out of the hole. After pulling the pipe to a depth of 739.0 m (8.0 mbsf), the drill string was hung off on the elevators while the driller laid out the previously pulled stand of drill pipe. At that time the bit contacted a ledge in the hole, causing the drill string to jump upward in the landing elevators. Using the still made up stand in the string above the rotary, the string was lifted off the elevators approximately 2.0 m where the bit was free from contacting the ledge. Two attempts were required before the drill string could be lowered past the ledge to the next tool joint. At that point, we laid out a double of drill pipe, and a full stand of pipe was then pulled allowing the bit to clear the HRB. The bit cleared the seafloor at 2115 hr 18 November 1997 under marginal weather and sea conditions. Winds were ranging from 33 to 41 kt gusting to 51 kt and swells were running 8-18 ft at 8-9 s periods. The vessel was heaving 10-18 ft. Because of extremely rough seas at the time, we decided to hang off the BHA/transition pipe below the keel and wait for the weather to abate. At 2215 hr 18 November 1997, the string was hung off on the elevators, and we commenced waiting on weather (WOW). By 0245 hr the following morning, conditions moderated enough to recover and nondestructive test (NDT) the BHA. We found no crack indications during the "magnaflux" inspection; however, two drill collars (#2 and #3) were bent. This was obviously the result of the incident that occurred at the seafloor and described above. The recovered bit was in excellent condition although (nose) cutter wear was much higher (50% of the carbide inserts worn away) than expected of a bit with less than 9 rotating hours. The ninth bit cored only 25.8 m but recovered an impressive 25.44 m (98.6%) in 8.8 bit rotating hours. The average ROP was 2.9 m/hr.
Core Bit Number 10
By 1500 hr 19 November 1997, our sea conditions had begun to moderate, and the captain felt confident that the storm system was both weakening and moving off location. We made up another new C-7 core bit and reentered at 1945 hr 19 November 1997. When the bit reached a depth of 1295.0 mbsf, it was again unable to pass, just as in bit run Number 8. As before, we picked up the top drive and reamed to bottom without incident. There was no indication of any fill on bottom. We proceeded to core beginning with Core 176-735B-198R at a depth of 1386.4 mbsf and continuing to a depth of 1508.0 mbsf. We took drift measurements at 1100 mbsf and 1400 mbsf determining hole deviation at those points to be 4.6° and 4.8°, respectively. We pumped 20 bbl bentonite mud sweeps after each of the first four cores, then increased the sweeps to 30 bbl per core because of the fairly rapid ROP. No fill was identified after any of the connections. We terminated coring after Core 176-735B-210R to short trip five stands of wear-knotted drill pipe out of the hole and replace it with standard 5-1/2" drill pipe. We could have proceeded coring using knobby drilling joints but the cores would have been shorter (9.2 m vs. 9.6/9.7) and it would have been more time consuming to handle the knobbies, whereas the wear-knotted pipe could be handled using the automated pipe racker. After pulling all but four joints of the wear-knotted pipe from the hole, we suffered a drill string failure. The pipe was landed in the elevators while the driller placed a double of drill pipe into the mousehole with the top drive. During this operation the drill string twice came into contact with a ledge in the hole when the vessel moved down because of heave, causing the drill string to rise approximately 0.5 m above the landing elevators. The driller made back up to the drill string as quickly as possible with the top drive, and the string was lifted off of the landing elevators. By that point, the damage had already been done, however, and the weight indicator showed a loss of 130K lb of string weight. Our calculations indicated that the string parted in the 5" drill pipe at or near the seafloor. We deployed the subsea camera to inspect the drill string/HRB to confirm that the failed pipe was not above the seafloor and then recovered the drill string. At 0600 22 November 1997, we identified the point of failure as the last engaged thread of a 5" drill pipe pin connection located at 739 m (8 mbsf). The fish we left in the hole was 1403 m in length and consisted of a 172-m-long BHA plus 43-2/3 stands of 5" drill pipe (1231 m).
