This document classifies and describes five types of reservoir fluids: black oils, volatile oils, condensate (retrograde gas), wet gas (rich gas), and dry gas. Black oils contain heavy hydrocarbons and have low gas-oil ratios. Volatile oils contain lighter molecules and higher gas-oil ratios than black oils. Condensate gas resembles volatile oil but exists as a gas in the reservoir. Wet gas contains significant heavy gas hydrocarbons. Dry gas is primarily methane with no condensate.
2. RESERVOIR FLUID TYPES
• Black Oils
• Volatile Oils
• Condensate (Retrograde Gas)
• Wet Gas (Rich Gas)
• Dry Gas
3. BLACK OILS
Black oils are made up of a variety of components including large, heavy,
and non-volatile hydrocarbons.
Black oils are dark in color indicating the presence of heavy
hydrocarbons.
It is characterized as having initial gas-oil ratios of 2000 scf/stb or less,
and the stock tank gravity is usually 45° API or less.
4.
5. VOLATILE OIL
Volatile oils contain fewer heavy molecules and more intermediate
components (ethane through hexane) than black oils.
Volatile oils generally have initial gas-oil ratios in the 2000 to 3300
scf/Bbl range, and the stock tank gravity is usually 40° API or
higher.
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.
6.
7. CONDENSATE (RETROGRADE GAS)
Condensate gas is very similar to volatile oils in terms of the colour
(green, orange, brown, even clear)
It is characterized as having initial gas-oil ratios is equal to 3300 scf/stb
or greater and stock tank oil gravity is in between (40° to 60° API).
However, the reservoir temperature of a condensate gas reservoir is
greater than the critical temperature of the fluid, and so where a
volatile oil is a liquid at original reservoir pressure and temperature, a
condensate gas is a gas.
8.
9. WET GAS (RICH 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, and other more complex
hydrocarbons, it is labelled as wet gas.
Wet GOR are very high ( > 50000 scf/stb) for wet gases. However, the gravity
of the stock tank liquid does not change during the life of the
reservoir(remains constant).
10.
11. DRY GAS
Natural gas that occurs in the absence of condensate or liquid
hydrocarbons, or gas that had condensable hydrocarbons
removed, is called dry gas.
It is primarily methane with some intermediates.
The hydrocarbon mixture is solely gas in the reservoir and there is
no liquid (condensate surface liquid) formed either in the reservoir
or at surface.