Module 2
What Flows: Products & Properties
Before you can move a barrel of oil or a cubic foot of gas, you have to know what it is. This module gives you the handful of numbers and units that engineers use to describe every drop and puff in the network.
What you'll be able to do
- Read an API gravity number and say whether a crude is light or heavy — and whether it floats or sinks.
- Tell sweet from sour, and wet/rich from dry/lean gas.
- Explain why gas is sold on energy (Btu) and not just volume (cubic feet).
- Decode the units that trip up beginners: Mcf vs MMcf, psia vs psig, therms, dekatherms, and inches of water column.
- Spot the places where "standard" values actually vary — so you always ask "which convention?"
The vocabulary of what flows: how we describe crude, gas, their energy, and the units that measure both.
Crude oil: how light, how sour
Crude oil is a soup of hydrocarbons. You don't need its full recipe — two numbers describe a crude well enough to price and route it: how dense it is, and how much sulfur it carries.
Density, measured as API gravity
API gravity (American Petroleum Institute gravity) is an upside-down density scale: a higher API number means a lighter (less dense) oil. It is defined from specific gravity (SG, density relative to water), both measured at 60 °F.
🧮 The formula
API gravity = (141.5 / SG) − 131.5
Plug in water (SG = 1.0): 141.5 − 131.5 = 10° API. So water sits at exactly 10°. Oil above 10° floats; oil below 10° sinks.
| Class | API gravity | Floats? |
|---|---|---|
| Light crude | > ~31.1° | yes |
| Medium crude | ~22.3°–31.1° | yes |
| Heavy crude | ~10.0°–22.3° | barely |
| Extra-heavy / bitumen | < 10° | no — sinks |
⚠️ Thresholds VARY
The 31.1 / 22.3 cut-offs are typical industry conventions, not a single codified standard. USGS and others use slightly different numbers. Treat them as conventional, not legal, boundaries.
Sulfur: sweet vs sour
Sweet crude is low in sulfur; sour crude is high. Sulfur is corrosive and demands more refining, so sweet crude is worth more. (The word "sweet" comes from the mild smell and taste of low-sulfur oil.)
⚠️ The sweet/sour threshold genuinely disagrees
The common trading convention is sweet < 0.5% sulfur by weight, sour ≥ 0.5% (some use 0.42%). But the EIA uses 1.0% sulfur as its cut. Always say which convention you mean.
Light-sweet crude (think WTI, Brent) is the prize: it yields more high-value fuels with the least processing.
Natural gas: mostly methane, plus the good stuff
Pipeline-quality natural gas is overwhelmingly methane (CH₄) — typically ~95–96%+ after processing. The rest is a mix of heavier hydrocarbons and a few unwanted gatecrashers.
The heavier hydrocarbons (ethane through pentanes-plus) are collectively the NGLs — Natural Gas Liquids. Contaminants to remove include water vapor, H₂S (hydrogen sulfide), CO₂, and nitrogen.
Wet vs dry, sour vs sweet
Wet / rich gas
- Carries significant recoverable NGLs / condensate
- Rule of thumb: ≥ ~5 gallons of liquid per Mcf ("GPM")
- Those liquids are valuable — worth stripping out
Dry / lean gas
- Mostly methane, few recoverable liquids
- Already close to pipeline spec
- Heads-up: "dry" can also mean water-free
ℹ️ Sour gas = has H₂S
Gas with H₂S (hydrogen sulfide) and/or high CO₂ is sour; gas that meets the pipeline H₂S limit is sweet. H₂S is acutely toxic, so it must be removed before the gas enters a pipeline.
Energy & heating value
Gas is sold on energy, not just volume — because a cubic foot of rich gas carries more heat than a cubic foot of lean gas. The measure is heating value: how many Btu you get from burning one standard cubic foot.
Btu = British thermal unit, the base energy unit. scf = standard cubic foot. HHV = Higher (Gross) Heating Value. Pipeline gas runs slightly above pure methane because of its ethane/propane content; tariff bands are commonly ~950–1,150 Btu/scf.
HHV vs LHV
Burning a hydrocarbon makes water vapor. Whether you count the heat released when that vapor condenses back to liquid is the whole difference between the two heating values.
HHV — Higher / Gross
- Counts the latent heat recovered when combustion water condenses
- The bigger number
- US gas is sold on HHV
LHV — Lower / Net
- Leaves the water as vapor (its heat unrecovered)
- The smaller number
- HHV ≈ 1.11 × LHV (~10–11% higher)
The Wobbe Index: are two gases interchangeable?
🔀 Same Wobbe = same flame
Two gases with the same Wobbe Index deliver the same heat through a given burner orifice at the same pressure — so a furnace or stove can swap one for the other without re-tuning.
Wobbe Index = HHV / √(specific gravity relative to air)
That is why pipelines hold an interchangeability spec, not just a Btu spec: the gas you receive has to burn like the gas your appliances were set up for.
