Well Test Separator Calculators
Four calculators for well test separator readings: orifice gas rate, liquid corrected to stock-tank conditions, GOR, and net oil and water cut. Results update instantly, in field or SI units.
Educational quick-check calculators: not an approved procedure and not a substitute for your company software or a qualified engineer’s review.
Gas rate from an orifice meter
Fundamental flow equation with the Reader-Harris/Gallagher discharge coefficient (ISO 5167-2), the same basis as AGA-3.
How it is calculated
qm = C · Ev · ε · (π/4) · d² · √(2 · ΔP · ρ1)
ρ1 = P1 · M / (Z · R · T) ; M = 28.9625 · SG (g/mol)
ε = 1 − (0.351 + 0.256β⁴ + 0.93β⁸) · [1 − (p2/p1)^(1/κ)]
C = Reader-Harris/Gallagher (ISO 5167-2:2003), iterated with ReD = 4·qm/(π·μ·D)
Qb = qm / ρb , ρb = Pb · M / (R · Tb) (Zb = 1)
Assumptions
- Bore and pipe diameters at reference temperature (no thermal expansion correction).
- Ideal gas at base conditions (Zb = 1); about 0.2% difference for natural gas. Enter flowing Z from an equation of state or a PVT report.
- Static pressure measured at the upstream tap. Steady single-phase flow with enough straight pipe upstream and downstream.
Worked example
D = 4.026 in, d = 2.000 in (β = 0.4968), ΔP = 50 inH₂O, 500 psig + 14.696 psia, 80°F, SG = 0.65, Z = 0.92, μ = 0.011 cP, κ = 1.3, flange taps → C = 0.60290, ε = 0.99900, ReD ≈ 1.21×10⁶, qm = 2.3645 lb/s, Qg = 4.1086 MMscf/d (116,388 Sm³/d). Matches the Python “fluids” ISO 5167 implementation to the last digit.
Sources: ISO 5167-1:2003 and ISO 5167-2:2003 (orifice plates: Reader-Harris/Gallagher C, expansibility ε, limits of use) · API MPMS Chapter 14.3 / AGA Report No. 3, Part 1 (concentric square-edged orifice meters; same fundamental flow equation) · Bell, C. — “fluids” Python library, ISO 5167 orifice solver (used to verify the example)
Educational quick-check tool, not an approved procedure. Follow your company procedures and the manufacturer’s data.
Liquid rate: meter factor and shrinkage to stock tank
From the oil-leg meter reading over the test period to the stock-tank oil rate.
How it is calculated
Q_water = Q_gross × BS&W
Q_net,sep = Q_gross × (1 − BS&W)
Q_STO = Q_net,sep × SF
Assumptions
- MF from the latest meter proving with the same liquid and similar conditions.
- SF from a shrinkage tester or a PVT report; it covers gas liberation and thermal shrinkage from separator to stock tank, so no separate CTL is applied.
- BS&W from a representative oil-leg sample (centrifuge).
Worked example
250 bbl in 12 h, MF = 1.012, BS&W = 2%, SF = 0.93 → gross = 250 × 1.012 × 24/12 = 506.0 bbl/d, water = 10.1 bbl/d, net oil = 495.9 bbl/d, stock tank = 495.88 × 0.93 = 461.2 STB/d.
Sources: API MPMS Chapter 12.2 (calculation of petroleum quantities using meters; meter factor) · API MPMS Chapter 4.8 (operation of proving systems) · API MPMS Chapter 20.1 (allocation measurement; shrinkage factors) · API MPMS Chapter 10.4 (water and sediment by centrifuge, field procedure)
Educational quick-check tool, not an approved procedure. Follow your company procedures and the manufacturer’s data.
Gas-oil ratio (GOR)
GOR from separator gas and stock-tank oil, with GLR and water cut.
How it is calculated
GLR = Qg / (Qo,ST + Qw)
WC = Qw / (Qo,ST + Qw)
1 Sm³/Sm³ = 5.6146 scf/STB
Assumptions
- Separator-gas to stock-tank-oil GOR; stock-tank vent gas is not included unless you add it to Qg.
- MMscf/d ↔ Sm³/d is a volume conversion only (1 scf = 0.0283168 m³); the base-condition difference (about 0.2%) is ignored.
Worked example
Qg = 1.2 MMscf/d, Qo = 460 STB/d, Qw = 120 bbl/d → GOR = 1,200,000/460 = 2,609 scf/STB (464.6 Sm³/Sm³), GLR = 2,069 scf/bbl, water cut = 20.69%.
Sources: McCain, W.D. — The Properties of Petroleum Fluids, 2nd ed. (GOR definitions) · API MPMS Chapter 20.1 (allocation measurement) · SPE Metric Standard / ISO 80000 (unit conversions)
Educational quick-check tool, not an approved procedure. Follow your company procedures and the manufacturer’s data.
BS&W, water cut and net oil
Combines oil-leg water (from BS&W) with the water-leg rate to give the well water cut.
How it is calculated
Q_net = Q_gross − Qw,oil leg
Qw,total = Qw,oil leg + Qw,water leg
WC = Qw,total / (Q_gross + Qw,water leg)
Assumptions
- Both volumes at the same separator conditions; use the previous calculator to correct net oil to stock tank.
- BS&W includes both water and sediment; it is treated here as water.
Worked example
506 bbl/d oil leg, BS&W = 2%, water leg 120 bbl/d → oil-leg water = 10.12, net oil = 495.88, total water = 130.12, total liquid = 626 bbl/d, water cut = 130.12/626 = 20.79%.
Sources: API MPMS Chapter 10.4 (S&W by centrifuge, field procedure) · API MPMS Chapter 20.1 (allocation measurement)
Educational quick-check tool, not an approved procedure. Follow your company procedures and the manufacturer’s data.