Reservoir types. Classification methodology

Reservoir types. Classification methodology
The objective of this lecture is to review the basic principles of reservoir fluid phase behavior and illustrate the use of phase diagrams in classifying types of reservoirs and the native hydrocarbon systems.
7.1. Classification of reservoirs and reservoir fluids
Petroleum reservoirs are broadly classified as oil or gas reservoirs. These broad classifications are further subdivided depending on:
• The composition of the reservoir hydrocarbon mixture
• Initial reservoir pressure and temperature
• Pressure and temperature of the surface production
The conditions under which these phases exist are a matter of considerable practical importance. The experimental or the mathematical determinations of these conditions are conveniently expressed in different types of diagrams commonly called phase diagrams. One such diagram is called the pressure-temperature diagram (figure 7.1).
F igure 7.1. Typical P-T diagram for a multicomponent system
These multicomponent pressure-temperature diagrams are essentially used to:
• Classify reservoirs
•Classify the naturally occurring hydrocarbon systems
• Describe the phase behavior of the reservoir fluid
To fully understand the significance of the pressure-temperature diagrams, it is necessary to identify and define the following key points on these diagrams:
• Cricondentherm (T_ct) is defined as the maximum temperature above which liquid cannot be formed regardless of pressure (point E).
• Cricondenbar (P_cb) is the maximum pressure above which no gas can be formed regardless of temperature (point D).
• Critical point is for a multicomponent mixture is referred to as the state of pressure and temperature at which all intensive properties of the gas and liquid phases are equal (point C). At the critical point, the corresponding pressure and temperature are called the critical pressure P_c and critical temperature T_c of the mixture.
• Phase envelope (two-phase region) - the region enclosed by the bubble-point curve and the dew-point curve (line BCA), where in gas and liquid coexist in equilibrium, is identified as the phase envelope of the hydrocarbon system.
• Quality lines - the dashed lines within the phase diagram are called quality lines. They describe the pressure and temperature conditions for equal volumes of liquids. Note that the quality lines converge at the critical point (point C).
• Bubble-point curve - the bubble-point curve (line BC) is defined as the line separating the liquid-phase region from the two-phase region.
• Dew-point curve (line AC) is defined as the line separating the vapor-phase region from the two-phase region.
In general, reservoirs are conveniently classified on the basis of the location of the point representing the initial reservoir pressure P and temperature T with respect to the pressure-temperature diagram of the reservoir fluid. Accordingly, reservoirs can be classified into basically two types. These are:
• Oil reservoirs if the reservoir temperature T is less than the critical temperature T_c of the reservoir fluid, the reservoir is classified as an oil reservoir.
• Gas reservoirs if the reservoir temperature is greater than the critical temperature of the hydrocarbon fluid, the reservoir is considered a gas reservoir.