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doi:10.2204/iodp.pr.328.2010

Site U1364 ACORK details

The primary components of the ACORK system deployed in Hole U1364A are shown in Figs. F13–F16. Four screens are centered at 156.3, 203.2, 243.6, and 303.6 mbsf, two above and two below the gas/gas hydrate boundary at ~230 mbsf and all within the accretionary prism lithologic unit (Fig. F7). Details of the various components are described as follows:

Screens

Hydrologic access to the formation was provided by 2.03 m long screen filters on 6.08 m long casing joints (Fig. F13). Carbolite granulate is packed in a 1.55 cm annulus between the outside of a solid section of 10¾ inch casing and a screen formed of wire wrapped on radial webs. Hydraulic lines leading to deeper intervals pass straight through the filters. The monitoring line accessing each filter is perforated along its length within the corresponding screen. Carbolite, an aluminum oxide ceramic, was used for the filter fill, with a grain size of 400–800 µm, a porosity of ~30%, and a permeability of 5 × 10^–10 m². The screen was wound with triangular 316 stainless steel wire with a surface width of 2.8 mm and a gap width of 0.4 mm, providing an effective open cross section of 12.5%. 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.

Hydraulic tubing

Formation pressure signals are transmitted to seafloor sensors via ¼ inch outer diameter 0.035 inch wall 316 stainless steel hydraulic tubes laid in a flat format and jacketed with polyurethane to form a single, robust umbilical (Fig. F14). Swaged connections were made during deployment between the umbilical and the tubes leading through or from each screen. Unused tubes in each section of umbilical between screens were capped at their bottom ends to prevent hydraulic communication between screens and left open at their tops to prevent collapse and damage to the rest of the umbilical. The umbilical was banded to the outside of the casing sections at roughly 4 m intervals. The tubing was chosen on the basis of what was felt to be a reasonable compromise between hydraulic capacitance, resistance, and robustness. Capacitance and resistance of the formation (the inverse of the product of these being equivalent to the hydraulic diffusivity) are likely to be high in parts of the sediment section. Hence, to transmit pressure variations over a broad range of frequency with no distortion requires the observation system to have very low resistance and capacitance. The quartz pressure sensors are essentially incompressible, and the compressibility of the thick-walled tubing can also be ignored; the water filling the tubing is the primary source of the observing system compliance. Reducing the internal diameter of the tubing is advantageous in that lesser amounts of water are required to flow in and out of the formation to transmit pressure signals, but only up to the point when the translation of fluid begins to be frictionally influenced by the tube wall. Given expected formation signal amplitudes, the dimension of the tubing chosen is much larger than necessary; the limiting factor was the size that could be handled without fear of clogging with either fine sediment that might invade the screens or grease and constrictions at the tube joints that might occur at the time of deployment. A quantitative discussion of the transmission of signals in context of formation permeability is provided in Davis and Becker (2007).

Wellhead configuration

The ACORK head is a 30 inch diameter cylindrical frame fabricated from ⅜ inch steel around a section of 10¾ inch casing. It provides space for instruments, wiring, and plumbing in each of three 120° wide, 78 inch high bays bounded at the top and bottom by circular horizontal bulkheads and divided from one another by radial webs (Fig. F15). All components are contained in a single bay in the Hole U1364A installation, including the sensor/logger/underwater-mateable connector assembly on its demountable frame and three-way pressure sensor valves (Fig. F16). The lowermost bulkhead is positioned ~16 inches above a submersible landing platform that covers the main 4.5 m diameter reentry cone. The ACORK running tool receptacle fitting at the top of the ACORK head doubles as a 23¾ inch diameter reentry funnel for bridge-plug and wireline instrument installations. A larger auxiliary reentry funnel was deployed to facilitate reentry into the 10¾ inch ACORK casing, but it failed to seat and fell to the seafloor after the bottom-hole assembly (BHA) was removed (see "Operations summary").

Pressure monitoring instrumentation was installed in the wellhead frame on the ship and deployed with the ACORK casing string (Fig. F16). Pressure-balanced underwater-mateable hydraulic connectors allow the instrument package to be removed and replaced by submersible or remotely operated vehicle (ROV) in the event that repairs or service are ever required. Three-way valves connect the umbilical lines to the instrumentation. In the "interval" position, these connect the formation to the sensors; in the "seafloor" position, the formation lines are closed and the sensors are connected to the ocean, allowing periodic checks on drift. A critical step in the assembly operation of every deployment is to purge air from all lines. In quantities too large to be absorbed by the local volume of water, trapped air will greatly increase the compressibility of the system and thus reduce the fidelity of the response to high-frequency formation pressure variations. Purging is done through lockable check valves at the highest point of the wellhead plumbing by submersing the fully assembled ACORK system below the moonpool immediately prior to deployment. Lines and couplers between the sensors and the three-way valves were purged when the instrument system was mounted to the wellhead frame. Pressure tests were carried out to test against leakage; results of those tests are shown in Figure F17.

