ODP Technical Note 10


A. Introduction

Tools to sample fluids in situ in oceanic boreholes were first built during DSDP and have continued to evolve during ODP. Barnes (1979) pioneered this effort, and has developed two generations of tools designed to sample sedimentary pore fluids through a probe that extends about 1 m ahead of the bit. The first-generation tool was modified during DSDP to allow concurrent measurements of temperature (Yokota et al., 1980; Uyeda and Horai, 1982) and/or fluid pressure (von Huene, 1985). These capabilities were integrated into the redesigned second-generation tool (Barnes, 1988), which could accomplish all three objectives. This design was refined before Leg 139 to facilitate sampling of high-temperature (200°C), highly reactive fluids. The Leg 139 iteration of the WSTP did not have the pressure-sensing capabilities available with the previous generation, as the section of the tool which contained the pressure transducers was refitted with a high-temperature, high-pressure electrical feed-through to hydraulically isolate the tool electronics from the probe tip. Discovery of corrosion on some of the high-temperature fittings (which were difficult to clean) during Leg 143 led to the retrofitting of the WSTPs with some lower temperature components. The tools will probably remain outfitted with a mixture of high- and lower temperature components and single thermistors for the next few years, until scheduled high temperature work requires their upgrading.

Barnes' (1979) goals were to sample sedimentary pore fluids in situ in order to (1) complement and calibrate the chemical analyses of pore fluids squeezed from cores, and (2) provide undisturbed samples for determination of in-situ concentrations of dissolved gases. The tools developed for these purposes have also proven useful in sampling borehole fluids that have partially re-equilibrated with pore fluids in reentered basement holes, such as Holes 418A and 504B. However, the borehole fluids that the WSTP samples in the vicinity of the bit are often displaced and/or contaminated by the massive drill string. When it is essential to avoid the effect of the drill string on borehole fluids in a reentry hole, ODP has available Kuster water samplers that can be run on the coring line through the bit, to sample fluids from open, undisturbed holes. ODP has also borrowed more sophisticated borehole samplers for some applications, and additional sampling projects are being developed (see Chapter VIII).

For a variety of reasons, the WSTPs are not presently scheduled to be refitted with pressure sensing capabilities. Pore pressure and permeability are crucial parameters that reflect and influence hydrogeologic and tectonic processes, but they are difficult to measure in situ because the emplacement of a measuring tool often disturbs the formation and fluid pressures in the vicinity of the tool. Perhaps in part for this reason, pressure data collected with the WSTP during DSDP Leg 84 and ODP Legs 110 and 112 have never been interpreted quantitatively. Numerical modeling suggests that in clay-rich sediments, decay of enough of the pressure disturbance resulting from probe insertion to make possible extrapolation to in-situ conditions and/or calculation of permeability is likely to require leaving the tool in bottom for hours to tens of hours (W. Fang and M. Langseth, pers. comm., 1992), which is not possible during ODP operations. In more permeable sediments, this decay may occur more quickly, but more permeable sediments are also more likely to be charged with fluids (surface seawater) which are pumped into the formation at high pressure during coring and drilling. Finally, in any kind of sediments, drilling requires putting substantial weight on the bottom of a hole. Insertion of the WSTP also requires setting the bit on bottom for a few seconds during deployment with an APC/XCB BHA, or leaving the bit on bottom in the case of RCB deployment. As the drill-string heave compensator can control bit weight to no better than ±5000 lb, sediments and pore pressures immediately below the bit are sure to be modified by drilling and probe insertion to such an extent that interpretations of WSTP pressure data are likely to be of questionable value.

To In-Situ Fluid Sampling, Section B.
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