Primary Pressure Derivative (PPD)In order to properly interpret pressure data for welltest analysis it is important to be able to differentiate between reservoir and wellbore effects. The primary pressure derivative (PPD) was developed to identify wellbore effects within a pressure response so they can be separated from reservoir effects.Some examples of wellbore effects: Phase redistribution due to liquid influx Gauge malfunction Wellbore cleanup operations Geo-tidal or seismic effects