ODP Technical Note 10


C. Overview of Downhole Measurements Provided by ODP

ODP provides tools and technical support to allow downhole sampling of pore fluids from soft and semilithified formations, borehole fluids, fluids and core under pressure, and the measurement of several basic physical parameters: temperature, borehole pressure, bulk permeability, and APC and RCB core orientation. For some parametric measurements, several different tools are available for use in different situations, e.g., while coring sediments, between cores, or in open hole. In addition, the same tool often can be deployed in different configurations, or using several different techniques or strategies; a method that works well under some circumstances may fail in others. Some tools work well only under ideal hole conditions, some only in basement. Downhole tools scientists are advised to consult scientists and marine specialists who have experience running these tools to plan for particular legs and environments.

The ODP tools are generally not used with the logging cable, but instead are deployed using the drill string and/or coring line in several possible modes: (a) lowered on the coring line to either seat in the bit or run beyond the bit in open hole, (b) lowered to the bit as part of a coring assembly, (c) dropped down the drill pipe under "free-fall" conditions and later retrieved using the coring line, (d) built into the drill string, or (e) installed for long times in the seafloor. Often, the use of a particular downhole tool or technology (perhaps a combination of tools) requires careful advance planning in terms of the bottom-hole assembly (BHA) run at the end of the drill pipe. There are many options for using these tools, and the focus of this section and the next is to outline these options, leaving other details for subsequent chapters.

A sample ODP data sheet for downhole tools is shown in Figure I-1. This data sheet is used to maintain a deployment record for the WSTP and APC-tool; other downhole tools may require more or less complicated record keeping.

1. Fluid sampling
Two or three water-sampling temperature probes (WSTP) are routinely available for sampling fluids ahead of the bit in sediments soft enough for the probe to penetrate without cracking. As the name implies, the WSTP is also capable of measuring temperature; an earlier version of the WSTP was equipped to measure hydrostatic and formation pressure as well. The WSTP is sometimes referred to as the Barnes/Uyeda probe or the Barnes sampler, after the original proponents and developers, R. Barnes and S. Uyeda.

The WSTP is run on the coring line between cores and therefore requires a break in coring and drilling operations (perhaps as long as 2-4 hours, depending on water and hole depth and measurement plan) to collect a sample. The probe lands in the BHA so that it extends up to 1 m into the sediment ahead of the bit; thus, the sediment to be cored next is disturbed. The WSTP presently collects three distinct fluid samples of varying size and quality. The sample chambers are arranged in series, and because of the sampling method (fluid is driven into the chamber by the contrast between ambient [hydrostatic] pressure in the formation and surface atmospheric pressure initially trapped in the sampling system), only a single sampling attempt can be made during each deployment. Fluid is drawn into the WSTP through a series of titanium, stainless-steel, and/or polyester filters, and through the end of a 1/16-in.titanium sampling tube mounted near the tip of the probe.

In open hole, the WSTP also can be used to sample borehole fluids ahead of the bit. Also, a large volume borehole sample (0.5 L) can be taken with a tool made by Kuster Company, which is run on the coring line to a distance ahead of the bit deemed safe by the ODP Operations Superintendent. On recent ODP Legs 137, 139, and 148 third-party tools were borrowed "at cost" from Sandia National Laboratory and the Lawrence Berkeley Laboratory, for sampling high temperature, potentially corrosive fluids. (In addition, Schlumberger tools for sampling open-hole fluids may be available through LDEO.)

2. Formation temperature measurements
Two types of ODP tools are routinely available for measuring temperatures in sediment and in open hole: the WSTP mentioned above, and miniature instruments deployed within special APC cutting shoes. These tools are normally used separately to obtain single measurements of temperature at discrete depths during a sequence of coring a hole through sediments. In addition, they can be configured together for simultaneous use without coring. Either tool or the combination can be washed and pushed through soft sediments or run into open hole, to measure temperatures at multiple depths during a single lowering.

The first generation of APC temperature recorders was developed by R. Von Herzen and colleagues during the last several years of DSDP; ODP recently purchased 10 new instruments designed and built by Adara, a Canadian geotechnical firm. Like its predecessor, the new device monitors the temperature of a single sensor within the APC cutting shoe. The tool contains its own microprocessor, which is programmed to cycle through a series of sleeping and sampling periods. On penetration, the coring shoe must be held in the sediments for 10-15 minutes to obtain enough data to allow extrapolation beyond the thermal disturbance of insertion to in-situ temperature. The APC tool is relatively easy to operate, and with sufficient personnel can be run on successive cores in an APC hole. The tool is limited to depths where the force required for pullout is within safe limits (usually less than 80k-100k lb). The pullout force is generally greater when the APC temperature tool is run, compared to when an the ordinary APC shoe is run, because leaving the core barrel in the bottom for several minutes for the temperature measurement allows sediments to become packed in around the barrel. In practice, this pullout limit is often reached within the upper 100-150 m of sediment below the seafloor.

