Naturally Occurring Petroleum Fluids

GNN | May 30, 2017, 3:15 PM IST|

Naturally Occurring Petroleum Fluids
The naturally occurring petroleum fluids are composed of organic chemicals. Liquids obtained from different petroleum reservoirs have widely different characteristic. Broadly, it can be classified in five types of reservoir fluids based on its properties. These are usually called as
1. black oil
2. volatile oil
3. retrograde gas
4. wet gas
5. dry gas
The type of reservoir fluid should be determined at the early life of the reservoir as fluid type is the deciding factor in many of the decisions which must be made regarding the development of the reservoir. This will be a deciding factor in planning the types and sizes of surface equipment, the calculation procedures for determining oil and gas in place, the techniques of predicting oil and gas production profile and the plan of depletion and exploitation of the reservoir.
Identification of Fluid Type
For the identification of the reservoir fluid, the fluid is sampled in-situ condition and analysed in the laboratory for its physical and chemical properties. The physical properties like initial gas-oil ratio, the gravity of the stock tank liquid, bubble point pressure etc provides an idea about the fluid but to obtain its true characteristics, the mole percentage of different hydrocarbon components present needs to be determined and its phase diagram needs to be generated.
Phase Diagram: A graph of pressure plotted against temperature showing the conditions under which various phases of substances will be present.
Bubble Point: The temperature and pressure at which the first few molecules leave the liquid and form a small bubble of gas is called bubble point.
Dew Point: The temperature and pressure at which the first drop of liquid drops out from gas is known as dew point.

The kind of fluid produced from KG reservoir is considered mainly gas condensate and that is the reason to discuss about the properties of such fluids.
Retrograde Gas Condensate
The retrograde gas condensate reservoir is typically a gas reservoir where the formation is saturated with gas till the reservoir pressure is above the dew point pressure. In such reservoirs the critical temperature of the gas composition is less than the reservoir temperature. The typical phase envelop is shown below:



A typical phase diagram of Gas condensate
Typical composition of such fluid is
Gas Condensate
Component Mole %
N2 0.29
CO2 1.72
C1 79.14
C2 7.48
C3 3.29
iC4 0.51
nC4 1.25
iC5 0.36
nC5 0.55
C6 0.61
C7+ 4.8

The initial producing gas oil ratio approximately ranges between 3300 SCF/STB to 50000 SCF/STB and produced GOP is constant. Stock tank liquid gravity is between 50 degree API to 60 degree API. The fluid is usually transparent and water-white to orange and greenish coloured. The fraction of C6+ components has to be greater than 5.5% and C12+ is mostly absent.

A typical condensate sample