Page 252 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Pressure Transient Testing of Oil and Gas Wells 221

EARLY MI DDLE LATE
TIMES TIMES TIMES
t /

i
pws

t +At

log t

Figure 5-1 37. Early-, middle-, and late-time regions [228].

plot of an LTR drawdown can be useful in determining reservoir limits. Care
should be taken to ensure that LTR data are not included in the MTR analysis.
The ETR region is dominated by skin and storage effects close to the wellbore.
Storage effects occur when surface and sandface flow rates are unequal such
as an injection well that goes on vacuum when shut in or a gas well where gas
compresses in the tubing due to afterflow following surface shut in. Conventional
pressure transient analysis is dependent on a constant sandface flow rate during