Page 261 - Drilling Technology in Nontechnical Language

252 Drilling Technology in Nontechnical Language Second Edition

2,500 psi in the bottom of the well and 3,000 psi in the formation, the
drawdown is 500 psi. The greater the drawdown, the faster the well flows.

When the well is first opened up, it takes a little while for the flow and
the downhole pressures to stabilize. It is a bit like increasing the throttle
setting on a big truck; it takes some time for the truck to settle down at a
steady speed.

The time taken for the well to reach a stable drawdown indicates the
permeability of the reservoir. The greater the permeability, the faster a
stable drawdown is reached. However, analysis of the drawdown data is

difficult because the flow rates are not stable until drawdown stabilizes.

It is therefore preferred to analyze the buildup curve once the well is
shut in again to evaluate permeability and wellbore damage. The faster
the pressure in the well builds up to reservoir pressure, the higher
the permeability.
If the well is flowing at a stable rate and a sudden change in rate is

made (by changing the choke size), a pressure disturbance is created
within the reservoir. This disturbance moves like a shock wave away from
the wellbore through the reservoir. This shock wave may be reflected

off internal disruptions to the reservoir or off the outer boundaries of
the reservoir. If it hits a gas cap, the shock wave may simply dissipate.
Any reflections can be recognized by a change in the measured pressure

once the reflected wave comes back to the wellbore. This type of test is

called a transient test. Transient testing, made possible by the extreme
accuracy of modern gauges and very powerful computers, allows well
test interpretations that provide a description of the internal geometry of
the reservoir.
To analyze a transient test, two curves are produced of pressure vs.
time using log-log axes. One curve simply plots pressure against time (a
pressure curve), and the other plots the rate of pressure change against

time (a derivative curve). The shape of the derivative curve identifies
features that would be too subtle to be recognized from the pressure curve
alone. Early transient curve shapes were compared to a library of curves
that were characteristic of various types of reservoir, but using computers,
it is possible to compare a vast number of reservoir model shapes to the
observed data.

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