Below is a Bourdet analysis of CSG Well in Australia, The data is very clear, as this was a build-up analysis. Build-up data tends to be very high resolution, and low noise resulting a very clear definition of radial flow.
In this example, some observations are:
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The end of the data is chosen as radial flow as zero slope was not observed.
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Radial flow could be occurring much later as the test was too short; perhaps a the well is still transitioning from high wellbore storage.
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Radial flow may be masked a pressure support (I.e., a constant pressure boundary); however, the test is very short which mean a very very close pressure source (likely? unlikely?)
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Any estimate of permeability, skin, drainage area, etc. is very difficult with this data.
Without additional information (such as detailed cartesian plot), another alterative is that the downward trend is due PTA: Super Charging? or Pressure Interference? .