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doi:10.2204/iodp.proc.327.107.2011

Motivation, background, and overview of experimental design

Long-term borehole observatory systems

Subseafloor borehole observatory systems (“CORKs”) are designed to (1) seal one or more depth intervals of a borehole so that thermal, pressure, chemical, and microbiological conditions can equilibrate following the dissipation of drilling and other operational disturbances; (2) facilitate collection of fluid and microbiological samples and temperature and pressure data using autonomous samplers and data logging systems; and (3) serve as long-term monitoring points for large-scale crustal testing, including formation response to perturbation experiments (Becker and Davis, 2005; Davis et al., 1992a; Wheat et al.). The development of CORK systems was motivated by the desire to address a broad range of scientific objectives that have been the focus of scientific ocean drilling for decades, particularly those associated with submarine hydrogeology, lithospheric and hydrothermal fluid evolution, tectonic processes and geohazards, and the nature of the deep microbial biosphere. In many cases, CORKs provide the only available technology for collecting data and samples that are essential for developing a process-based understanding of these subseafloor systems.

The CORKs developed for Integrated Ocean Drilling Program (IODP) Expedition 327 share features with systems deployed during earlier drilling expeditions, particularly Ocean Drilling Program (ODP) Leg 205 and IODP Expedition 301. However, substantial modifications were made to the Expedition 327 CORK designs on the basis of specific technical and scientific goals of planned experiments and on experience gained since Expedition 301. In this paper, we describe the design and deployment of two new CORK systems during Expedition 327. We also provide a status update on CORK systems deployed during ODP Leg 168 and IODP Expedition 301, reporting on how they were configured as of the start of Expedition 327. Finally, we describe plans for using these systems over the next several years during a series of multidisciplinary experiments. We begin with a brief overview of the motivation for Expedition 327 and related (earlier) drilling and nondrilling expeditions and experiments.

ODP Leg 168 and IODP Expeditions 301 and 327 and related experiments

During ODP Leg 168 a transect of eight sites was drilled across 0.9–3.6 m.y. old seafloor east of Juan de Fuca Ridge to collect sediment, rock, and fluid samples; determine thermal, geochemical, and hydrogeologic conditions in basement; and install four CORK observatories in the upper crust (Davis, Fisher, Firth, et al., 1997). Two of these CORKs were placed in 3.5–3.6 m.y. old volcanic oceanic crust near the eastern end of the drilling transect in Holes 1026B and 1027C (Fig. F1). Expedition 301 returned to this area, drilled deeper into basement at Site U1301 (Fig. F1), and conducted single-hole experiments; sampled sediment, basalt, and microbiological materials; replaced the observatory in Hole 1026B; and established two multilevel observatories in Holes U1301A and U1301B for use in long-term cross-hole experiments (Fisher et al., 2005). The primary goals of Expedition 327 were to (1) drill two new basement holes at Site U1362 (Fig. F1), core and wireline log one of these holes across a depth range of 100–360 meters subbasement (msb), conduct a 24 h pumping test, initiate a tracer-injection experiment, and install multilevel CORKs; (2) recover the existing CORK installed in Hole 1027C, deepen the hole by 40 m, and install a new multilevel CORK with instrumentation; and (3) recover and replace the instrument string deployed in the CORK in Hole U1301B. Work in Hole 1027C could not be completed during Expedition 327, but other primary and secondary objectives were achieved (see the “Expedition 327 summary” chapter).

Considerable time and resources were applied between Expeditions 301 and 327 to service the CORK observatories in Holes 1026B, 1027C, U1301A, and U1301B. The samplers and loggers deployed in the Expedition 301 CORKs had operational life spans of 4–6 y; all of the seafloor and subseafloor instruments originally installed in these systems had to be replaced prior to the start of Expedition 327 so that these systems could be used to monitor single- and cross-hole response for several years following drillship operations. In addition, the CORKs installed in Holes U1301A and U1301B during Expedition 301 were not sealed as intended (Fisher et al., 2005), and data and samples collected during subsequent remotely operated vehicle (ROV) and submersible servicing operations showed that both observatory systems were leaking (Davis et al., 2010; Fisher et al., 2008). Several attempts were made to seal these systems using cement delivered initially with a submersible, and eventually with the R/V JOIDES Resolution during IODP Expedition 321T (Fisher and IODP Expedition 321T Scientific Party, 2010).

