Page 468 - Petrophysics

436 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Integrating Equation 7.55d and solving explicitly for the volumetric flow
rate, q, results in the following equation:

(7.56)

Expressing all the terms in practical oilfield units, this relationship
becomes:

(7.57)

where: qsc = surface production rate at Tsc = 60°F and psc = 14.7 psia,
STB/D.
k = formation permeability, mD.
h = formation thickness, ft.
p = oil viscosity, cP.
Pe = external pressure, psia.
pw = well pressure, psia.
r, = well bore radius, ft.
re = external radius, (43,56OA/n)'/*, ft.
A = drainage area, acres.
Bo = oil formation volume factor, bbl/STB.

This equation is only valid for the case where the well is located at the
center of a circular drainage area.
The external pressure Pe is generally approximated by the static
pressure of the reservoir, especially in the case of an infinite reservoir.
In strong water-drive reservoirs, Pe is equivalent to the initial reservoir
pressure pi. If the pressure pe cannot be determined with some rea-
sonable accuracy, Equation 7.57 should be expressed in terms of the
average reservoir pressure, p, which can be easily obtained from a
pressure buildup or drawdown test [20].
Inasmuch as re >> r,, the volumetric average reservoir pressure may
be expressed as [ 11 :

(7.58)

where p is the reservoir pressure at any radius r. From Equation 7.56:

p=pw+- 9' In(;) (7.59)
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