Production of hydrocarbons from heavy oil reservoirs often requires large investments both in capital and in operation and despite the best efforts portions of the reservoir may be bypassed or injected fluids may be lost. Time-lapse geophysical methods have shown promise in monitoring such reservoirs, particularly in the case of weakly consolidated sands. The seismic response can depend on a variety of factors that include temperature, pore pressure, stress, fluid saturation state, and material damage. These effects are briefly reviewed in the context of Gassmann's relation that predicts the compressibility, rigidity, and density of a saturated porous rock. While Gassmann's relation is widely used in fluid substitution studies, it is not yet clear whether it can be appropriately applied in the context of viscous heavy oil reservoirs.

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