Page 360 - Fundamentals of Reservoir Engineering

REAL GAS FLOW: GAS WELL TESTING 295

For a gas well test, this part of the program should also analyse the transient pressure
fall-off during the flowing periods, giving a re-determination of the permeability and two
separate skin factors, at different flow rates, which facilitate the calculation of the rate
dependent skin (exercise 8.3).

For an initial test in either an oil or gas reservoir (exercises 7.6 and 8.3, respectively)
the analysis would cease at this point. In the more common instance of routine
pressure surveys made throughout the producing lifetime of the well, the following
steps are necessary which incorporate the boundary condition in the analysis.
Digitized MBH charts

This set of data is the main-spring of the special analyses part of the program. As
already mentioned, the MBH charts (figs. 7.11-15), which are equally appropriate for
dimensionless pressure or pseudo pressures, have al ready been digitized by
15
Earlougher , for dimensionless time in the range 0.001 ≤ t DA ≤ 10. From this data bank,
values of the constant terminal rate solution β D, equ. (8.70), can be extracted for any
fluid type, geometry of the drainage area and degree of well asymmetry.

Special analyses

This part of the program should be quite flexible with the main aims being to determine
the average pressure within the drainage boundary at the time of the survey and to
gain some information about the magnitude and shape of the drainage area and well
position with respect to the boundary. For instance, the program could be structured,
as described in exercise 7.7, to match the equation of the observed linear buildup,
obtained in the preliminary analysis, with a theoretical straight line for various assumed
area sizes and shapes. Alternatively, an attempt could be made to match the entire
buildup by evaluating both β D (t D + ∆t D) and β D (∆t D), and plotting the difference
between the two, also as described in exercise 7.7.

Kazemi iteration

Iteration over the entire test analysis may be required for surveys conducted in gas, or
in gas saturated oil reservoirs, as described in sec. 8.11. This measure is necessary to
refine the estimate of the average pressure within the drainage area at the time of the
survey.

REFERENCES

1) Russell, D.G., Goodrich, J.H., Perry, G.E. and Bruskotter, J.F., 1966. Methods of
Predicting Gas Well Performance. J.Pet.Tech., January: 99-108.Trans. AIME.

2) Al-Hussainy, R., Ramey, H.J.,Jr. and Crawford, P.B.,1966. The Flow of Real
Gases Through Porous Media. J.Pet.Tech., May: 624-636. Trans. AIME.

3) Al-Hussainy, R. and Ramey, H.J., Jr., 1966. Application of Real Gas Flow Theory
to Well Testing and Deliverability Forecasting. J.Pet.Tech., May: 637-642.
Trans. AIME.