CAPE ROBERTS PROJECT : Report on CRP-2 drill hole (1998), with comments on plans for CRP-3
This document is primarily a report on the CRP-2 drill site operation and the science support manager's responsibilities during October - December 1998, but includes comments that may be useful for drilling CRP-3 in 1999.
Section 1 of this report is structured to provide a brief background on previous relevant drilling experience in the area, and to show how preparations for the Cape Roberts Project drilling have evolved. Notes on drilling aspects of CRP-1 and CRP-2 are presented to show the lessons that have been learned and how these have been developed to achieve the scientific drilling goals. Notes on scientific support for the drilling operation, including the winter sea ice remote sensing, are also presented, with comments on CRP-3 preparations for 1999. Preliminary notes have been presented to the ISC meeting, Rome, February 1999.
1. GEOLOGY - DRILLING STRATEGY
1.1 Background - The CIROS-1 experience.
The drilling programme for CRP was based primarily on drilling experience and conditions found at CIROS-1 in 1986.
The CRP drilling programme adopted a sea riser and two coring drill string option in comparison to the three coring strings used at CIROS- 1. An increase in maximum operating water depth from 200 m at CIROS- 1 to 500 m for CRP sites was the main constraint in developing the current sea riser system. Cost and the belief that drilling could be successful with only 2 coring strings, HQ and NQ, led to the adoption of the present system. It was assumed from the sea floor conditions at CIROS-1 that 10-20 m of relatively soft Quaternary sediment could be washed through, and that the underlying older lithified strata would be suitable for embedding the sea riser to a depth of at least 20 m bsf with under-reaming techniques.
Drilling equipment purchases (drill rod etc) were based on the drilling strategy of four holes up to 500m deep in water depths of 150-500m as outlined in the CRP Workshop Report of December 1992. Mike Blong (Baroid, New Plymouth) analysed the CIROS--1 mud usage and developed a simple mud programme for CRP. Drill rod, sea riser and drill fluid products that were expected to be sufficient for all four holes were supplied in December 1994 for the January 1995 ship off-load at Cape Roberts.
Our Experience in 1997 while drilling CRP-1 identified inadequacies in the riser system designed and supplied by Austoil NZ Ltd. These were remedied for CRP-2 with a detailed engineering study of theoretical riser performance, improved deep-water rigid flotation and a sophisticated air bag tensioning system at the top of the riser. A new "off the shelf two arm under-reamer was also purchased to assist riser embedment. CIROS-1 found no significant down hole fluid over pressure problems although traces of methane were analysed and a page break bitumen residue found in sandstone.
The drilling of CRP-1 showed that the Quaternary sediment was more than twice as thick as expected from the available seismic survey data, and much more time was spent on seating the sea riser than had been planned. As a consequence it was decided that it was unrealistic to expect to have the time to drill two 500-m-deep holes in a season, and that we should instead set a single 700-m-deep hole as a goal. The drilling of CRP-2 has confirmed the wisdom of this view. A consequence of this decision has been that, following ODP advice, fluid control hardware has been procured for low over-pressure conditions (<1500 psi). For drilling below 500 mbsf this calls for the H casing string to be set in impermeable strata and cemented to withstand down hole pressures.
1.2 CRP-1 and CRP-2 - Results and Lessons
- Sea floor surface sediments consisting of unconsolidated young [Quaternary?] glacial diamicts, possible enriched in clasts, and even clast supported. Unlithified muddy/sandy matrix. Unknown thickness but likely to be between 20 and 50 metres. Embedding the sea riser in such sediments to a depth of around 20 m bsf is a slow and difficult process and likely to take around 10 days of the 45 day drilling window. This time includes cementing the sea riser, though experience indicates this is worth doing only when it has been fully embedded (i.e. at least 12 m bsf). Experience also shows that these sediments can be cored some tens of metres ahead of the sea riser once it is supported a few metres into the sea floor.
