One frequently used model for miscible displacement in carbonates uses a capacitance mass exchange process to model both concentration profiles as a function of position and effluent profiles as a function of time. The capacitance effect in carbonates is quantified as a concentration driven mass exchange between a poorly connected system (stagnate fraction) and the miscible flowing fraction.

In a simple yet elegant experiment we have been able to expand this model to carbonate rocks that contain bi-modal pore size distributions. By combining experimental single phase dispersion tests with continuous nuclear magnetic resonance (NMR) T2 distribution measurements we observe directly velocity and concentration gradients. Results illustrate that the micro-porous system in select skeletal-oolitic rock types is preferentially connected and that intra-pore system fluid transport is more pronounced than that occurring between micro and macro pore systems. Using a combination of effluent and in-situ measurements we are able to model time dependent capacitance effects not apparent with the conventional capacitance dispersion model. The new model accurately captures early micro-macro system fluid exchange and late time capacitance effects modeled using effluent profiles.

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