Overview
Crude oil is a naturally occurring hydrocarbon mixture extracted from underground reservoirs. Its physical properties vary significantly by source, depending on the distribution of hydrocarbon chain lengths (C₁ to C₆₀+), sulfur content, and dissolved gases. The most common classification is by API gravity (an inverse density scale). Density and viscosity are the two properties that most affect pipeline transport, storage, and pumping.
API Gravity Classification
| Classification | API Gravity | Density (kg/m³) | Typical Source |
|---|---|---|---|
| Extra heavy oil | <10° | >1000 | Bitumen, oil sands (Venezuela, Canada) |
| Heavy oil | 10-22° | 920-1000 | Heavy sour (Venezuela, Mexico) |
| Medium oil | 22-31° | 870-920 | Standard crude (most world production) |
| Light oil | 31-40° | 825-870 | West Texas Intermediate, Brent |
| Condensate | >40° | <825 | NGL, associated gas liquids |
API gravity = (141.5 / specific gravity at 60°F) − 131.5. Higher API = lighter oil = more valuable (yields more gasoline/diesel).
Physical Properties (typical medium crude, 20°C)
| Property | Value | Notes |
|---|---|---|
| Density | 870 kg/m³ | (API 31°) — ranges 800-950 by grade |
| Dynamic Viscosity | ~7 × 10⁻³ Pa·s (7 cP) | Light: ~2 cP; Heavy: 50-500 cP |
| Kinematic Viscosity | ~8 cSt | Strongly temperature dependent |
| Specific Heat | 2000 J/kg·K | |
| Thermal Conductivity | 0.14 W/m·K | Low — insulator |
| Flash Point | -7 to 65°C | Light crude is flammable; heavy is combustible |
| Pour Point | -30 to +15°C | Wax content dependent |
| Sulphur content | 0.1-3.0% | Sweet <0.5%; Sour >0.5% |
| Boiling range | 30-550°C | Multi-component mixture |
Viscosity vs Temperature
Crude oil viscosity drops dramatically with temperature, which is critical for pipeline transport of heavy crudes:
| Temperature | Light Crude (cSt) | Medium Crude (cSt) | Heavy Crude (cSt) |
|---|---|---|---|
| 10°C | 2 | 20 | 500 |
| 20°C | 1.5 | 8 | 200 |
| 50°C | 0.9 | 3 | 40 |
| 80°C | 0.6 | 1.8 | 15 |
Engineering Applications
Pipeline Transport
- Pressure drop calculations use actual crude density (NOT 1000 kg/m³)
- Reynolds number depends on kinematic viscosity — heavy crude often laminar
- Design for batch operations between different crudes in multi-product pipelines
- Pressure surge (water hammer) slower in crude than water (lower acoustic velocity)
Storage Tanks
- Tank capacity tables reference observed density; gross/net (volume/mass) calculations
- Thermal expansion: crude expands ~0.07% per °C — tank ullage design accounts for this
- Vapor recovery for light crude tanks (VOC emissions, environmental compliance)
- Stratification: heavy crudes can settle; light ends rise
Pumping
- Centrifugal pump power ∝ density × head × flow — use actual crude density
- NPSH requirements higher for light/hot crudes (higher vapor pressure)
- Positive displacement pumps for high-viscosity heavy crude service
- Viscosity corrections for pump performance curves above ~10 cSt
Safety Considerations
- Classified hazardous areas (API/IP zoning) around tanks and loading
- Hydrogen sulfide (H₂S) hazard in sour crude — toxic and corrosive
- Wax deposition in cold pipelines requires pigging programs
- Flammability varies: light crude is Class 1B (highly flammable)
Frequently Asked Questions
What is the density of crude oil? Typical medium crude is about 870 kg/m³ (API ~31°). Light crude can be as low as 800 kg/m³, while heavy crude ranges from 920 to 980 kg/m³. Always use the actual assay value for design.
What is API gravity and how is it calculated? API gravity = (141.5 / specific gravity at 60°F) − 131.5. Higher API means lighter, less dense oil. Water has an API gravity of 10°; values above 10° float on water, values below 10° sink.
Why does crude oil viscosity matter for pipeline transport? Viscosity determines the Reynolds number and pressure drop. Heavy crude is often laminar at ambient temperature and may be too viscous to pump economically — requiring heating, diluent blending (dilbit), or emulsion transport.
What is the difference between light and heavy crude? Light crude (API above 31°) has lower density and viscosity, higher value, and yields more gasoline and diesel. Heavy crude (API below 22°) is dense, viscous, and requires more processing and special transport.
Why can't I use water density for oil calculations? Using 1000 kg/m³ instead of 870 kg/m³ overestimates pump power by about 15%. Conversely, heavy crude at 950 kg/m³ needs 9% more power than water density suggests. Always use the actual measured density.
What is the flash point of crude oil? It ranges from about -7°C for light crude to +65°C for heavy crude. Light crude is Class 1B (highly flammable); heavy crude is combustible rather than flammable.
How does temperature affect crude oil viscosity? Viscosity drops dramatically with temperature — heavy crude can fall from 500 cSt at 10°C to 15 cSt at 80°C. This is why heating is the primary strategy for transporting heavy crudes.
Summary
Crude oil density (~870 kg/m³ for typical medium crude) and viscosity vary widely by grade and temperature — classified by API gravity from heavy (high density, viscous) to light (low density, fluid). Pipeline and pump design must use actual assay values rather than assuming water density. Heavy crude requires heating, diluent, or emulsion transport. Viscosity is highly temperature-sensitive and the key design parameter for heavy oil systems.
Related Guides and Tools
- Fluid Density Calculator — density of crude and other fluids
- Pipe Flow Calculator — flow and velocity in oil pipelines
- Pump Power Calculator — hydraulic power using actual crude density
- Tank Volume Calculator — storage tank sizing
- Tank Weight Calculator — tank shell and content weight
- Pressure Drop Calculator — friction loss in crude pipelines
- Reynolds Number Calculator — laminar/turbulent regime for viscous crude
- Pump Selection Calculator — pump sizing for oil service
- Tank Design Basics — storage tank fundamentals
- Centrifugal Pump Fundamentals — pump curves and NPSH
- Pipe Flow Engineering — pipe sizing for viscous fluids
- Water Hammer Analysis — surge in liquid pipelines
- Water Properties — comparison with the most common process fluid