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doi:10.2204/iodp.proc.301.101.2005

Operational strategy and overview of expedition achievements

Operational strategy

Second Ridge Site U1301 was positioned ~1 km south-southwest of Site 1026 (Table T2). Both sites are located above a buried basement ridge (Figs. F2, F3, F4), where sediment thins to 250–265 m. The plan originally proposed for Site U1301 was to complete all operations in a single hole, so as to minimize the possibility of creating a hydrogeologic "short circuit" between basement and the ocean. However, discussion with engineering and operations personnel during initial planning stages for Expedition 301 in Fall 2003 led to development of a two-hole basement strategy for achieving primary technical and scientific objectives. One hole was intended to penetrate through the sediments and into uppermost basement, with a maximum penetration of 100 m into basement. The second hole was planned to be cased through the least stable parts of upper basement, allowing coring and downhole measurements at greater depths. This hole was originally proposed to extend 600 m into basement, but once the time requirements for the two-hole strategy became clear, the depth objective for the deeper Site U1301 hole was reduced to 300–400 m into basement.

Both holes were designed to be sealed with CORK observatories, the first monitoring uppermost basement and the second monitoring three additional zones at greater depths in the crust. These two holes were planned to be as close together as operationally feasible so that monitoring of conditions at different depths would be a true test of vertical connectivity within the crust. The actual distance between Holes U1301A (shallow) and U1301B (deep) ended up being just 36 m, which we believe to be the smallest intended spacing between adjacent reentry basement holes drilled in the history of DSDP and ODP.

The standard approach for establishing basement reentry holes is to first drill one or more exploratory holes to determine sediment properties and thickness. We decided to proceed directly with reentry holes at Site U1301 for several reasons. First, we already had a good understanding of sediment thickness and properties based on extensive site survey data and drilling at Site 1026 during Leg 168. Additional seismic coverage across Second Ridge provided clear indication of the depth to the sediment/basement contact. Second, we began at-sea operations <24 h after leaving port and we were concerned that there would be insufficient time following remobilization of the JOIDES Resolution to prepare the laboratories and train shipboard scientists for core handling, particularly that involving microbiological sampling and analysis. Third, we wanted to save sediment coring options for times later in the expedition, perhaps when packer testing or observatory installation operations could not be completed because of weather or sea state and/or until we had confidence of achieving higher-priority basement and observatory objectives. Fourth, delaying sediment coring operations gave the scientific party a chance to learn more about regional geology and decide where such coring should take place. Finally, we did not want any holes adjacent to the observatories that might penetrate the sediment/basement interface and risk a hydrologic "short-circuit" between basement and the seafloor.

Basement drilling and coring plans were unconventional in other ways. First, we planned to drill the deeper hole first. This would allow us to assess the depth extent of unstable basement with a bottom-hole assembly (BHA) well suited for this environment, including a tricone (noncoring) bit. This decision meant that we would not have core or logs from uppermost basement to help with assessing where to set casing and would rely instead upon drilling parameters and other qualitative indicators of formation stability. However, core recovery and log quality are notoriously poor in upper basement, and rapid drilling would provide the best chance for establishing a stable hole and installing casing. Also, by taking this approach we would have two opportunities for casing off unstable upper basement; if the first attempt failed, that hole could be used for shallow basement monitoring and we could offset and start a second hole.

Upper basement coring on Second Ridge had been attempted at Site 1026 during Leg 168 but yielded low recovery and an unstable hole that required a liner (5½ inch drill pipe) to keep the hole open. As it turned out, the first attempt to penetrate the upper 100 m of basement with a tricone bit during Expedition 301 was successful, but we failed to land the long 10¾ inch casing string necessary to keep the hole open (see discussion below about casing seals and cementing). This hole became our shallow basement completion, with casing installed across only the upper 15 m of basement, and a second hole was started nearby for deeper basement penetration.

