DESCRIPTION OF CORK-II COMPONENTS

OsmoSamplers were designed and built to sample fluid continuously throughout the duration of each deployment. Significant design modifications to previously deployed OsmoSamplers (Wheat et al., 2000b; H. Jannasch, pers. comm., 2001) and those deployed at Sites 1024 to 1027 of ODP Leg 168 (Wheat et al., 2000a) allow the samplers to be recovered and redeployed within pressurized horizons. A newly developed OsmoSampler seat permits the OsmoSamplers to be replaced without losing formation pressure.

An OsmoSampler (Figs. F1, F2) consists of an osmotic pump originally described by Theeuwes and Yum (1976) and a sampling coil (Figs. F3, F4). The pump does not require any electrical power and has no moving parts. Flow within the pump is driven by the osmotic pressure gradient across a semipermeable membrane that separates solutions of different salinity. Membranes are chosen with an appropriate pore size to allow water to diffuse through the membrane while restricting the passage of dissolved salts. The difference in the salt concentration between the two solutions bounding the membrane drives a net diffusion of water, and thus flow, through the membrane from the fresher to the more saline side. The magnitude of the salt gradient, a function of the concentration difference and membrane thickness, controls the osmotic pressure and resulting flow rate. Rates of water flow across the membrane are dependent on membrane area, thickness, porosity, and number of membranes in the pump, as well as the osmotic pressure gradient, temperature, and the diffusion coefficient of water. The osmotic pressure is maintained by keeping a saturated salt (NaCl) solution with excess salt on one side of the membrane while maintaining a solution with no salt (distilled water) on the other side. Housings of the osmotic pumps are made of acrylic because its thermal expansion coefficient is similar to that of water. A discussion of technical details can be found in Jannasch et al. (1994).

The osmotic pump draws sample fluid into and through a long spool of small-bore (0.8 to 1.2 mm inside diameter [ID]) Teflon tubing. The tubing is initially filled with degassed distilled water with the lower end open to the formation fluid to be sampled. Up to several hundred meters of Teflon tubing are wrapped onto spools that are then connected in series for long-term sampling (Fig. F5). Samples for gas analyses are drawn into copper tubing with an ID of 1.2 mm. Coils used during Leg 205 all contain ~300 m of Teflon or copper tubing with an ID of 1.2 mm for the fluid samplers and 0.8 mm for the flowmeter spools. Ten membranes deliver a pump rate of 4 mL per week at a temperature of 10°C. After retrieving the OsmoSamplers, the formation fluids are obtained by cutting the Teflon tubing into sections of desired length based on required fluid volume and desired temporal resolution, extracting the fluid, and analyzing it for chemical species of interest. Copper tubing is frozen, crimped, and cut. A vacuum line is then used to extract dissolved gases from the sample and partition those gases into several containers for future gas analyses. Typically, sections are cut into 1-m-long sections, which provide ~0.5 and 1.0 mL of sample from an 0.8- and 1.2-mm-ID tubing, respectively. Laboratory tests in which sample input alternated between seawater and modified seawater confirm that dispersion resulting from diffusion and peak smearing is not significantly greater than that calculated from molecular diffusion alone (Jannasch et al., unpubl. data).

The OsmoSampler packages for Hole 1255A (Leg 205) are capable of being recovered and redeployed without losing pressure within the sampling environment. This is accomplished with a new custom OsmoSampler seat, which has a plunger-seal design that is penetrated by a titanium sampling probe. The OsmoSampler seat is located within 4-in casing (4 in outside diameter [OD], 10-lb/ft K-55 casing), which, along with the packer on the outside, completely seals the formation pressure. This seat has a 1-in hole sealed with a plunger that can be pushed down by the sampling probe to expose a 12-in section of screen open to the pressurized formation. An inner screen on the OsmoSampler seat is located within the tightly fitting outer-perforated 4-in casing, which is located below a packer (Fig. F6). The outer screen is 8 m long to concentrate any fluid flow within its depth range to the 12-in inner screen. The solid vertical support ribs keep flow from evading the casing and bypassing the OsmoSampler intakes. The outer casing is filled with Carbolite and should seal to the formation after its collapse (Fig. F6). Without an OsmoSampler (e.g., when the OsmoSamplers are being exchanged), the plunger is pulled up inside the OsmoSampler seat and isolates the pressurized zone from the overlying hydrostatic pressure.

