Use of vertical equilibrium in 2-D simulation of 3-D reservoir performance

The use of the Vertical Equilibrium (VE) concept in simulating heterogeneous reservoirs is discussed. Where VE criteria are met, this technique allows 2-dimensional simulation of 3-dimensional problems with equivalent accuracy and with attendant substantial savings in data preparation and machine time. The VE concept itself and a new dimensionless group are presented as criteria for the validity of VE as applied to thick reservoirs or to reservoirs where the capillary transition zone is a small fraction of thickness. A description of the generation of the appropriate pseudo relative permeability and capillary pressure curves is presented. In addition to the dimensionless group criterion, an actual comparison of the results of an x-z cross section and a 1-dimensional areal run with VE illustrates the validity of the VE concept. Numerical results of such a comparison along with the attendant machine time requirements are presented. More than an order of magnitude difference in machine time requirements was experienced. Finally, an actual field case example shows the utility of VE as applied to a reservoir containing one or multiple gas pools residing on a common aquifer.