Reservoir fluids range from gaseous to very heavy liquids. Heavy crudes refer to liquid hydrocarbons with very high viscosity.
Generally, reservoir fluids are characterised by their physical state – gas or liquid. An extra classification is used for crude oil in terms of their API gravity.
In addition to hydrocarbons reservoir fluids or Petroleum may contain other constituents such as carbon dioxide(CO2), Hydrogen sulphide(H2S) and other complex compounds of nitrogen, sulphur and oxygen. So crude oils may be classified as :
- Paraffin based – Chief components are paraffins which when distilled leaves a solid residue of wax
- Asphatin based – Chief components are cyclic compounds like naphthenes which after distillation leaves solid residue of asphalt
- Mixed base crude – Mixture of paraffin and alphatene crudes.
Crude Oil
Quality of crude oil or liquid petroleum is measured in terms of its API gravity defined as
API =(141.5/ SG) – 131.5
As oil is produced from reservoir to wellbore, the pressure decreases to wellbore flowing pressure. Further reduction occurs as the fluid flows up the production tubing to the surface facilities at stck tank conditions. Changes occur in the fluid as discussed above as a function of the following:
- Bubble point pressure
- Formation volume factor, Bo
- Solution gas-oil ratio, Rs
In a given reservoir, fluids are distributed in accordance with their specific gravity.
Thus we have gas on top of crude oil and crude oil above saline water.
