The generalized Impes method applies to simulation models involving any number of conservation equations. The Impes pressure equation is a linear combination of the linearized conservation equations. This paper shows the generality, simplicity, uniqueness, and derivational brevity of that equation. The associated Impes reduction vector leads directly to the value of total compressibility in a multiphase grid block. That compressibility in turn gives several error checks on black oil pvt data.

The Impes-type compositional models of Young and Stephenson [7], Acs etal [8], and Watts [9] are compared and found to be very similar. The former model is found to be more general and more efficient than the latter two, excluding the partial Jacobian update [7] and sequential [9] features.

An example problem serves two comparative purposes related to Impes model efficiency.

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