8.2.1 Rock Physics Experiments
In 2001 Michael Helgerud presented a com pre- hensive PhD thesis at Stanford where he analysed wave velocities in gas hydrates and sediments containing gas hydrates. For pure gas hydrate samples made in the
laboratory he measured P-wave (compressional) velocities around 3700 m/s and S-wave (shear) velocities around 1950 m/s. Only small variations with temperature and confining pressure were observed. Te ratio between P-wave velocity of pure
hydrate versus ice is 0.98. For the S-wave velocity the corresponding ratio is close to 1.0. Tis means that, acoustically, pure ice and pure methane hydrate are very similar. Tis is maybe as expected since the dominant crystalline structures of the two are similar.
8.2.2 Fluid, Frame or Cement?
Is hydrate a part of the fluid, the frame or is it cement? We have seen that pure hydrate and ice are
very similar with respect to traditional seismic parameters, such as velocities and densities. However, in nature the hydrate is found as a part of a rock, making the rock physics relations more complex. What happens when methane hydrate enters into a sedimentary rock? Hydrate is found both in clay-rich sediments and in sands. Several models have been proposed for this, varying from regarding the hydrate as a part of pore fluid fill, to it being a part of the rock frame or acting as cement between sand grains. Helgerud found that the P-wave velocity is slightly higher when assuming that the hydrate is a part of the rock frame. If we want to estimate geophysical parameters
for a gas hydrate rock, there are two ways (at least!) to investigate it further: either by observing geophysical parameters from wells being drilled through a hydrate-bearing rock, or by injecting methane hydrate into a rock sample in the laboratory. In the following sections we discuss examples of both and put particular emphasis on a recent lab experiment performed in China.
8.2.3 Seismic Attenuation
Like the presence of free gas, the presence of gas hydrate affects seismic attenuation. Attenuation has the potential to map hydrate concentrations through the effect of local blanking of sediment stratigraphic reflectivity. However, there are few studies related to seismic attenuation in hydrate- bearing sediments and it remains an open topic for future studies. Te attenuation of seismic energy by gas hydrates is likely to depend on the concentration of hydrate, the thickness of hydrate, the mechanism of hydrate formation, and the dominant frequency of the seismic measurements, in addition to the lithology changes. VSP data in the Mackenzie Delta in Canada indicate that
Figure 8.8: Compressional and shear velocity versus time. The figure shows the response to a warming of the sample from 5 to 20°C followed by a cooling back to 5°C again. Confining pressure: 9,000 psi.
Figure 8.9: Compressional and shear wave velocities versus confining pressure for methane hydrate. The horizontal scale is from 4,000 to 9,000 psi.
Figure 8.10: Compressional and shear velocity versus porosity for ice.
quite thick hydrate-bearing zones have significant attenuation at seismic frequencies of 10–200 Hz (Q-values, which describe inverse attenuation, of around 10).
8.2.4 Observations from Wells
In a field example from a well drilled at the Blake Ridge (ODP site 995) offshore South Carolina, USA, Helgerud found that the rock physics model that assumes hydrate is part of the rock frame gave a reasonable fit between hydrate concentrations estimated from P-wave well log measurements and those obtained from the resistivity log. Te deviation between the two models is not huge, but based on the well log observation it is clear that the model which assumes hydrate is part of the rock itself and acts as a kind of cement explains the well log data best. A relative good fit between observed
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