In this configuration, OsmoSamplers are encapsulated in a pressure housing that sits above the OsmoSampler seat and is rigidly attached to the titanium sampling probe. A total of nine ¹/₁₆-in OD tubes connect the various sampling ports within the probe to the OsmoSamplers within the pressure housing. The pressure housing is maintained at the pressure of the sampling zone by an additional ¹/₈-in OD pressure equilibration line that expels a pressure equilibration fluid within the pressure housing at the same rate as water is being sampled throughout deployment. This eliminates any pressure gradient that the OsmoSamplers would need to pump against. The chemistry of the pressure equilibration fluid was chosen to mimic that of the in situ pore fluid in the uppermost 50 m of the underthrust sediment (Site 1040 of Leg 170) plus 10% of the sum of the major component concentrations used (Cl, SO₄, Mg, Ca, K, and Na) added as NaCl. The overall density of the pressure equilibration fluid is ~3% higher than that of the average underthrust in situ pore fluid, and it is injected below the sampling horizons for fluid chemistry and flow rate determinations. This minimizes any possible mixing or contamination. Cesium is used as a tracer to quantify any mixing that may occur between the pressure equilibration and the formation fluids. Cesium was chosen as the tracer for this fluid because of its low concentration in seawater and the accurate and precise determinations of low Cs concentrations at nanomolar levels possible by inductively coupled plasma-mass spectrometry (ICP-MS). A concentration of 12.118 mM, equivalent to 2000 ppm Cs, was chosen (5.4 x 10⁶ seawater concentration), which ensures analytical determination after dilution upon introduction into the sampling probe.

The OsmoSampler packages for Site 1253 also sample the pressurized formation but use a simpler pressure seal above two OsmoSampler packages that are suspended within the hole on a line with a sinker bar. This permits sampling at two depth horizons, but the seal will be broken during replacement of the OsmoSamplers. The upper OsmoSampler package directly below the plug seal is located within a section of screened pipe to ensure retrieval in case the hole collapses. This is similar to the CORK deployed at Site 1200 (Shipboard Scientific Party, 2002b), where OsmoSamplers were protected with a section of screened casing. This change was initiated because hole instabilities at ODP Sites 1025 and 1026 entombed OsmoSamplers that had been deployed there. The lower OsmoSampler package and the sinker bar hang near the bottom of the open hole below the screen. Two weak links (steel plates sewn together with ¹/₈-in polypropylene rod to avoid corrosion) located below each OsmoSampler package ensure that in case of collapse as much of the system as possible, and at a minimum, the upper OsmoSampler package, will be retrieved.

An OsmoFlowmeter was incorporated into the titanium sampling probe for Hole 1255A (Leg 205). The flowmeter consists of two perpendicular -in holes bored through the sampling probe. A tracer solution is added directly at the center, and four OsmoSamplers sample 1 cm away from the tracer input in each of the four branches (Figs. F7, F8) to determine both direction and flow rate of pore waters through the formation. The tracers are injected continuously from a coil connected to the brine output of one of the OsmoSamplers. Thus, the tracer is injected at the same rate as the intake of the four-membrane OsmoFlowmeter samplers (1.6 mL/week). These OsmoFlowmeters will measure the direction by presence and concentration of the tracer, and the relative flow rate will be calculated from the dilution factor. The directional flow will be limited to relative directions, since the true geographical orientation of the OsmoSampler package would require a compass.

The tracer fluid consists of an artificial pore fluid spiked with rubidium and iodate (IO₃^-) tracers. Rubidium was chosen as the primary tracer because of its low concentration in seawater and ease of measurement by way of ICP-MS. Iodate was chosen as a redundant tracer because of its conservative behavior in pore fluids and ability to measure low concentrations colorimetrically.

The bulk chemistry and, thus, density of the artificial pore fluid spiked with tracers was chosen to match that of the décollement and underthrust sediments at Sites 1254 and 1255 (Sites 1040 and 1043 of Leg 170, respectively). Interstitial water chemistry data obtained during Leg 170 were used to calculate an average pore water chemistry for both horizons. Chloride, SO₄, Mg, Ca, K, and Na concentrations in Hole 1040C of Leg 170 were averaged between the depths of 229.33 and 357.73 meters below seafloor (mbsf) for the décollement and between the depths 401.18 and 452.18 mbsf for the upper section of the underthrust sediments. These representative concentrations were used to estimate the mass of salt needed to add to the flowmeter output volume to produce the same chemistry and relative density as the Site 1254 pore fluids. This ensured that the artificial pore fluid would not sink or diffuse away upon injection. For the tracer compounds, higher concentrations than those in the pore fluid and seawater are desired so that they are not masked by the pore fluid concentrations and are measurable at high precision if diluted by fluid flow in the borehole. The following concentrations were thus used: 23.99 µM for Rb (~17x seawater) and 100 µM for IO₃^- (221.7x seawater). Even with 100x dilution of the tracers upon injection into the sampling probe, their concentrations can be determined with high precision.

