## Petroleum Engineering Equations & Formulas

#### Oil density

Oil density at surface conditions is commonly quoted in °API.

#### Formula:

API = 141.5/γ_{o}-131.5 Where: γ_{o}is the specific gravity of oil (relative to water = 1, measured at STP).

The downhole density of oil (at reservoir conditions) can be calculated from the surface density equation using:

ρ_{orc}* B_{o}= ρ_{o}+ R_{s}* ρ_{g}Where: ρ_{orc}: is oil density at reservoir conditions (kg/m^{3}) B_{o}: is the oil formation volume factor (rm^{3}/stm^{3}) ρ_{o}: is oil density at standard conditions (kg/m^{3}) R_{s}: is the solution gas : oil ratio (sm^{3}/stm^{3}) ρ_{g}: is gas density at standard conditions (kg/m^{3})

### $$ API = \frac{141.5}{\gamma_{o}} - 131.5 $$

Where:

- $\gamma_{o}$ is the specific gravity of oil (relative to water = 1, measured at STP).

### $$ \rho_{orc}*B_{o} = \rho_{o} + R_{s} * \rho_{g} $$

Where:

- $\rho_{orc}$ is the oil density at reservoir conditions $(kg/m^{3})$
- $B_{o}$ is the oil formation volume factor $(rm^{3}/stm^{3})$
- $\rho_{o}$ is the oil density at standard conditions $(kg/m^{3})$
- $R_{s}$ is the solution gas : oil ratio $(sm^{3}/stm^{3})$
- $\rho_{orc}$ is the gas density at standard conditions $(kg/m^{3})$