Page 256 - Introduction to Petroleum Engineering
P. 256
GAS WELL DELIVERABILITY 243
q 1 q 2 q 3 q 4
Flow rate
q
p 1 p 2 p 3 p 4
p res
Pressure
p
t
FIGURE 12.7 Conventional backpressure test.
q 1 q 2 q 3 q 4
Extend
Flow rate q
q
p 1 p 2 p 3 p 4
p res
Pressure
p
t
FIGURE 12.8 Isochronal test.
The isochronal test can be conducted more quickly than the conventional
backpressure test and is especially useful in low‐permeability reservoirs with long
stabilization times. The isochronal test also uses equal duration flow periods, and the
flow rate is increased from one production period to the next. The difference between
the isochronal test and the conventional backpressure test is the shut‐in period that
follows each flow period shown in Figure 12.8. The shut‐in period provides time for
the pressure in the well to return to p observed at the beginning of the test. An
res
extended flow rate is maintained after the last equal duration flow period.
Production time is lost during the isochronal test while the well is shut in.
Another approach, the modified isochronal test, is faster but less accurate than
the isochronal test. In this case, the flow periods and shut‐in periods are the same
length of time, as shown in Figure 12.9. The shut‐in periods are too short in the
modified isochronal test to allow shut‐in pressures to return to the pressure p at
res
the beginning of the test.
