Reservoir Fluid Properties - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 17

Fig. 3.2 Three-dimensional diagram of a single-component system. Courtesy Schlumberger
temperature the dew-point line and the bubble-point line coincide at the critical
point . In summary, a single-component hydrocarbon (methane, ethane, etc.) can
exist as a gas, a liquid, or a gas-liquid mixture depending on the pressure and
temperature to which it is subjected.
Multicomponent Hydrocarbon System
In actual reservoirs, the hydrocarbons found are never single-component sys-
tems. Rather, they are mixtures of several different hydrocarbons and the behav-
ior of the mix is different from that of any single component. In particular, there
is no single vapor-pressure line. Rather, an envelope exists between the bubble-
point line and the dewpoint line within which gas and liquid coexist. Figure 3.3
illustrates this concept. Note that on the PV plane the bubble-point and the dew-
point are found as discontinuities and no straight-line portion exists on the PV
graph (see Fig. 3.4 ). What, therefore, distinguishes one type of reservoir from
another? What kind of production may be expected from a multicomponent
hydrocarbon system? The answers lie in the starting and ending points on a
pressure-temperature plot and their positions relative to the envelope between
the bubble-point and dew-point lines.