Types of Petroleum Reservoirs
According to the matter we are willing to study, we can first classify a reservoir according to its pressure and how this affects the fluid characteristics.
If we think of Oil and gas as its main components, we can also talk about how present is each of them in this complex hydrocarbon mixture called Petroleum. This will depend on something called PHASE BEHAVIOR, which is a description of the state of the matter (proportions of liquids and gases), which is simply a relationship between the variations on Pressure, Temperature and Volume (PVT) and the behavior of Oil.
From this phase behavior, we can define a type of fluid occuring in an specific time inside a reservoir rock storing it. There are 5 types of fluids generally named in petroleum industry as follows:
1. Black Oil
2. Volatile Oil
3. Gas Condensate Retrograde
4. Wet Gas
5. Dry Gas – approximately 100% methane, which is always gaseous at atmospheric and also at reservoir conditions.
When we have defined the type of fluid present inside our reservoir, we need to define the natural forces that will be involved in the oil production, also called Drive Mechanism. We also have to take into account all the fluids that could interact with the fluid, as the connate water or water inside nearby aquifers. We can classify the drive Mechanisms into three rough categories that can also be appearing ensembled:
1. Solution Gas Drive
Some of the associated characteristics are:
- A constant Volume (volumetric reservoir) – No foreign fluid encroaching (i.e. water)
- Two Phase flow at pressures under the bubble point.
- The Gas comes out of solution but never forms a gas cap, so the gas bubbles remain in the oil phase while it flows.
- This is related and can be simulated in a laboratory with a flash gas-liberation process
2. Water Drive (also called ‘water encroachment’)
Some of the main features are:
- The reservoir volume does change while water encroaches
- There is a displacement of oil by water
- Gas phase could also be present, meaning a combination with the gas drives
- The rate of production must be optimized
- According to the geometry and spatial distribution of water and hydrocarbons, it can be classified into Edge-Water Drive of Bottom Water Drive
3. Gas Cap Expansion (also called segregation or gravity drainage)
This also occurs when the pressure has allowed some gas to be released and removed from the oil phase. For it to exist, the vertical permeabilityof the reservoir rock should be high enough to allow the counterflow of the released gas (i.e. more than 100 md).
As production increases, the gas cap lying on top of the reservoir will expand
This process can also be simulated in the lab with a differential gas liberation process.
According to the types of Drive Mechanisms, the production will vary on its characteristics and with it, the economical aspects of the projects.