Page 138 - Geology and Geochemistry of Oil and Gas

PHYSICAL PROPERTIES OF NATURAL GASES 109
and

p =p ¼ expðL=144Z av NRT av Þ (6.18)
bc
s
or
p bc ¼ p expðL=144Z av NRT av Þ (6.19)
s
where Z av is the average compressibility factor computed at T av and p av , N is the
number of moles of gas, R the universal gas constant, which is equal to 10.7 for 1 lb.
mol of gas, p s is the pressure at surface in psia, p bc is the pressure at bottom of gas
column in psia, L is the length of gas column in ft, and T av the average temperature
in gas column in 1R.
On simplifying the exponent and considering 1 lb of gas,
p bc ¼ p expðLG g =53:3Z av T av Þ (6.20)
s
where G g is the gravity of gas as compared to that of air (SG air ¼ 1).
The average temperature in the gas column can be computed from the following
equation:
T av ¼ T s þ ðD=2ÞðdT=dLÞ þ 460 (6.21)
where T s is the temperature at the surface in 1F, D is the vertical length of gas column
in ft, and dT/dL is geothermal gradient, which is around 21F/100 ft. Geothermal
gradient, however, varies with locality.

6.3.5. Gas viscosity

Gas viscosity increases with temperature. The reason is the increase in speed of
molecules and in the number of their collisions. If, however, the pressure also in-
creases, the gas viscosity stops increasing and then even declines at pressures specific
for each gas. At high pressures, viscosity of the natural gas increases with increasing
molecular weight. At the same pressure and temperature, the hydrocarbon gases
possess lower viscosity than the non-hydrocarbon gases. The viscosity of hydrocar-
bon gases increases with increasing pressure at the same temperature (1001C) as
shown in Fig. 6.2. In the presence of N 2 , CO 2 , and H 2 S, the viscosity of hydrocarbon
gases slightly increases.

6.3.6. Hydrate formation

At certain pressure and temperature, hydrocarbon gases combine with water,
forming crystalline hydrates. Crystalline hydrates are solid solution of gas in water.
Water molecules form a 3D frame invaded by very mobile gas molecules. Only light
hydrocarbons through pentane are capable of forming gas hydrates. The following
empirical formulae were established for the gas hydrates: methane, CH 4 7H 2 O;
ethane, C 2 H 6 8H 2 O; propane, C 3 H 8 18H 2 O. Critical hydrate-formation temper-
ature for methane is 21.51C; for ethane, 14.51C; for propane, 5.51C; for i-butane,
2.51C; and for n-butane, 11C. The heavier the hydrocarbon gas, the easier it forms