Page 210 - PVT Property Correlations

184 PVT Property Correlations

The little gas production is usually flared at these quantities for economic
reasons. Additionally, at these low levels of bubble point pressure, the reser-
voir pressure may not ever reach the bubble point pressure (the reservoir may
be abandoned before reaching the bubble point pressure). In some cases (e.g.,
shallow reservoirs that are developed with the high density of production
wells), little free gas is generated in the reservoir when the pressure declines
below the bubble point pressure. Even in those cases, gas production will not
increase significantly, and the concept of low GOR oils can be justified.
Low GOR oils usually exist at shallow and medium depths. One can
argue that oil trapped near the surface may not contain large amounts of gas
as the volatile components of oil may have escaped the oil and found their
way to the surface over geologic ages. Low GOR oils are usually discovered
at low initial reservoir pressures.
As discussed in Chapter 2, Reservoir-Fluid Classification, many classifi-
cations for reservoir fluid types have been suggested. The most widely used
classification is that of McCain (1994). This classification divides oils into
two types: black and volatile. The low GOR oil is included in the black-oil
classification of McCain (1994). However, low GOR oils, as a class of fluids
on their own, differ from typical black oils as follows:
1. Low GOR oils produce very little gas, which is often ignored in field
measurements.
2. Production remains above the bubble point for the entire life of the
reservoir.
3. Calculation of PVT properties is reduced to the undersaturated portion of
the PVT properties curve.
4. Solution GOR can be neglected for reservoir engineering calculations.
However, it can be assumed to be a constant value for production engi-
neering calculations (e.g., vertical-lift performance calculations of pro-
duction wells).
A typical phase diagram of low GOR oil is shown in Fig. 8.2.This
figure shows that at reservoir temperature, bubble point pressure is low and
may not be reached during reservoir depletion. The oil, therefore, remains in
the single-phase liquid region for the entire life of the reservoir. The phase
diagram of low GOR oil also shows that even if the reservoir pressure
declines below the bubble point pressure, little gas will be formed in the res-
ervoir. Efficient recovery of the oil from the reservoirs will require a signifi-
cant number of wells and/or water injection or enhanced oil recovery.
Fig. 8.3 represents three different phase diagrams plotted using an EOS
program for three different low GOR oil samples taken from the same
region. The three samples represent oils that are produced from stacked reser-
voirs. The depth of the three samples ranges from 5000 to 7000 ft TVDss.
The composition of the three samples is given in Table 8.1. Although the
three samples differ in composition (values of C7 1 mole percent range