Reservoir fluids range from light gases to very heavy liquids. Knowing which type of fluid a reservoir holds decides how the field is developed, how the wells are completed, and how the fluids are processed at surface. This article explains how reservoir fluids are classified by physical state, by chemical makeup, and by API gravity.
Classification by Physical State and Behavior
Generally, reservoir fluids are characterised by their physical state, gas or liquid, at reservoir and surface conditions. Heavy crudes are liquid hydrocarbons with very high viscosity. The type of fluid is confirmed by reservoir fluid sampling. In practice, five main fluid types are recognized:
| Fluid type | Typical behavior | Typical producing GOR (scf/STB) |
|---|---|---|
| Black oil | Liquid at reservoir conditions; some dissolved gas | Below about 2,000 |
| Volatile oil | Light oil with a lot of dissolved gas, shrinks strongly at surface | About 2,000 to 3,300 |
| Gas condensate | Gas in the reservoir that drops out liquid as pressure falls | About 3,300 to 50,000 |
| Wet gas | Gas at reservoir conditions; liquid forms only at surface | Above about 50,000 |
| Dry gas | Mostly methane; no liquid at surface | Practically no liquid |
The GOR (gas-oil ratio) values are typical guideline ranges and vary between sources. The fluid type is confirmed from samples and PVT laboratory tests.
Classification by Chemical Composition
In addition to hydrocarbons, reservoir fluids (petroleum) may contain other constituents such as carbon dioxide (CO₂), hydrogen sulphide (H₂S) and complex compounds of nitrogen, sulphur and oxygen. Crude oils may be classified by their chief components as:
- Paraffin base: the chief components are paraffins, which leave a solid residue of wax when distilled.
- Asphalt (naphthene) base: the chief components are cyclic compounds such as naphthenes, which leave a solid residue of asphalt after distillation.
- Mixed base: a mixture of paraffin-base and asphalt-base crudes.
Crude Oil Quality: API Gravity
The quality of crude oil, or liquid petroleum, is measured by its API gravity, defined as:
API = (141.5 ÷ SG) − 131.5
where SG is the specific gravity of the oil at 60°F relative to water. Heavier oil has a lower API gravity. Example: an oil with SG = 0.85 has API = (141.5 ÷ 0.85) − 131.5 ≈ 35°.
| Class | API gravity (approximate) |
|---|---|
| Light | Above 31.1° |
| Medium | 22.3° to 31.1° |
| Heavy | 10° to 22.3° |
| Extra heavy | Below 10° |
Lighter oils (higher API) are generally easier to produce and refine, and usually sell at a higher price.
What Happens to the Fluid During Production
As oil moves from the reservoir to the wellbore, pressure falls to the wellbore flowing pressure. It falls further as the fluid flows up the tubing to the surface facilities at stock tank conditions. The fluid changes along the way, and these changes are described by:
- Bubble point pressure: the pressure at which the first gas comes out of the oil.
- Formation volume factor, Bo: the volume of oil at reservoir conditions per unit volume at stock tank conditions.
- Solution gas-oil ratio, Rs: the volume of gas dissolved in the oil at a given pressure.
How Fluids Are Distributed in the Reservoir
In a given reservoir, fluids are distributed according to their specific gravity. Thus gas lies on top of crude oil, and crude oil lies above saline water. The shape of the rock that holds these layers is the trap; see reservoir rocks for the main trap types.
Frequently Asked Questions
What is the difference between black oil and volatile oil?
Volatile oil contains much more dissolved gas and shrinks far more when it reaches surface conditions.
What does a higher API gravity mean?
A lighter oil. Water is 10° API, so oil with a higher value is lighter than water and floats on it.
How do we know which fluid a reservoir has?
From fluid samples analysed in a PVT laboratory, together with production data such as the producing gas-oil ratio.
Key Takeaways
- Reservoir fluids are grouped as black oil, volatile oil, gas condensate, wet gas and dry gas.
- Crude oils are paraffin, asphalt or mixed base.
- API gravity measures oil quality: higher means lighter.
- Bubble point, Bo and Rs describe how the fluid changes as pressure falls.
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