Undersaturated Volatile Oil Reservoir

An undersaturated volatile oil reservoir is a type of oil reservoir that contains oil with high gas content but has not yet reached its bubble point pressure.

Here’s a breakdown of the concept

• Volatile Oil  -  This type of oil is characterized by having a high gas-oil ratio (GOR) and high API gravity (usually between 40° to 60° API).  It contains a significant amount of dissolved gas that can be released when the pressure drops.  The oil tends to be more fluid and lighter compared to other types of crude oil.
• Undersaturated  -  This term refers to the pressure condition of the reservoir.  An undersaturated reservoir is one where the reservoir pressure is higher than the bubble point pressure, the pressure at which gas begins to come out of solution and form a free gas phase within the reservoir.  In an undersaturated condition, all the gas remains dissolved in the oil.
• Reservoir Behavior  -  In an undersaturated volatile oil reservoir, as long as the reservoir pressure remains above the bubble point pressure, the oil will not release any gas.  The oil is in a single-phase liquid state.  As production occurs and pressure drops, the reservoir may eventually reach the bubble point pressure, at which point the reservoir will become a two-phase system (oil and gas), with gas coming out of solution.

Key Characteristics

• High Gas Oil Ratio (GOR)  -  The oil has a significant amount of gas dissolved in it.
• High API Gravity  -  The oil is lighter and more fluid.
• Pressure above Bubble Point  -  The reservoir pressure is still high enough that the gas remains dissolved in the oil.

Understanding whether a volatile oil reservoir is undersaturated is crucial for predicting production behavior and managing the reservoir effectively.