Compositional Simulation

Part 13, Chapter 13: Frontier Topics in Reservoir Simulation

When Two Components Are Not Enough

Black-oil tracks only oil and gas pseudo-components through Rs and the formation volume factors. It fails when the composition itself changes: gas injection, a volatile oil, or a gas condensate exchange individual components between the phases.

Compositional simulationliquidvaporbubbledewlight-component fractionpressureInside the two-phase loop the feed splits along the tie-line into a light-rich vapor and a heavy-rich liquid; the lever rule gives a vapor fraction near 0.23.

The Equation-of-State Flash

Compositional simulation carries the real components and, in every cell each step, flashes the mixture with an equation of state into coexisting liquid and vapor. On a pressure-composition diagram the feed lies in the two-phase loop and splits along a horizontal tie-line.

The Lever Rule

The lever rule sets the vapor fraction from where the feed sits between the liquid and vapor compositions, V=z−xLyV−xLV = \dfrac{z - x_L}{y_V - x_L}. The cost is steep: many components and a flash per cell make compositional runs far heavier than black-oil.

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