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doi:10.2204/iodp.proc.327.107.2011

Configuration of Expedition 327 CORKs

In this section we document the configuration of two new CORKs and associated instrumentation deployed during Expedition 327. In the next section we summarize the status of CORK systems deployed during Leg 168 and Expedition 301 and subsequently modified through drillship and/or seafloor servicing using a submersible or ROV.

Hole U1362A

The configuration of the CORK deployed in Hole U1362A is summarized in Figure F13 and Tables T1, T2, T3, T4, and T5. An engineering drawing showing hole, casing, and CORK configurations in Hole U1362A is included with this volume (see Fig. F3 in the “Site U1362” chapter and COMPLTN in “Supplementary material”). The Hole U1362A CORK was designed to monitor two basement intervals: a deeper interval extending from the top of the “in-gauge” section of hole to total depth and a shallower interval extending from the base of 10¾ inch casing to the top of the packers at the top of the deeper interval. The initial design for Hole U1362A was based on experience gained drilling Hole U1301B (800 m to the south-southwest; Fig. F1), which suggested that although the uppermost ~200 m of basement would be rubbly, hole conditions should improve at greater depth. In fact, drilling conditions and the rate of penetration, core recovery, and wireline logging data from Hole U1362A (see the “Site U1362” and “Expedition 327 summary” chapters) all indicated that the shallow interval of rubbly basement extended to a somewhat greater depth in Hole U1362A than in Hole U1301B. The interval of consistently better conditions began near 450 mbsf (~220 msb), but when this was determined there was not enough open hole below this depth to confidently deploy the complete CORK system. For this reason, the open hole CORK packers that make up the boundary between deeper and shallower basement intervals were placed in an in-gauge section at ~419–429 mbsf (~183–193 msb) (Fig. F13). When the Hole U1362A CORK was deployed there was ~49 m of open hole below the end of the CORK bullnose.

Downhole OsmoSamplers, microbiological experiments, and temperature loggers were deployed inside the Hole U1362A CORK. The geochemical and microbiological instruments are located entirely within perforated and coated drill collars and casing, where they are monitoring the deeper of the two isolated intervals. Temperature loggers are located within this interval and at shallower depths inside nonperforated casing in basement and sediments (Fig. F13; Tables T3, T4).

In addition to monitoring two basement intervals, the pressure valves in the Hole U1362A CORK wellhead were configured at deployment to continuously monitor seafloor pressure as a means to assess sensor drift (Fig. F14A). The gauge currently monitoring seafloor pressure can be switched to spot-check pressure within the annular gap between 16 and 10¾ inch casing during CORK servicing. This gap should be sealed at the top by a casing hanger seal and a swellable casing packer (Fig. F4A, F4B) and at the base of the 10¾ inch casing by cement (Fig. F2). Even if the cement seal at depth is incomplete, the top seals should still be sufficient to determine pressure conditions in upper basement.

Geochemical sampling valves on the Hole U1362A CORK wellhead were deployed closed (Fig. F14B), as were the large-diameter flowmeter valve and microbiology sampling valves (Fig. F14C). Seafloor OsmoSampler systems will be attached in the geochemistry bay and these valves will be opened during future CORK servicing expeditions, and short-term or long-term sampling using the microbiology valves can also be initiated. We are currently not planning to open the ball valve in the flowmeter bay on the Hole U1362A CORK during summer 2011 operations, but a final determination will be made during these dives once the status of the full CORK network is verified.

Hole U1362B

The configuration of the CORK deployed in Hole U1362B is summarized in Figure F15 and Tables T1, T2, T3, T4, and T5. An engineering drawing showing hole, casing, and CORK configurations in Hole U1362A is included with this volume (see Fig. F4 in the “Site U1362” chapter and COMPLTN in “Supplementary material”). The Hole U1362B CORK was designed to monitor a single basement interval that partly overlaps the shallowest interval monitored in Hole U1362A (Fig. F13; Table T1). Because Hole U1362B was not planned to penetrate to a depth sufficient to create an in-gauge section of 9⅞ inch hole, packers were set only inside 10¾ inch casing, but perforated collars and casing extended beyond this depth into basement drilled using 14¾ and 9⅞ inch tricone bits (Fig. F15). When the Hole U1362B CORK was deployed there was ~36 m of open hole below the end of the CORK bullnose.

Downhole OsmoSamplers and microbiological experiments were deployed entirely within perforated and coated drill collars and casing. Temperature loggers are located within this interval and at shallower depths inside nonperforated casing in basement and sediments (Fig. F15; Tables T3, T4).

In addition to monitoring a single basement interval, the Hole U1362B CORK was configured on deployment to continuously monitor pressure within the annular gap between 16 and 10¾ inch casing (Figs. F15, F16A). Both gauges are run through valves that can be switched to spot-check seafloor pressure during periodic servicing as a means to assess instrument drift. In addition, a third (dedicated) gauge was configured in the Hole U1362B CORK to monitor seafloor pressure.

Geochemical sampling valves on the wellhead were deployed closed (Fig. F16B), as were the large-diameter flowmeter valve and microbiology sampling valves (Fig. F16C). OsmoSampling systems will be attached in the geochemistry bay and these valves will be opened during future CORK servicing expeditions, and short-term or long-term sampling using the microbiology valves can also be initiated. We currently plan to open the large-diameter ball valve on the Hole U1362B CORK during summer 2011 operations and attach an autonomous flowmeter to determine the rate of fluid egress from the wellhead for the following 1–2 y. The pressure response in basement to this free-flow perturbation will be monitored using CORKs at Sites 1026, 1027, and U1301 and in Hole U1362A.