Page 135 - Hydrocarbon Exploration and Production Second Edition

122 Reservoir Fluids

gas, at which point the reservoir pressure is ‘blown down’ as for a wet gas reservoir.
The sales profile for a recycling scheme consists of early sales of condensate liquids
and delayed sale of gas. An alternative method of keeping the reservoir above the dew
point but avoiding the deferred gas sales is by water injection, but this is rarely done as
gas trapped behind an advancing gas–water contact represents a significant loss.
Figure 6.20 shows that as the pressure is reduced below the dew point, the
volume of liquid in the two-phase mixture initially increases. This contradicts the
common observation of the fraction of liquids in a volatile mixture reducing as
the pressure is dropped (vaporisation), and explains why the fluids are sometimes
referred to as retrograde gas condensates.

6.2.3.6. Volatile oil and black oil
For both volatile oil and black oil, the initial reservoir temperature is below the
critical point, and the fluid is therefore a liquid in the reservoir. As the pressure drops,
the bubble point is eventually reached, and the first bubble of gas is released from the
liquid. The composition of this gas will be made up of the more volatile components
of the mixture. Both volatile oils and black oils will liberate gas in the separators,
whose conditions of pressure and temperature are well inside the two-phase
envelope.
A volatile oil contains a relatively large fraction of lighter and intermediate
components which vaporise easily. With a small drop in pressure below the bubble
point, the relative amount of liquid to gas in the two-phase mixture drops rapidly, as
shown in the phase diagram (Figure 6.20) by the wide spacing of the iso-vol lines. At
reservoir pressures below the bubble point, gas is released in the reservoir, and is
known as solution gas, since above the bubble point this gas was contained in solution.
Some of this liberated gas will flow towards the producing wells, whilst some will
remain in the reservoir and migrate towards the crest of the structure to form a
secondary gas cap.
Black oils are a common category of reservoir fluids, and are similar to volatile
oils in behaviour, except that they contain a lower fraction of volatile components
and therefore require a much larger pressure drop below the bubble point before
significant volumes of gas are released from solution. This is reflected by the position
of the iso-vol lines in the phase diagram, where the lines of low liquid percentage
are grouped around the dew point line.
Volatile oils are known as high shrinkage oils because they liberate relatively large
amounts of gas either in the reservoir or the separators, leaving relatively smaller
amounts of stabilised oil compared to black oils (also called low shrinkage oils).
When the pressure of a volatile oil or black oil reservoir is above the bubble
point, we refer to the oil as undersaturated. When the pressure is at the bubble point
we refer to it as saturated oil, since if any more gas were added to the system it could
not be dissolved in the oil. The bubble point is therefore the saturation pressure for
the reservoir fluid.
An oil reservoir which exists at initial conditions with an overlying gas cap must
by definition be at the bubble point pressure at the interface between the gas and the
oil, the gas–oil contact (GOC). Gas existing in an initial gas cap is called free gas,
whilst the gas in solution in the oil is called dissolved or solution gas.