Classification of reservoir fluids

           The reservoir fluids are classified into five types based the pressure and temperature behavior of the reservoir and subsequent production

• Black oil
• Volatile oil
• Condensate gas
• Wet gas
• Dry gas

Black oil:

·         Black oils are made up of heavy large and non volatile hydrocarbons.

·         When the pressure lies  anywhere on the line 1-2, then it is said to be undersaturated oil  which means the oil could dissolve more gas if more gas present.

·         If the pressure lies @ point 2 then it is said to be saturated oil which means the oil contains  the maximum amount of dissolved gas and can’t hold any more gas

·         If  the pressure reduction occur below the point 2, then gas will get evolved from the oil which forms free gas phase

·         Additional gas evolved from the oil as it moves from the reservoir to the surface

·         Thus black oil is also said to be low shrinkage crude oil

                                   

Volatile oil

·        Volatile oils contain fewer heavy molecules and more intermediate components (ethane through hexane) than black oils.

·        The color is generally lighter than black oil – brown, orange, or green.

·        Gas associated with volatile oils tends to be very rich and similar to retrograde condensate gas.

·        The reservoir temperature is always lower than the critical temperature of the fluid.

·        At point 2 the gas is get evolved from oil due to pressure reduction.

·        Compared to black oil, volatile oil produce more gas phase inside the reservoir and during production.

·        Thus volatile oil is said to be high shrinkage crude oil.

   

Condesate gas(Retrograde)

·        Condensate gas is very similar to volatile oils in terms of the colour of the produced oil.

·        The reservoir temperature of a condensate gas reservoir is greater than the critical temperature of the fluid.

·        The retrograde gas exists completely in a gaseous state inside the reservoir at point 1.

·         As the pressure decreases, the condensate exhibits a dew point at point 2.

·        As the reservoir further depletes and the pressure drops, liquid condenses from the gas to form a free liquid inside the reservoir.

                                       

Wet gas

·        Natural gas that contains significant heavy hydrocarbons such as propane, butane and other liquid hydrocarbons is known as wet gas or rich gas.

·        The general rule of thumb is if the gas contains less methane (typically less than 85% methane) and more ethane it is considered as wet gas.

·        Wet gas exists as a gas in the reservoir throughout the reduction in reservoir pressure .No liquid is formed inside the reservoir.

·        However, separator conditions lie within the phase envelope, causing some liquid to be formed at the surface. This surface liquid is normally called condensate, and the reservoir gas is sometimes called condensate-gas.

·         Note that the pressure path line does not enter the phase envelope, meaning no liquid is ever formed inside the reservoir.

             

   

Dry gas

·        Natural gas that occurs in the absence of condensate or liquid hydrocarbons, is called dry gas.

·        It is primarily methane with some intermediates.

·        The gas  present in the reservoir and there is no liquid formed either in the reservoir or at surface.

·        The pressure path line does not enter into the phase envelope in the phase diagram, thus there is only dry gas in the reservoir.

·        Note, the surface separator conditions also fall outside the phase envelope hence no liquid is formed at the surface.

                  

REFERENCES:

 https://ihsmarkit.com/btp/fekete.html

By: Mohammed Zubair Ahmed M

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