Page 255 - Introduction to Petroleum Engineering

242 TRANSIENT WELL TESTING
such as permeability and original gas in place. The quality of the match depends on
the quality of data, which can be affected by noise in the data and the frequency of
data sampling. Data noise is introduced by mechanisms that are not included in RTA,
such as multiphase flow, interference from other wells, and the allocation of rates to
different geologic layers.
Stabilization time and radius of investigation depend on permeability. Tight gas
reservoirs and gas shales have permeabilities that are orders of magnitude smaller
than conventional gas reservoirs. It is often too expensive to conduct a pressure
transient test in a tight gas well or shale gas well until the reservoir boundary is
reached because the stabilization time is too long or the radius of investigation is too
short for a realistic well test duration.

12.3.6 Two‐Rate Test
The turbulence factor D is determined by conducting two transient well tests at two
,
different flow rates {q , q }. Each test yields a skin value SS 2 . The two equations
1
1
2
for skin are solved for the two unknowns S and D in the expression S S Dq
,
given flow rates {q , q } and skins SS .
1 2 1 2
12.4 GAS WELL DELIVERAbILITY
The gas well deliverability test is used to predict gas flow rate as reservoir pressure
declines (Canadian Energy Resources Conservation Board, 1975; Beggs, 1984;
Ahmed, 2000; Lee, 2007). The result of a deliverability test is a relationship between
pressure measurements and corresponding flow rates.
Changes in gas flow rate cause changes in pressure measured at the well. The
choice of a deliverability test depends on the length of time needed to stabilize
pressure changes. Stabilized reservoir pressure p is obtained by shutting in the well
r
until reservoir pressure stops changing. Stabilization time is estimated as

r 2
t s 1000 e (12.31)
kp r
where the variables are stabilization time t (hr), stabilized reservoir pressure p
r
s
(psia), porosity ϕ (fraction), gas viscosity μ at p (cp), outer radius of drainage area r
r
e
(ft), and effective permeability k (md). Stabilization time t increases as reservoir per-
s
meability decreases.
Gas deliverability tests for a single well include the conventional backpressure
test, the isochronal test, and the modified isochronal test. The conventional back-
pressure test consists of a few equal duration flow periods. The test begins at res-
ervoir pressure p . A reasonable number of flow periods is four, as shown in
res
Figure 12.7. The flow rate is increased from one production period to the next.
The flow rate is maintained in each flow period until the flowing pressure at the
well stabilizes. The pressure at the well does not return to p until the test is
res
completed.