Acoustic properties

P-wave signals are recorded at numerous times during testing (e.g., Fig. 4). P-wave velocities, Vp, are calculated routinely also. Figure 6 shows velocity changes induced in different materials by gas hydrate formation, freezing of the pore water, and by increasing consolidation stress. Knowledge of the material occupying the pore space and the consolidation stress are critical to understanding the implications of the various velocity measurements. Vp values typically increase with consolidation stress because of the greater grain to grain contact stress (primarily) and increased density. This is illustrated by Figure 7 for a Mackenzie Delta sediment, but the rate of velocity change decreases with increasing pressure. Sample shortening during consolidation could also create an apparent velocity increase due to reduced travel distance for the acoustic pulse (if sample height is not accurately measured).

Initial results indicate that samples obtained from the Mackenzie Delta containing natural-gas hydrate acoustically behave as part of the sediment frame or as disseminated throughout the pore fluid, but not as cements between grains.

4.3. Strength properties of samples from Mallik 2L-38 Well

Preserving field samples that contain natural gas hydrate for subsequent laboratory strength testing is inherently difficult. Unless a pressurized coring system that can maintain in situ P-T conditions is used, some gas hydrate may be lost during sample transfer into transportation vessels in the field or later during transfer into a laboratory test system.

If gas hydrate behaves similarly to that of ice in sediment then gas-hydrate-bonded sediment should in most cases be stronger than similar material that does not contain gas hydrate. Andersland and Anderson (1978) demonstrate that the presence of ice can substantially increase the strength of sediment. Frozen sediment exhibits a wide range in strength properties, because strength is influenced by a number of factors: strain rate, temperature, confining pressure, grain size, and density. The strength of sediment containing gas hydrates is probably influenced by these and other factors such as gas hydrate-cage-occupancy.

Mallik Sand (Reconstituted) Ottawa Sand Ottawa Sand Water/Ice

Mallik Sand (Intact)

P-wave velocity (km/s)

Figure 6: Comparison of changes in P-wave velocity for different materials tested in GHASTLI by forming gas hydrate, freezing pore water, and increasing consolidation stress. The arrows represent an initial material velocity before the indicated process began, followed by a gradual increase in Vp until a final, maximum value is reached.

_Freezing ^

Gas Hydrate Formation

Consolidation (0.25 to 8.0MPa) i-r

Undrained triaxial shear tests were performed on samples from the Mallik Well both before and after natural-gas hydrate was dissociated (Figure 8). Plots of the shear stress vs. axial strain for the four test samples show that the strength of the sample containing gas hydrate is indeed much higher. The gas-hydrate-containing sample exhibited higher negative pore pressures during shear. Andersland and Ladanyi (1994) indicate that frozen samples are typically plotted with respect to total stresses because of the difficulty in measuring

Figure 7: Effective consolidation stress, cr'c, versus Vp for a sample from the Mallik 2L-38 well.

intergranular stresses. Evidently, in the gas-hydrate-containing Mallik 2L-38 specimen, the pores contained enough free water to transmit pore pressure. The large difference in strength between the specimen containing gas hydrate and the other samples may be related to the fact that many of the pores of the gashydrate sample contained a solid that increased the tendency for dilation during shear. Hence the higher negative pore pressure and corresponding strength values.

The maximum effective friction angle (Holtz and Kovacs, 1981) for the hydrate sample was also the highest value of any of the tests (44.4°). The friction angles for the non-hydrate bearing samples (33.8 - 38.6°) are within the typical range of sandy sediment (Hunt, 1984). Depending upon the amount present, gas hydrate has the potential to greatly affect the mechanical properties of the host sediment.

The dissociation of methane hydrate in GHASTLI produces an excess volume of gas which is related to the test pressure and temperature conditions. Typically during a test the internal sample pressure is maintained constant and the additional gas produced by dissociation is measured in the collector.

However, in situ, the production of excess gas may have a destabilizing effect because of the increase in pore pressure accompanied by a subsequent reduction in sediment effective stress. The amount of pressure generated in situ by dissociation depends upon a number of factors, some of which include: P-T conditions, amount and distribution of gas hydrate present, percent of cage occupancy of the hydrate by gas molecules, inherent sediment permeability, rate of dissociation, and faulting present in the region. Measuring sediment and gas hydrate properties is only the first step in predicting in situ behavior.

Figure 8: Comparison of triaxial strength plots for a sample containing natural gas hydrate to other samples from the Mallik 2L-38 well after gas hydrate was allowed to dissociate.

Figure 8: Comparison of triaxial strength plots for a sample containing natural gas hydrate to other samples from the Mallik 2L-38 well after gas hydrate was allowed to dissociate.

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