The synthesis methods discussed in this chapter are well suited for experiments measuring thermal properties such as conductivity or diffusivity. Techniques developed by von Herzen and Maxwell (1959), for instance, can be adapted to measure thermal conductivity by use of a needle probe design that approximates an infinitely long, continuous line source of heat in an infinite medium. Cylindrical sample geometry can be modeled approximately as an infinite medium, and a needle probe placed along the axis of the sample can be approximated as a continuous line source of heat. Thermal conductivity of a sample can then be calculated directly by measuring the rise of temperature measured by the probe for a given heat input per unit length of wire per unit
Figure 10. Schematic of sample chamber outfitted for hydrate growth followed by in situ thermal conductivity measurement using the needle probe technique. The probe is inserted through the base of the sample chamber prior to packing it with seed ice, and is supported from below by a metal brace to prevent probe expulsion from the chamber at elevated pressure. An O-ring seal around the probe prevents gas leakage through the insertion hole. The needle probe consists of an epoxy-filled hypodermic tube containing a 4 kii thermistor and a heater wire running the length of the probe. The probe has essentially the same length as the sample vessel, so departures from ideal cylindrical geometry during the heating phase of the experiment are negligible. Needle probe diameter is not drawn to scale.
The primary adjustment to the standard synthesis apparatus for such measurement involves replacing the standard sample vessel base with an endcap modified to accommodate the thermal probe, as shown in Figure 10. Preliminary success with this design has been reported by deMartin et al., (1999), in measuring thermal conductivity of pure methane hydrate samples and hydrate + quartz sand aggregates. Further discussion of the context of these measurements is provided by Ruppel et al., (this volume). Not only does in situ gas hydrate growth with the probe already in place ensure excellent contact between the hydrate and the probe, which is crucial to the success of this technique, but this method also avoids structural and stability problems inherent to drilling holes in quenched material for probe insertion. As this method measures thermal parameters of a porous aggregate, however, it is necessary to either establish the influence of the pore pressure medium on the measurements, or design a hydrostatic compaction capability around the sample to carefully compact and fully densify the material without damaging the probe. Such apparatus and procedural developments are currently in progress.
Compaction of as-molded, porous material can be achieved by either uniaxial or hydrostatic compaction procedures. Our method for uniaxial compaction utilizes an apparatus that permits in situ synthesis, compaction, and elastic wave speed measurement (Fig. 11; see also Waite et al., 2000). This apparatus incorporates pistons at each end of the sample chamber, both of which are outfitted with a transducer assembly for wave speed measurement. The time, and for a given probe radius.
moving piston is advanced hydraulically to axially shorten the sample after synthesis, and the length change of the sample is monitored by the linear conductive plastic (LCP) transducer as shown in Figure 11.
Figure 11. Gas hydrate synthesis apparatus equipped for subsequent in situ uniaxial compaction and measurement of acoustic wave speeds (from Waite et al., 2000).
(A) Pressure vessel schematic. Polycrystalline gas hydrate is synthesized directly in the sample chamber by the methods described in this chapter, then uniaxially compacted by hydraulically advancing the moving piston.
(B) Transducer assembly. Both pressure vessel pistons house a 1 MHz center-frequency piezoelectric transducer (either p- or s- wave), allowing pulsetransmission wave speed measurements to be made throughout the compaction process.
Prior to uniaxial compaction, the sample is vented to a pressure just sufficient to maintain it in its stability field; it is therefore beneficial to begin the compaction sequence at cold temperatures where hydrate is stable at lower pressures. Following this cold compaction, samples are partially repressurized and further compacted while raising the sample's temperature through the ice point. Compaction at high temperature facilitates elimination of the last percent of porosity, although the high strength of hydrate makes it increasingly difficult to achieve full compaction. Monitoring the wave speed profile throughout the compaction process provides independent determination of the presence or lack of any unreacted ice or water in the sample, as a measurable discontinuity in the wave speed can be measured if even trace amounts of H2O freeze or melt.
For hydrostatic compaction (Fig. 12), a sample of gas hydrate is initially sealed in a soft jacket (such as indium metal) between two end caps or between an end cap and a force gauge, and pressurized from the outside with a confining medium gas. The sealing procedure must be performed at temperatures sufficiently cold to ensure stability of the hydrate. Hydrostatic compaction is best performed in an apparatus in which a piston can be advanced to touch and square the base of the sample, such as with the triaxial deformation apparatus shown in Figure 12. The confining pressure (Pc) is then slowly "stepped" up to 50 or 100 MPa in increments of roughly 20 MPa, taking care not to induce a high-pressure phase transformation in the samples at pressures just above 100
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MPa (Chou et al., 2000). Following each compaction step, the piston is advanced to touch and square the bottom of the sample, then advanced just sufficiently to lightly compress the sample in order to compact it with minimal deformation. Optimally, the top end of the sample is attached to a gas line that can be either fully vented or maintained with a low CH4 back pressure on the sample. Compaction can then be performed at either very cold conditions in the vented configuration, or at warmer temperatures with a regulated pore pressure.
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