CORK: A Hydrologic Seal and Downhole Observatory for Deep-Ocean Boreholes

Abstract

A new tool has been developed and successfully deployed that provides a means by which cased reentry holes drilled by the Ocean Drilling Program can be hydrologically sealed and instrumented. The seal prevents flow of water into or out of a hole that would otherwise cause severe thermal and chemical disturbance to the formation drilled. The seal is capable of withstanding both positive and negative differential pressures that may be present in hydrothermally or tectonically active environments, and can be removed for later drilling operations. The instrumentation, developed for initial deployments during Ocean Drilling Program Leg 139, was designed to monitor the formation temperatures and pressures for up to 2 years during and following the recovery from drilling disturbances. A recording gauge measures absolute pressure in the borehole below the seal. A sensor string containing 10 thermistors and a fluid-sampling tube hang in the hole below the recording package. The sampling tube extends through the seal to a port, allowing the differential pressure to be measured and fluids to be tapped from deep within the hole. Data recovery and fluid sampling can be done via submersible or remotely operated vehicle.Two units were deployed in the sediment-filled Middle Valley rift of the northern Juan de Fuca Ridge. The first was installed in Hole 857D, which was drilled to a total depth of 936 meters below seafloor, through 470 m of turbidite sediment and into a highly permeable sediment-sill complex that is inferred to be a sediment-sealed hydrothermal "reservoir." The second was deployed in Hole 858G, which was drilled to a total depth of 433 meters below seafloor into a buried volcanic edifice that underlies a hydrothermal vent field. Data were recovered successfully from both holes about three weeks after deployment. Temperatures and pressures recorded in Hole 858G show that formation conditions are severely affected by drilling and by downward fluid flow into the formation through a nearby uncased exploratory hole that was not adequately sealed with cement. The differential pressure across the seal in Hole 858G was initially -0.24 MPa, and at the time of the data recovery was becoming increasingly negative. Temperatures and pressures recorded in Hole 857D appear to be recovering slowly from drilling disturbances. Initially, the differential pressure across the seal in this hole was -1.10 MPa; at the time of the data recovery, the differential had reduced to -0.55 MPa. The instruments will continue to monitor formation pressures and temperatures once per hour for 2 years.

Long-term observations of pressure and temperature in Hole 948D, Barbados accretionary prism

“…1; Table 1), was suspended in Hole 948D from a circulation obviation retrofit kit (CORK; Davis et al, 1992) boreseal. This string uses two serial lines to communicate digitally between the sensors and the data logger, allowing for a larger number of sensors to be connected than in previous CORK experiments.…”

Nitrogen cycling in the deep sedimentary biosphere: nitrate isotopes in porewaters underlying the oligotrophic North Atlantic

“…Sediment cores were retrieved using the advanced piston corer, which penetrated the seafloor sediments until contact with basement, followed by extended core barrel coring of the upper section of basement rock. Site 2B (∼ 90 m sediment thickness, depth to basement) is located in the deeper part of the pond, approximately 25m away from DSDP (Deep Sea Drilling Project) legacy hole 395A, which was instrumented as a CORK (Circulation Obviation Retrofit Kit) observatory (Davis et al, 1992). Site 3D (∼ 42 m sediment thickness, depth to basement) lies in the northeastern region towards the edge of North Pond (∼ 5.9 km away from U1382A), whereas site 4A (∼ 95 m sediment thickness, depth to base- ment) is located on the northwest side in a deeper part of the basin, approximately 3.9 and 6.2 km in distance from U1383 and U1382, respectively (Fig.…”

Design, deployment, and status of borehole observatory systems used for single-hole and cross-hole experiments, IODP Expedition 327, eastern flank of Juan de Fuca Ridge

“…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 longterm monitoring points for large-scale crustal testing, including formation response to perturbation experiments Davis et al, 1992a;. 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 evolu-…”