An equation of state approach for estimating fluid acoustic properties for 4D seismic applications

We show a practical and general approach to estimating the acoustic fluid properties required in 4D seismic applications involving complex fluid mixtures, such as the ones encountered during CO2 injection in depleted oil and gas reservoirs and saline aquifers for carbon sequestration (CS), H2 storage, and in enhanced oil recovery (EOR) projects with gas and thermal injection.Our approach to computing fluid properties overcomes a major limitation of the empirical correlations which cannot deal with complex fluid compositions. Our solution is physics-based, practical, and general, and it does not rely on empirical correlations or require implementation within reservoir flow simulation code. To compute fluid properties we use well-established equations from thermodynamics and a PVT-tuned reservoir fluid equation of state (EoS). The EoS is always available as input to the compositional reservoir flow simulation models built by reservoir engineers to model and manage complex CS and EOR field projects. Our approach ensures full consistency between the fluid properties in the reservoir flow simulator and the fluid acoustic properties in the seismic petroelastic model. We show an example of applying our approach to 4D seismic modeling in a CO2 injection project for carbon sequestration in a depleted gas condensate reservoir in the North Sea.