Feasibility of monitoring gas hydrate production with time-lapse VSP Page: 4 of 35
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hydrate accumulations in the vicinity of this exploratory well are thought to represent
technically promising targets for production since the bottom of the HBL corresponds to the
bottom of the hydrate stability zone, such that dissociation may be induced in the system with
relative ease (Moridis and Reagan, 2007).
Figure 1 shows the geometry of the system considered in this study (including the
overlapping regions of the production and seismic simulations). The seafloor is at a depth of
0 m. Under the seafloor is 466 m of overburden, which overlies an aquifer of approximately
33.5 m thickness. The HBL, which occupies the upper 18.5 m of the aquifer, initially contains
only water and hydrate (hydrate saturation Sh = 0.7, water saturation Sa = 0.3, and gas
saturation Sg = 0). The remaining lower 15 m of the aquifer is initially water saturated (Sa =
1). Below the aquifer is 100 m of underburden.
The grid for the production simulation spans a vertical distance of 100 m (between depths
of 433 m and 533 m), and a horizontal radius of 800 m. For the purposes of the production
simulation, the overburden and underburden are assumed to allow heat exchange while being
impermeable to fluid flow. The production simulation assumes cylindrical symmetry around
the production well. Grid spacing in the radial direction is fine (0.25 m) near the well, and
increases (to 30 m) at larger distances from the well, while vertical grid spacing is fine (0.25
m) in the HBL and increases (to 7 m) in the overburden and underburden.
An equilibrium reaction model was employed in the simulation in accordance with
previous studies that indicate that this reaction model-instead of the more computationally
intensive kinetic one-is justified in geological settings similar to the one considered here
(Kowalsky and Moridis, 2007).
As a first step in the simulation, an equilibration procedure was performed, based on the
process described by Moridis et al. (2007), to obtain appropriate initial conditions for the
pressure and temperature distributions (Moridis and Reagan, 2007). The simulation of
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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/m1/4/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.