ODP Technical Note 10


C. Borehole Fluid Sampling with WSTP and Kuster Tools

In addition to sampling pore fluids in sediments, the WSTP can also be used to sample borehole fluids, as can fluid samplers made by Kuster Company of Long Beach, California. The WSTP is described in the previous section; Figure II-3 shows the Kuster sampler, and Table II-3 gives its operating specifications. ODP purchased three of the 500-mL Kuster samplers for Leg 111. The Kuster tools were never operated successfully during Leg 111, primarily because the tools tended to trigger prematurely. Kuster has refurbished one of these tools with a more robust triggering mechanism, and that tool is now available for use on future cruises, although it has never been field tested. Kuster provided relatively brief operating instructions, which are available from ODP and are summarized in the following paragraph.

The Kuster sampler is a simple tool consisting of a sample bottle with an inlet valve, two non return valves, a locking device, and a mechanical clock. In operation, the clock is wound and the tool is lowered into open hole on the coring line. When the clock reaches the programmed sampling time (60 or 150 minutes after being wound), it releases the locking mechanism, allowing the inlet valve to open and the sample chamber to fill. When the fluid pressure inside the chamber plus the spring pressure of the non-return valve reaches in-situ fluid pressure, the non-return valves close and seal the sample chamber. Once the sample has been taken, the sampler should not be lowered any deeper into the hole, as increasing the external pressure will re-open the valves and allow the sample to be contaminated. There may also be leakage if warm sampled fluids are cooled during ascent, resulting in a significant reduction in fluid specific volume. The associated reduction in pressure inside the sampler may be sufficient to overcome the strength of the spring which seats the sampler valve.

When the tool is retrieved to the surface, the pressurized sample must be released using a special extractor body, to which a system of high-pressure valves and hoses is attached. There is no "standard" valve/hose system available, and a custom system may need to be constructed for each leg, depending on such factors as anticipated sample pressures, requirements to preserve dissolved gases, etc. It is the responsibility of the co-chief scientists, in consultation with participating fluid chemists, to specify any special requirements for such a sample-extraction system at the pre-cruise meeting.

During ODP Legs 137, 139, and 148 third-party borehole fluid samplers were deployed from the JOIDES Resolution. Scientists interested in using sophisticated third-party fluid samplers during ODP legs should discuss this possibility with ODP personnel as soon as possible after a leg is scheduled (see Chapter VIII for a description of sampler operations during Leg 137).

1. Operational considerations for sampling borehole fluids
Immediately after drilling, borehole fluids are composed primarily of the surface seawater used to flush cuttings from the hole. Thus it is normally of interest to sample borehole fluids only if considerable time has elapsed since the hole was drilled, so that the fluids have partially reequilibrated with in-situ pore fluids - a situation that occurs only when a reentry hole is revisited. (While it is conceptually possible for a "producing" formation to rapidly displace drilling fluid to the extent that a formation-fluid sample could be collected in the borehole, this process has never been documented during an ODP cruise.) If borehole fluids are to be sampled when a reequilibrated borehole is reentered, it is essential that this be done before any significant disturbance to the borehole fluids, as might be produced by reaming the hole, or circulating to condition for logging, or even running a sinker bar on the coring line to verify that the hole is open.

Both the WSTP and Kuster tool can be run out into open hole. If the WSTP is run with the colleted delivery system and landed in an APC/XCB bottom-hole assembly, the tool will scope out about 6 m ahead of the bit. If the collet is left off, and the WSTP is connected directly to the coring line, the WSTP can be run out into open hole, just like the Kuster tool. Running the WSTP or Kuster tool into open hole might be facilitated by running a logging or other BHA for reentry, as the opening at the end of a coring BHA could be small enough to upset either sampler as it is being retrieved into the drill string. However, if there is any chance of obstructions, it is safer to reenter the hole with a cleanout bit and run the drill string to the bottom of the hole, rotating and circulating where necessary to clean the hole. As the bit is worked down the hole during such a cleanout process, it is possible to pause at nearly any depth in the hole, to lower the WSTP into the bit and sample borehole fluids just ahead of the bit, which may be affected to some degree by any required rotation and/or circulation.

There will probably be concerns about hole conditions while reentering holes into sedimentary formations, holes into rubbly basement, and basement holes in which the sediments are not completely cased off. Such concerns should be considered carefully by the co-chief scientists and ODP engineers at the pre-cruise meeting, where the decision should be made as to what apparatus is to be run first into a reentered hole. These concerns will be relatively minor in reentering a properly cased basement hole with a good drilling history and no indications of hole stability problems.

To In-Situ Temperature Measurements, Section A.
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