Recovery Factor Formula

Recovery factor (RF) is the fraction of original oil in place (OOIP) that can be economically produced. It is the single most important number in reserves estimation. RF is estimated using volumetric methods (OOIP × RF), empirical API correlations, and analogy to similar reservoirs. Typical oil RF r...

Overview

Theory

Volumetric OOIP

OOIP = 7758 * A * h * φ * (1 - Swi) / Boi

where 7758 = bbl per acre-ft, A = area (acres), h = net pay (ft), φ = porosity, Swi = initial water saturation, Boi = initial oil FVF.

Ultimate Recovery

EUR = OOIP * RF

Formulas

API Primary Recovery Correlation (1967)

For solution-gas drive:

RF = 0.41815 * (k*μw/(μo*Boi))^0.1611 * (k*krw/μw)^0.0979 * (Swi)^(-0.3722) * (φ)^0.1741 * (Pb/Pa)^0.0854

(Simplified form — actual API statistical correlation uses multiple regression on 80+ reservoir data points.)

Typical RF by Drive Mechanism

Drive Mechanism	Typical RF (%)
Solution gas drive	5 – 30
Gas cap drive	20 – 40
Water drive (strong)	35 – 75
Gravity drainage	40 – 80
Combination drive	25 – 50

Displacement Efficiency

RF = ED * EA * EV

where:

ED = microscopic displacement efficiency = (Soi - Sor) / Soi
EA = areal sweep efficiency (typically 0.5–0.9)
EV = vertical sweep efficiency (typically 0.4–0.9)

Gas Recovery Factor

RF_gas = 1 - (Bg_i / Bg_a) * (P_a / P_i) * (Z_i / Z_a)

where subscript i = initial, a = abandonment.

For strong water drive gas reservoirs, RF_gas can be lower (50–70%) due to water trapping gas.

Worked Example

Given: A = 640 acres, h = 30 ft, φ = 0.18, Swi = 0.25, Boi = 1.30 RB/STB, strong water drive.

Step 1: OOIP:

OOIP = 7758 * 640 * 30 * 0.18 * (1 - 0.25) / 1.30
     = 7758 * 640 * 30 * 0.18 * 0.75 / 1.30
     = 7758 * 2,592 / 1.30
     = 7758 * 2,592 * 0.769
     = 15,465,216 STB ≈ 15.5 MMSTB

Step 2: With RF = 45% (strong water drive):

EUR = 15.5 * 0.45 = 6.98 MMSTB

Step 3: Sensitivity — if RF ranges from 35% to 55%:

EUR_low  = 15.5 * 0.35 = 5.4 MMSTB
EUR_high = 15.5 * 0.55 = 8.5 MMSTB

Valid Ranges

Parameter	Typical Range
RF (oil, primary)	5 – 40%
RF (oil, waterflood)	25 – 55%
RF (oil, EOR)	40 – 70%
RF (gas, volumetric)	70 – 90%
RF (gas, water drive)	50 – 70%
φ	0.05 – 0.35
k	1 – 5,000 md

Key Factors Affecting RF

Mobility ratio (M = μokrw / μwkro): M < 1 favorable, M > 1 unfavorable
Reservoir heterogeneity: Layering, fractures, permeability contrast
Oil viscosity: Heavy oil (μ > 100 cp) → low RF without thermal EOR
Drive mechanism: Water drive >> solution gas drive
Well spacing: Tighter spacing improves sweep but may not be economic

References

API (1967). "A Statistical Study of Recovery Efficiency." API Bulletin D14.
Arps, J.J. et al. (1967). "A Statistical Study of Recovery Efficiency." API Bulletin D14.
Dake, L.P. (1978). Fundamentals of Reservoir Engineering. Elsevier. Chapter 1.
PetroWiki — Recovery factor: https://petrowiki.spe.org/Recovery_factors

Recovery Factor