TABLE OF CONTENTS The primary objective of initialization is to establish the correct amount and distribution of the original reservoir fluids. The majority of simulations are initialized based on static gravity capillary pressure equilibrium. This works well in the majority of cases. However, there is no requirement the fluids in the earth are static and, in truth, this is rarely completely correct.
A number of different methods are used to enter fluid saturations.
If the grid blocks are smaller than the capillary pressure transition zone, then the saturations of grid blocks can be accurately estimated from the capillary pressure curve at the midpoint of the grid block. This also requires the depth of the relevant contacts be specified.
This was the method used by early reservoir simulators. The distribution of fluids in the grid blocks was assumed to be uniform.
Later, it became possible to use a specialized technique to simplify reservoir problems that involved capillary pressure transition zones smaller than the grid block height. Known as vertical equilibrium (VE), this method required the saturations in the grid block be averaged. Since the transition zone is assumed to be of negligible thickness, the saturation of the block can be calculated using a linear relationship based on the distance from the specified gas-oil contact (GOC) or water-oil contact (WOC). This is shown in Figure 9-1.
Recall from chapter 8 that VE generated straightline relative permeabilities. Capillary pressure was ignored...