The units engineers actually use
This is where beginners get burned. The units below are old, regional, and occasionally booby-trapped. Learn these and most of the field stops looking like alphabet soup.
⚠️ The big trap: M = 1,000
In this industry "M" is the Roman numeral for 1,000 — so MM = a million (1,000 × 1,000). This collides head-on with SI, where M (mega) already means million. So Mcf = thousand cubic feet, MMcf = million, Bcf = billion, Tcf = trillion.
| Unit | Means | Notes |
|---|---|---|
| bbl | barrel of oil | exactly 42 US gallons (~158.99 L) |
| BOE | Barrel of Oil Equivalent | gas converted to oil-energy; basis varies ~5,800 cf (energy) vs 6,000 cf = 6:1 (reporting) |
| scf | standard cubic foot | gas volume at standard conditions — which vary (see below) |
| Mcf / MMcf | thousand / million cf | M = 1,000 (Roman), MM = million; then Bcf, Tcf |
| Btu / MMBtu | British thermal unit / million Btu | MMBtu ≈ the energy in ~1 Mcf of gas |
| therm | 100,000 Btu | ~the energy in ~100 cf; common residential billing unit |
| Dth | dekatherm = 10 therms | = 1 MMBtu; common wholesale gas energy unit |
| psia / psig | psi absolute / gauge | psig = psia − ~14.7 (atmospheric ≈ 14.696 psia) |
| in WC | inches of water column | 1 psi ≈ 27.7 in WC; very low pressures |
🏠 A number to feel
Gas arrives at your home at only about ~7 in WC — that's roughly ¼ psi (since 7 ÷ 27.7 ≈ 0.25). A gentle breath, not the hundreds of psi out in the transmission line.
⚠️ There is no universal "standard cubic foot"
US gas always uses a temperature base of 60 °F, but the pressure base differs: 14.73 psia (the dominant interstate / GPA business standard), 14.696 psia (one true atmosphere, the scientific basis), or 14.65 psia (Texas Railroad Commission). Always read the tariff/contract.
Go deeper: is 1 Mcf really 1 MMBtu? optional
"1 Mcf ≈ 1 MMBtu" is a handy back-of-envelope rule, not an identity. For typical US gas the true value is ~1.036–1.037 MMBtu per Mcf — because real pipeline gas runs a little above 1,000 Btu/scf.
This small gap is exactly why custody transfer measures both volume and energy instead of trusting one approximation.
Why gas is sold on BOTH volume and energy
A pipeline physically moves volume — it cares about pressure and flow rate. But customers buy energy. Since Btu-per-cubic-foot changes with composition, you cannot get one from the other without measuring.
Meter ──measures──► VOLUME (cubic feet)
Gas chromatograph ──measures──► COMPOSITION → heating value (Btu/scf)
│
VOLUME × HEATING VALUE ─┘─► ENERGY (MMBtu) ◄── what you're billed
🧭 The big idea
Operators meter volume and analyze composition, then multiply: volume × heating value = energy. Both numbers matter — which is why every custody-transfer point reports cubic feet and Btu.
Key takeaways
- API gravity = (141.5 / SG) − 131.5. Higher = lighter; 10° = water (above floats, below sinks).
- Crude bands (light/medium/heavy) and the sweet/sour sulfur cut VARY by convention — always say which.
- Pipeline gas is ~95–96%+ methane; heavier C₂–C₅+ are NGLs. Sour = has H₂S; wet/rich = has recoverable liquids.
- Gas energy: ~1,000–1,050 Btu/scf (HHV); HHV ≈ 1.11 × LHV; same Wobbe Index = interchangeable.
- Units: M = 1,000 → MM = million; bbl = 42 gal; psia = psig + 14.7; 1 psi ≈ 27.7 in WC; standard conditions vary.
- Gas is billed on volume × heating value = energy — both numbers are measured.
A crude has SG = 1.0 (same density as water). What is its API gravity, and does it float?
Why: 141.5/1.0 − 131.5 = 10° API. Water is exactly 10°; oil above 10° floats, below 10° sinks.
In gas-industry units, how many cubic feet is "MMcf"?
Why: "M" is the Roman numeral for 1,000, so MM = 1,000 × 1,000 = a million. (Bcf is the billion.)
US natural gas is sold on which heating value, and how does it compare to the other?
Why: US gas is sold on HHV (Higher/Gross), which counts the latent heat of condensing water. HHV ≈ 1.11 × LHV.
Residential gas is delivered at about 7 in WC. Roughly what is that in psi?
Why: 1 psi ≈ 27.7 in WC, so 7 in WC ÷ 27.7 ≈ 0.25 psi — a very gentle pressure.
Why does the industry measure gas by both volume and energy?
Why: Pipes move volume but customers buy energy, and energy density changes with composition. So operators meter volume and analyze composition, then bill volume × heating value.