In addition to installing two CORK borehole observatories at Site U1301, we also planned to replace the earlier-generation CORK systems in Holes 1026B and 1027C. Replacing the CORK in Hole 1026B was a higher priority than that in Hole 1027C for several reasons. First, the Hole 1026B CORK began leaking soon after it was emplaced during Leg 168. The cause for the leak is not known, but it may have resulted from broken hydraulic tubing. As a result, it was not possible to determine with confidence the formation pressure in upper basement because an unknown amount of excess pressure leaked past the seafloor seal. Second, the instruments originally deployed in Hole 1026B had been removed years before Expedition 301, and there was no data logger installed for recording formation pressure.

In contrast, Hole 1027C remained well sealed below a Leg 168 CORK at the start of Expedition 301, and a pressure monitoring system was in place and working well. However, Hole 1027C includes a single monitored interval comprising two distinct hydrogeologic regions: uppermost ("true") oceanic crust and a shallower section of sills and sediment. Also, Hole 1027C may offer excellent potential for geochemical and microbiological monitoring of upper basement once casing packers are installed in the open hole because this will separate the long section of metal casing above from the monitored intervals. Replacement of the Hole 1027C CORK was scheduled for the end of Expedition 301 with the intent that time necessary for this operation might instead be needed for other CORK activities.

We also planned 2 days of sediment coring during Expedition 301, and an additional 1.5 days was scheduled as contingency time at the end of the expedition in case we had problems with CORK deployments or other basement operations. The primary focus of sediment coring was on microbiological and geochemical objectives, particularly in the interval close to basement, and we knew that rotary core barrel (RCB) and extended core barrel (XCB) coring would likely lead to significant contamination and poor recovery. We began Expedition 301 with a good lithostratigraphic record of Second Ridge from nearby Site 1026, so we were free to select a strategy intended to maximize other scientific (mainly sampling) objectives. Thus, we elected to attempt APC spot-coring through the sediment section at Site U1301, an approach that was largely successful. Sediment coring also provided an opportunity to assess the thermal state of the sediment column at Site U1301, which was important to hydrogeologic and observatory objectives.

Overview of expedition achievements

The most important objectives of Expedition 301 were achieved. We created two new basement holes, Holes U1301A and U1301B, that penetrate 108 and 320 m into basement, respectively, and instrumented each of these holes with multilevel CORK observatories (Fig. F8). We also replaced the CORK observatory in Hole 1026B. All of the holes have multiple isolated intervals to monitor and sample pressure, temperature, chemistry, and microbiology and will serve as observatory points for planned cross-hole experiments. In Holes 1026B and U1301A, the uppermost of two intervals comprise entirely cased hole—these intervals are to be monitored to help assess how well the CORK systems are operating and the extent to which the packers are sealed. Lower zones in these holes monitor conditions in basement. In Hole U1301B, we isolated basement zones comprising both the uppermost (brecciated and highly fractured) crust and the underlying, more massive rock. We cored upper basement in Hole U1301B with ~30% recovery overall, typical for basaltic crust. Samples were collected for alteration, microbiology, paleomagnetism, and physical property studies, most of which will occur postcruise. Approximately 9% of recovered basement rocks were sampled as whole rounds and dedicated to microbiological analysis, a first for scientific ocean drilling. Logging data from the lower interval of Hole U1301B indicate that the hole is to gauge and that the crust is highly layered. Comparison to other upper crustal holes shows that we achieved our fundamental basement objective: separately isolating the upper and lower parts of the extrusive section of the crust. Packer experiments completed in Holes U1301A and U1301B show that the upper crust is highly permeable, but preliminary analysis suggests that there may be a decrease in bulk permeability with depth.

Actual operations differed from the plan outlined in the Expedition 301 Scientific Prospectus (Fisher, Urabe, Klaus, et al., 2004) in two significant ways: we did not replace the CORK in Hole 1027C and did not achieve as much basement penetration in Hole U1301B as originally hoped. Both of these deviations from the plan outlined in the Expedition 301 Scientific Prospectus primarily resulted from not being able to set 10¾ inch casing deep into the crust with a seafloor seal inside 16 inch casing, as was planned, and from difficulties that ensued as a result of unsuccessful cementing operations that were attempted in the absence of a seafloor casing seal.

