Page 69 - Introduction to Petroleum Engineering

DEFINITIONS 53
where M is apparent molecular weight. Apparent molecular weight is calculated as
a

a ∑
M = N c yM i (3.7)
i
i=1
where N is the number of components, y is the mole fraction of component i, and M i
c
i
is the molecular weight of component i. Gas density is calculated from the ideal gas
equation of state as
pM γ pM
,
,
ρ = agas = g a air (3.8)
g RT RT
where p is the pressure, T is the absolute temperature, R is the gas constant, and mole
fraction n is the mass of gas m divided by the apparent molecular weight (or molar
g
mass) of gas.
Oil specific gravity is calculated at standard conditions using the density of
freshwater as the reference density. The American Petroleum Institute characterizes
oil in terms of API gravity. API gravity is calculated from oil specify gravity γ at
o
standard temperature and pressure by the equation

141 5 .
.
°API = −131 5 (3.9)
γ o
If specific gravity γ greater than 1, the oil is denser than water and API less than 10.
o
If specific gravity γ less than 1, the oil is less dense than water and API greater than
o
10. Heavy oils with API less than 20 do not contain much gas in solution and have a
relatively large molecular weight and specific gravity γ . By contrast, light oils with
o
API greater than 30 typically contain a large amount of volatile hydrocarbons in
solution and have a relatively small molecular weight and specific gravity γ . The
o
equation for API gravity shows that heavy oil has a relatively low API gravity because
it has a large γ , while light oils have a relatively high API gravity.
o
Gas–Liquid Ratio. The gas–liquid ratio (GLR) is the ratio of a volume of gas
divided by a volume of liquid at the same temperature and pressure. The choice of
GLR depends on the fluids in the reservoir. Two commonly used GLR are gas–oil
ratio (GOR) and gas–water ratio (GWR). The GWR is the ratio of gas volume to
water volume at the same temperature and pressure. GOR is the ratio of gas volume
to oil volume at the same temperature and pressure. The ratios can be calculated
using volume ratios or ratios of flow rates.
Viscosity. Viscosity is a measure of resistance of a fluid to shearing. Fluids like
honey and heavy oil have a very high viscosity, while fluids like water have a
relatively low viscosity. In the oil industry, viscosity is often expressed in centipoise,
which equals 0.01 poise. One centipoise (1 cp) equals 1 millipascal second
(1 mPa s = 0.001 Pa s), which is the metric unit for viscosity.
Compressibility. Compressibility is a measure of the change in volume resulting
from the change in pressure applied to the system. The fractional volume change of