- CRP-1 and the upper part of CRP-2 were younger than expected (16-20 instead of 28-30 Ma). Some strata were poorly lithified, and included soft running sands. These sediments are normally difficult to drill, recover quality core and maintain a stable hole. The problem is compounded when these strata are windowed to the sea floor up-dip [shown by down-hole logging in CRP-2], allowing almost complete loss of drilling fluid and also the incursion of sea water at depth causing uncompacted sands to fluidise. The zones of most concern in the older strata were at ~ 80, 120, 160, 270 and 460 mbsf. Two of the zones lay below the cemented H casing, which had been cemented at 200 m bsf. Tests indicated that the casing was not set deep enough to provide adequate control for over-pressured fluids. CRP-2 had minor inflammable gas shows detected by the gas monitoring system on two occasions which only lasted a few minutes each. No effervescence was observed in the core on these occasions or at other times and the gas shows were considered minor and transitory Of more concern, however, is that the well returned significant volumes of drilling fluid when the inner tube was pulled to recover the core. On only one occasion did the returning fluid appear to be cut with other down-hole fluid. A pressure test showed that at the initiation of flow the locked in fluid pressure was in order of 160 psi and minor flow was still evident after 36 hours of down-hole logging. It was not clear at the time if this effect was a result of gas pockets or due to the formation relaxing after expansion caused from drill fluid pressures during the coring process. We are seeking advice from other experts familiar with slim-hole coring in gas-bearing sediments, but for the moment we need to consider the possibility of closed gas pockets that may exist up-dip in the formation.
Under ice buoyancy for CRP-2/2A.
The two five tonne air bags were deployed under the sea ice below the Drill Rig with the assistance of Crary Lab Divers. About 9 tonnes of buoyancy was applied under the 17 tonne drill rig and this maintained the freeboard in the ice page break hole to 180 mm (210 mm when unloaded) by the end of drilling. No buoyancy was applied under the Mud Hut complex which is also a significant load especially when the mud tanks are full (about 9 tonnes) and at the end of drilling this freeboard was reduced to less than 100 mm. Both air bags have been recovered but will require an improved mooring system to prevent freeze-on under the sea ice and facilitate recovery at the end of drilling.
Sea Riser & Under-reamer for CRP-2/2A.
The two new flotation systems of the refurbished sea riser worked well this season. The syntactic "foam" rigid floats provided reliable service this season. Cracks were observed in a few floats immediately after arrival in Antarctica at WINFLY, when they were subjected to rapid temperature change from the aircraft to lower than minus 40 degrees Celsius, but these did not appear to cause any failure during deployment. The floats were assembled in 4 unit modules and 7 modules deployed on the sea riser at the CRP-2 site. On one or two modules the clamps appear to have slipped during operations but this did not adversely affect the riser operation.
The new inflatable flotation also performed well and enabled precise control of the tension in the riser even when the riser was not anchored into the sea floor. Both flotation systems will require minor modification for operations in 1999.
The riser was initially embedded into the "hard" sea floor to a depth just over 6 m bsf by slow drilling ahead with a 4 inch roller bit and 100% loss of drill fluid to the sea floor. The sediments appeared to be a clast-supported diamict with a soft dark grey muddy matrix. At this point two attempts were made to cement the riser, but the cement did not set up properly on either attempt, and we continued to core ahead to 57 m with minimal top tension on the riser. There was 100% loss of drill fluid thoughout most of this period.
The initial purpose of coring was to provide a better understanding of the strata we were drilling so that the two armed under-reamer could be deployed, but this also provided useful core from the youngest sediments for the science community. Coring was slow, but good recovery was achieved (70%). It should be noted that with this slim hole drilling system that down-hole progress is generally as fast by coring as drilling with a roller bit even with the drill collars that were hired specifically for the roller bit drilling.
The riser was bumped down with the main winch to where we thought we had clast-poor sediments that were suitable for deploying the under-reamer. The process of bumping down the riser is very slow as you are using the hardened riser shoe to cut clasts only millimetres at a time and also flush out the cuttings to the sea floor. The under-reamer was deployed but became disconnected from the drill string even though the standard preparation and torquing up of the tool was carried out. It is still not clear how disconnection occurred but probably resulted from the combination of the following.
