Strength measurements

Gas hydrate samples fabricated by the methods given in this chapter are also well suited for deformation testing in the type of apparatus shown in Figure 12. Because the composition and grain characteristics are well known and reproducible from sample to sample, such material is appropriate for rheological measurement and flow law characterization.

Figure 12. (A) Schematic of triaxial gas deformation apparatus used for hydrostatic compaction and subsequent deformation testing of gas hydrate test specimens. A jacketed sample (B) can be compacted and then measured for strength in compression tests, over the T range 77 to 300 K, at confining pressures up to 0.6 GPa, and strain rates 10"4 to 10"8 s'1. The sample is attached to the internal force gauge, and a sliding piston moves through dynamic seals from below to impose axial shortening. A gas collection system or pore pressure line can be attached at the top of the apparatus to monitor possible methane loss during compaction and deformation, or to prevent gas hydrate decomposition.

The strength of methane hydrate samples has been measured in this laboratory in a suite of constant-strain-rates tests in compression, at conditions ranging from T = 140 to 287 K, confining pressure (Pc) of 50 to 100 MPa, and strain rates from 3.5 x 10"4 to 10"8 s"1. The apparatus shown in Figure 12 is a 0.6-GPa gas deformation apparatus outfitted for cryogenic use (Heard et al., 1990) in which or He gas provides the Pc medium. The pressurized column within the apparatus consists of an internal force gauge, the jacketed sample, and a moving piston that compresses the sample axially against the internal force gauge at a fixed selected displacement rate. The soft indium jackets in which the samples were grown serve to encapsulate them during compaction and testing to exclude the Pc medium, and provide the additional benefit of superbly replicating the outer surface of the deformed sample, thus enabling subsequent microstructural study at room conditions (Fig. 10 in Stern et al., 1998a, for example). All gas hydrate samples tested in this apparatus were initially subjected to the hydrostatic pressurization and compaction sequence described above.

In tests conducted at T < 200 K, methane hydrate samples displayed strengths that were measurably but not substantially different from steady-state strengths of H20 ice (Stern et al., 1996, 1998a). Recent tests conducted at warmer test conditions (>250 K), however, show that methane hydrate is enormously strong relative to water ice (Zhang et al., 1999, Durham et al., work in progress), a result which was not previously expected as water ice has commonly been assumed as a general proxy for the mechanical properties of gas hydrates. Figure 13 shows the relative strength differences between methane hydrate and pure water ice deformed in one of our first tests at 260 K, roAE-PttES&pneUNe

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illustrating the dramatic strength differences. Further testing is currently underway to resolve better the strength and flow behavior of methane hydrate, and particularly to determine if possible shear instabilities or other solid-state processes are inherent to the deformation behavior (Stern et al., 1996, Durham et al., work in progress). Such instabilities not only increase the ice content of the sample with increasing strain, but mask the true strength of the pure hydrate due to the increasing ice contamination in the samples.

Figure 13. Comparison of the stress-strain histories of a mixed-phase sample of polycrystalline methane hydrate plus H20 ice (approximately 3:1 volume ratio hydrate to ice), vs pure polycrystalline H2O ice, at comparable test conditions. At lower temperature test conditions (< 200 K), the strength differences between methane hydrate and water ice are not significantly different, due to the high strength of both hydrate and ice at those conditions. At elevated temperatures as shown here, however, the strength contrasts are dramatic. Recent tests show that the strength of pure methane hydrate, even at T > 273, is at least several times stronger than that of the mixed-phase sample shown here (Durham et al., work in progress).

Figure 13. Comparison of the stress-strain histories of a mixed-phase sample of polycrystalline methane hydrate plus H20 ice (approximately 3:1 volume ratio hydrate to ice), vs pure polycrystalline H2O ice, at comparable test conditions. At lower temperature test conditions (< 200 K), the strength differences between methane hydrate and water ice are not significantly different, due to the high strength of both hydrate and ice at those conditions. At elevated temperatures as shown here, however, the strength contrasts are dramatic. Recent tests show that the strength of pure methane hydrate, even at T > 273, is at least several times stronger than that of the mixed-phase sample shown here (Durham et al., work in progress).

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