In contrast the WSTP has been run to depths greater than 400 mbsf, but it may require several hours per run on the coring line between cores. The WSTP data logger contains no microprocessor, and instead continuously samples at a fixed time interval the resistance of one or more thermistors mounted in a cylindrical probe. The WSTP is attached to the front end of a special core barrel, which functions as a pressure case for the electronics, batteries, data logger, and hydraulics, and is lowered down the drill string and pushed into the sediments. Again, the probe must be held in place for 10-15 minutes to obtain enough data to allow extrapolation to in-situ temperature.

3. Permeability measurements using packers
A packer produces a hydraulic seal in a borehole, allowing the hydrologic properties of the formation to be tested by applying differential fluid pressures to the isolated section. One packer is available for regular use in ODP, a drill-string straddle packer developed by K. Becker with an NSF grant to the University of Miami. This packer was manufactured by TAM International, and uses inflatable rubber/steel elements to seal the borehole. Several other packers have been brought aboard the Resolution as part of ODP cruises over the past several years, although none has been deployed successfully. These other tools included a rotatable drill-string packer and a wireline sampler, both of which have been retired, and the Geoprops probe (which has not yet undergone full field testing, on land or at sea).

With sufficient advance notice, the drill-string straddle packer is available for regular ODP use. The straddle packer can be run only as part of a noncoring BHA in reentry holes that penetrate reasonably stable formations, or possibly in less stable formations which have been "stabilized" with casing and a perforated liner. Permeability measurements in holes that penetrate unstable formations or in single-bit holes are not yet possible, although it was hoped that the WSTP (when it was equipped with pressure sensors) or the Geoprops probe might provide this information.

Any use of the drill-string packer requires considerable preparation, a significant amount of ship time, and the cooperation of several key shipboard personnel. The experienced ODL (SEDCO) core technicians take responsibility for preparing the drill string packer for deployment: the Operations Superintendent is responsible for finalizing the layout of the BHA, and technical support staff will assist with data logging. On some legs there may be a dedicated special tools engineer who can also assist with preparation and planning. The important task of directing the packer experiments (choosing depths from core and log data, supervising the test sequence, setting pump rates and volumes) falls to the packer scientist. The procedures for formation testing with the drill-string packers are somewhat complicated; interested scientists will need to become familiar with numerous aspects of ODP operations and procedures. A more complete discussion of packer measurements and data analysis is presented in Becker (1990a).

The prime data in packer testing are fluid pressures measured with gauges that are run from the rig floor and within the isolated zone when the packer is inflated. ODP supports the use of mechanical Kuster pressure recorders attached to "go-devils" that are dropped down the drill string into the packer and also enable the mechanisms for inflating the packer. The Kuster pressure records are scratched (quite accurately) onto small metal charts, which are read with a Kuster chart reader. Becker owns a Kuster chart reader, as well as two digital pressure gauges built by Geophysical Research Corporation, which were deployed successfully during recent legs.

Formation-testing procedures at each setting depth typically run 6-8 hours, including the time required to drop and retrieve the go-devils. For permeability measurements using the straddle packer after coring a reentry hole, up to 24 hours may be required for a pipe trip (depending on water depth) to build the packer into a logging BHA. In some cases this operation will not actually involve extra time, as the packer is compatible with logging and it may be possible to accomplish a full program of logging and packer measurements during the single pipe trip required to deploy a logging BHA into a reentry hole. In the oil field, drill-string packers are often used in conjunction with borehole-fluid samplers (sometimes in the form of "sampling go-devils") to draw fluids from the isolated portion of the borehole. To date, this technology has not been applied in ODP, but it should be possible to do so in the future, provided there is interest and resources are allocated for the development of special samplers. Probably the greatest problem with sampling borehole fluids during packer testing is that the formation is likely to be charged with surface seawater which has been pumped down the pipe during drilling and earlier packer operations.

4. Orientation of APC and RCB cores
In the past, the azimuth and deviation from vertical of (nonrotary) cores taken with the APC were routinely measured using an Eastman-Christensen magnetic "multishot" camera. This device records photographs of a magnetic compass and pendulum taken at pre-selected intervals. It is installed in a nonmagnetic sinker bar/pressure case that connects the coring line to the APC barrel, and must be used with a special nonmagnetic drill collar in the BHA.

An electronic replacement, or "digital multishot", has been developed for ODP use by Tensor, Inc. This instrument uses two accelerometers and three orthoganal magnetometers in combination with the nonmagnetic BHA and running hardware listed above. The tool records hole inclination (drift), azimuth, and magnetic tool face (core alignment) direction, either in single-shot or continuous modes.

