1. What is BS&W?
Basic sediment & water (BS&W) is the combined volume percentage of water and suspended sediment carried in a crude oil sample. It is measured by spinning a sample (often with solvent and demulsifier) in a centrifuge tube and reading the water-and-sediment layer that separates at the bottom — the method of API MPMS Chapter 10.3 / ASTM D4007. Because that water and sediment are not saleable hydrocarbon, BS&W is deducted from the gross volume to credit only net oil.
2. Free Water vs Entrained BS&W
Two different water quantities are handled separately. Free water is the bulk water that settles to the bottom of a tank and is gauged directly (the difference between the total and oil/water-interface gauges). BS&W is the water and sediment still entrained in the oil column above the free water, measured by centrifuge as a percentage. In the custody chain, free water is removed first as a volume; BS&W is then applied as a percentage of the remaining volume.
Free Water
Gauged (bbl)
Bulk water at the tank bottom; removed as a volume.
BS&W
Centrifuge (%)
Entrained water + sediment in the oil; applied as a percentage.
Net Oil
What remains
Saleable hydrocarbon after both deductions.
3. The Net Oil Deduction
This deduction is the final step of the net standard volume chain (API MPMS Ch. 12). BS&W may be entered as a single combined percentage or split into separate water and sediment percentages, which sum to the total BS&W.
4. Worked Example
Gross standard volume 1,000 bbl, free water 5 bbl, BS&W 0.5%:
Of the 1,000 bbl gross, about 990 bbl is net oil and ~10 bbl is water and sediment — the difference between what is delivered and what is paid for.
5. Standards & References
| Standard | Scope |
|---|---|
| API MPMS Ch. 10.3 / ASTM D4007 | Water & Sediment in Crude Oil by the Centrifuge Method |
| API MPMS Ch. 10.1 | Sediment & water by distillation (high water / emulsions) |
| API MPMS Ch. 12 | Calculation of Petroleum Quantities (net oil) |