CO2 gas injection is an interesting EOR technique for mature oil fields and can also be a way to decrease emission of a greenhouse gas. When used as EOR gas an optimum recovery will only be achieved if the injected CO2, possibly after a number of contacts, becomes miscible with the oil. Whether miscibility develops in a reservoir is determined by reservoir conditions, permeability and reservoir fluid composition.
A number of different PVT experiments have been designed to deal with gas injection including swelling, multi-contact, equilibrium contact and slimtube tests. A slimtube experiment gives a measure of the percent recovery as a function of reservoir pressure and a measure of the minimum miscibility pressure (MMP), while the other mentioned experiments provide information on the changes in phase properties and phase compositions after one or more contacts between gas and oil.
The injected CO2 may cause the reservoir fluid to split into two separate CO2-rich liquid phases. A liquid-liquid split can be difficult to account for in reservoir evaluations, since compositional reservoir simulators only handle one liquid phase.
Gas injection processes are modeled with compositional reservoir simulators carrying out EoS phase equilibrium calculations in each grid block in each time step. The quality of the simulation results are dependent on a reliable fluid characterization (EoS model) matching the EOR PVT data.
Dedicated tie-line MMP simulations are becoming increasingly popular as a fast alternative to full 3D or 1D compositional reservoir simulation studies. The MMP options further have the advantage that it is possible to tune the EoS model to match a particular minimum miscibility pressure.
The targets of the JIP are:
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