Page 245 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

414 Reservoir Engineering

PRESSURE TRANSIENT TESTING OF OIL AND GAS WELLS

Production rates depend on the effectiveness of the well completion (skin
effect), the reservoir permeability, the reservoir pressure, and the drainage area.
Pressure transient analysis is a powerful tool for determining the reservoir
characteristics required to forecast production rates. Transient pressure data are
generated by changing the producing rate and observing the change in pressure
with time. The transient period should not exceed 10% of the previous flow or
shut-in period. There are a number of methods to generate the transient data
available to the reservoir engineer.
Single-well tests such as buildup, falloff, drawdown, injection, and variable-
rate describe the isotropic reservoir adjacent to the test well while multiple well
tests such as long term interference or short term pulse describe the char-
acteristics between wells. Buildup and falloff tests are most popular because the
zero flow rate is readily held constant. Drawdown and injection tests are run
less frequently due to problems with maintaining a constant rate. Variable rate
tests are useful when wellbore storage is a problem. Multiwell testing for
characterizing anisotropic reservoirs has been popularized by the increased use
of sophisticated simulation software.

Definitions and Concepts
Several excellent references on well test analyses are available [ 13,66,228], and
a good discussion of difficulties in interpretation of data is available in a recent
text [ 1971. From information in these references several definitions will be given,
and the basic concepts of well test analysis will be summarized. More advanced
concepts can be found in the foregoing references or in the extensive literature
on this subject that has appeared in recent years.

Definitions

Transient Region. Flow regimes that occur at different flow times are shown
in Figure 5-132 for a well flowing at a constant rate. The flowing bottomhole
pressure is shown as a function of time on both linear and semilog plots. In
the transient region, the reservoir is infinite-acting, and the flowing bottomhole
pressure is a linear function of log At. This region is amenable to analysis by
transient methods. and occurs for radial flow at flow times up to approximately
t = $pcr:/O.O0264k, where field units are used t is time in hours, Q is porosity
as a fraction, p is viscosity in cp, c is compressibility in psi-l, re is the external
radius in ft, and k is permeability in md [131.

Late-Transient Reglon. At the end of the transient region and prior to the
semisteady-state period, there is a transitional period called the late-transient
region (see Figure 5-132). There are no simple equations that define this region,
but the late-transient period may be very small or practically nonexistent.

Semisteady-State Region. If there is no flow across the drainage boundary
and compressibility is small and constant, a semisteady- or pseudosteadystate region
is observed in which the pressure declines linearly with time (see Figure 5-132).
Pressures in the drainage area decrease by the same amount in a given time,
and the difference between reservoir pressure and wellbore pressure remains
constant during this period. For radial flow, semisteady-state flow conditions