Page 337 - Fundamentals of Reservoir Engineering

REAL GAS FLOW: GAS WELL TESTING 272

EXERCISE 8.2 MULTI-RATE GAS WELL TEST ANALYSED ASSUMING
UNSTABILIZED FLOW CONDITIONS

A gas well is tested by producing at four different rates for periods of one hour, followed
by a four hour pressure buildup. The rates, times and observed pressures during the
flow test are listed in table 8.5.

Rate (Q) Cum. Flowing Time (t) p wf m (p wf)
2
Mscf/d hrs psia psia /cp
10 × 10 3 1 4160 1023.35 × 10 6

20 × " 2 3997 967.00 "
30 × " 3 3802 899.59 "

40 × " 4 3577 821.81 "

TABLE 8.5

The reservoir temperature and gas properties are again the same as detailed in
table 8.1 and therefore, the pressure-pseudo pressure relationship for p > 2800 psia is
given by equ. (8.48). The reservoir and well data are listed below.

-6
= 4290 psia = 3.6 × 10 cp/psi
p i (µc) i
A = 200 acres φ = .15

4 h = 50 ft
Geometry 1
r w = .3 ft
This exercise requires the evaluation of m D(MBH) (t DA) for values of t DA < 0.01, which is
the lower limit of the dimensionless time scale in each of the MBH charts, figs. 7.11-15.
For t DA < 0.01, values of m D(MBH),for the 4:1 geometry, can be obtained from fig. 8.11,
which has been drawn from the tabulated values of m D(MBH) versus t DA presented by
15
Earlougher . (N.B. There appears to be a typographical error in the latter reference in
that the data listed for the 4:1 geometries

and in table 2, p. 203, should be interchanged).

The pressure buildup is first analysed from which it is determined that the permeability
is 50 mD. (This technique will be fully described in sec. 8.11 and illustrated in
exercise 8.3).
Analyse this multi-rate test to determine values of k, S and F using

1) m D evaluated for transient conditions, equ. (8.32)
2) m D evaluated using the general expression, equ. (8.40)