Reservoir Types

Once the data acquisition studies are concluded, the analysis of the oil and gas accumulated in the reservoir begins. In this phase, the different types of oil and gas are identified and the quantities are estimated with a greater degree of precision.     

An oil and gas reservoir type is inferred from the data analysis that correlates, among other things, the pressure, temperature, and chemical composition conditions of the reservoir fluids. Schematically, the behavior of a mixture of hydrocarbons is represented through a phase diagram, in the figure below.

Each mixture will have its own phase diagram, calculated through equations of state, when there is knowledge of the substances that constitute it and their respective quantities – fluid composition. This is determined in a lab through the chromatography of samples extracted from the reservoir through production testing (bottom sampling), wireline testing, surface collecting (separator), etc.  

According to the observed characteristics, oil and gas reservoirs are classified into five groups (two oil groups and three gas groups).

Oil Reservoirs

Oil reservoirs contain mixtures of hydrocarbons in which the initial pressure and temperature condition is a point to the left of the critical point (CP), above in the phase diagram.

Black Oil Reservoir

Black oil reservoirs contain large proportions of heavy hydrocarbons in their composition. They are also known as low-shrinkage oil reservoirs. They present high recovery on the surface of the reservoir’s produced liquid, due to the reduced quantity of dissolved gas. This is the type of reservoir found in the Atlanta and Oliva fields, in the main fields of the Campos basin, and in international fields like the Bentley field, in the North Sea. “Black Oil” is a definition that covers an enormous amount of oil reservoirs, and generally includes reservoirs containing heavy and medium oil. Typical density values are between 10 and 40°API, with a GOR bellow 200 m³std/m³std.

Volatile Oil Reservoir

This fluid contains greater proportions of light and intermediate hydrocarbons in comparison to black oil. Volatile oil releases a relatively larger volume of gas during its production, leaving a smaller quantity of recovered liquid on the surface. Therefore, they are known as high-shrinkage oil. They are generally found in  reservoirs containing light oil, including the Brazilian pre-salt fields and the Brent field. They typically present a density of up to 50°API and a GOR between 200 and 1000 m³std/m³std.

Gas Reservoir

Gas reservoirs contain mixtures of hydrocarbons in which the initial pressure and temperature condition is a point to the right of the critical point (CP), above the phase diagram.

Retrograde Gas Condensate Reservoir

The mixture of hydrocarbons is originally found in the reservoir in the gas phase. Due to fluid production and original pressure reduction of the accumulation, a liquid phase is formed in the reservoir itself, through gas condensation. Oil and gas are produced on the surface. These types of reservoirs present a GOR below 10.000 m³std/m³std and the liquids produced on the surface (condensate) are similar to water, or have a light coloration. Economically, they are more attractive than dry gas reservoirs, because, in addition to gas, they produce condensate, a liquid with high commercial value.

Wet Gas Reservoir

A wet gas reservoir does not present liquid formation. This means that the liquid produced on the surface is condensed in the distance covered during production, in a later point to the passage of the gas from the reservoir to the well. This is the type of reservoir that characterizes the Manati field. Generally, they present a GOR over 10.000 m³std/m³std, the API of the liquid produced on the surface (condensate) is greater than 50° and its appearance is similar to water. As in the retrograde gas condensate reservoirs they produce condensate on the surface, although in smaller quantities, therefore their commercial value is greater than that of dry gas reservoirs.

Dry Gas Reservoir

The mixture of hydrocarbons in dry gas reservoirs is presented solely in the gas phase, both in the reservoir and on the surface. The fluid composition and original and operational conditions of the field impede any condensation of the gas. They are generally lean gases, composed essentially of methane. The only liquid associated to a dry gas reservoir is water. 

Last updated on 2015-04-30T16:21:37

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