Petroleum Reservoir Classification and Phase Behavior
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Petroleum Reservoir Classification
Petroleum reservoirs are broadly classified as oil or gas reservoirs. These 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
Key Phase Behavior Definitions
Cricondentherm (Tct)
The cricondentherm is defined as the maximum temperature above which liquid cannot be formed, regardless of pressure.
Cricondenbar (pcb)
The cricondenbar is the maximum pressure above which no gas can be formed, regardless of temperature.
Critical Point
The critical point for a multicomponent mixture is the state of pressure and temperature at which all intensive properties of the gas and liquid phases are equal.
Phase Envelope (Two-Phase Region)
The region enclosed by the bubble-point curve and the dew-point curve (line BCA), wherein 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.
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
The dew-point curve (line AC) is defined as the line separating the vapor-phase region from the two-phase region.
Reservoir Categorization
- Oil Reservoirs: If the reservoir temperature (T) is less than the critical temperature (Tc) 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.