Fishing Attempt Number 1
For our first fishing attempt, we made up a 9-1/2" overshot dressed with a 6-7/8" basket grapple and mill control. This was deployed on a fishing BHA consisting of five 8-1/4" drill collars, one tapered drill collar, and two stands of 5-1/2" heavy wall drill pipe. We reentered the hole at 1030 hr 22 November 1997. While lowering the pipe to 12.0 mbsf, the overshot started hanging up on a ledge, preventing the drill string from being hung off on the elevators for the next connection. We pulled the pipe clear of the seafloor and added a 10-ft pup joint to space the string out lower and, we hoped, past the bad spot. We reentered the HRB at 1130 hr and lowered the overshot to the top of the fish at 838 m (107 mbsf). The fish was engaged in short order, and we were proceeding to lift it to its total weight of 130K lb when it parted leaving only 35K lb of weight suspended below the overshot. We pulled the fishing string to the surface, where we recovered a total of 497 m of 5" drill pipe by 1930 hr 22 November 1997. The fish parted in the 5" drill pipe tube two feet below the box tool joint. This was at a point where the tube had buckled when the string impacted the bottom of the hole after the initial failure. The portion of fish remaining in the hole (906 m) consisted of 26 stands (734 m) of 5" drill pipe plus the coring BHA of 172 m.
Fishing Attempt Number 2
For our second fishing attempt, we assembled an 8-7/8" overshot dressed with a 5" basket grapple and mill control. We used the same fishing BHA with the addition of a 10-ft drill collar pup joint. After a brief trip to the seafloor, we reentered the hole at 0000 hr 23 November 1997. We contacted the fish at 0630 hr that same day at a depth of 1337 m (606 mbsf). After nearly 3 hr, however, no engagement was accomplished, and we elected to pull the drill string out of the hole. The fishing tools cleared the rig floor by 1200 hr and significant damage was observed on the lip guide of the overshot, including a large chunk of missing material. We could find no indication that the fish had ever contacted the mill control or basket grapple.
Milling Attempt Number 1
Based on the recovered piece of failed drill pipe tube, we suspected that the top of the fish was bent over and elongated at the top. Therefore, we made up a 9-5/8" flat-bottomed junk mill and ran to the seafloor. We reentered the hole at 1545 hr 23 November 1997, and by 2030 hr the fish was contacted at the same depth of 1337 m (606 mbsf) as before. Milling continued until 0330 hr 24 November 1997 when a depth of 1339 m (608 mbsf) was reached. It was difficult to tell if the mill was on top of the fish or just sidetracking down the side of the pipe; however, we hoped for the former. When the tools were pulled out of the hole and cleared the rig floor (0600 hr), we could find no evidence that the bottom of the mill had ever contacted the top of the fish. The sides of the mill, however, were worn back ~2" where obvious metal-on-metal contact had occurred.
Milling Attempt Number 2
We then made a second milling attempt with a concave junk mill. This mill had more carbide cutting structure on the sides as well as the bottom. We elected to run an additional stand of 8-1/4" drill collars this time because of the anticipated heavy swell and attendant large heave. We reentered the hole at 0900 hr 24 November 1997, and by 1245 hr the fish was contacted at approximately the same depth (1338 m or 607 mbsf). As before, milling continued until 1530 hr 24 November 1997 when we reached a depth of 1340 m (609 mbsf). The tools were pulled out of the hole and cleared the rig floor at 1830 hr. This time we did find some evidence that the bottom of the mill had contacted the top of the fish, although there was also significant side wear.
Fishing Attempt Number 3
For fishing attempt Number 3, we reentered the hole at 2230 hr 24 November 1997 with an 8-7/8" overshot dressed with a 5" basket grapple and mill control. By 0330 hr 25 November 1997, we had contacted the fish at a depth of 1340 m (609 mbsf). By this time, there were large long-period swells generating 3 m heave at times, aggravating our attempts to engage the fish. Nothing worthwhile was accomplished, and the drill string was recovered to the rig floor by 0445 hr that same day. This time we found the lip guide bent over to such a degree that it would not have gone over the fish. The damage was likely the result of the high heave experienced during the fishing period. As before, there was no indication that we had ever contacted the fish with the mill control or basket grapple.
