Page 412 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

378 Production

constant pressure by injection mercury. The volume of the free gas is displaced
and the oil remaining in the cells are thus measured at cell conditions. This
procedure is repeated for all the pressure increments until only oil remains in
the cell at reservoir temperature and atmospheric pressure.
In contrast to flash vaporization, differential vaporization is undertaken with the
decreasing mass participating in the process. Equation 6-15 cannot be applied
because F is not constant anymore at the given set pressure. and temperature.
The question arises: Which process occurs in reservoir conditions? Some
specialists, e.g., Moses [8], assume that the reservoir process is a combination
of differential and flash. Such statements are incorrect. They produce misunder-
standing and confusion. The following is meant to classify and straighten out
this problem:
1. In reservoir conditions, differential process always occurs.
2. In production tubing and surface pipeline flow and in the separators the
flash process takes place (subject to some limiting assumption).
Flash process refers to the conditions where the mass of the considered system
does not vary with changes in pressure and temperature. Hash process in two-
phase regions (vaporization or condensation) can be defined in terms of total
system composition. The total system composition (zi) can be measured at any
point outside of saturation line, e.g., points A, B and C (Figure 6-3). As a
substitution the following treatment can be used; the total system composition
in two-phase region flash process remains constant. The flash process may ensue
for a composition z, that separates into two phases for the values of pressures
and temperatures inside the saturation curve area. After the temperature and
pressure are chosen, all the gas is in equilibrium with all the oil. In other words,
a change of pressure or temperature, or both, in a flash process can change
the equilibrium conditions according to the Gibbs phase rule. This rule provides
the number of independent variables that, in turn, define intensive properties.
Flash vaporization may be a batch or a continuous process. Treating two-phase
flow in tubing as a steady state, neglecting the gas storage effect, and gas
slippage result in a flash process. In a horizontal flow, and in separators, a
similar flash process comes about.
The same kind of equilibrium, but with its fluid mass decreasing dmerentially,
is called a differential process (liberation or condensation).
In reservoir conditions the hydrocarbon pore volume (HCW) remains constant
if the expansion of interstitial water and rock compressibility are neglected. For
such constant HCW it must be made clear that differential process occurs always
as diffkrential vaporization or differential condensation. Differential vaporization
takes place when the reservoir temperature is less than critical temperature of
solution (TmS < TJ, and also it takes place during retrograde gas reservoir
depletion, but only in the region pressure and temperature where the retrograde
liquid is vaporized.
In differential condensation, the oil reservoir pressure is maintained constant
or almost constant-for example, by gas injection. Differential condensation can
also occur just below the dew point in a gas-condensate reservoir.
Above the bubble point and the dew point curves, the virtual (apparent) value
of vaporization and/or condensation is zero, but because the mass of the fluid
in a depleted reservoir is changing as a result of decreasing pressure, the process
could be assumed to be differential. One important statement has to be add&
there is no qualitative difference between the reservoir fluid in either differential
or flash process, if pressure and temperature fall into the area outside of the