Page 21 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 9

1.9.2 Classification of Reservoir Based on Reservoir Fluid
Generally, reservoirs are classified into oil and gas reservoirs from the reservoir fluid
point of view. Based on the phase diagram plot and the point representing reservoir
condition (reservoir pressure temperature), the following subdivisions are drawn for
oil and gas reservoirs:
Oil reservoirs:
• Ordinary black oil
• Volatile crude oil
• Near-critical crude oil
Gas reservoirs:
• Retrograde gas reservoirs
• Wet gas reservoirs
• Dry gas reservoirs

1.9.3 Natural Gas Properties

Gas is defined as a light fluid with low viscosity and density having significant compress-
ibility. Natural gas is composed of hydrocarbon and nonhydrocarbon compounds.
Hydrocarbons composing gas fluid are chiefly the lightest, including methane, ethane,
propane, butane, pentane, and a small amount of heavier components. Nitrogen, carbon
dioxide, and hydrogen sulfide are the most nonhydrocarbon compounds found in natu-
ral gas. In order to simulate reservoirs and investigate different scenarios, modeling res-
ervoir fluid behavior is essential. In other words, pressure volume temperature study
and physical properties for reservoir fluids should be carried out. The required physical
properties in gas reservoirs for solving problems are: apparent molecular weight, density,
specific gravity, compressibility factor, gas formation volume factor, and gas viscosity.

1.9.3.1 Apparent Molecular Weight
Apparent molecular weight for gases is mathematically defined as follows:
X
MW a 5 y i MW i
where MW a is apparent molecular weight of mixture, y i is mole fraction of compo-
nent I, and MW i is molecular weight of component i.

1.9.3.2 Density
In order to analyze fluid phase behavior, studying the relationship between pressure,
volume, and temperature is necessary. For ideal gases the mathematical relationship is
expressed as:

PV 5 nRT