- clasts in the sediment causing intermittent cutting,
- the two arm design which could experience high torque down hole,
- the API thread which only requires 3 1/4 turns to make up and store rotary energy ("twisting") in the HQ drill string.
The under-reamer was however recovered after some very careful work by the drillers.
In the process of embedding the riser by the bumping down procedure we found that the scanned images of the core obtained from drilling ahead to be an extremely useful tool to help plan the embedment. We now know that we can drill by rotation in the riser for a time with minimal top tension while coring ahead. Coring ahead is vital to plan the embedment process, and bumping the riser into the sea floor appears to be the only reliable way to penetrate clast-rich sediments near the sea floor without redesigning the lower part of the sea riser and using down hole hammer techniques.
The initial cementing operations were not successful. Previous cementing in CIROS- 1 and page break CRP-1 appeared to work satisfactorily, but none of this cement had been cored to confirm that cementing techniques and setting times were appropriate. Expert advice and cementing references had suggested that we would need to use high concentrations of Calcium Chloride accelerator in the cement for low temperatures. However, our situation was more complex than the "textbook examples" because we are also using sea water containing Sodium Chloride which is also an accelerator. We also knew that high concentrations of both accelerators can react as a retardant and or weaken the cement
At the time of the first cementing attempt for CRP-2 we therefore had not been able to confirm that the theoretical cementing techniques and previous cementing attempts were working properly at sea floor temperatures of −1.8 degrees Celsius. Placement of the cement was carried out with proprietary cementing plugs directly within the riser for the first two cementing attempts. We hoped these plugs would speed up the operation but the plugs were unsatisfactorily in the modified riser string. The third cementing of the sea riser at about 13 m bsf appeared to work, although we could not positively confirm this by recovering strong cement core.
The cementing of the HQ drill barrel at 199.31 m, where down-hole temperatures were theoretically +5 degrees Celsius worked well, with several metres of hard cement recovered. This confirmed that the cement/accelerator mix was appropriateand indicated maximum setting times. A long setting time was deliberately allowed for this primarily because it was vital that the HQ barrel landing ring and diamond coring bit were locked firmly in place to be successfully drilled through with the NQ drill string. Major delays could have occurred or even hole abandonment if drilling out of the barrel was not successful. However, even with this latest CRP-2 experience we still do not have positive confirmation of the setting times required for initial cementing at the sea floor at −1.8 degrees Celsius.
Down Hole Logging.
The down-hole loggers had two opportunities to log parts of the hole. They were not allowed access to the uppermost 60 m of hole because this part had already shown itself to be unstable, with some of the hole drilled twice. In the first logging session in the upper part of the hole (HQ-96 mm hole) the loggers were requested not to use their radioactive source tool in case this was lost. It is common practice in many countries not to allow a radioactive tool in an open hole. Loss of this tool could have meant the early abandonment and cementing of the hole because fishing operations could have been hazardous.
In the second logging session (NQ-75.7 mm hole) all tools were available for use at the loggers' discretion but unfortunately the hole became bridged around 440m after running some of the tools to the hole bottom then running the calliper tool. This was the first bore hole wall contact tool, and it's use presumably caused the bridge to form. The Drilling Manager and Science Manager agreed to attempt to try to clear the bridge with wire line tools. We were not prepared to re-run the NQ drill string and chase the bridge which could have meant re-drilling the lower 200m of hole because this could have taken several further drilling shifts. The Drilling Manager pointed out that even running the wire line tools in an open hole represented a risk because the tools would be operated below the casing. Subsequently the wire line tools and much wire line were lost down hole and two days were spent in clearing the hole to 130 mbsf. . We now have confirmed that the current wire line and winch used for the normal core inner tube recovery is inadequate for the operation of the ODP piston and Shelby soft sediment samplers. The remaining deep logging tool runs were now restricted to the middle third of the hole. Further bridging restricted the VSP study to the interval above 130 m bsf. The VSP logging was then treated as an experiment with 24 shots fired in marginal blowing snow conditions. The initial VSP results appeared to be good, although smaller charges will be required for data acquisition in the upper part of future holes.
