Page 101 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 40

the aim should be to operate at above dew point pressure until dry gas breakthrough
occurs in the producing wells. After this, the injection is terminated, and the remaining
dry gas produced.

The dry gas material balance equations can also be applied to gas condensate
reservoirs if the single phase Z−factor is replaced by the, so-called, two phase Z−factor.
This must be experimentally determined in the laboratory by performing a constant
volume depletion experiment.
A volume of gas, G scf, is charged to a PVT cell at an initial pressure p i, which is above
the dew point, and at reservoir temperature. The pressure is reduced in stages as gas
is withdrawn from the cell, and measured as G′ scf, without altering either the cell
p
volume or the temperature. This simulates the production of the reservoir under
volumetric depletion conditions and therefore, applying the depletion type material
balance equation, (1.35), and solving explicitly for Z gives

p
Z 2 phase = (1.46)
p G ' p
−
i
Z i 1− G

Until the pressure has dropped to the dew point, the Z−factor measured in this
experiment is identical with the Z−factor obtained using the technique described in
sec. 1.5(a). Below the dew point, however, the two techniques will produce different
results.

The latter experiment, for determining the single phase Z−factor, implicitly assumes
that a volume of reservoir fluids, below dew point pressure, is produced in its entirety to
the surface. In the constant volume depletion experiment, however, allowance is made
for the fact that some of the fluid remains behind in the reservoir as liquid condensate,
this volume being also recorded as a function of pressure during the experiment. As a
result, if a gas condensate sample is analysed using both experimental techniques, the
two phase Z−factor determined during the constant volume depletion will be lower than
the single phase Z−factor. This is because the retrograde liquid condensate is not
included in the cumulative gas production G′ in equ. (1.46), which is therefore lower
p
than it would be assuming that all fluids are produced to the surface, as in the single
phase experiment.

Figure 1.15(b) shows a typical phase diagram for oil. As already noted, because oil
contains a higher proportion of the heavier members of the paraffin series, the two
phase envelope is more horizontally inclined than for gas.

If the initial temperature and pressure are such that the reservoir oil is at point A in the
diagram, there will only be one phase in the reservoir namely, liquid oil containing
dissolved gas. Reducing the pressure isothermally will eventually bring the oil to the
bubble point, B. Thereafter, further reduction in pressure will produce a two phase
system in the reservoir; the liquid oil, containing an amount of dissolved gas which is
commensurate with the pressure, and a volume of liberated gas. Unfortunately, when
liquid oil and gas are subjected to the same pressure differential in the reservoir, the