Hydrocarbon Reservoir Classification: Oil and Gas Fluid Types

Low-Shrinkage Crude Oil Properties

This crude oil type is characterized by quality lines that are closely spaced near the dewpoint curve in its phase diagram. The other associated properties of low-shrinkage crude oil include:

• Oil Formation Volume Factor: Less than 1.2 bbl/STB
• Gas-Oil Ratio (GOR): Less than 200 scf/STB
• Oil Gravity: Less than 35° API
• Color: Black or deeply colored

Volatile Crude Oil Characteristics

Volatile crude oil exhibits quality lines that are close together near the bubblepoint and become more widely spaced at lower pressures. This crude oil type is commonly characterized by a high liquid shrinkage immediately below the bubblepoint, as illustrated in Figure 1-7.

Other characteristic properties of volatile crude oil include:

• Oil Formation Volume Factor: Less than 2 bbl/STB
• Gas-Oil Ratio (GOR): Between 2,000 and 3,200 scf/STB
• Oil Gravity: Between 45° and 55° API
• Liquid Recovery: Lower liquid recovery at separator conditions, as indicated by point G on Figure 1-6
• Color: Greenish to orange

Retrograde Gas-Condensate Reservoirs

A reservoir is classified as a retrograde gas-condensate reservoir if its temperature (T) lies between the critical temperature (Tc) and the cricondentherm (Tct) of the reservoir fluid.

Key characteristics include:

• The gas-oil ratio for a condensate system increases with time due to liquid dropout and the loss of heavy components in the liquid.
• Condensate Gravity: Above 50° API
• Stock-Tank Liquid Color: Usually water-white or slightly colored

Wet-Gas Reservoir Properties

The temperature of a wet-gas reservoir is above the cricondentherm of the hydrocarbon mixture. Because the reservoir temperature exceeds the cricondentherm of the hydrocarbon system, the reservoir fluid will always remain in the vapor phase region as the reservoir is depleted isothermally, along the vertical line A-B.

Wet-gas reservoirs are characterized by the following properties:

• Gas-Oil Ratio (GOR): Between 60,000 to 100,000 scf/STB
• Stock-Tank Oil Gravity: Above 60° API
• Liquid Color: Water-white
• Separator Conditions: Separator pressure and temperature lie within the two-phase region

Dry-Gas Reservoir Definition

In a dry-gas reservoir, the hydrocarbon mixture exists solely as a gas, both within the reservoir and in surface facilities.

Typically, a system with a gas-oil ratio greater than 100,000 scf/STB is classified as a dry gas.