Phase Behavior and ThermodynamicsFluid Properties
Stock-Tank Oil Density from API Gravity Formula
Stock-Tank Oil Density from API Gravity calculates stock-tank oil density for fluid properties workflows in phase behavior and thermodynamics.
How engineers use this formula
Use this formula when the listed inputs (API, rho_w) are known and the assumptions behind the cited fluid properties relationship match the engineering case being checked.
Assumptions
- Input values are representative for the well, reservoir, fluid, or equipment case being evaluated.
- The declared units match the field-unit constants used in the formula.
- The cited formula applies to the selected petroleum engineering workflow.
Limitations
- The calculation does not replace a full engineering model or operating procedure.
- Accuracy depends on the source correlation, assumptions, input quality, and unit consistency.
Common mistakes
- Mixing unit systems without converting the inputs.
- Using default example values as field recommendations.
- Applying the formula outside the source assumptions.
Default example
Using the default inputs, rho_o_st equals 53.030631 lb/ft^3.
APIdeg API
35
rho_wlb/ft^3
62.4
Inputs
API
deg APIAPI Gravity
rho_w
lb/ft^3Water Density at Standard Conditions
Outputs
rho_o_st
lb/ft^3
Stock-Tank Oil Density
API
deg API
API Gravity
rho_w
lb/ft^3
Water Density at Standard Conditions
gamma_o
dimensionless
Stock-Tank Oil Specific Gravity
Source and review
reviewedU.S. Energy Information Administration API gravity glossary and ScienceDirect oil-gravity definition.
Source