Page 137 - Principles of Applied Reservoir Simulation 2E

122 Principles of Applied Reservoir Simulation

two-phase regions. If we consider pressures in the single-phase region and move
to the right of the diagram by letting temperature increase towards the critical
point, we encounter volatile oils. At temperatures above the critical point but
less than the cricondentherm, reservoir fluids behave like condensates. The
cricondentherm is the maximum temperature at which a fluid can exist in both
the gas and liquid phases. When reservoir temperature is greater than the cri-
condentherm, we encounter gas reservoirs. A summary of these fluid types is
given in Table 13-1. Notice that separator gas-oil ratio (GOR) is a useful
indicator of fluid type.

Table 13-1
Rules of Thumb for Classifying Fluid Types

Fluid Separator GOR Pressure Depletion
Type (MSCF/STB) Behavior in Reservoir

Dry gas No surface liquids Remains gas
Wet gas >100 Remains gas

Condensate 3-100 Gas with liquid drop out

Volatile oil 1.5-3 Liquid with significant gas
Black oil 0.1 - 1.5 Liquid with some gas

Heavy oil ~ 0 Negligible gas formation

Let us consider a reservoir containing hydrocarbons that are at a pressure
and temperature corresponding to the single-phase black oil region. If reservoir
pressure declines at constant temperature, the reservoir pressure will eventually
cross the bubble point pressure curve and enter the two-phase gas-oil region.
Similarly, starting with a single-phase condensate and letting reservoir pressure
decline at constant temperature, the reservoir pressure will cross the dew point
pressure curve to enter the two-phase region. In this case, a free-phase liquid
drops out of the condensate gas. Once liquid drops out, it is very difficult to
recover. One recovery method is dry gas cycling, but the recovery efficiency
will be substantially less than 100%. If we drop the pressure even further, it is