Page 100 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 39

Thus if n pound moles of liquid have been produced, of molecular weight M, then the
total mass of liquid is

nM = γ oρ w × (liquid volume)

where γ o is the oil gravity (water = 1), and ρ w is the density of water (62.43 Ib/cu.ft).
Since liquid hydrocarbon volumes are generally measured in stock tank barrels
(1 bbl = 5.615 cu.ft), then the number of pound moles of liquid hydrocarbon produced in
N p stb is

γ o N p
n = 350.5
M

Expressing this number of moles of hydrocarbon as an equivalent gas volume at
standard conditions, gives

nRT γ N 10.732 520
×
V = sc = 350.5 o p ×
sc
p sc M 14.7
γ o N p
or V = 1.33 10 5
×
sc
M
The correction in adding the equivalent gas volume to the cumulative gas production is
generally rather small, of the order of one percent or less, and is sometimes neglected.
If the initial reservoir pressure and temperature are such that the gas is at point C,
fig. 1.15(a), then during isothermal depletion liquid will start to condense in the
reservoir when the pressure has fallen below the dew point at D.

The maximum liquid saturation deposited in the reservoir, when the pressure is
between points D and E in the two phase region, is generally rather small and
frequently is below the critical saturation which must be exceeded before the liquid
becomes mobile. This phenomenon is analogous to the residual saturations, discussed
previously, at which flow ceases. Therefore, the liquid hydrocarbons deposited in the
reservoir, which are referred to as retrograde liquid condensate, are not recovered and,
since the heavier components tend to condense first, this represents a loss of the most
valuable part of the hydrocarbon mixture. It may be imagined that continued pressure
depletion below the dew point at E would lead to re-vapourisation of the liquid
condensate. This does not occur, however, because once the pressure falls below
point D the overall molecular weight of the hydrocarbons remaining in the reservoir
increases, since some of the heavier paraffins are left behind in the reservoir as
retrograde condensate. Therefore, the composite phase envelope for the reservoir
fluids tends to move downwards and to the right thus inhibiting re-vapourisation.

It is sometimes economically viable to produce a gas condensate field by the process
of dry gas re-cycling. That is, from the start of production at point C, fig. 1.15(a),
separating the liquid condensate from the dry gas at the surface and re-injecting the
latter into the reservoir in such a way that the dry gas displaces the wet gas towards
the producing wells. Since only a relatively small amount of fluid is removed from the
reservoir during this process, the pressure drop is small and, for a successful project,