Sensors and logging electronics

The logging instrumentation includes individual sensors (Paroscientific 8B4000-2 and 8B4000-1 quartz depth sensors) to monitor pressures at the seafloor and at each of the formation screens (Table T1). Frequency signals from these sensors are digitized with high-resolution (~1 ppb frequency or 10 ppb full scale pressure = 0.4 Pa, equivalent to 0.04 mm of water head) low-power Precision Period Counter (PPC) cards (Bennest Enterprises, Ltd.). The records shown in Figure F17, collected during a plumbing leakage test prior to deployment, give a sense of measurement resolution. Absolute accuracy is limited by sensor calibration and drift. Experience from previous multiyear deployments shows that drift is typically <0.4 kPa/y. Drift and calibration inaccuracy (~5 × 10^–4 of total pressure or roughly 10 kPa at Site U1364) are dealt with by intergauge hydrostatic checks immediately prior to installation and later at times of submersible visits using the wellhead three-way valves. Bottom water temperature is measured by a temperature-sensitive quartz oscillator of one of the Paroscientific sensors, as well as with a highly stable platinum thermometer. Time-tagged frequency and temperature data are stored by a MT-01 (Minerva Technologies, Ltd.) data logger that utilizes a low-power SanDisk 512 megabyte flash memory card. On-board power is supplied by 12 DD lithium-ion batteries having a total capacity of 210 A-h, sufficient to power the system for roughly 10 y at a sampling period of 1 min (user programmable). An onboard voltage detection circuit will automatically switch the system into a high-rate (1 Hz) sampling mode and idle the batteries when external power from a NEPTUNE connection is made (anticipated within the first year of operation). Serial RS422 communications with the instrument and an external power feed is accommodated via a seven-contact Teledyne ODI underwater-mateable connector (Fig. F16). The instrument pressure case is built of 4130 alloy heat-treated steel, pressure tested for use to a 2800 m water depth following deep submersible research vessel (DSRV) Alvin certification specifications. The total weight of the instrument assembly is ~43 kg in water.

Other downhole instruments planned for future installations will be wireline-deployed inside the 10¾ inch casing using a submersible or remotely operated vehicle. These will include a thermistor cable, tilt sensors, and a seismometer.

Data format and calibration

The CORK's MT-01 data logger is configured to make real-time data from the RTC/PPC (real-time clock + controller/precision period counter) measurement system available via an RS-422 serial interface and concurrently to store all readings in memory. Data transmission rates for real-time access as well as data download are configured to be 115,200 baud. Parameter files documenting the instrument's configuration can be reviewed at the CORK Observatory Software repository (www.corkobservatories.org/). Software packages mentioned below can also be downloaded.

An example of the real-time data as it will be received by NEPTUNE Canada once a cable connection is established is provided in Table T2. Binary data records, containing a timestamp, the logger ID, the logger housing temperature, the readings from the pressure sensors, and a trailing zero byte, are written to compact flash memory. These are converted to hexadecimal and sent to the RS-422 interface as lines of ASCII characters.

Data stored in the logger can be downloaded using a conventional communications terminal program, but for speed and ease of use reasons we highly recommend the use of the "mlterm" software available at the Web site given above. The downloaded *.raw files contain the complete custom file system of the logging unit, which must be stripped from the data records prior to any further processing. This is done using the "mlbin" software also available from the Web site.

Given the complex CORK setup, with four Type I (pressure only) and one Type II (pressure and temperature) Paroscientific pressure sensors, the "mldat" software that is usually used to convert raw data to engineering units cannot be used. Python code, which can handle the necessary calibrations of more complex CORK setups, was developed for this purpose (see the CORK Observatory Software repository). To ensure the usability of the data in the future, all necessary information to convert the raw data to engineering units is given below:

Timestamp (4 bytes)

Times are recorded in seconds since 1-Jan-1988 00:00:00.

Logger ID (1 byte)

The ID (08) is the hexadecimal representation of the ID byte; it is associated with the RTC electronics.

Internal temperature (logger housing) temperature (3 bytes)

The logger/RTC/PPC pressure housing temperature is measured using a platinum sensor mounted to one endcap with thermal contact compound. The conversion from readings (x) to housing temperatures (T_h) in degrees Celsius is done using

For the sensor used (serial number 92), the calibration coefficients are a = –2.95416e–006°C and b = 40.1418°C.