Milling Attempt Number 3
We made a third attempt at milling with a fresh concave junk mill, because we assumed that the top of the fish was likely severely damaged. The same BHA was made up, except we added a string stabilizer just above the first full drill collar. We hoped that the stabilizer would stiffen up the BHA and aid in keeping the mill straight in the hole, reducing its propensity to sidetrack. We reentered the hole at 0930 hr 25 November 1997, and it became immediately apparent that our BHA configuration would not work. With the bit at 19 mbsf, the torque was so severe that we decided the stabilizer would have to be removed from the string or we would risk sticking the BHA downhole. The drill string was recovered, and we laid out the stabilizer by 1230 hr. We reentered the hole without the stabilizer at 1515 hr and by 1915 hr we contacted the fish at a depth of 1340 m (609 mbsf). This time the milling parameters were excellent with all indications that the mill was grinding away on the top of the fish. We suspended milling at 2145 hr 25 November 1997 after reaching a depth of 1342 m (611 mbsf). The milling BHA was pulled out of the hole, clearing the rig floor at 0045 hr 26 November 1997. The wear pattern on the bottom of the mill appeared to be circular with a +5" diameter, giving us new hope that the fish may be recoverable.
Fishing Attempt Number 4
On the fourth fishing attempt we again ran the 8-7/8" overshot, this time with an extension to allow more of the fish to be swallowed. It was again dressed with a 5" basket grapple and mill control. We reentered the hole with this assembly at 0345 hr 26 November 1997. For the first time since initiating fishing operations, we observed back flow from the pipe during connections. At 0715 hr, we contacted the fish at a depth of 1342 m (611 mbsf). This time conditions were different. Each time we contacted the fish with the overshot, the pipe would torque up and stall the top drive instantaneously. Overpulls of 40K lb were required to pull the tool free. At no time did we see any pump pressure indication that the fishing tool had swallowed the fish. Our efforts to engage the fish continued, using no RPM, slow RPM, high RPM, light weight, heavy weight, etc.&151all;to no avail. We finally abandoned further attempts at 0815 hr and recovered the fishing assembly by 1200 hr 26 November 1997. After inspection, it was obvious that the fish had not entered the throat of the overshot.
Fishing Attempt Number 5
We suspected that because of its length, diameter, and rigidity, the overshot assembly may not have made it all the way down to the fish. We also theorized that the hole was possibly being sidetracked with the lost drill pipe acting as a whipstock; however, this was inconsistent with the junk mill data. We decided to try a shortened overshot assembly to see if it would work any better. We reentered the hole at 1600 hr 26 November 1997 and by 2000 hr 26 November 1997 we contacted the fish at a depth of 1342 m (611 mbsf). All of our attempts to engage the fish were unsuccessful, and the tools were once again retrieved to the rig floor arriving at 0045 hr 27 November 1997.
Tricone Drill Bit Deployment Number 2
All indications at this point were that the mill/fishing tools were sidetracking the fish rather than setting down on top of it. To verify this, we decided to run the same 9-7/8" Smith F57 tricone drill bit that was used earlier to clear the obstruction in the hole. We reentered the hole at 0330 hr 27 November 1997, and at 0645 hr the fish was contacted at a depth of 1337 m (606 mbsf) or the original depth prior to the milling operations. The bit was torquing as if it were either on top of the fish or possibly attempting to ream out the undergauged hole made by the sidetracking mills. Our suspicions confirmed, we recovered the drilling assembly, clearing the rig floor by 1000 hr 27 November 1997.
Fishing Attempt Number 6
The challenge at this point was to capture the fish without going down the sidetracked hole. To attempt straightening out the pipe in the hole, we fabricated a wall hook out of an 8-7/8" extension sub and a 9-1/2" extension sub. The larger extension sub would be less likely to follow the sidetrack hole. The smaller sub provided the threads necessary to make up to the 8-7/8" overshot body. The 8-7/8" was needed because there were no 5" basket grapples or mill controls aboard for the 9-1/2" body. With the new fishing tool, we reentered the hole at 1915 hr 27 November 1997, and by 2300 hr the fish was again contacted at a depth of 1337 m (606 mbsf). Engaging the fish was unsuccessful again. Each time we set down on the fish, the torque would jump up and the top drive unit would stall. We worked the tool for 2-1/4 hr before abandoning the effort and retrieving the tool. The fishing tool was back on the rig floor by 0345 hr 28 November 1997; however, the lower 0.5 m of the wall hook was left in the hole.
