"PVT Analysis for Compositional Simulation" by Steve Furnival

Generally, the fluid composition within the production system is not fixed for a variety of reasons. Within the reservoir, the following changes can take place:
• Composition varies with depth and areal location. The presence of high permeability streaks can then allow different fluids to mix.
• As fluid drops below saturation pressure, one phase – generally the gas – will flow in preference to the oil so the produced well composition changes with time. This effect is particularly important for Gas Condensates and Volatile Oils – near critical fluids.
• Gas injection for pressure maintenance or miscibility processes.
Within the production system:
• Fluids from different parts of the reservoir or reservoirs, can mix, i.e. Eastern Trough Area Project (ETAP).
• Gas injection for Gas-Lift.
• Changes in surface separation.
• Phase slippage in long pipe and flow lines can cause formation of liquid slugs.

All these cases, and more, point to the need for a compositional treatment of the fluid system. These methods are computational expensive. However, with the rapid increase in computer power at reducing cost, they are all now achievable on a high-end PC.