Pressure sensor temperature (4 bytes)

Pressure sensor temperature data are recorded for the seafloor sensor. The sensors for the formation are capable of recording only pressures; this provides enhanced pressure resolution, reduced power consumption, and reduced stored data quantity. The conversion of AD counts x_TF from the pressure sensor temperature channel is done in three steps. First, the counts are converted into the period X (µs) of the temperature oscillator (a conversion that is the same for all PPC measurement devices):

In a second step, the difference U between the measured period X and the period U0 at 0°C is computed:

U₀ is a calibration coefficient that differs for each pressure sensor (Table T3). The differential period U is used to compensate the pressure channel of the same sensor (see below) and to compute the sensor temperature (T_p) using the calibration coefficients Y_i (Table T3):

Formation pressures (4 x 4 bytes)

Formation pressures measured at screen #1 (deepest) to screen #4 (shallowest) are measured using Type I Paroscientific pressure sensors (no temperature-dependent frequency signal). For the temperature compensation of the pressure signal from these sensors, an external temperature measurement must be supplied. For this purpose, we use the temperature measured by the seafloor pressure sensor, which is collocated with the other pressure sensors and should have a similar thermal time constant. From our experience, the logger pressure housing temperature has a much smaller time constant than the pressure sensors—the quartz oscillators (both pressure and temperature) are by design thermally well isolated within the Paroscientific pressure sensor housings. Therefore, use of the logger housing temperature for temperature compensation would induce high-frequency noise in the calculated pressures. For Type I sensors, the compensation factor U equals the temperature in degrees Celsius we wish to compensate for. Otherwise, the same calibration procedure as for Type II sensors (see below) applies.

Seafloor pressure (4 bytes)

The seafloor pressure sensor is the only Paroscientific Type II sensor (temperature and pressure channel) of the CORK. The temperature compensation factor U for this sensor is the differential period X – U₀ and not the temperature in degrees Celsius as for Type I sensors (see above). If the temperature period for a Type II sensor is not available, U can be computed from an externally provided probe temperature T_p using the following equation (cf. Table T3 for calibration coefficients):

Given a temperature compensation factor U, the conversion of logged PPC counts x_PF from a sensor pressure channel is done in three steps. First the counts are converted into periods τ (µs) of the pressure oscillator, a conversion that is the same for all PPC pressure channels:

Next, a set of three compensation factors (C, D, and T₀) is computed based on the calibration coefficients C_i, D_i, and T_i, which are provided by Paroscientific Inc. for each pressure sensor (Table T3):

and

Finally, the temperature-compensated pressure P (in psia) is computed from the factors above:

To convert the compensated pressures into dBar or kPa, they must be multiplied by 0.6894757 dBar/psia or 6.894757 kPa/psia, respectively.

Trailing zero (1 byte)

As shown in Table T2, each sample record in the binary data is terminated by a zero byte. In the ASCII RS-422 output the records are terminated by carriage-return line-feed characters and thus appear on separate registered lines. The constant number of bytes between the ID byte and the trailing zero byte helps to identify the start and end points of data records, in the event that the structure of the binary data is disrupted (e.g., by incomplete data records).

Screen spacing

At the simplest level, the four monitoring points enumerated above will allow determinations of the average vertical pressure gradient generated by prism thickening and driving vertical fluid flow, along with the contrast in gradient between the section above and below the level of gas hydrate stability associated with a contrast in permeability if one exists. The combination of the 2.03 m length of the screens and their ~50 m separation should make such gradient determinations relatively insensitive to localized heterogeneities associated with fractures, turbidite layering, and lenses of massive gas hydrate accumulation.

Data from below and above the gas/gas-hydrate boundary will also constrain the contrast in mechanical properties of gas- and gas hydrate–bearing sediments and provide independent information about the effective permeabilities of the sections above and below the boundary. The way this can be done is summarized in Figure F18, which begins with a schematic illustration of how variable loading either at the seafloor (e.g., tides and ocean waves) or within the formation (tectonic strain and seismic waves) is transmitted to formation pore water and how local contrasts in loading response causes transient pressure gradients to be established (Fig. F18A). The instantaneous (elastic) response to seafloor loading = γ (referred to as the loading efficiency) (Fig. F18B) depends on porosity, Poisson's ratio, the compressibility of the solid grain constituents, the compressibility of the sediment or rock framework, and the compressibility of the interstitial fluid or fluid + gas mixture. With the first three of these being well known, absolute values and contrasts in observed loading efficiency can be used to constrain the effects of gas on the elastic properties of the fluid (and hence gas content) and the effect of gas hydrates on the elastic properties of the matrix (and hence average gas hydrate content).

In simple cases where a sharp mechanical properties contrast is present (e.g., at the seafloor or at the gas/gas hydrate boundary), a transient pressure gradient will be established and interstitial water will flow (Fig. F18A). A damped diffusional wave will propagate away from the boundary, adding a component to the signal (Fig. F18B) that decays with distance (Fig. F18C). At large distances, the response is purely elastic (γ) and constrains such things as the matrix compressibility, the gas content (Fig. F18D), and the coefficient that defines how tectonic deformation loads the interstitial water (Fig. F18E). At intermediate distances, the characteristic diffusion scale length, l (Fig. F18C), depends on the hydraulic diffusivity of the formation, η, and the period of the loading signal, P, as

The broad bandwidth of ocean wave and tidal loading, for which periods range from seconds to weeks, combined with the distribution of the screens around the gas/gas hydrate boundary, should allow much to be learned about variations in formation elastic and hydrologic properties.

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