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doi:10.2204/iodp.pr.328.2010

The Site U1364 ACORK observatory configuration

Background

Starting in the late 1980s, ODP engineers and scientists developed the CORK hydrologic observatory system to seal cased boreholes at the seafloor and allow the formation state to be monitored continuously for years after drilling (Davis et al., 1992). In 1991, the first CORKs were installed in two holes drilled through sediment and into igneous basement in the Middle Valley rift of the northernmost Juan de Fuca Ridge (Davis and Becker, 1994), with the goal of determining the natural hydrologic state of this hydrothermally active setting (i.e., after the large perturbations from drilling and open-hole conditions had dissipated). Other sites followed, including two in the Cascadia accretionary prism (Davis et al., 1995), two in the Barbados accretionary prism (Becker et al., 1997; Foucher et al., 1997), four on the eastern Juan de Fuca Ridge flank (Davis and Becker, 1998, 1999), one on the western Mid-Atlantic Ridge flank (Davis et al., 2000; Becker et al., 2001), and one in a Mariana forearc serpentinite diapir (Salisbury, Shinohara, Richter, et al., 2002; Wheat et al., 2008). Although the specific objectives of these installations have differed, all of the monitoring experiments have made use of the basic CORK capabilities to monitor seafloor and formation temperatures and pressures over long periods of time. Beginning with the Barbados installations, fluid samplers were added to some of the temperature sensor cables to allow continuous sampling of deep-formation water. At several of the CORK sites, additional sampling and hydrologic testing experiments have been completed using valves that allow controlled access to the sealed sections (e.g., Screaton et al., 1995, 1997).

These original CORKs succeeded in preventing borehole flow, allowed formation pressures to be measured accurately, and allowed temperatures to be determined at multiple depths. However, use of only a single seal meant that the formation pressure observed and fluid samples collected reflected averages over the open sections of the holes (i.e., either the entire length of open hole beneath casing in igneous basement or the length of perforated casing within sediments). Determinations of local pressure gradients were not possible. This limitation, along with the success of the initial CORK design, stimulated interest in enhancing the capabilities to include monitoring of multiple zones in a single hole and to permit a greater range of sensor capabilities.

These objectives were addressed with the ACORK (Mikada et al., 2002; Becker and Davis, 2005), designed for a variety of applications and installed for the first time during Leg 196 for a long-term monitoring experiment in the Nankai subduction zone. Other variations of the CORK system followed (e.g., Jannasch et al., 2003; Becker and Davis, 2005; Fisher et al., 2005). All take greater advantage of individual boreholes by utilizing packers and screens to allow pressures to be observed at multiple levels, just as is often done in terrestrial hydrologic experiments. The ACORK system consists of a 10.75 inch outer diameter, 10.05 inch inner diameter casing string with modular screened monitoring elements positioned at desired depths. The initial ACORKs included hydraulically inflated packer elements to isolate screens, although experience from Nankai monitoring showed that collapse of the formation provided good hydrologic isolation in intervals where no packers were used and experience elsewhere demonstrated that hydraulically inflated packers were ineffectual for long-term use. Pressure monitoring and/or fluid sampling is done via a multiline hydraulic umbilical strapped to the outside of the casing. Hydraulic lines from each level pass successively through packers and screens above and are then plumbed into a seafloor framework that houses sampling and testing ports, pressure sensors, and data loggers. Once the casing with its packers and screens is in place and a bridge plug is installed to seal the casing at its deepest point, the inside of the ACORK string becomes hydrologically isolated from the formation. In this way, access to the hole inside the sealed casing is provided for downhole instruments to the total depth of the installation without perturbing the pressure monitoring.

Most CORK installations to date have been configured to meet a broad suite of requirements, including passive geophysical monitoring, active hydrologic testing, and formation-fluid chemical and microbiological sampling (see reviews by Kastner et al., 2006; Becker and Davis, 2005; Fisher et al., 2005). Unfortunately, large perturbations can occur when fluids are produced from the formation, particularly when monitoring screens are situated in low-permeability material. Direct effects arise from any pressure drop associated with production, and indirect effects arise from thermal perturbations caused by flow (see discussion in Davis and Becker, 2007). The latter can be caused by the anomalous buoyancy of the water in the umbilical that connects the screens to the seafloor sensors and from transient thermal expansion of the fluid in the umbilical and screens that is confined by the low-permeability material surrounding the screens. To avoid these problems, the observatory at Site U1364 will be devoted to passive geophysical monitoring exclusively; fluid sampling and other formation-interactive experiments devoted to gas hydrates are being planned in separate devoted holes roughly 3 km to the southeast.

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