ACORK
BOREHOLE HYDROGEOLOGIC OBSERVATORIES
Starting in the late
1980s, ODP engineers and scientists developed instrumentation to hydrologically
seal-cased boreholes at the seafloor, simultaneously emplacing long-term
instrumentation in the sealed holes. These hydrogeologic observatories are
called CORKs (Davis et al., 1992; Davis and Becker, 1993). CORKs (Fig. F8)
allow the formation state to be monitored continuously for years after drilling,
with the aim of understanding in situ hydrological processes after recovery from
drilling-induced disturbances, which have been documented to persist for years
in unsealed reentry holes. In 1991, the first CORKs were installed during Leg
139 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). 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 most recently one in a Mariana forearc
serpentinite diapir (Salisbury, Shinohara, Richter, et al., 2002). While the
specific objectives of these installations have differed, all of the monitoring
experiments have made use of the basic CORK capabilities to monitor temperatures
at multiple depths and seafloor and formation 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 average formation pressures to be
measured accurately, and allowed temperatures to be determined accurately at
multiple depths. However, use of only a single seal meant that the formation
pressure observations and fluid samples were integrated over the open sections
of the holes, either from the entire open hole sections of igneous basement
beneath the casing or through sections of perforated casing within sediments. In
other words, determinations of local pressure or compositional gradients were
not possible other than for the total interval from the seafloor to the level of
the observation. This limitation, as well as the success of the initial CORK
design, stimulated great interest in enhancing the capabilities to include
monitoring of multiple zones in a single hole, corresponding more closely to
natural hydrogeologic structures, with a greater range of sensor capabilities.
These objectives have now
been addressed with the ACORK (Becker and Davis, 1998, 2000), designed for a
variety of applications and installed for the first time during Leg 196 for a
long-term monitoring experiment in the Nankai Trough accretionary prism. The
design takes greater advantage of individual boreholes by utilizing packers and
screens to allow pressures and compositions to be observed at multiple levels,
just as is often done in terrestrial hydrologic experiments. The system consists
of a 10.75-in outside-diameter (OD), 10.05-in inside-diameter (ID) casing string
with modular packer and screened monitoring elements positioned at desired
depths. Pressure monitoring and fluid sampling is done via a multiline hydraulic
umbilical strapped to the outside of the casing. The hydraulic lines from each
level pass successively through packers and screens above, then are plumbed into
a seafloor framework that houses sampling and testing ports, pressure sensors,
and data loggers. The numbers of packer and screen elements are limited only by
cost, deployment logistical considerations, and the space needed for the total
number of hydraulic lines that must pass through the uppermost element. 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, full-diameter access is
provided for downhole instruments to the total depth of the installation without
perturbing the pressure monitoring. A small-diameter reentry cone at the top of
the ACORK frame allows instrument installations at any time by wireline. It also
allows reentry from a drilling vessel for hole deepening or sensor installations
below the bridge plug and casing.
Summary
of Installation and Configuration
The sandy turbidites
deposited along the Nankai Trough axis are unstable, and casing was required to
prevent collapse of this portion of the holes, both for LWD and ACORK
operations. Hence, the first step in completing the ACORK installations was to
install reentry cones and 20-in casing to 121 mbsf in Hole 1173B and 157 mbsf in
Hole 808I (Fig. F9A).
LWD operations followed immediately, with penetration to 3 m below the top of
basement in Hole 1173B and to ~30 m below the décollement zone in Hole 808I
(Fig. F9B).
Although basement was targeted at Site 808, hole instability at the décollement
zone and the resistance to circulation caused by centralizers above the logging
tool assembly limited penetration to just below the décollement zone.
The 9
-in LWD holes completed during the first three weeks of Leg 196 served as
"pilot holes" and were enlarged to 17
in for the ACORK assembly (Fig. F9C).
