IODP

doi:10.2204/iodp.pr.321T.2009

Background

Ocean Drilling Program (ODP) Leg 168 completed a drilling transect of eight sites across 0.9–3.6 Ma seafloor east of the Juan de Fuca Ridge; collected sediment, rock, and fluid samples; determined thermal, geochemical, and hydrogeologic conditions in basement; and installed a series of CORK observatories in the upper crust (Davis, Fisher, Firth, et al., 1997). Two of the Leg 168 observatories were placed in 3.5–3.6 Ma seafloor near the eastern end of the drilling transect, in ODP Holes 1026B and 1027C (Fig. F1). Expedition 301 returned to this area and drilled deeper into basement; sampled additional sediment, basalt, and microbiological materials; replaced the borehole observatory in Hole 1026B; and established two multilevel observatories at IODP Site U1301 for use in long-term, three-dimensional hydrogeologic experiments.

Hole 1026B was drilled to 295 meters below seafloor (mbsf), cased across the sediment/basement interface, and extended to 48 meters subbasement (msb) during Leg 168 (Davis, Fisher, Firth, et al., 1997). The original CORK installed in Hole 1026B included a data logger, pressure sensors, thermistors at multiple depths, and a fluid sampler, all of which (except for the fluid sampler, which fell deeper into the hole) were recovered in 1999. The Hole 1026B CORK was never completely sealed after being installed in 1996, and because crustal fluids are overpressured with respect to ambient hydrostatic conditions at ODP Site 1026 (e.g., Davis, Fisher, Firth, et al., 1997), this hole discharged fluid for years until it was replaced during Expedition 301. As of the start of Expedition 301, warm (~64°C) altered basement fluid vented freely through the top of the wellhead. The original Leg 168 CORK installed in Hole 1026B was replaced successfully during Expedition 301.

Site U1301 was positioned 1 km south-southwest of Site 1026, where sediment thickness is 260–265 m above a buried basement high (Fig. F1). Hole U1301A was drilled without coring to 370 mbsf (107 msb). The casing was extended into the upper 15 m of basement, but poor hole conditions prevented installation of longer casing, coring, or deeper drilling. 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.

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 casing was installed to 85 msb. Basement was cored from 86 to 318 msb, with mean recovery of 30%, a value typical for upper basement rocks from young crust. The upper 100 m of the cored interval in Hole U1301B was irregular in diameter, often much larger than the maximum inflation diameter of packers used for hydrogeologic testing and CORK observatories. However, the lower 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.

Both of the Site U1301 boreholes contained four nested casing strings: 0.50 m (diameter = 20 inches) casing in the uppermost sediments, 0.41 m (16 inches) casing extending just across the sediment/basement interface, 0.27 m (10¾ inches) casing extending into basement, and a 0.11 m (4½ inches) inner CORK casing that houses instrument strings and plugs (Fig. F2). The two largest casing strings were sealed by collapse of unconsolidated sediments, and the 0.41 m string was also cemented across the sediment/basement interface. The annulus between 0.41 and 0.27 m casing strings at Site U1301 was supposed to contain a rubber mechanical casing seal near the seafloor, but this component was not available for use during Expedition 301 as planned. An attempt was made to seal the 0.27 m casing strings at depth with cement, but rubbly basement prevented this cement from sealing between the casing and borehole wall. Operations were additionally complicated in Hole U1301B by the separation of the unwelded 0.27 m casing string into two sections, leaving a gap just above the sediment/basement interface (Fig. F2B). The CORK installed in Hole U1301A included a casing packer (as part of the 0.11 m inner casing) that was set inside 0.27 m casing. In contrast, the CORK installed in Hole U1301B included two casing packers set in open hole, intended to hydraulically isolate sections of the upper crust (Fig. F2B).

Expedition 301 CORKs and the preexisting CORK in Hole 1027C were visited with the ROV ROPOS soon after the drilling expedition in September 2004 and again in September 2005 with the submersible Alvin. Data recovered during these dives showed that the Hole 1026B observatory was sealed and operating as intended, although the pressure in Hole 1026B was recovering slowly from the thermal perturbation associated with 8 y of upflow of warm formation fluid (from when this CORK observatory had been unsealed, prior to Expedition 301). The CORKs in Holes U1301A and U1301B were incompletely sealed, allowing cold ocean-bottom water to flow into the formation following CORK installation. The flow of cold water into the crust at Site U1301 caused a measurable pressure perturbation at ODP Site 1027, 2.4 km away, comprising an inadvertent crosshole test (Fisher et al., 2008).

Attempts to seal Hole U1301B using a cement delivery system with the submersible Alvin in summer 2006 and 2007 were unsuccessful. The submersible could not deliver a sufficient quantity of cement to the cone. Shimmering fluid was observed discharging from Hole U1301A during and after summer 2007 dive operations. No such evidence for upflow from the borehole was observed during earlier visits, suggesting that Hole U1301A must have "turned around" sometime between 2006 and 2007 servicing operations. In fact, downhole temperature loggers recovered from Hole U1301A in summer 2008 provide a detailed record of this flow reversal. Remarkably, Hole U1301B continued to draw fluid rapidly into basement as of summer 2008, even though it is located just 36 m from Hole U1301A, which is vigorously discharging warm formation fluid to the ocean. Understanding of the pressure and thermal interactions between Holes U1301A and U1301B, and implications for local and regional crustal hydrogeology, will require additional investigation. Sealing both observatory systems at the seafloor is important for completing the experimental and monitoring program begun with Expedition 301 (Fisher et al., 2005).

The ROV platform installed at Hole U1301A was based on an older design, with a solid surface perforated by eight 12 inch diameter holes (Fig. F3A). Screens were welded below these holes prior to platform installation so that instrumentation being deployed or manipulated by a submersible or ROV would not fall through the holes. In contrast, the ROV platform at Hole U1301B has a series of radial support arms covered by the same screen material as was welded to the bottom of the platform at Hole U1301A (Fig. F3B). By the time the ROV platform was deployed at Hole U1301B, Expedition 301 participants realized that it would likely be necessary to cement the cone to achieve a hydraulic seal around the CORK, and a slot was cut through the screen to facilitate this operation. Several barrels of cement were pumped into the cone near the end of Expedition 301, after passing the end of the drill string through the slot in the ROV platform, but the quantity of cement available at that time was insufficient to achieve a seal.