During planning for the expedition, we developed a two-hole strategy at Site U1301 to give us two chances to achieve our deeper crustal objectives. If the first hole became unstable, we could set shallow casing for the uppermost basement observatory and attempt a second hole to achieve the deeper objectives. As part of this strategy, we should have brought to sea two sets of mechanical casing seals, allowing 10¾ inch casing to seal inside 16 inch casing, so (1) we would not have to depend on cement to achieve a seal at the 10¾ inch casing shoe in fractured and broken uppermost basement and (2) we could maintain the two-hole strategy and be able to make either hole the shallow or deep observatory. Despite extensive Expedition 301 planning, only one seal was fabricated for the expedition. Ultimately, we decided not to use the single mechanical seal. This seal appeared to have been built with incorrect tolerances and had not been test fit. The shipboard operations and engineering staff decided that it was best not to risk the installation by attempting to deploy the seal. Thus, completion of expedition goals at Site U1301 required successful cementing in upper basement.

One difficulty with using a conventional cementing approach in upper basement is that it requires pausing to attach the cementing swivel and manifold when running the 10¾ inch casing into the open basement hole. After drilling Hole U1301A to target depth, we returned to the hole to run casing, but when we stopped advancing the casing to attach the cementing manifold (an operation requiring ~16 min), we were subsequently unable to lower the casing the last few meters into the hole. We eventually shortened the casing string in Hole U1301A by 85 m, to case off only uppermost basement, and then attempted to cement this casing into place with a substantial length of open hole below. We could have tried to remove a single joint (~14 m) of casing for the second attempt, but we decided to shorten the casing string by 85 m to increase our chances of being able to install the string (the area where we were stuck appeared to be well above the bottom of the hole) and to increase our chances of being able to get a good cement seal back up into the base of the 16 inch casing. We had similar difficulties running casing in Hole U1301B, but after considerable hole conditioning and two attempts with running long strings of 10¾ inch casing, we finally landed a full-length casing string. However, the cementing job at the base of the 10¾ inch casing here was not successful, in that the cement did not form a solid bond with the formation around the casing shoe. Subsequent drilling out of the cement in the casing and RCB coring operations caused the casing to unscrew from the bottom, and ~100 m of the casing fell 5–6 m to the bottom of the hole, leaving a gap in the 10¾ inch casing string. This led to numerous difficulties during pipe trips and forced an early end to coring operations in Hole U1301B to attempt remedial cementing.

In addition, the 10¾ inch casing gap led us to run dozens of bowspring centralizers on the 500 m long CORK casing string initially run into Hole U1301B in order to centralize the CORK casing/packer/screen assembly as it passed through the casing gap. After reentering the hole with the bottom of the CORK, something hung up, preventing the CORK from passing freely into the hole. The 4½ inch casing failed, and the rest of the CORK casing was run out onto the seafloor. We suspect that the failure was caused by the bowspring centralizers hanging up in the throat of the reentry cone or by accumulating too much friction once the first six to eight were run into the 10¾ inch casing. It is clear that the failure happened long before the end of the CORK encountered the gap in the base of the 10¾ inch casing. The fundamental problem was that the CORK casing string was deployed without sufficient weight at the bottom to "pull" the casing into the hole and keep the casing string in tension. This lack of weight also made it difficult for the drillers to observe a weight loss when it hung up.

We used the CORK head from the first CORK attempt in Hole U1301B and other parts intended for use in Hole 1027C to build a new CORK observatory for Hole U1301B. For this string we added 10,000 lb of drill collars on the bottom and used only three spring centralizers, and it was deployed into the hole with no major problems. We finished operations in Hole U1301B by cementing the CORK observatory into the reentry cone in an attempt to seal between the 16 inch and 10¾ inch casing strings. We noted that CORK deployment in Hole 1027C would benefit from limited deepening of the hole so that weight might be added to the bottom for its CORK casing string, but we lacked both time and components to complete work in Hole 1027C during Expedition 301.

With primary goals achieved during Expedition 301, we are ready to press forward with the second half of the experimental program, including multidisciplinary, cross-hole experiments. A second drilling expedition will also replace the CORK in Hole 1027C and complete additional remedial cementing in the cones around the CORKs in Holes U1301A and U1301B to assure that these systems are fully sealed for the subsequent 5+ y of hydrogeologic experiments.

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