Experimental borehole configurations

During Leg 168, Hole 1026B was drilled to 295 meters below seafloor (mbsf), cased across the sediment/basement interface, and extended to 48 msb (Shipboard Scientific Party, 1997). Upper basement was unstable in this location, and it was necessary to deploy an old piece of drill pipe as a liner across uppermost basement to help keep the hole open for subsequent monitoring and fluid sampling. The original CORK installed in Hole 1026B included a data logger, pressure sensors, thermistors at multiple depths, and a fluid sampler; all were recovered in 1999 except for the fluid sampler and sinker bar, which were stuck in the hole. Prior to the initial CORK installation in 1996, the hole produced water at a rate of ~100 L/min (Fisher et al., 1997); after the CORK installation, flow persisted at ~3 L/min until a ½ inch valve was opened for microbiological sampling in 1997, at which time flow increased to ~8 L/min (Davis and Becker, 2002). When the original data logger was removed in 1999, warm (~64°C) altered basement fluid flowed rapidly (at an unknown rate) up through the 4 inch inside diameter (ID) logger landing seat until the full CORK body was replaced during Expedition 301 in summer 2004. The instrument string deployed in the new Hole 1026B CORK was recovered and replaced with a new thermistor string, fluid samplers, and microbiological growth experiments in 2008. The loggers for the thermistor string and the pressure gauges were connected to the NEPTUNE cable system in 2009, and both are providing real-time data that are accessible through the Internet (www.neptunecanada.ca/). The Hole 1026B CORK is currently leaking with an uphole flow rate of ~0.8 L/min. The cause of the present leak is unknown.

Also during Leg 168, Hole 1027C was drilled 2200 m east of Hole 1026C, where sediment thickness is 575 m above buried basement (Fig. F1). Hole 1027C penetrated to 632 mbsf. The upper part of the hole was cased through sediments and into uppermost basement, leaving 54 m of open hole. The open interval near the base of Hole 1027C comprises a diabase sill, intercalated sediments, and basalt breccia overlying 26 m of extrusive volcanic rocks (Shipboard Scientific Party, 1997). The CORK installed in Hole 1027C during Leg 168 included a data logger, pressure sensors, thermistors at multiple depths, and a fluid sampler. These instruments were retrieved in 1999, and the pressure logging system was replaced. In contrast to Hole 1026B, Hole 1027C was naturally underpressured with respect to ambient hydrostatic conditions (Davis and Becker, 2002). Hole 1027C was fully sealed and recorded formation pressure until 2008, when an attempt was made to connect a new-generation pressure data logger to the hole using the Aeroquip connector on the fluid sampling line. The hydraulic coupling was incomplete, causing hydrostatic pressures to be recorded until the sampling valve was closed in 2009.

Site U1301 was positioned 1 km south-southwest of Hole 1026B, where sediment thickness is 260–265 m above a buried basement high (Fig. F1) (Expedition 301 Scientists, 2005b). Hole U1301A was drilled without coring to 370 mbsf (107 msb). Casing of 10¾ inch outside diameter (OD) was extended into the uppermost 15 m of basement, and the hole reached a maximum total depth of 107.5 msb. A depth check prior to CORK deployment in Hole U1301A revealed that much of the lower part of the hole had filled in with rocks from the rubbly formation around the hole (Expedition 301 Scientists, 2005b).

Hole U1301B was positioned 36 m away from Hole U1301A and penetrated to a total depth of 583 mbsf (318 msb). Uppermost basement was drilled without coring, and 10¾ inch steel casing was installed to 85 msb. Basement was cored from 86 to 318 msb. The uppermost 100 m of the cored interval in Hole U1301B was irregular in diameter, often much larger than the maximum inflation diameter of packers to be used for hydrogeologic testing and CORK observatories. However, the lowermost ~100 m of the hole was stable and to gauge, allowing collection of high-quality wireline logs and providing several horizons suitable for setting drill string and CORK casing packers (Expedition 301 Scientists, 2005b).

Hole U1362A was positioned 235 m south-southwest of Hole 1026B (Fig. F1), where the sediment thickness is 236 m. As at Site U1301, both sediment and uppermost basement were drilled and cased without coring, with 10¾ inch casing installed to 72.5 msb. Hole U1362A was cored to 260 msb and then drilled to 292 msb to make sure there was sufficient open hole to emplace a multilevel CORK system. Prior to CORK installation this hole was logged with a single string of geophysical instruments and tested with a drill string packer to assess near-hole hydrogeologic properties (see the “Site U1362” chapter).

Hole U1362B was positioned 311 m south-southwest of Hole U1362A and 514 m north-northeast of Hole U1301B (Fig. F1), where sediment thickness is 242 m. Hole U1362B was originally planned to be drilled close to Hole U1362A (as with Holes U1301A and U1301B), but because Hole U1301A continued to vent formation fluid as of the start of Expedition 327, the location of Hole U1362A was shifted to the south, closer to Site U1301. This should provide a better chance for recovery in Hole U1301A of fluids and tracers injected in Hole U1362B and will allow crustal conditions between Holes U1362A and U1362B (and between Holes U1362A and 1026B) to be assessed. Sediment and uppermost basement in Hole U1362B were drilled and cased without coring, with the base of the 10¾ inch casing placed at 30 msb. Basement below the casing was drilled without coring to 117 msb prior to a 24 h pumping and tracer injection experiment (Fisher, Cowen, et al.), followed by installation of a single-level CORK system with instrumentation (see the “Site U1362” chapter).

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