Classification of Reservoir Using Phase Diagram

     A petroleum reservoir or oil and gas reservoir is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. Petroleum reservoirs are broadly classified as conventional and unconventional reservoirs. In case of conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability. While in unconventional reservoirs the rocks have high porosity and low permeability which keeps the hydrocarbons trapped in place, therefore not requiring a cap rock.

Types of reservoir:

         Based on phase diagram the reservoirs are classified into:

·        Single phase gas reservoir
·         Retrograde condensate – gas reservoir
·         Dissolved gas reservoir
·         Two phase gas reservoir

       In order to understand the phase diagram we need to understand some key concepts which includes:

Cricondentherm: It is defined as the maximum temperature above which liquid cannot be formed regardless of pressure.

Cricondenbar: It is defined as the maximum pressure above which no gas can be formed regardless of temperature.

Phase Envelope (two-phase region): The region enclosed by the bubble-point curve and the dew point curve wherein gas and liquid coexist in equilibrium.

Bubble-Point curve: The bubble-point curve is defined as the line separating the liquid-phase region from the two-phase diagram.

Dew-Point Curve: The Dew-point curve is defined as the line separating the vapor-phase region from the two-phase region.

Critical point: The point at which bubble point and dew point curve meets. The corresponding pressure and temperature at that point is said to be critical pressure and critical temperature.

Single phase gas reservoir:

·         If reservoir pressure and temperature are such that they:

          - lie outside the two phase region

          - lie on right hand side of the cricondentherm;

then the reservoir is a single gas phase reservoir.

·         During production the reservoir pressure drops and the reservoir temperature remain constant. So the reservoir fluid follows the path AA₁ during production.
·         Both temperature and pressure decline along the wellbore while producing to the surface, the reservoir fluid follows the path AA₂.
·         The path AA₂ crosses the two phase region, the produced fluid may be into two phases - gas or liquid. But the fluid composition will be the same.
·         Hence condensate liquid produced at the surface, but the reservoir fluid remains as single phase gas

Retrograde condensate – gas reservoir:

•If reservoir pressure and temperature are such that they:

    -lie outside the two phase region

    -lie on left hand side of the cricondentherm and right hand side of critical point.

then the reservoir is a Retrograde condensate gas reservoir.

·          The reservoir fluid is initially in single phase gas state. During production reservoir pressure drops thus, the reservoir fluid follows a path BB₁B₂B₃.
·         The reservoir fluid enter the two phase region at B₁.Below that pressure the liquid condenses out from the gas. The condensed liquid adheres at the walls of rock pores.
·         The liquid is immobile until certain saturation is achieved. The gas produced at the surface will have lower liquid content thus producing gas oil ratio rises.
·         At point B₂ the liquid content is maximum. The retrograde condensation process continues till B₂.
·          The retrograde condensation is the process in which some of the gas condenses into a liquid under isothermal conditions instead of expanding or vaporizing when pressure is decreased.
·         The composition of produced fluid changes after the dew point and the composition of remain reservoir fluid also changes. Hence there is small right shift in the two phase region.
·         From B₂ to B₃ the vaporization of liquid takes place, thus gas oil ratio decreases.

Dissolved gas reservoir:

·         If reservoir pressure and temperature are such that they:

    -lie outside the two phase region

    -lie on left hand side of the critical point

then the reservoir is a dissolved gas reservoir.

·         The reservoir fluid is initially in single phase liquid state. As the pressure declines it reaches the bubble point. Below this point bubbles and free gas will appear.
·         The free gas evolved will flow in to the wellbore. The flow of gas will be increasing and the flow of oil will be decreasing as the pressure drops during production.
·         On the phase diagram, the reservoir fluid path is CC₂. At CC₁ it crosses the bubble point curve and enters in to the two phase region.
·         Thus the volume of oil will be decreasing as the pressure drops.

Two phase reservoir:

     • If reservoir pressure and temperature are such that they:

-          lie inside the two phase envelop in P-T phase diagram

           then it would be a two phase reservoir.

·         It will consist of a liquid oil zone overlain by a gas cap. The compositions of oil and gas will be different.
·         The liquid or oil zone will be at its bubble point where as the gas cap will be at its dew point and it may be retrograde.

Summary of Reservoir Types:

	Type A Single phase gas	Type B   Gas condensate	Type C Dissolved gas	Type D  Two phase
Typical primary recovery mechanism	Volumetric gas drive	Volumetric gas drive	Depletion drive, water drive	Volumetric gas drive, depletion drive, water drive
Initial reservoir conditions	Single phase: Gas	Single phase: Gas	Single phase: Oil	Two phase: Oil and gas
Reservoir behavior as pressure declines	Reservoir fluid remains as gas.	Liquid condenses in the reservoir.	Gas vaporizes in reservoir.	Saturated oil releases additional gas.
Produced hydrocarbons	Primarily gas	Gas and condensate	Oil and gas	Oil and gas

    Reference: Applied petroleum reservoir engineering. 
 
  by: Mohammed Zubair Ahamed M  

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