Feasibility of monitoring gas hydrate production with time-lapse VSP Page: 14 of 35
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defined as the volume of hydrate within the remaining hydrate-free pore space (i.e.,
Sh=Vh/(Va+Vg+Vh)), in Figure 3 for comparison with the previous cases.
Since the purpose of this paper is to examine the effect of the HBL on seismic
measurements, we are neglecting the effect of mineralogy and of the pressure and porosity
gradients on the elastic moduli of the overburden and underburden. It should be noted that, in
reality, velocities of HBS may follow some combination of the rock physics models
described above. In addition, when calculating the effective pressure-defined as the
difference between the lithostatic and pore pressure-within the HBL, we neglect variation in
the lithostatic pressure throughout the 18 meter thick HBL, setting the value equal to that at
the top of the HBL. Also, we are assuming that the materials can be modeled with isotropic
(not anisotropic) viscoelastic properties, and that conditions on the ocean bottom are the same
for all survey times.
Overall, Figures 3-5 indicate that during production of natural gas from HBS, in which gas
is released and the various phase saturations change, the elastic moduli (and therefore wave
velocities) can change considerably. However, the degree of change predicted by the different
rock physics models varies significantly. Case A serves as the lower limit of the expected
velocity values, and Case D serves as the upper limit.
Evolution of the P-wave and S-wave velocity profiles during production are shown-for a
vertical slice located 650 m from the production well-in Figure 6 for rock physics model
Cases A-D. As production from the HBL progresses, S-wave velocities within the entire
reservoir (i.e., from the top of the HBL at 466 m to the bottom of the underlying aquifer at
500 m) increase. The overall increase is caused by the increasing effective stress from
reservoir depressurization. The ratio of the velocity increase in the HBL for Cases A and B
(-40%) is larger than for Cases C and D (-25%), because the sediment frame stiffness14
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Kowalsky, M.B.; Nakagawa, S. & Moridis, G.J. Feasibility of monitoring gas hydrate production with time-lapse VSP, article, November 1, 2009; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1014155/m1/14/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.