Physical properties of gas-hydrate-bearing sediments depend on the pore-scale
interaction between gas hydrate and porous media as well as the amount
of gas hydrate present. Well log measurements such as proton nuclear
magnetic resonance (NMR) relaxation and electromagnetic propagation tool
(EPT) techniques depend primarily on the bulk volume of gas hydrate in
the pore space irrespective of the pore-scale interaction. However, elastic
velocities or permeability depend on how gas hydrate is distributed in
the pore space as well as the amount of gas hydrate. Gas-hydrate saturations
estimated from NMR and EPT measurements are free of adjustable parameters;
thus, the estimations are unbiased estimates of gas hydrate if the measurement
is accurate. However, the amount of gas hydrate estimated from elastic
velocities or electrical resistivities depends on many adjustable parameters
and models related to the interaction of gas hydrate and porous media,
so these estimates are model dependent and biased. NMR, EPT, elastic-wave velocity, electrical resistivity, and permeability measurements acquired
in the Mallik 5L-38 well in the Mackenzie Delta, Canada, show that all
of the well log evaluation techniques considered provide comparable
gas-hydrate saturations in clean (low shale content) sandstone intervals
with high gas-hydrate saturations. However, in shaly intervals, estimates
from log measurement depending on the pore-scale interaction between
gas hydrate and host sediments are higher than those estimates from measurements
depending on the bulk volume of gas hydrate.
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