Page 142 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 127

Both saturated and undersaturated curves are included as functions of pressure
only. Phase changes occur at the saturation pressures. Single-phase oil becomes
two-phase gas-oil when pressure drops below the bubble point pressure (P 6),
and single-phase gas becomes two-phase gas condensate when pressure drops
below the dew point pressure (P d).
Simulators run most efficiently when fluid property data are smooth
curves. Any discontinuity in a curve can cause numerical difficulties. Ordinarily,
realistic fluid properties are smooth functions of pressure except at points where
phase transitions occur. As a practical matter, it is usually wise to plot input PVT
data to verify the smoothness of the data. Most simulators reduce the nonlinearity
to interpo-
of the gas formation volume factor B g by using the inverse b g = l/B g
late gas properties.
Oil properties from a laboratory must usually be corrected for use in a
black oil simulator [Moses, 1986]. Flow in the reservoir is a relatively slow
process that corresponds to a differential process in the laboratory. A differential
process is one in which pressures are allowed to change in relatively small
increments. For comparison, a flash process allows pressures in the experiment
to change by relatively large increments. The production of oil up the wellbore
to surface facilities is considered a flash process. Oil is flashed to the surface
through several pressure and temperature regimes. The corrections applied to
oil property data are designed to adjust the data to more adequately represent
fluids as they flow differentially in the reservoir prior to being flashed to surface
conditions. The corrections alter solution gas-oil ratio and oil formation volume
factor. The effect of the correction is illustrated by the case study in Chapter 20,
The oil property correction is often significant.
Water properties must also be entered in a simulator. Ideally water
properties should be measured by performing laboratory analyses on produced
water samples. If samples are not available, correlations are often sufficiently
accurate for describing the behavior of water.
In the absence of reliable fluid data for one or more of the reservoir fluids,
it may be necessary to use correlations. McCain [ 1991 ] reviewed the state of the
art in the use of correlations to describe fluid properties. New correlations for