Technical Paper: Fluid Composition Equilibrium; a Proxy for Reservoir Connectivity

Society: SPE
Paper Number: 145703
Presentation Date: 2011
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Assessing reservoir connectivity during the earliest stages of reservoir evaluation is highly desirable for successful field development. Nevertheless, it has long been problematic to assess reservoir connectivity prior to production. Recently, downhole fluid analysis has enabled facile assessment of fluid compositional gradients vertically and latterally. Using equations of state, the extent of fluid compositional equilibrium can be established.

Only a process that stretches across the entire age of the reservoir is likely to capture geologic events that cause compartmentalization. Fluid composition equilibration requires mixing of the entire content of the reservoir which occurs only on the geologic time scale. Restrictive flow barriers are not compatible with thorough mixing of fluids throughout the reservoir. Fluid composition equilibration provides a tight constraint to test connectivity.

In this paper, the time constants for fluid composition equilibration are evaluated in numerical simulations. Equilibration processes are simulated in a simplified model over geologic timescales at isothermal conditions where diffusion and gravity are the active mechanisms. A variety of initial conditions and reservoir fluid types are considered. The effect of barriers on the equilibration time is investigated for single and multiple barriers. The results are compared with analytical calculations. Longer equilibration times correspond to tighter constraints on connectivity.

This work shows the progression of compositional gradients over geologic time until all components have reached zero mass flux. It investigates the foundation of connectivity studies that rely on fluid composition. Determination of fluid equilibrium should become part of the standard procedure for reservoir connectivity evaluation.

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