Formation conditions were well defined by the experience gained through both the
LWD and hole-opening operations, and the combination of LWD data with coring and
wireline logging results from Legs 131 and 190 defined the targets for ACORK
monitoring intervals and the total depths that could be reached. The scientific
rationale for the choice of specific monitoring zones and packer depths for the
installations in Holes 808I and 1173B is described in the site chapters in this
volume.
After each hole was
opened, the ACORK string, including screens, packers, hydraulic lines, seafloor
frame, sensors, and data logger (described in detail below), was made up for
reentry. The string was hung off on a BHA that included an ACORK running tool,
pipe, drill collars, a mud motor, and a pilot bit plus underreamer with
collapsible arms, positioned just below the deepest section of the ACORK string,
to clear constrictions in the hole and fill that may have accumulated at the
bottom. This assembly was lowered to the seafloor, the hole was reentered, and
the ACORK string was drilled into place (Fig. F9D).
Once the target depth was reached, the packer inflation sequence was completed
(Fig. F9E), the
running tool was decoupled (Fig. F9F),
a remotely operated vehicle (ROV) platform was lowered into place, and the drill
string was recovered, leaving a fully installed ACORK (Fig. F9G).
Packers
The inflatable packers
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
initial diameter of 15 in to a maximum of 22 in. A differential inflation
pressure of ~150 psi (1 MPa) is required to overcome the rigidity of the
elements.
The bladders are filled
with the -in inflation
tube in the hydraulic umbilical. Flow is passed from the tube into a plenum that
feeds both the continuation of the inflation line and 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. This pressure is set with
shear wires selected for each packer such that the inflation occurs in a desired
sequence. We chose to inflate from bottom to top, assuming that the annulus
would probably remain at near-hydrostatic conditions until a packer was inflated
overhead. Attempting a different inflation sequence would cause uninflated
packers to be trapped in confined intervals, which could inhibit inflation or
cause hydrofracturing. Bottom-up inflation optimizes the chances for annular
water to escape as packers are filled and for the packers to inflate in a
hydrostatic environment.
The final inflation
pressures are set by a second shear-wire valve that when activated locks in a
closed position. No flow into or out of the packer can occur after this point.
For the Leg 196 ACORKs, these valves were set to 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., roughly 3 MPa above the pressure required to
expand the packer itself), with the exception of the shallowest two packers at
Site 1173, which were inflated to a lower total of 400 psi (2.75 MPa) to avoid
fracturing the weaker formation at that level. The fracture limit, defined by a
combination of the overburden, tensile strength, deviatoric stress, and fluid
pressure, is difficult to assess with any confidence. The inflation values were
believed to be a conservative compromise between the risks of fracturing (under
conditions of high formation pressure and confined conditions in the hole
annulus) and incomplete inflation.
The inflation procedure is
done in stages, with line pressure (defined at the rig floor downstream of the
circulation pumps) increased initially to a pressure between the first- and
second-element filling valve activating pressures, then to a second pressure
between the second and third, etc. Pressure is held long enough at each stage to
allow sufficient flow through the -in
inflation line to fill each packer. If the annular pressure is close to
hydrostatic, the sequence proceeds as planned. If any of the packers becomes
isolated in an overpressured zone by hole closure above, packer filling is
delayed until the line pressure allows the differential across the opening valve
(line pressure to annulus) to exceed the preset shear-wire threshold. Filling
then proceeds until the bladder-to-annulus differential reaches the closing
valve threshold. With this possible eventuality in mind, line pressure is
increased and held at a level equivalent to lithostatic pressure for 30 min at
the end of the filling sequence.
Screens
Hydrologic access to the
formation was provided by 7.6-m-long screen filters on 11.24-m casing joints,
manufactured by Houston Wellscreen, Inc. Sand is packed in a 2-cm annulus
between the outside of a solid section of 10
-in
casing and a screen formed of wire wrapped on radial webs. Hydraulic lines
leading to deeper intervals pass straight through the filters. The sampling
and/or monitoring line accessing each filter is terminated in a separate,
smaller (2-cm OD x 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 x
10-10 m2. The screen was wound with 0.085-in wire with
0.10-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 action of spool valves activated during the packer inflation, as
described below.
