Abstract
A rock physics model was established to calculate the P-wave velocity dispersion and attenuation caused by the squirt flow of fluids in gas hydrate-bearing sediments. The critical hydrate saturation parameter was introduced to describe different ways of hydrate concentration, including the mode of pore filling and the co-existence mode of pore filling and particle cementation. Rock physical modeling results indicate that the P-wave velocity is insensitive to the increase in gas hydrate saturation for the mode of pore filling, while it increases rapidly with increasing gas hydrate saturation for the co-existence mode of pore filling and particle cementation. Meanwhile, seismic modeling results show that both the PP and mode-converted PS reflections are insensitive to the gas hydrate saturation that is lower than the critical value, while they tend to change obviously for the hydrate saturation that is higher than the critical value. These can be interpreted that only when gas hydrate begins to be part of solid matrix at high gas hydrate saturation, it represents observable impact on elastic properties of the gas hydrate-bearing sediments. Synthetic seismograms are calculated for a 2D heterogeneous model where the gas hydrate saturation varies vertically and layer thickness of the gas hydrate-bearing sediment varies laterally. Modeling results show that larger thickness of the gas hydrate-bearing layer generally corresponds to stronger reflection amplitudes from the bottom simulating reflector.
Highlights
Elastic behaviors of gas hydrate-bearing sediments can be modeled using various rock physics methods by considering different ways of concentration for the gas hydrate in sediments. Lee et al (1996) used the weight equation to estimate the amount of gas hydrate from seismic velocities where the gas hydrate exists as pore filling
We introduced the critical hydrate saturation parameter Sc in the rock physics model to describe various patterns of gas hydrate concentration
It is considered that for the coexistence mode of hydrate both as pore filling and particle cementation, the cementation reduces the porosity of the formation by φ = φ0 [1 − (S − Sc)], where S is the total saturation of the gas hydrate in the sandstone, and φ0 is the porosity of the gas hydrate-bearing sandstone when the hydrate has not attached to the solid minerals yet
Summary
Elastic behaviors of gas hydrate-bearing sediments can be modeled using various rock physics methods by considering different ways of concentration for the gas hydrate in sediments. Lee et al (1996) used the weight equation to estimate the amount of gas hydrate from seismic velocities where the gas hydrate exists as pore filling. We proposed a rock physical modeling method using poroelastic theories to quantify the P-wave velocity dispersion and attenuation of gas hydrate-bearing formations.
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