Page 257 - Introduction to Petroleum Engineering

244 TRANSIENT WELL TESTING

q 1 q 2 q 3 q 4
Extend
Flow rate q
q

p 1 p 2 p 3 p 4
p res

Pressure
p

t
FIGURE 12.9 Modified isochronal test.

Two methods can be used to evaluate gas deliverability tests: simplified backpres-
sure analysis (SBA) and laminar–inertial–turbulent (LIT) analysis. The SBA method
uses pressures, while the LIT method uses real gas pseudopressures. According
to Lee (2007, page 758), the SBA method is valid at low pressures (<2000 psia).
The LIT method is valid for all pressures.

12.4.1 The SbA Method
The SBA method uses the backpressure equation
n n
q Cp 2 p 2 C p 2 (12.32)
sc r wf
where C and n are empirical parameters, q is the gas flow rate at standard conditions
sc
(MMSCFD), p is the stabilized reservoir pressure (psia), and p is the flowing well
r
wf
pressure (psia). The logarithm of the backpressure equation for measurement i yields
logq n log p 2 logC (12.33)
sci i
A plot of log q versus log(Δp ) yields a straight line. The values of C and n are
2
sci
i
determined by using least squares analysis to fit Equation 12.33 to deliverability test
data. The slope of the line is n and the intercept is log C. The absolute open flow
(AOF) of a gas well is the rate at flowing well pressure p = 0.
wf
Example 12.7 SBA Method
Application of the SBA method to data from a gas well deliverability test gives
C = 5.89 × 10 and n = 0.3477. Estimate the AOF for the well at stabilized
−2
reservoir pressure equal to 1815 psia. The deliverability test uses well rates in
MMSCFG/day.