| THEORY & FORMULAE |
Gas well deliverability test (also referred as conventional backpressure test, flow-after-flow test), consists of a series of three or more flow periods, with pressures, rates and other data recorded as a function of time. It is used primarily to estimate the AOF (absolute open flow) potential of the well.
The AOF potential is the rate corresponding to atmospheric pressure at the sandface. The AOF value is used to compare the producing potential of different wells, deliverability schedule, design surface equipment and by government agencies to set allowables. The most common of the backpressure tests (the 4-point test) is implemented here.
The backpressure equation is given as:
q = C[pR2-pwf2]n
where
q = gas flow rate, Mscfd
q* = AOF rate, Mscfd
pR = average reservoir pressure at well shut-in, psia
pwf = flow wellbore pressure @ sandface, psia
C = performance coefficient, (Mscfd)/[1000psia]2n
n = an exponent (1/n = slope of the deliverability straight line)
θ = angle which the deliverability line makes with the vertical
tanθ = n
To analyse the test, a log-log plot is made of [pR2-pwf2] versus q2, and a best-fitting straight line is drawn through the plot points. Thus, the unknowns - C, n, θ and q* - can be determined.
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