Page 68 - Introduction to Petroleum Engineering

52 PROPERTIES OF RESERVOIR FLUIDS

Example 3.1 Temperature of the Earth’s Crust
The temperature in some parts of the Earth’s crust increases by about 1°F for
every 100 ft of depth. Estimate the temperature of the Earth at a depth of 8000 ft.
Assume the temperature at the surface is 60°F. Express your answer in °F and °C.

Answer
 F ° 1 
Temp = 60 F ° +(8000 ft)   =140 F °
 100 ft 
5
32
T = (140 − ) = 60 ° C
C
9

Composition. The composition of a fluid refers to the types and amounts of mole-
cules that comprise the fluid. Oil is a mixture of hydrocarbon compounds and minor
amounts of inorganic molecules such as carbon dioxide and nitrogen. In situ water
usually contains dissolved solids and dissolved gases.
The relative amount of each component in a mixture may be expressed in such
units as volume fraction, weight fraction, or molar fraction. The unit of concentration
should be clearly expressed to avoid errors. The elemental composition of oil is
primarily carbon (84–87% by mass) and hydrogen (11–14% by mass). Oil can con-
tain other elements as well, including sulfur, nitrogen, oxygen, and various metals.
The symbols x and y are often used to denote the mole fraction of component i in
i
i
the liquid and gas phases, respectively. The mole fraction of component i in a gas
mixture is the number of moles n of the component in the gas (vapor phase) divided
iV
by the total number of moles in the gas phase:
n
y = iV
i ∑ N c n (3.4)
j=1 jV
where N is the number of components in the mixture. The mole fraction of component
c
i in an oil mixture x is similarly defined as the number of moles n of the component
i
is
in the liquid phase divided by the total number of moles in the liquid phase:
n
x = iL (3.5)
i
∑ N c n jL
j=1
Specific Gravity. Specific gravity is the ratio of the density of a fluid divided by
a reference density. Gas specific gravity is calculated at standard conditions using air
density as the reference density. Taking the ratio of gas density to a reference gas
density and canceling common terms, lets us write the specific gravity of gas as

M M
,
γ = agas ≅ agas (3.6)
,
g
M aair 29
,