Gas Well Deliverability Testing
Fri, 9 Sep 2005
What is Deliverability Testing?
The "Deliverability" of a
gas well can be defined as the well's capacity to produce against the
restrictions of the well bore and the system into which the well must
flow. These restrictions are barriers which must be overcome by the
energy in the reservoir. Reducing the size of the well bore or
increasing the pressure of the system into which the well must produce,
increases the resistance to flow and therefore reduces the
"Deliverability" of the well. The Deliverability Test allows prediction
of flow rates for different line and reservoir pressures.
testing goes under several names such as "Back-Pressure Testing",
"4-Point Testing" , "Open Flow Potential Testing", and "AOF Testing".
The terms "Open Flow Potential" and "Absolute Open Flow" refer to the
theoretical maximum flow rate from the reservoir if the sand face
pressure were reduced to atmospheric. A "Deliverability Test" usually
requires the well to be produced at several rates. The flowing pressure
at the sand face for each rate and the sand face pressure after
build-up are then determined. The pressure/flow data are used to
determine AOF or Deliverability. Deliverability tests are used by
regulatory agencies to allocate production quotas and by pipeline
operators to contract for gas purchases.
SPIDR Deliverability Tests
testing requires accurate measurement of well-head pressures and flow
rates under rapidly changing conditions. The SPIDR is ideally suited
for this activity. It's high accuracy internal pressure transducer
records instantaneous pressures, even though they may be changing
rapidly. This is impossible with a dead-weight tester. The SPIDR is
also more accurate than a field dead-weight tester. The two-pen
circular chart recorder normally used for flow measurement is difficult
to read and relatively inaccurate. The SPIDR interfaces with an
electronic d/p cell to measure differential pressure across an orifice
plate. The meter-run may be located several hundred feet from the
well-head where the SPIDR is located. A diagram of the SPIDR d/p cell
installation can be seen here. It is not necessary to replace the
two-pen recorder when using the electronic d/p cell.
The SPIDR system permits continuous digital display of flow rates and well head pressures during the test.
key to accurate determination of "deliverability" is the ability to
reliably convert well-head pressures to down-hole pressures for both
static and flowing conditions. This is especially important for wells
with significant fluid production. The SPIDR software for converting
well head pressure to bottom hole pressure has been proven over
thousands of tests. The conversion to bottom-hole pressure is accurate
for wells producing up to 150 barrels of fluid per million cubic feet
of gas. The software can also generate the AOF and Deliverability plots
shown in this Technical Alert. Use of the SPIDR system reduces the work
load associated with Deliverability testing thereby reducing the cost.
The flow of gas to the well bore can be described by the equation:
BHPsi is the shut-in bottom-hole pressure and BHPwf is the flowing
bottom hole pressure at flow rate Q. The coefficient "C" is a constant
that includes the drainage radius, radius of the well bore, reservoir
permeability, formation thickness, gas compressibility and viscosity,
and reservoir temperature. The exponent "n" accounts for non-ideal gas
behavior and nonsteady state flow. Under ideal conditions, "n" equals 1.
The gas flow equation can be rewritten by taking the log of the equation:
this equation it is evident that a plot of the log of the flow rate
against the log of the bottom hole pressure differences squared, will
yield a straight line of reciprocal slope "n" as shown at right.
intersection of the straight line with the square of the shut-in
bottom-hole pressure yields the theoretical flow from the reservoir
(AOF) if the sand face pressure were reduced to zero. For a given well,
the terms "C" and "n" may often be considered as constants. However,
for wells with low permeability, "C" will decrease with increasing flow
time. It will then be necessary to use the Isochronal or Modified
Isochronal Deliverability Tests described below.
The value of
"n" will normally fall between .5 and 1.0. Values outside this range
are not considered valid. A value of "n" equal to 1 indicates steady
state viscous flow and a value of .5 indicates steady state turbulent
Once the value "n" has been determined for a given well,
subsequent tests may use the "one-point" method. This technique assumes
that the slope of the deliverability curve does not change with time.
The "one-point" test requires the well-head pressures at one stabilized
flow rate and after a stabilized shut-in. The well-head pressures are
converted to bottom-hole pressures and the AOF is calculated from the
find the "DELIVERABILITY" of the well against any line pressure, the
equation is modified by substituting surface pressures for bottom-hole
"DELIVERABILITY" is calculated after substituting the flow rate (Qwf)
of the well with its corresponding well-head pressure (WHPwf), and a
recent shut-in well-head pressure (WHPsi). The deliverability can then
be calculated for any line pressure (Pline).
Deliverability Test Procedures
"DELIVERABILITY" test requires that the well be produced at several
different rates, usually four. As a general rule, the rates should be
high enough to create drawdowns of 5, 10, 15, and 20%, of the shut-in
well-head pressure. The rates must also be sufficiently high to
continuously unload produced fluids. The flow rate and the flowing
well-head temperature should be accurately recorded at the end of each
flow period. The flow periods must be of sufficient duration to achieve
stabilized flow which is defined as pressure changes of less than 0.1%
of the shut-in well-head pressure over 15 minutes. The figure at right
is a diagram of the conventional Deliverability test.
flowing well-head pressures (Pwfi) at the end of each flow rate(Q1-Q4)
are converted to bottom-hole pressures and squared. The squared
pressures are then subtracted from the square of the shut-in
bottom-hole pressure (Psi). These differences are plotted against the
flow rates on a log-log scale as shown in the previous graph.
it is difficult or impractical to achieve stabilized flow because of
low reservoir permeability, the Isochronal or Modified Isochronal
multipoint test isused. In the Isochronal test, care is taken that the
flow periods are of equal duration. At the end of each flow period, the
well head pressure is allowed to return to the initial shut-in pressure
(Psi). The last flow in the sequence is of extended duration in order
to achieve stabilized flow. A diagram of the isochronal test is shown
Four sets of flow rate/WHP values should be taken
during each flow period. For the sake of clarity, the four data sets
are only shown for flow period Q2. After converting the well-head
pressures to BHP values, the plot shown below and to the right is
slope of each line should be similar. The flowing well-head pressure at
the end of the extended flow period is converted to bottom-hole
pressure and used to locate the Stable Flow point on the previous plot.
A line of the same slope is drawn through the Stable Flow point to
obtain the AOF.
To further shorten the test period for low
permeability wells, the Modified Isochronal Test is used. This test
differs from the Isochronal test in that the flow periods and shut-in
periods are of equal duration. The well is not allowed to build back to
its pretest shut in pressure. The next plot shows the Modified
Isochronal deliverability test.
plotting the data, care should be taken that the build-up pressure
before each flow rate is used when calculating (Psi2 - Pwf2) for each
flow. The plot is constructed and the AOF determined in the same manner
as described for the Isochronal Deliverability Plot.