The OsmoFlowmeter design assumes that the formation will collapse around the outer screen; otherwise, much of the pore fluid would flow around the outer screen. This collapse should be observable in the data. Solid vertical bars between the outer screen and 4-in casing are intended to focus the flow from the 8-m section of screen to the 12-in section of inner screen and sampling area. The 4-in casing under the outer screen (8 m long) is only perforated in the area of the inner screen (~12 in long). The tight tolerance between the inner and outer screens, as well as -in flowmeter holes in the sampling probe, should further focus the flow. The holes for the sampling and pressure equilibration tubes are, therefore, only ¹/₈ in.

Inside the end caps of the OsmoSampler housings, we installed a miniaturized temperature data logger (MTL) (Pfender and Villinger, 2002). The MTL consists of a 140-mm-long x 15-mm-OD cylindrical data logger housing with a thin-walled tip (20 mm long with an OD of 4 mm) containing the temperature sensor (Fig. F9). The pressure housing consists of high-strength corrosion-resistant steel and withstands a pressure equivalent of 6000 m water depth. Programming the logger and downloading the data are performed without opening the pressure case. A readout unit contacts the logger's tip and end cap with a voltage delivered by an RS232 interface from a PC. A high-strength plastic washer isolates the tip and main body to allow a two-point connection for data transfer.

The electronics of the logger consist of a microprocessor, a 16-bit analog-to-digital converter, a real-time clock, and nonvolatile memory for up to 64,800 measurements. The sample interval can be varied from 1 s to 255 min. The complete system is powered by a standard 3-V lithium battery. A thermistor (interchangeability of 0.1 K) is used as a sensing element. The characteristics of the sensor provide a temperature range from -5° to +60°C and a resolution of 1 mK at typical deep-sea temperatures of 2°C. The absolute accuracy of the logger after calibration with a high-precision thermometer in a well-stirred water bath is <5 mK, which is adequate for the expected temperature fluctuations.

All loggers installed in the OsmoSampler packages have been calibrated in an absolute sense before deployment. They were programmed to take a measurement every 17 min, which allows recording of temperatures over a period of 2 yr. Only limited experience exists with long-term deployments of the MTLs. However previous deployments of over ~1 yr in shallow water in the Gulf of Mexico did not reveal significant problems with drift of the electronics or the internal clock. It is yet to be seen how long the batteries will last and how well the pressure housing will withstand corrosion over a period of 2 yr.

The package for pressure measurements and data logging is very similar to the one described in the "Explanatory Notes" chapter of the Leg 196 Initial Reports volume (Shipboard Scientific Party, 2002a). Therefore, we will describe it only very briefly. The package, designed to be removable and serviceable if required, weighs 115 kg in water and includes the data logger in its pressure case, two pressure sensors connected to the monitoring lines and one for monitoring seafloor pressure variations, an eight-pin underwater-mateable electrical connector, and interconnecting cables (Figs. F10, F11). The pressure sensors, manufactured by Paroscientific, Inc., employ matched pairs of quartz crystals, one sensing pressure and the second adding temperature compensation (Fig. F12). The total range of the sensors used during Leg 205 is 70 MPa (7000 m equivalent water depth). The actual pressure resolution realized is dependent upon the integration time of the pressure gauge, which is set to 10.7 s to achieve a resolution of 1 ppm (equivalent to 70 Pa). Absolute accuracy is limited by sensor calibration and drift. Experience from previous multiyear deployments shows that drift is typically <0.4 kPa/yr. This and an absolute calibration inaccuracy (~5 × 10^-4 of total pressure or 23 kPa at the Costa Rica sites) are dealt with well by the intergauge hydrostatic checks both prior to final installation and later at times of submersible visits.

Temperature is measured with a thermistor mounted to the inside of the pressure housing of the data logger. Its temperature range is from 0° to 150°C, with a sensitivity of ~5.6 counts/mK at 2°C. Data are acquired during an interval of 1 hr, which is adequate to observe low-frequency temperature fluctuations at the seafloor with a sensor that has a large time constant resulting from its position at the inside of a thick-walled pressure case.

