Page 132 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 119

When the two components are mixed together (say in a mixture of 10% ethane,
90% n-heptane), the bubble point curve and the dew point curve no longer
coincide, and a two-phase envelope appears. Within this two-phase region, a mixture of
liquid and gas exists, with both components being present in each phase in
proportions dictated by the exact temperature and pressure, that is the composition
of the liquid and gaseous phases within the two-phase envelope is not constant. The
mixture has its own critical point C m3 .
Using this mixture as an example, consider starting at pressure A and isothermally
reducing the pressure to point D on the diagram. At point A the mixture exists
entirely in the liquid phase. When the pressure drops to point B, the first bubble of
gas is evolved, and this will be a bubble of the lighter component, ethane. As the
pressure continues to drop, the gaseous phase will acquire more of the heavier
component and hence the liquid volume decreases. At point C, the last drop of
liquid remaining will be composed of the heavier component, which itself will
vaporise as the dew point is crossed, so that below the dew point the mixture exists
entirely in the gaseous phase. Outside the two-phase envelope the composition is
fixed, but varies with pressure inside the two-phase envelope.
Moving back to the overall picture, it can be seen that as the fraction of ethane in
the mixture changes, the position of the two-phase region and the critical point
change, moving to the left as the fraction of the lighter component (ethane) increases.
The example of a binary mixture is used to demonstrate the increased
complexity of the phase diagram through the introduction of a second component
in the system. Typical reservoir fluids contain hundreds of components, which
makes the laboratory measurement or mathematical prediction of the phase
behaviour more complex still. However, the principles established above will be
useful in understanding the differences in phase behaviour for the main types of
hydrocarbon identified.

6.2.3.2. Phase behaviour of reservoir fluid types
Figure 6.20 helps in explaining how the phase diagrams of the main types of
reservoir fluid are used to predict fluid behaviour during production and how this
influences field development planning. It should be noted that there are no values
on the axes, since the scales will vary for each fluid type. Figure 6.20 shows the
relative positions of the phase envelopes for each fluid type.
The four vertical lines on the diagram show the isothermal depletion loci for the
main types of hydrocarbon: gas (incorporating dry gas and wet gas), gas condensate,
volatile oil and black oil. The starting point, or initial conditions of temperature and
pressure, relative to the two-phase envelope are different for each fluid type.

6.2.3.3. Dry gas
The initial condition for the dry gas is outside the two-phase envelope, and is to the
right of the critical point, confirming that the fluid initially exists as a single-phase
gas. As the reservoir is produced, the pressure drops under isothermal conditions, as
indicated by the vertical line. Since the initial temperature is higher than the
maximum temperature of the two-phase envelope (the cricondotherm – typically less