At the end of the logging the HQ casing was cut and recovered, exposing about the upper 60 page break m of hole for logging that was obscured by casing in the first logging session. Most of this part of the hole immediately bridged, confirming the original drilling decision not to expose this part of the hole for logging prior to cementing the HQ casing. I believe that the drillers were impressed with the professionalism of the down-hole logging operations. Of minor concern was the time that some tools remained at the bottom of the hole where they were susceptible to being sedimented in. I think that the down hole logging was very successful considering the unstable nature of a major part of the CRP-2 hole. In retrospect, the attempt to clear the bridged hole was a mistake but the other decisions to allow access to only the stable parts of the hole have been proven correct.
CRP-2 Site completion.
The drilling plan called for coring to cease on November 23, allowing for 2 days of logging, 5.5 days for returning the drilling system to Cape Roberts, and 7 days for breaking down the camp and storing on Cape Roberts. This presumed a normal ice year in which heavy plant could move safely on the sea ice until the first week in December. In 1998 down-hole progress was much slower than expected early in the season. With the hole 200 m short of the target depth on November 21 and down-hole progress at 30 m/day, it was decided that drilling could be continued a little beyond the schedule date.
- Down hole logging requirements
- Sea ice conditions at the drill-site including observations of surface ice conditions.
- Horizontal Sea Ice Offset
- Sea ice conditions on the supply route and the transition onto Cape Roberts
- Time required to complete the Camp Recovery and storage on Cape Roberts
- Aircraft availability for all returning CRP personnel to Christchurch
By the time the drill site was abandoned significant surface melt had occurred in the immediate vicinity of the Drill Rig and Mud Huts primarily due to the reflection of sunlight of the buildings. After the removal of the drill rig it also became apparent that the ice platform immediately under the drill rig was in very poor condition and had been strongly corroded by accumulated minor spills of salt rich drill fluid. We took extreme care in the operation of the heavy plant close to the drill site area during site abandonment. The surface prediction of the timing of surface melting is extremely difficult but can occur within a period of 2-3 days during warm temperatures and bright sunlight. We also hope to reduce the quantity of spilt drill fluid that accumulates around the drill rig in the future with better capture proceedures.. Sea ice offset was evident from the angle of the sea riser at the surface by 18 November, when a chain hoist was fitted to the top of the riser (Annular Diverter) to reduce the angle at the entry point of the NQ drill string. At this time horizontal ice offset from spud-in was 9 m. By the time coring finished on November 25 the offset was at least 11 m and some difficulty with rotation was evident during high current flow. This offset is about 6% of water depth and corresponds well with the latest engineering study and this can now be used to predict part of the system's operational constraints.
The transition route onto Cape Roberts had become flooded with sea water in the tide crack zone by 22 November. Other less suitable routes were available but these crossed more highly fractured ice in the 30-m-wide tide crack zone. The nature of the sea ice and ice foot transition at Cape Roberts varies from season to season which can influence the timing of flooding, melting and cracking and it's safe use.
1.3 Science support of the Drilling Operation
- DMSP. Infra red weather satellite images were processed and supplied primarily by Andy page break Archer of ASA HQ. Denver every few days throughout the 1998 winter months when good cloud-free images were available. These images have a spatial resolution of about 500 m per pixel and are useful for providing a history of the fast ice limit through the winter, thus allowing a judgement to be made on whether the sea ice is likely to be suitable for drilling in the spring.
- RADAR SAT. A series of these images were processed and made available to CRP by Bob Onstott, ERIM Michigan in 1998. These images have a resolution in the order of 35 m per pixel and are used to determine the WINFLY reconnaissance sea ice routes to Cape Roberts and drill sites areas. The timing of the successful acquisition of these images can be a problem but they were once again available at the beginning of the WINFLY period. I believe that with GPS navigational control data now to hand that future images will become an even better navigational tool vital for the early season sea-ice reconnaissance operations.
Drill Site Science operations
- Submarine Video. The umbilical of the video camera system was stressed and damaged this season. The system has a vital role in the sea riser embedment process and subsequent monitoring of the riser and sea floor as drilling proceeds. It is not clear at this time if the umbilical can be reliably repaired or if it will have to be replaced. Some other minor repairs will be required for 1999 operation including the building of umbilical/guide wire separators to reduce the risk of future umbilical damage.