Tubing
Transmission of pressure
signals to the seafloor sensors was accomplished using thick-walled, 316-L
stainless steel tubing of 
-in
OD and 0.035-in wall thickness. One smaller diameter (
-in OD, 0.028-in wall thickness) line was provided for sampling fluids from
screens in the décollement zone or stratigraphic equivalent of the décollement
zone. All lines were laid in a gentle helical wind around the -in
OD (0.049-in wall thickness) packer inflation line, then 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. In intervening sections the
umbilical was banded to the outside of the casing sections.
The tubing was chosen to
satisfy a variety of requirements. The packer line diameter was chosen to allow
individual packers to be filled in ~10-15 min at the typical line pressures
applied. Monitoring lines were chosen on the basis of what was felt to be a
reasonable compromise between capacitance and resistance. Capacitance and
resistance of the formation (the inverse of the product of these being
equivalent to the hydraulic diffusivity) are likely to be very high in parts of
the sediment section. Hence, to transmit pressure variations with no distortion
over a reasonable range of frequency 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 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 in the
tube begins to feel the effects of the tube wall. Simple calculations suggest
that the selected inside diameter is large relative to when frictional effects
would be significant. Semidiurnal tidal signals of 10 kPa amplitude would result
in a maximum displacement of only 4 mm in the longest tube installed (940 m).
Ultimately, the limiting factor was the size that could safely be handled
without fear of clogging with either fine sediment invading the screens and
filters or grease and constrictions at the tube joints. Given the volume of
water in the line to the deepest screen and the volume of water in the filter
itself, the volumetric displacement required to transmit a 10-kPa signal is 0.2
cm3. Distributed over the surface of the screened interval, the fluid
displacement would be only 3 x
10-5 mm. The period at which signals would begin to suffer a phase
lag would be far shorter than we are planning to monitor initially (10 min) or
are capable of monitoring (10 s).
Once undisturbed
monitoring proceeds for a few years, an attempt will be made to sample fluid
from basement and from the décollement zone. It is anticipated that basement
will be sufficiently permeable that sampling can be done using the -in-OD
(0.46-mm ID) monitoring line without significant perturbation of the pressure
signal. This will not be the case in sedimentary units that have low
permeability. To minimize the pressure perturbation and formation flux and to
achieve a reasonable flow velocity up the décollement zone sampling tube, it
was necessary to reduce the tube diameter. The -in-OD
tubing used was felt to be a reasonable compromise for this first attempt at an
ACORK installation in light of competing goals of minimizing handling difficulty
and clogging risks while maximizing flow velocity for a given flux.
ACORK
Head Physical Configuration
The ACORK head is a
30-in-diameter cylindrical frame fabricated from 
-in
steel around a section of 11-in
casing. It houses components in each of 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. F10).
The bays contain the following components described in more detail below: (1)
the sensor/logger/underwater-mateable connector assembly on a demountable frame,
(2) the spool valves and pumping/sampling valves and ports, and (3) the
three-way pressure sensor valves and the geochemical sampling valve and port.
The lowermost bulkhead is positioned ~16 in above the submersible landing
platform that covers the reentry cone. Pairs of 3-in-OD, 2.75-in-ID docking
tubes, 12 in center to center, are welded immediately beneath the lower bulkhead
to provide an aid for maintaining submersible or ROV stability during site
visits. Numerous cutouts on the vertical webs can be used as manipulator
"hand holds" for the same purpose. At the top of the ACORK head is a
30-in reentry cone for drill-bit, subcasing, or wireline tool delivery systems.
Plumbing
at the ACORK Head
At the ACORK head, the
packer, monitoring, and sampling lines are routed to several destinations (Fig. F11).