Because of specific BHA requirements, the decision to obtain oriented piston cores at a given site must be made before the BHA is made up and the pipe run into the hole. An extra 5 to 10 minutes per oriented core is required for handling the multishot assembly. Oriented piston cores can be obtained only to depths that are safe to core with the APC, typically on the order of 100-300 mbsf, depending on the force required for pullout.

The orientation of hard-rock cores is an exciting new ODP development. This technology has four independent components: electronic core orientation, sonic core monitor, core-scribing cutting shoe, and bit-depth indicator. The first two components listed above are presently digital, while the latter two are analog. Core orientation is accomplished with the electronic multishot, as described previously. The sonic core monitor is deployed as part of the core-barrel assembly; it records the time at which individual pieces of core enter the core barrel. The core scribe marks the core as it enters the barrel, with reference to a fixed direction. The bit-depth indicator records the position of the bit relative to the rig floor. Because hard-rock cores tend to break off during drilling, forming pieces centimeters to meters in length, each piece of core must be marked and oriented separately. At present the merging of all four data types described above is done manually.

5. Long-term observatories (CORK)
For some forms of downhole measurements, it is necessary to hold instrumentation in place for days to years, longer than is practical with the drill ship. In addition, there are occasions when having the borehole open to the overlying ocean compromises the critical parameters of interest

(e.g., formation pressure or borehole-fluid chemistry). In these instances, it will sometimes be desirable to initiate a long-term experiment using the CORK system, developed jointly by T. Pettigrew of ODP engineering, and collaborating scientists E. Davis, K. Becker, and B. Carson (Davis et al., 1992).

The CORK system is a combination of modified seafloor-reentry hardware, a hydraulic seal, and an instrumented sensing and logging package. Systems deployed thus far include the seal (consisting of a standard reentry cone with a modified casing hanger, plus the plugging device itself), a thermistor and pressure-sensor string, a fluid-sampling tube and a means of passing borehole fluid through the CORK, and a data logger with a long-term power supply and the capability of dumping data to a submersible or remotely operated vehicle (ROV). Future systems may include data telemetry capabilities. The seal comprises two parts, an outer section which must be deployed and recovered by the drillship, and an inner section with the data logger and pressure case which can be recovered by the drillship or with an ROV.

The reentry system with cone and casing is first deployed as part of regular operations. A reentry hole is drilled and cased to the necessary depth with enough open hole left below the casing as required for the experiment (perforated casing or a liner also can be installed). The CORK seal itself is then made up, with one or more drill collars hanging below the CORK to hold the tool in place inside the reentry cone before the CORK is latched in. The CORK is lowered to the seafloor attached to the drill pipe and positioned with the drill collars in the throat of the cone. Borehole instrumentation and a data logger are lowered down inside the drill string on a coring line. The sensors pass through the CORK and into the cased hole, the data logger is latched inside the CORK, and the coring line is retrieved. The CORK is then set inside the reentry cone, and a ball is pumped down the drill pipe to land in the CORK running tool and allow the drill pipe to become pressurized. The pressure in the drill pipe is used to activate a latch ring in the CORK, which mates with grooves near the top of the casing inside the cone. This latching mechanism prevents the CORK from being forced out of the hole if the borehole below the cone becomes overpressured. After the CORK is latched in, a submersible/ROV platform is made up around the drill pipe and dropped to the seafloor. The drill pipe is then disconnected from the CORK and brought back to the surface, ending CORK installation.

6. Pressure core sampler (PCS)
The PCS is a free-fall deployable, hydraulically actuated, wireline-retrievable sampling device intended to return formation sediment and fluid samples at hydrostatic pressure. The system is fully compatible with standard APC/XCB drilling and coring hardware, allowing pressurized samples to be collected at any time from the mud line down to the depth of indurated formations or hard rock. The PCS is of particular interest to scientists studying frozen hydrates, gases, or other pressure-sensitive materials which tend to degrade during conventional core retrieval and rig-floor handling. Unlike the WSTP, which draws fluid and gas into an evacuated chamber and can therefore alter the chemistry of the sample, the PCS is intended to trap ambient pressure at the time that the sample is collected; also unlike the WSTP, the PCS returns a solid sample.

The PCS comprises several components: a detachable sample chamber, a ball-valve subassembly that traps pressure in the chamber, an actuator assembly that catches the ball valve and pulls the core tube into the sample chamber, a latch assembly that directs torque to the PCS cutting shoe and serves other standard coring functions, an accumulator that maintains pressure in the sample chamber, and a manifold assembly that allows fluid and gas within the PCS to be monitored and accessed under pressure. The PCS can recover a 42-mm-diameter core sample, 0.86 m long, at pressures up to 68.9 MPa (10,000 psi). A detailed description of the PCS with tool drawings is available as ODP Technical Note 17 (Pettigrew, 1992).

To Introduction, Section D.
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