### The default well test derivative may be misleading !

##### In some programs such as Kappa Saphir, there is a default smoothing on the well test derivative that could mask a particular problem with the data. Worse, it could create some erroneous features in the default derivative that could be misinterpreted as flow regimes or a wrong well behaviour.

##### Saphir, for example, uses a default smoothing coefficient of 0.1. So the data are not the raw ΔP plot and derivative but smoothed responses. As a result, the analyst may not spot some data quality issues, tidal effects, wellbore phase redistribution that could corrupt the entire PBU, etc…

##### Tidal effects may not be observed with the default derivative:

##### And below is the derivative that we should see instead. This derivative -with no smoothing- shows some problem with the data, due to the tidal effects.

##### These effects mask the pressure transient response and need to be removed from the data in a proper way.

The plot below shows another example.

##### While the change in the derivative shape is a bit abrupt, this could be mis-interpreted as a decrease in reservoir properties, a boundary or a baffle.

##### If we set the smoothing coefficient to 0, the derivative becomes:

##### The data after 48 hours are affected by a valve handling issue and is therefore not representative of the well and reservoir. Similarly wellbore phase redistribution may not be detected.

##### On some cases, the default smoothing could create some artificial features that could be misleading. In this next example, we see a fall in the derivative instead of the correct stabilization.

##### We need to be cautious with the derivative plot when a default smoothing is applied.

##### Poor quality data, tides, noise, wellbore phase redistribution may then be difficult to spot. Worse, the derivative may be misleading and not representative of the well and reservoir.

**Conclusion: always set the smoothing coefficient to 0. Better to check an “unprocessed” derivative first. **