Production System: Why There are Different Petroleum Types in Reservoirs

This post is an elaboration of the "Petroleum Production System" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Petroleum Production System" slide at page 7

The layer of formation which has been drilled will be classified as reservoir if it traps sufficient amount of hydrocarbon. Hydrocarbon, or commonly known as oil and gas, is a molecule with composition of chemical elements hydrogen (H) and carbon (C). Oil and gas made up of these two elements, with very diverse proportions. The Oil deposits found in one place will very rarely be found in other places with the same composition.
There are many factors effecting the composition of the hydrocarbon, including the history of the maturation. Hydrocarbon itself consists of a liquid phase (oil) and a gas phase, depending on the reservoir condition it takes place (pressure and temperature). Changes in reservoir conditions will result in a phase change as well as the physical properties change of reservoir fluids.

Because there are very different composition of hydrocarbon, also because of the reservoir conditions are vary from one reservoir to other reservoir, that is why we cannot expect type of hydrocarbon in the reservoir we found. The hydrocarbon properties we need to measure extensively will be about the density and specific gravity, viscosity, formation volume factor, gas solubility, compressibility, and bubble /dew point pressure. The measurement can be done in the field or in laboratories.

In the petroleum industry, there are 5 types of fluid reservoirs that have different types and characteristics. 5 types of reservoir fluid are

A. Dry gas

In the dry gas, the main component is methane, which is very light.  That is why the hydrocarbon state in the reservoir will be gas. in fact, it remains in gas state from reservoir to the surface. All properties from the reservoir to the surface does not change. Based on field data, this reservoir has an initial GOR ≥ 100,000 scf / STB and content of heptane plus of 0.7% mole

B. Wet gas

The main content of the reservoir is generally similar to the dry gas, just more portion of intermediate hydrocarbon content (C2 - C4). In reservoir, the state of hydrocarbons is gas, but at the time it flow to the surface, condensation is happened due to the decrease in pressure and temperature. Condensate that forms on the surface from  the wet gas is fairly expensive because  it contain  short-chain hydrocarbons which have a greater heating value. Based on the results of field data, this reservoir has a GOR of 70000-100000 scf / stb with more than 50 degrees API

C. Retrograde gas

The components are mostly filled with methane and intermediate hydrocarbon. The phase in reservoir is gas. Retrograde gas is unique because the liquid phase will be increased as the temperature and pressure decreased (whether the pressure in reservoir decreased or because it flow to the surface), but if the pressure continues to fall, then some of the liquid back to gas. That is why the properties in reservoirs will be different than on the surface. The GOR is from 8000 to 70,000 scf / stb with initial Specific Gravity Stock Tank Oil> 40 API.

D. Black Oil

The majority of the oil reservoir is in the form of black oil. The other name for black oil is low shrinkage oil, which means a slight reduction of pressure resulted in a slight decrease in the percentage of liquid phase.

E. Volatile Oil

The other name is high shrinkage oil which means a slight reduction of pressure resulted in a large decrease in the percentage of liquid phase. The GOR is from 2000 to 3300 scf / stb, The oil SG is about 30-50 API.

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