Page 454 - Petrophysics

422 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

If the mean flow rate is expressed in terms of ft3/day at the mean pressure
p and mean temperature T and other variables are expressed in oilfield
units, Equation 7.14 becomes:

The following equation is useful in determining the outlet volumetric
flow rate q2 at the pressure p2, which is generally the atmospheric
pressure in a laboratory experiment:

kA - (P1 - P2)
q2 = -p (7.20)
PgL P2

where q2 is in cm3/sec. If practical oilfield units are used in Equation
7.20, where 92 is expressed in ft3/day:

6.33 x io3~ (pl - p2)
-
q2 = P (7.21)
PgL P2
In general, most equations used to study steady-state flow of incompre-
ssible fluids may be extended to gas flow systems by simply squaring the
pressure terms, and expressing the gas flow rates as SCFD and the gas
formation volume factor in bbl/SCF.

EXAMPLE

A horizontal pipe having 2 in. inside diameter and 12 in. long is filled
with a sand of 24% porosity. This sandpack has an irreducible water
saturation of 28% and a permeability to gas of 245 mD. The viscosity of
the gas is 0.015 cP.

(a) What is the actual velocity of the gas (in cm/sec) under 1OOpsi
pressure differential?
(b) What is the average flow rate of the gas in ft3/D and cm3/sec?

SOLUTION

(a) The actual velocity, Va, of the flowing gas can be calculated from
Equation 7.3 where $I = 0.24, SWi = 0.28, and the apparent velocity,
v, can be obtained from Darcy’s law. Inasmuch as k = 0.245 Darcy,