Milling Run Number 4
With the additional junk in the hole, another milling run was required. We ran in with a 9-1/2" flat bottomed junk mill, reentering the hole at 0645 hr 28 November 1997. By 1030 hr, we again contacted the fish, this time slightly higher at a depth of 1336.6 m (605.6 mbsf). Milling on the junk went surprisingly well with constant torque and very slow ROP. After 2-1/2 hr, we halted the milling and recovered the tools to the surface. By 1615 hr 28 November 1997, the mill was back at the rig floor showing definite signs of being on top of the drill pipe fish. A very discernable concentric wear pattern ~5.0" in diameter was identifiable on the mill face. This gave everyone encouragement that the top of the fish had centralized in the hole before the wall hook had failed.
Fishing Attempt Number 7
Once again we ran in with an 8-7/8" overshot assembly, reentering the hole at 1900 hr 28 November 1997. Our seventh fishing attempt began at 2300 hr 28 November 1997 at a depth of 1337 m (606 mbsf). Enthusiasm soon turned to frustration as our latest fishing attempt was characterized almost exactly as the previous one. Immediate stall torque was experienced upon contact with the fish. We worked the overshot for 2 hr; however, at no time was there any indication of increased pump pressure or overpull that might suggest engagement with the fish. At this point, we decided to stop fishing and complete the wireline logging and VSP experiments, as this would give us a little contingency time in the schedule should there be unforeseen problems with the logging program or deterioration in the weather, thus necessitating an earlier than scheduled departure from the drill site. We recovered the fishing tools, clearing the rig floor by 0515 hr that same day. All drill collars were laid down except those required for logging.
Wireline Logging and VSP Experiment
We used a shortened BHA for logging, consisting of a reentry cleanout bit, five each 8-1/4" drill collars, one each tapered drill collar, and two stands of 5-1/2" transition drill pipe. We installed a landing saver sub with a "special" landing ring two drill collars up from the bit to provide a landing point for the Kinley Crimper/Cutter assembly should it be required. The landing ring inside diameter (ID) had to be opened up to 3.918" to accommodate the oversized Schlumberger VSP tool. Details of this modification are included later in the report under "Downhole Logging and VSP." We reentered the hole at 0745 hr 29 November 1997, and the pipe was placed at 780 m or 49 mbsf with a 30-ft drilling knobby through the guide horn. At 0915 hr, we began to rig up the Schlumberger logging sheaves and wireline compensator.
The first tool string consisted of HLDT, caliper, APS, and HNGS probes. The second logging run consisted of NGT, DSI, GPIT, and FMS probes. The third tool string was composed of NGT, GPIT, and DLL probes. We rigged down the VSP tools and logging sheaves by 0415 hr 1 December 1997 and proceeded to pull the drill string, clearing the seafloor at 0430 hr and reaching the rig floor by 0530 hr.
Fishing Attempt Number 8
While the logging/VSP efforts were under way, we fabricated another fishing assembly. An 8-7/8" overshot assembly was shortened as much as possible, bringing the mill control and basket grapple closer to the open end of the overshot. In addition, "Cutrite" hard facing was applied to the lip guide, creating a "milling capable" overshot assembly. We hoped that the combination of the shorter fishing assembly and the limited milling ability would allow the basket grapple to engage the drill pipe fish.
After making the BHA up with the new fishing tool, we round-tripped the drill string one last time for a final fishing attempt. The fish was contacted ~2 m deeper than before at 1339 mbrf or 608 mbsf. During subsequent working of the pipe, we contacted the fish at the shallower (607 mbsf) depth; however, all attempts to engage the fish proved futile. We abandoned our final fishing attempt at 1400 hr 1 December 1997 and began preparations to get under way for Cape Town, South Africa. Upon recovery of the fishing tool, the overshot showed obvious indications that a fish (drill pipe, wall hook, ???) had at some time been inside the overshot as far up as the mill control assembly.
To 176 Transit to Cape Town , South Africa
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