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doi:10.2204/iodp.sp.327.2010

CORK configurations

The CORKs to be deployed during Expedition 327 differ in configuration depending on their intended use and the expected hole geometry, but they have several design elements in common (Fig. F9). CORKs are designed to seal open holes so that thermal, pressure, and chemical conditions can equilibrate after the dissipation of drilling disturbance. CORKs also facilitate the collection of fluid and microbiological samples as well as temperature and pressure data using autonomous samplers and data logging systems and serve as long-term monitoring points for large-scale crustal testing. Expedition 327 CORKs will include a seafloor reentry cone and casing hanger(s); concentric (nested) casing strings that penetrate through sediments and allow access to underlying basement; a series of seals (both between casing strings and between casings and the formation) that hydraulically isolate the open crustal interval at depth from the overlying ocean; downhole and seafloor instrumentation for collection of samples and data; and a seafloor wellhead that includes valves, fittings, electrical connections, and a landing platform so that the observatory can be serviced by submersible or ROV, allowing samples and data to be retrieved without recovery of the complete observatory assembly.

Expedition 327 CORKs differ somewhat from earlier systems. All CORKs will use a two-packer system for each borehole seal at depth. A hydraulic packer (inflated by pumping during deployment) will be supplemented with a swellable packer; adjacent hydraulic and swellable packer elements will be run in tandem. The hydraulic packer will provide an immediate seal to monitor short-term formation pressure response and limit the continued inflow of cold bottom water, whereas the swellable element will provide assurance that the seal lasts for at least several years. All Expedition 327 CORKs will be configured with a mixture of sample and monitoring lines with different diameters and construction materials to meet particular purposes. These CORKs will include three perforated drill collars at the bottom of the inner CORK casing to provide ~10,000 lb of weight that will help to pull the CORKs into the holes. The perforated collars and a section of perforated 5½ inch casing above the collars will be coated with nonreactive material to reduce the extent of contamination resulting from interactions between borehole fluids and steel. The CORKs to be installed at proposed Site SR-2 include a lateral casing section that extends at an angle from below the seafloor seal to a 4 inch ball valve placed in a wellhead instrument bay above the seafloor seal ("L-CORK") (Fig. F9). This ball valve will be opened by submersible or ROV after a year of system equilibration to allow free flow of overpressured basement fluids, which will permit large-volume sampling and a cross-hole hydrogeologic experiment.

The Hole SR-2A CORK will have two monitored intervals at depth, extending to a maximum depth of ~515 meters below seafloor (mbsf) (~260 msb) (Fig. F10). Both basement intervals will be monitored for pressure, and a three-way valve at the seafloor will permit spot measurement of the interval isolated within the annular gap between the 10¾ and 16 inch casing strings. Fluids from both depth intervals in basement will be sampled using seafloor OsmoSampler systems installed on the wellhead, and a separate polytetrafluoroethylene (PTFE, a Teflon variant) sampling line will be dedicated for microbiological sampling. A landing seat will be placed at depth inside the 4½ inch inner CORK casing for future deployment of a bottom plug, but no bottom plug will be deployed during Expedition 327 so that this hole can be used for the long-term free-flow experiment by opening the wellhead ball valve.

The Hole SR-2B CORK is preferred for use in the long-term cross-hole experiment, but the L-CORK design used here will also be used in Hole SR-2A for redundancy. The Hole SR-2B CORK will have one monitored interval at depth, extending to a maximum depth of ~325 mbsf (~70 msb) (Fig. F11). The basement interval will be monitored for pressure, and a three-way valve at the seafloor will permit spot measurement of the interval isolated within the annular gap between the 10¾ and 16 inch casing strings. Fluids from depth in basement will be sampled using seafloor OsmoSampler systems installed on the wellhead, and a separate PTFE sampling line will be dedicated for microbiological sampling. A landing seat will be placed at depth inside the 4½ inch inner CORK casing for future deployment of a bottom plug, but no bottom plug will be deployed during Expedition 327 so that this hole can be used for the long-term free-flow experiment using the wellhead ball valve. The CORK instrument string that hangs inside the 4½ inch casing will contain a combination of OsmoSamplers for collection of fluids and gases, microbial growth substrate, and autonomous temperature loggers.

The Hole 1027C CORK will have two monitored intervals at depth, extending to a maximum depth of ~675 mbsf (~60 msb) (Fig. F12). The basement intervals will be monitored for pressure, and a dedicated pressure gauge will also be used to monitor the interval isolated within the annular gap between the 10¾ and 16 inch casing strings (to evaluate mechanical system compliance). Fluids from depth in basement will be sampled using seafloor OsmoSampler systems installed on the wellhead, and a separate PTFE sampling line will be dedicated for microbiological sampling. Two landing seats will be placed at depth inside the 4½ inch inner CORK casing, and two internal plugs will be deployed to assist with isolation of the different depth intervals in basement. Both basement intervals will be monitored using instruments inside the 4½ inch CORK casing, with perforated and coated casings providing borehole fluid access to the samplers deployed on the instrument string. The CORK instrument string will contain a combination of OsmoSamplers for collection of fluids and gases, microbial growth substrate, and autonomous temperature loggers.

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