Page 253 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

222 Reservoir Engineering

the test period and since zero is an easy constant flow rate to maintain, wellbore
storage is a frequent problem. The length of the storage period is increased
by skin damage since damage acts as an area of reduced permeability around
the wellbore.
A logarithmic plot of the change in pressure, pi - pwF versus the test time,
dT, provides a practical means of determining the end of the ETR and the
beginning of the MTR semilogarithmic straight line. A logarithmic plot with a
unit-slope line (a line with 45' slope) indicates storage effects. The proper MTR
semilogarithmic straight line begins at 50 times the end of the unit slope, that
is, wellbore storage effects cease at about one and a half log cycles after the
disappearance of the unit-slope line.
A logarithmic plot that exhibits a half-slope line (a line with a slope of 26.6')
indicates a fractured wellbore. The proper straight line begins at 10 times the
end of the half-slope line if the fractures are unpropped. Pressure drop at the
start of the straight line is twice that at the end of the unit-slope line. Injection
wells, acid jobs, or naturally fractured reservoirs are typical examples of the
uniform flux, unpropped fractures.
A well that has short, propped (infinite capacity) hydraulically induced
fractures will exhibit the proper straight line at 100 times the end of the half-
slope line. The pressure drop will be about 5 times that at the end of the half-
slope line.
Hydraulically stimulated wells in tight formations (~0.01 md) with long (finite
conductivity) fractures never exhibit the proper straight line during a con-
ventional transient test time period. As a practical matter, all production from
tight gas wells occurs during the ETR. Type curves or computer simulation are
required to successfully analyze this type of ETR data. The ETR can range to
hundreds of years in tight gas wells with finite-conductivity fractures.
Use of the logarithmic data plot to determine the start of the semilogarithmic
straight line of course means that the ETR data must be recorded. Pressure
changes need to be monitored by the minute and bottomhole pressure at the
time of shut-in must be precisely determined. Occasionally a great deal of
emphasis is placed on the accuracy of the pressure-measuring equipment when
the emphasis should be on the clock.
A problem in determining the initial pressure frequently arises when pressure
buildup data from pumping wells are analyzed. A Cartesian plot of the early-
time bottomhole pressure versus shut-in time should result in a straight line with
the proper initial pressure at the intercept.

Important Pressure Transient Analysls Equations

,
SClB
permeability, k = 162.6 - md (5-134)
mh

radius of investigation, ri = (5-135)

k
skinEactor (buildup), s = 1.151 1og(-)+3.23] (5-136)
$Pclr: