Page 152 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 137

of the blocks in Figure 14-3. Thus, if the block mid-point is above the gas-oil
contact (GOC), the entire block is treated as a gas cap block (single-phase gas
with irreducible water saturation), even if much of the block extends into the
oil column. A more accurate representation may be obtained by decreasing the
thickness of the gridblocks, but this often results in a substantial increase in the
cost of making computer runs. The relative benefits of incremental accuracy
versus incremental cost must be considered when modeling transition zones.

Gas Cap -^^
Oil Column
Water Leg

Figure 14-2. Case 1: Neglect transition zones.

Gas Cap
*
Gas Cap
GOC

s Oil Col.
Oil Col. Oil Col.
Oil Col.

woe

Water Leg
Water Leg

Figure 14-3. Initial fluid distribution in model without transition
zone.

The inclusion of a transition zone in the model requires specifying a
capillary pressure (P c) curve as a function of saturation for whatever transition
zone is being modeled: oil-water, gas-oil, or gas-water. The height h tz of the
transition zone above the free water level (the level corresponding to P c=0 psia)