CORK-Lite: Bringing Legacy Boreholes Back to Life
Abstract
The primary goal for Integrated Ocean Drilling Program (IODP) Exp 336 to North Pond, on the western flank of the Mid-Atlantic Ridge at 22°N, is to elucidate the microbial community structure, origin, and activity within a defined geochemical, hydrological, and geological setting (Expedition 336 Scientists, 2012). The goals are to be met by conducting downhole and well-head microbiological experiments utilizing borehole observatories. The plan to achieve this goal included the modernization of an existing borehole observatory (CORK; Davis et al., 1992) at Deep Sea Drilling Project (DSDP) Hole 395A, the deployment of two new CORKs (one shallow and one deep), and the recovery and analysis of sediment and basalt. After the deployment of two CORKs, one in DSDP Hole 395A and the other in a shallow crustal hole tens of meters away (Hole U1382A), the next operation was to drill, case, and instrument a deep borehole at Site U1383, about 5 km to the north-northeast. The planned deep hole at U1383B began as designed (Expedition 336 Scientists, 2012). The 20-inch casing and re-entry cone were deployed and an 18.5-inch hole was drilled through 52.8 m of sediment and into basement (67.8 meters below seafloor (mbsf)). The 16-inch casing was deployed and the borehole deepened with a 14.75-inch tri-cone bit. Unfortunately, the tri-cone bit failed at 89.9 mbsf. Given the uncertainty of successfully recovering the bit and continuing drilling of this hole, a new hole (U1383C), about 25 m away, was started and later instrumented. With the successful deployment of the CORK at Hole U1383C, it was critical to seal Hole U1383B so that the open borehole would not "short circuit" the natural regional hydrologic flow. While on IODP Exp. 336 the plan quickly evolved from a simple seal for Hole U1383B to a newly designed CORK that could be deployed by a remotely operated vehicle (ROV). The design and operations for this new "CORK-lite" were conceived at sea. The first step was to modify an ROV platform with strength members below the platform to guide it in place when deployed from the ship in a "free-fall" mode. The second step was to design a CORK system that could seal the borehole and in consultation with ROV pilots, make sure the design was compatible with ROV operations. Operations were successful and a new instrumented "legacy hole" are now in place in the seafloor at North Pond. The platform was built and deployed during Expedition 336; the seal and instrumentation for long-term pressure monitoring and possible fluid sampling were built later and deployed successfully with the ROV Jason four months after the expedition. The success of this program has implication for instrumentation of numerous other "legacy" holes drilled as part of DSDP/ODP/IODP for combined hydrogeological, microbiological, and geochemical experimentation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS24B..03E
- Keywords:
-
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0450 BIOGEOSCIENCES / Hydrothermal systems;
- 0456 BIOGEOSCIENCES / Life in extreme environments;
- 0460 BIOGEOSCIENCES / Marine systems