Page 295 - Fundamentals of Reservoir Engineering

OILWELL TESTING 231

Since F is a constant, dependent upon the well design and fluid compressibility, then
by simple proportionality

(T/F )
f
T = × T w
f
(T/F )
w
or

(T/F )
f
k = × k w
f
(T/F )
w
Apart from the two methods mentioned in this text, afterflow analysis techniques relying
22
on the use of type curves have also been presented by Ramey and Earlougher and
23
Kersch . Which, of all these methods, is the most reliable is a question which is still
unanswered. One point on which all the authors of papers on the subject agree is that
afterflow analysis techniques should not be used for pressure buildup analysis when
there is a clearly defined linear portion of the conventional Horner plot which can be
subjected to the analysis techniques described in sec. 7.7. This is because the physical
and mathematical concepts involved in the description of afterflow are vastly more
complex than for the simple pressure buildup theory and therefore, the analysis results
may be less reliable.

Nevertheless, it is recommended that engineers should experiment with one or all of
these methods in their own fields to determine which, if any, is suitable. To do this,
pressure buildup tests which provide both a linear trend on the conventional Horner
plot and also a significant deviation from this trend due to afterflow, for small values of
∆t, should be analysed using both the normal and afterflow analysis techniques, and
the results compared. If the comparison is favourable and statistical confidence is built
up in one of the afterflow methods, then the engineer can use this method for test
analysis on such occasions as when buildup surveys are recovered which are
dominated by the effects of afterflow.

EXERCISE 7.9 AFTERFLOW ANALYSIS TECHNIQUES

A twelve hour pressure buildup test was conducted in a flowing oil well from which the
pressure-time record was recovered, as listed in table 7.17.

The production data and reservoir and fluid properties are as follows:

= 30,655 stb = 0.2
N p φ
q = 231 stb/d µ = 0.6 cp
h = 10 ft B o = 1.3 rb/stb

-6
= 0.3 ft c = 20 × 10 /psi
r w