Sensors are activated at a user-specified interval, and data are recorded with a logger built by Richard Brancker Research, Ltd., of Ottawa, Canada. The logger has modular capability to store up to 32 channels of pressure and temperature data. Time-tagged pressure data are recorded in 8-MB-capacity flash memory. Logging rates are programmable at sampling intervals ranging from 10 s to 1 day. With three gauges being logged at 10-min intervals at Leg 205 sites and temperature being logged once an hour, memory capacity will provide 6 yr of operation. Data recovery and reprogramming of the logger is possible via a serial link and the underwater-mateable eight-pin connector. Power is supplied by lithium sulfuryl chloride battery packs at 7.4 V with a capacity of 360 A·hr, sufficient at this rate of logging for more than the shelf life of the batteries (15 yr). Logging can be extended beyond the life of the batteries by applying power from an external source through the underwater-mateable connector.

The inflatable packers used to seal the outside of the 4-in casing to the formation are essentially identical to those used during Leg 196 except for diameter. They were constructed by TAM International, Inc., around standard-diameter 8.48-m-long casing sections. The elements themselves consist of 3-m-long steel-reinforced (vertical stave) nitrile composition rubber bladders, rated to 100°C and effective to 140°C in this application. The elements are attached to the casing core at the bottom of the packer and to a sealed sliding sleeve at the top. The sleeve rides on an annular volume through which the monitoring and packer inflation lines pass. The bladders are designed to expand from their ID of 8 in to a maximum of 14 in. A differential inflation pressure of ~150 psi (1 MPa) is required to overcome the rigidity of the elements. The bladders are filled using the -in inflation tube in the hydraulic umbilical. Flow is passed from the tube into a plenum that feeds the bladder of each packer through a pair of valves. The first valve is in an initially closed state and opens when a critical pressure in the plenum (relative to the local annular pressure) is reached, at which point filling begins. The inflation pressure is set by a shear-wire valve to a value of 600 psi that locks in an open position when activated. For the Leg 205 CORK-IIs, these valves were set too close when the internal pressure in the packer bladders rose to a total of 600 psi (4.1 MPa) relative to the local annular pressure (i.e., ~3 MPa above the pressure required to expand the packer itself).

To inflate the packer, the drill string is pressured up to 800 psi (5.5 MPa) and pressure is held for 30 min. After 30 min, the drill string pressure is increased to 1800 psi (12.4 MPa) and held for 10 min to activate the spool valves to connect the pressure sensors in the wellhead to the downhole screens (see discussion below). The last step consists of bleeding off the pressure through the rig floor standpipe manifold relief valve.

Hydrologic access to the formation was provided by 7.6-m-long screen filters on 11.68-m casing joints, manufactured by Houston Wellscreen, Inc. Granular fill is packed in a 2-cm annulus between the outside of a solid section of 4-in casing and a screen formed of wire wrapped on radial webs (Fig. F13). The OsmoSampler is centered inside the screened section. The pressure monitoring line accessing the filter is terminated in a separate, smaller (2 cm OD × 1 m length) wire-wrapped screen located within the screened section. Carbolite, an aluminum oxide ceramic, was used for the filter fill, with a grain size of 400-600 µm, a porosity of ~30%, and a permeability of ~2 × 10^-10 m². Laboratory experiments indicated that the Carbolite does not interfere with the fluid chemical data. The screen was wound with 0.085-in wire with a 0.01-in wire-to-wire spacing and provided an effective open cross section of 15%. The design was intended to provide good hydrologic communication to the formation with maximum effective contact area and permeability while preventing sediment from invading and clogging the sampling or monitoring lines. The risk of clogging during installation was further reduced by having the monitoring lines closed to prevent flow through the screens. The monitoring lines open by the action of the spool valves following packer inflation, as described below.

Transmission of pressure signals to the seafloor sensors is accomplished using thick-walled, 316-L stainless steel tubing of in OD and 0.035 in wall thickness. One smaller diameter ( in OD, 0.03125 in wall thickness) line was provided for sampling fluids from screens in the horizon of interest. All lines, including the tube for packer inflation, are jacketed with polyurethane to form a single robust umbilical, provided by Cabett Services, Inc. Connections were made during deployment between the umbilical and the tubes leading through or from each packer or screen (Figs. F14, F15). In intervening sections, the umbilical was banded to the outside of the casing sections. A detailed discussion of the tubing dimensions chosen can be found in the "Explanatory Notes" chapter of the Leg 196 Initial Reports volume (Shipboard Scientific Party, 2002a).

The CORK-II head is a 30-in-diameter cylindrical frame fabricated from steel around a section of 11-in casing. It houses components in two of the three 120°-wide, 60-in-high bays that are bounded above and below by circular horizontal bulkheads and divided from one another by radial webs (Fig. F11). Two bays contain (1) the sensor/logger/underwater-mateable connector assembly on a demountable frame and (2) the spool valves, pumping/sampling valves, three-way pressure sensor valves, and geochemical sampling valve and port. The lowermost bulkhead is positioned ~16 in above the submersible landing platform that covers the reentry cone. Numerous cutouts on the vertical webs can be used as manipulator "handholds" for the same purpose. At the top of the CORK-II head is a small reentry cone for wireline tool delivery systems.

