Page 247 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

216 Reservoir Engineering

start at a flow time of about t = ~pcr~/O.O0088k, in field units as already
specified [13]. This region is suitable for reservoir limit tests in which reservoir
size and distance to boundaries can be estimated. The most useful test to
estimate reservoir limits is the drawdown test [ 1971; interpretation of reservoir
limit tests can be difficult as discussed in the literature [197].
Steady-State Flow. At a constant flow rate for steady-state flow, the pressure
at every point in the reservoir will remain constant with time. This condition is
rarely encountered in most well test analyses; steady-state flow may be approached
in reservoirs with strong water drives or in cases where reservoir pressure is
maintained by gas injection or waterflooding.

Buildup Tests. Pressure buildup tests are conducted by: (1) producing an oil
or gas well at a constant rate for sufficient time to establish a stabilized pressure
distribution, (2) ceasing production by shutting in the well, and (3) recording
the resulting increase in pressure. In most cases, the well is shut in at the surface
and the pressure is recorded downhole. In pumping wells, buildup tests can be
made by: (1) pulling the rods and running a pressure bomb in the tubing, (2)
by measuring pressure in the annulus from sonic measurements obtained with an
echodevice, or (3) occasionally by using surfaceindicating gauges. The pressure
buildup curve is analyzed for wellbore conditions such as damage or stimulation
and for reservoir properties such as formation permeability, pressure in the
drainage area, reservoir limits or boundaries, and reservoir heterogeneities.

Drawdown Tests. Pressure drawdown tests are conducted by: (1) having an oil
or gas well shut in for sufficient time to establish a stabilized pressure distribu-
tion, (2) putting the well on production at a constant rate, and (3) recording
the resulting decrease in bottomhole pressure. An ideal time to run a drawdown
test is when the well is initially put on production because in addition to
obtaining information on wellbore conditions and formation permeability,
estimates of reservoir volume can be made also. A long, constant flow rate
is required.
Falloff Tests. Pressure falloff tests are conducted in injection wells and are
analogous to the pressure buildup tests in producers. A falloff test consists of:
(1) injecting fluid at a constant rate, (2) shutting in the well, and (3) recording
the decrease in pressure. As long as the mobility ratio between the injected fluid
and in-situ fluids is near unity, the analysis of pressure transient tests in injection
wells is relatively simple. The equations used in producing well tests are
applicable with the exception that the flow is taken to be negative for injection
whereas flow is positive for production.
Multiple-Rate Tests. The preceding tests apply to conditions in which the flow
rate eithezf has been or is constant. In some cases, maintaining a constant flow
rate may not be possible or practical. In other cases, regulatory agencies may
require that wells, especially gas wells, be tested at various flow rates. Multiple-
rate tests may be conducted at variable flow rates or a series of constant rates,
and are applicable to buildup or drawdown tests in producers or falloff tests in
injectors. If accurate flow rate and pressure data are obtained, information on
permeability, skin, and reservoir pressure can be deduced.
Interference Tests. In the prior tests, the pressure and flow rate applied to
only one well at a time. With interference tests, two wells are involved. Inter-