- Gas Detection. The sensors for this system will require calibration and minor changes made including a new larger gas sampling line with a heated section installed to reduce freezing problems and false readings. The current system detects both inflammable and hydrogen sulphide gases and does not distinguish the composition of the inflammable components. The current system is sufficient for the safety of the drilling operation and records gas events along with weather data at the drill site. If more sophisticated gas analysis is proposed I would consider this as a scientific study not required for general safety and drilling operations.
- Core processing. Core processing and packaging worked well, with the split core- tube sections being used succesfully for retaining the integrity of unlithified or crumbly core when boxed. Minor modifications are planned for the core splitting saws. Consumables including plastic splits for soft core and diamond saw blades will have to be restocked for 1999. We hope to be able to package all CRP 3 core in plastic splits to reduce core damage and improve handling proceedures at the Crary Lab and core repositories.
- Core orientation tool. This tool will require some modification to operate reliably at the bottom of a deep hole. The tool was only run once this season and damaged the core face at the bottom of the hole so that orientation marks were not recorded and the tool did not trigger. It also requires good hard core that is later continuously cored without loss and this is hard to predict. The deployment of this tool poses a problem in that it requires someone with time and appropriate skills to operate but more importantly requires a dedicated wire-line trip each time and so competes with coring time. I am aware that this allocation of time and the tools operation was not resolved well in 1998 and we intend to improve this in 1999 in conjunction with other experiments related to the in-situ stress studies- Core scan images. These images were provided as a service to the project by Dr Terry Wilson's group and were invaluable at the drill site especially during the process of sea riser advancement. The scanned images provide an immediately useable product that conventional photography cannot. The scanned images will also be reproduced in the initial reports.
- GPS surveying. The GPS surveying of the drill site position in general went very well. A real-time correction signal was broadcast from the temporary Cape Roberts Hut Base Station (CRHBS) for the initial navigation and site selection during the WINFLY period. Subsequently all data was post processed against CRHBS data to give drill site positions with errors normally less than 0.5 m for sea ice offset calculations. The base station computer developed an intermittent fault during November resulting in three days of non-processable data at the end of drilling. This computer will have to be replaced for future page break work
1.4 CRP-3 in 1999 - Scientific and Drilling preparations
- Satellite imagery. The same level of service and supply of product that was achieved in 1998 will be required for the analysis and prediction of any planned drilling season in 1999. The first Radar Sat images hopefully can be supplied by 1 August and navigation control will be improved.
- Currents. We collected a good suite of current data near the expected CRP-3 site. This data will be worked on in the following few months and will be used to help refine the drilling strategy for the CRP-3 hole.
- Riser Deployment. We now have more confidence in operating the riser with minimum top tension for short periods and are prepared to core ahead as a matter of course to assist riser embedment. We expect to use the main winch bump-in method to embed the riser into clast-rich sediments and plan to use a more robust riser cutting shoe. The use of the two-arm under-reamer is not necessarily ruled out but appropriate strata will be required if its deployment is considered. The proper embedment of the riser is still paramount to achieving good down-hole progress. The available embedment techniques are limited and relatively slow but have now been shown to be successful. CRP-3 will be in deeper water, about 350 m, and although we have gained valuable experience in 1998 we should expect drilling operations to take about the same time as for CRP-2. - Drilling prognosis. CRP-3 should begin coring in the time equivalent of the lowest 200 m of CIROS- 1 then into strata of unknown fades and age. In general we can expect the formation to increase in hardness, but circulation was lost in a conglomerate at the base of CIROS-1, and we should expect intervals of circulation loss deep in CRP-3 also. Increase in the expanding clay content of the strata can be expected as we drill into the warmer conditions of the Eocene. This will require maintenance of the potassium content of the drill fluids to avoid expanding clay-induced problems down hole. The drill fluid programme planned for CRP-3 is based on the consumption of fluid in CRP-2/2A and will consist of basically the same mud compositions.