The packer filling line is connected to a sliding sleeve valve contained within
the ACORK running tool. During drilling-in operations and after completion of
the installation, this valve routes the packer line to a check valve that
prevents loss of line pressure but allows flow into the packers to prevent
collapse during lowering. Drill pipe circulation is directed to the mud motor at
the bottom of the ACORK assembly. After the hole is reamed and the assembly has
reached the target depth, a wireline retrievable tool ("go-devil") is
used to shift the sleeve valve to a second position that diverts drill string
pressure and flow from the mud-motor to the packer inflation line. At this point
the stepwise sequence of packer inflation begins.
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 sand-packed screens. The spool valves are set to shift when the packer
inflation line pressure reaches 1800 psi (12.4 MPa) (i.e., once all packers are
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.
Above the three-way valves
(further downstream from the screens), the monitoring lines are routed to the
pressure sensors via underwater-mateable couplers mounted at the base of the
framework holding the data logger and pressure sensors. The sliding-sleeve
couplers are pressure balanced so that the line pressures are radial only. No
piston force is generated under conditions of high screen-line pressure. The
downhole ends of the couplers are closed by spring-loaded sleeves when they are
unmated, and thus provide additional safety against leakage if the logger unit
is ever removed.
In addition to the lines
leading from the screens to the spool valves and onto the pressure sensors,
there are also lines leading to valved pumping/sampling ports where fluid
samples can be collected and pumping tests performed. In the Nankai Trough
ACORKs, one is a devoted geochemical sampling port connected to the
small-diameter line that leads directly to its own rolled-screen manifold in the
décollement zone screen. Other pumping/sampling ports lead from local
"T" junctions in the monitoring lines. 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.
A critical step in the
assembly operation 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. The packers are filled with water
at the moonpool immediately before they are deployed, and the packer line is
purged via the check valve in the running tool. The monitoring lines in the
umbilical are purged through manually closed ports at the spool valves, and the
lines running from the spool valves to the pressure sensors are precharged with
water.
Pressure
Sensors and Electronics
Pressure sensors and data
loggers (Figs. F10,
F11) are housed
in frames that can be lifted from the ACORK heads in the event that service of
any of the components is required. This monolithic subassembly weighs 115 kg in
water, and includes the data logger in its pressure case, multiple pressure
sensors connected to the monitoring lines and one for monitoring seafloor
pressure variations, an eight-pin underwater-mateable electrical connector, and
interconnecting cables. The frames are supported on four pins mated into
tapered-throat sockets welded to the ACORK head. O-rings
and a valved hydraulic line on the upper pair provide a hydraulic lock for
deployment, and allow the frame to be pumped from its mount in the ACORK head if
necessary. The valve and hydraulic coupler are mounted to the ACORK head
directly above the data logger bay (Fig. F10).
The pressure sensors,
manufactured by Paroscientific, Inc., employ matched pairs of quartz crystals.
Line or hydrostatic pressure is applied to one via a Bourdon tube, and the other
is isolated from stress to provide temperature compensation. Pressure is
calculated on the basis of the resonant frequency of the loaded quartz crystal
(with temperature effects determined from the second crystal removed). The total
range of the sensors used during Leg 196 is 70 MPa (7000 m equivalent water
depth). Data are logged as 24-bit values; the actual pressure resolution
realized is dependent upon the time over which the frequency was determined
(determined as the time-integrated cycle counts), which in turn affects the
power consumption. The integration time used (0.7 s) provides a resolution of
~10-6 of full scale, or 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 calibration inaccuracy (~5 x
10-4 of total pressure or 25 kPa at the Nankai Trough sites) are
dealt with well by the intergauge hydrostatic checks both prior to final
installation and later at times of submersible visits.
Sensors are activated at a
user-specified interval and data are recorded with a logger built by Richard
Brancker Research, Ltd. The logger is controlled by a Zilog Z80S153 processor
and has modular capability to store up to 32 channels of pressure and
temperature. 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 seven gauges being logged at 10-min intervals at Site 808, memory
capacity will provide 3 yr of operation. Power is supplied by lithium
sulfurylchloride 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.
