Page 265 - Fundamentals of Reservoir Engineering

OILWELL TESTING 202

2) The slope of the linear section of the buildup plot is m = 24.5 psi/log cycle. Therefore,
since the well is fully penetrating the effective permeability of the formation is

162.6q µ B oi 162.6 123 1 1.22
×
× ×
k = = = 50 mD
mh 24.5 20
×
3) The skin factor can be evaluated using equ. (7.52) in which the hypothetical value of
p ws(LIN) 1-hr = 4752 psi is obtained from the extrapolation of the linear buildup trend to
∆t = 1 hour, fig. 7.23, therefore,

(p − p ) k
S = 1.151 ws(LIN) 1 -hr wf − log + 3.23
m φµ cr 2
w (7.52)
−
= 1.151 ( (4752 4506 ) − log 50 + 3.23
6
24.5 .2 1 20 10 × .09
−
××
×
4) The additional pressure drop due to the skin, while producing, can be calculated as
qµ
∆ p = × S atm.
2kh
π
2m
= × S = 0.87mS psi
2.303
= 128 psi

*
5) The assumption that p i = p relies entirely on the fact that both p D functions in the
theoretical buildup equation (7.32) can be evaluated under transient flow conditions so
that the equation can be reduced to the simple form equ. (7.54) for the infinite reservoir
case. As already noted for a well at the centre of a circular bounded reservoir, there is
a fairly sharp change from transient to semi-steady state flow for a value of t DA ≈ 0.1.
Therefore, for the effective flowing time of 97.6 hours, the minimum area for which the
assumption is valid is

0.000264kt 1
A min = ×
0.1φµ c 43560
0.000264 50 97.6 1
×
×
A = × ≈ 74 acres
min
0.12 120 10 − 6 43560
×
×× ×
*
and since the estimated area is 300 acres the assumption that p = p i is perfectly valid
as the test is conducted entirely under transient flow conditions.
EXERCISE 7.7 PRESSURE BUILDUP TEST ANALYSIS: BOUNDED DRAINAGE
VOLUME

A pressure buildup test is conducted in the same well described in exercise 7.6, some
seven and a half months after the start of production. At the time, the well is producing
400 stb/d and the cumulative production is 74400 stb. The only change in the well data
presented in the previous exercise is that B o has increased from 1.22 to 1.23 rb/stb.
The closed-in pressures as a function of time are listed in table 7.7.