At the CORK-II wellhead, the packer inflation line, pressure monitoring lines, and fluid sampling lines are all routed to several destinations (Figs. F11, F16, F17, F18, F19, F20). The packer inflation line is connected to the CORK-II running tool by a hydraulic hose terminated by a quick release with integral check valve. When the quick release is engaged, the integral check valve is held open, maintaining an open hydraulic circuit through the CORK-II running tool to the inside of the drill string. During deployment the packer inflation line and the internal voids of the packer body are thus allowed to fill and equalize pressure with the increasing hydrostatic pressure by way of the open quick-release check valve. After the assembly has reached the target depth, a wireline retrievable tool (go-devil) is used to divert all drill string flow and pressure into the packer inflation line. Once the go-devil is in place, the drill string pressure is increased inside the CORK-II running tool, thereby increasing the packer inflation line pressure. At a predetermined pressure (~800 psi), an "open" valve (integral to the packer) opens, allowing inflation of the packer element to occur. Once a predetermined packer element inflation pressure (~600 psi) has been reached, a "close" valve (integral to the packer) closes, preventing any fluid from either escaping or entering the packer element. The hydraulic hose quick-release check valve also acts as a redundant "close" valve in that when the quick release is disconnected from the CORK-II wellhead, the check valve automatically closes, preventing any loss of fluid or pressure from the packer inflation line.

Another branch of the packer inflation line leads from the sliding sleeve valve to a manifold that passes the packer inflation line pressure to a bank of locking spool valves, one for each screen. Two positions of the spool valves, controlled by the packer line pressure, provide two different routings among screen lines, pressure sensor lines, and a local hydrostatic port. During deployment, drilling, and packer filling operations, lines from the screens are closed and lines from the pressure sensors are routed to a local hydrostatic port. Venting the sensor lines prevents damage to pressure sensors from any excess pressures that might be produced during drilling and packer inflation and allows a local hydrostatic calibration point to be established before the sensors are connected to the monitoring lines. Keeping the screen lines closed prohibits flow through the screens and minimizes the potential for infiltration of fine-grained material into the Carbolite-packed screens during installation. The spool valves are set to shift when the packer inflation line pressure reaches 1125 psi (7.8 MPa) (i.e., once the packer is inflated). Once they shift and lock, the spool valves close the hydrostatic port and interconnect the screen and pressure sensor lines for monitoring. The spool valves are designed to shift with no volumetric change so that no pressure pulse is generated that could potentially damage the pressure sensors.

A manual means for pressure sensor protection and calibration is provided by three-way valves plumbed into the pressure sensor lines downstream from the valved pumping and sampling ports described below. In their normal position, pressure from the screens (via the spool valves) is routed directly to the pressure sensors. In the second position, the sensor lines are opened to hydrostatic pressure and the screen lines are closed. Thus, these serve the same function as the spool valves but with manual control. The sensors can be isolated temporarily if pumping experiments are ever performed. Hydrostatic reference checks can be done at the time of any submersible or ROV visit, and the screen lines can be closed to prevent drainage of the formation if the logger/sensor unit is ever removed.

In addition to the lines leading from the screens to the spool valves and into the pressure sensors, there are also lines leading to valved pumping/sampling ports where fluid samples can be collected and pumping tests performed. One other pumping/sampling port leads from a local "T" junction in the sampling port bay to a monitoring line. If any pumping tests are ever carried out in the future, the three-way pressure sensor isolation valves would need to be switched to protect the sensors.

Before deployment of the CORK-II, all sampling valves and bleed valves on the wellhead are opened and the wellhead is lowered into the water to purge the hydraulic lines of air. The ¹/₁₆-in hydraulic lines connecting the pressure sensor control valves to the pressure sensors are filled with water prior to picking up the wellhead. Following immersion, the wellhead is raised back to the moonpool level where all valves are closed. Large rubber bands are then attached to the individual valve handles such that they would hold the valves in the closed position during the deployment. This is done to prevent the valves from partially opening during the deployment, as happened with the CORK-II valves deployed during Leg 196. With all hydraulic lines purged and all valves closed and after a last-minute inspection of the wellhead completed, the CORK-II assembly is ready to be lowered to the seafloor. In a final step, the CORK-II is latched permanently into the reentry cone and the packer is inflated, and the spool valves are shifted to connect the pressure sensors to the pressure ports in the screens.