Page 251 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

9120 Reservoir Engineering

t

P

IW %
f-
Figure 5-136. Skin region [228].

include: horizontal flow, negligible gravity effects, a single fluid of small
and constant compressibility, a homogeneous and isotropic porous medium,
the Darcy equation is obeyed, and several parameters (including porosity,
permeability, viscosity, and compressibility) are independent of pressure.
Pressure transients arriving at the well following a rate change move through
three regions on their way to the wellbore. Nearest the wellbore is the early-
time region, ETR, where storage and skin effects dominate; next is the middle-
time region, MTR, where the formation permeability is determined, and most
distant is the late-time region, LTR, where drainage boundaries are sometimes
observed (see Figure 5-137a for examples of a buildup test and Figure 5-137b
for a drawdown test) [228]. As discussed earlier, the transient flow region (see
Figure 5-132) is amenable to analysis by transient flow methods; this region
consists of both the ETR and the MTR. The LTR can include the latetransient
and the pseudosteady-state or semisteady-state regions. The crux of the analysis
involves selecting the proper data to analyze.
Middle-time data will plot as a straight line on semilogarithmic paper. The
slope and the intercept of the MTR straight line are used to calculate reservoir
permeability, skin factor, and average reservoir pressure. Semilogarithmic straight
lines can occur in the ETR and the use of their slopes and intercepts results in
unrealistic reservoir characteristics. Typically, improper use of injection well ETR
data indicates a tight, fractured reservoir while use of producing well ETR data
indicates damaged permeable rock when that is not at all the case.
Flow conditions in the pseudosteady-state LTR occur when transients reach
the no-f low drainage boundary during producing-well transient-tests. Flow
conditions in the steady-state LTR occur when transients reach the constant
pressure boundary in secondary recovery operations. The slope of a Cartesian