Page 343 - Fundamentals of Reservoir Engineering

REAL GAS FLOW: GAS WELL TESTING 278

periods. In each case the solid line represents the correct analysis technique, in which
m D is evaluated using equ. (8.51),while the dashed line is derived for the same
observed pressure data but with the m D function evaluated under the assumption that
transient flow conditions prevail throughout the test. Using the correct analysis
technique always gives the same values of k and S, but the results obtained from the
transient analysis are open to severe misinterpretation. One may either treat the
dashed lines in figs. 8.13 (b) and (c) as being approximately linear, which would result
in the calculated values of both k and S being too small. Alternatively, since both the
dashed lines have a distinct upward curvature one may be tempted to linearize them
either by increasing the non-Darcy flow coefficient (even though the correct value of
F = .05 has been used in the analysis to produce figs. 8.13 (b) and (c) ), or by reducing
the initial pressure as demonstrated in exercise 8.1.

The majority of gas well test analyses described in the literature, for non-stabilized flow
conditions, are for wells which have very low permeabilities; a few millidarcies or less.
Under these circumstances, the transient analysis technique of Essis and Thomas,
which is usually applied in one form or another, is probably quite justified. The
foregoing exercise illustrates what can go wrong in applying transient analysis
techniques in a moderately high permeability reservoir and, of course, at the time of
planning the test the permeability is unknown. It is therefore very difficult to estimate in
advance just how long the total test duration should be for the application of transient
analysis to be still valid.

As a safeguard, it is recommended that all sequential flow tests be followed by a
pressure buildup, which under normal circumstances should provide a more reliable
value of the permeability and one which can be determined independently of the
boundary condition or flow condition (refer Chapter 7, sec. 7). If the permeability
derived from the multi-rate test, assuming the transient flow condition, is less than that
from the buildup it is likely that the multi-rate test analysis is incorrect and should be re-
analysed, attempting to make allowance for the boundary condition and discarding the
assumption of transient flow. The latter is easier said than done, however, for although
the value of k, obtained from the buildup, facilitates the determination of t DA, required in
the analysis, there always remains an ambiguity in the estimation of the correct shape
of the drainage area which, as shown in exercise 7.8, cannot be resolved by
conventional analysis techniques.
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Odeh et al. have also described an analysis technique for multi-rate flow tests which
are followed by a buildup. The obvious drawback to this method of testing is that it
negates one of the main aims of multi-rate testing which is to avoid well closure.

8.11 PRESSURE BUILDUP TESTING OF GAS WELLS

Just as in the case of oilwell testing, pressure buildup tests in gas wells, if analysed
correctly using the Horner buildup plot, can provide the most reliable values of the
permeability and skin factor. The only difference is that a buildup in a gas well must be
accompanied by two separate flow periods, one before and one after the buildup, as
shown in fig. 8.14. This measure is necessary in order to determine both components
of the skin factor, S and DQ.