Page 475 - Petrophysics

RADIAL FLOW SYSTEMS 443

Equation 7.76 is very general and more accurate than Equations 7.60
and 7.64 because it includes the effect of wellbore condition, as well as
the effect of the outer boundary of the reservoir. Equation 7.76 is valid
for both steady and pseudosteady states, depending on the value of the
parameter f. The skin factor is best obtained from pressure transient tests
[ 18, 201.

EXAMPLE

(a) Calculate the production rate for an oil well in a 160-acre drainage
area where the average pressure is partially maintained at 1,850 psia
by water injection at the boundary. The following parameters are
available :

rw = 0.5ft p = 2.2cP
h= l6ft Bo = 1.1 bbl/STB
k = 180mD pw = 1,230psia
s=2 f = 0.25

(b) What is the ideal production, i.e., no skin damage?

SOLUTION

In oilfield units, Equation 7.76 can be expressed as

O.O0708kh(I', - p,)
qsc = (7.77)
yB,[ln (r,/r,) - 0.75+0.25f + SI

(a) The radius of the drainage area is:

re = (43,56OA/~)'.~ = (43,560 x 160/~)~,~ = 1,489ft

The production rate of this well is equal to:

(0.00708)( 1 SO)( 16)( 1,850 - 1,2 30) STB
= 561 -
'"= (2.2)(1.1)[ln(1,489/0.5) -0.75+(0.25)(0.25)+2] D

(b) The ideal production rate of this well is obtained by letting s = 0
in Equation 7.77, which gives qsc = 715 STB/D. Thus, if this well is
treated to remove the skin damage, an additional 154 STB/D will be
produced, an increase of approximately 27%.