Oil & Gas Engineering Glossary
Essential definitions across reservoir engineering, drilling hydraulics, fluid thermodynamics, and petroleum asset economics.
API Gravity
A measure of how heavy or light a petroleum liquid is compared to water. Light crude > 31.1° API, medium 22.3°–31.1°, heavy 10°–22.3°, extra-heavy < 10°.
Arps Decline
Empirical production decline model developed by J.J. Arps (1945), encompassing exponential (b=0), hyperbolic (0<b<1), and harmonic (b=1) decline curves.
Boi (Oil Formation Volume Factor)
Ratio of the volume of oil at reservoir conditions (including dissolved gas) to the volume of oil at standard stock tank conditions (STB).
Bg (Gas Formation Volume Factor)
Volume of gas at reservoir temperature and pressure required to produce one standard cubic foot (scf) of gas at standard surface conditions.
Colebrook-White Equation
Implicit relationship giving the Darcy friction factor f as a function of Reynolds number and pipe relative roughness (ε/D).
DAK (Dranchuk-Abou-Kassem)
An 11-constant Benedict-Webb-Rubin modified equation of state widely used in petroleum engineering to accurately calculate the gas deviation factor (Z).
ECD (Equivalent Circulating Density)
The effective density exerted by drilling fluid against the formation while circulating, accounting for hydrostatic mud weight plus annular friction pressure losses.
EUR (Estimated Ultimate Recovery)
Total cumulative volume of hydrocarbons expected to be economically recovered from a well or reservoir throughout its productive life.
Fann Viscometer
Rotational coaxial cylinder viscometer standardly used in oilfield mud testing at speeds of 600, 300, 200, 100, 6, and 3 RPM.
GOR (Gas-Oil Ratio)
Ratio of standard cubic feet of gas produced per stock tank barrel of oil (scf/STB).
Herschel-Bulkley (Yield Power Law)
A three-parameter rheological model describing drilling fluids that possess a yield stress (τ0) followed by shear-thinning behavior.
IRR (Internal Rate of Return)
The discount rate at which the Net Present Value (NPV) of all upstream capital outlays and operating cash inflows equals zero.
Minimum Curvature Method
Standard 3D directional drilling survey integration algorithm that assumes wellbore trajectory segments follow a smooth circular arc between survey stations.
NPV (Net Present Value)
Sum of the present values of all incoming and outgoing cash flows over the project lifecycle, discounted at a hurdle interest rate (e.g. 10% or 15%).
OGIP (Original Gas in Place)
Total volume of gas initially contained in a reservoir prior to production, standardly calculated deterministically or via Monte Carlo.
P90 / P50 / P10 (Petroleum Convention)
In upstream petroleum resource classification: P90 denotes the conservative estimate (90% exceedance probability), P50 is the median/best estimate (50%), and P10 is the upside case (10%).
STOIIP (Stock Tank Oil Initially in Place)
Total volume of crude oil originally present in a hydrocarbon reservoir at discovery, expressed in stock tank barrels (STB).
TDH (Total Dynamic Head)
Total equivalent height that a pump must lift fluid, equal to static elevation change plus friction head losses plus pressure differential.
VCF (Volume Correction Factor)
Temperature adjustment multiplier (ASTM D1250) used to correct oil volume measured at field temperatures back to standard 60°F reference conditions.
Z-Factor (Gas Deviation Factor)
The ratio of actual gas volume to the volume predicted by ideal gas law at identical pressure and temperature (Z = PV / nRT).