Detect a fluid breakout

 

A derivative plot can be used to identify the flow regimes and in particular the radial flow regime. The evolution of the derivative shape by overlaying the data from different PBU tests on the same log-log plot (called derivative overlay) will help to identify these flow regimes and to notice any changing flowing conditions in the near wellbore region.

 

The plot below shows the downhole gauge pressure and rate data versus time for a (fictitious) oil well.
production PBUs
If we plot the pressure data from the different PBU tests on the log-log plot, the derivative overlay below is obtained:
gas breakout in well test analysis
As shown by the red arrow above, a small derivative stabilization is visible at early times, i.e. near wellbore, around 2 hrs. As we plot data from more recent PBUs, this early time “plateau” is extending towards later times, i.e. away from the well.
The derivative overlay shows a dynamic change, a region near wellbore at lower mobility is gradually extending and moving away from the well. In this case, this is consistent with a gas breakout: as the wellbore pressure is probably below bubble point pressure, a zone at lower effective permeability to oil is being created and gradually extending further away from the well (the well is not choked back during this period).
During this observation, no change was detectable at surface because, early on, gas saturation was still lower than critical value and because of uncertainty on rate measurement. Unfortunately, as the well wasn’t choked back, this was later confirmed at surface, and the situation became irreversible. 
Pressure transient analysis will help you to detect and diagnose production problems, and to act before the situation becomes irreversible.
detect production problems
Also note in this example the derivative V-shape and discontinuity at 1hr which is visible in the late PBU tests (shown with the green arrow on the derivative plot). This is indicative of phase redistribution in the reservoir as the gas concentration increases in the wellbore.
In general, a fluid breakout would be first noticed with an increase in mechanical skin as the fluid breaks out at the perforation level. Then later on, the derivative would show the presence of a multiphase region around the wellbore (higher stabilization level at early times indicative of an inner region at lower effective permeability). After some time, total compressibility would also be affected, which would also impact the derivative and the data-plot.
 
The derivative overlay helps to understand other causes of performance deviation, monitor damage, detect water front and predict water breakthrough, quality control the rate data, etc…
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