Pressure difference between the current bottom-hole pressure and the bottom-hole pressure at time moment when flowrate has changed :

\delta p (t) = p_{wf}(t_0) - p_{wf}(t)

Pressure drop is:

positive		when production rate is increased or injection rate is decreased
negative		when production rate is decreased or injection rate is increased

On log-log plots the pressure drop is always pictured as positive.

Fig. 1. A sample of typical DTR (pressure drop P in blue and log-derivative P' in red) presented in log-log plot for PTA type-library analysis

In particular case when the well has been shut-in for a long time and bottom-hole pressure has stabilised by the time moment and when well is opened for flow the pressure drop will be equal to pressure drawdown:

{\rm pressure } \, \, {\rm drop} = \delta p (t) = p_{wf}(t_0) - p_{wf}(t) = p_e - p_{wf}(t) = \Delta p(t) = {\rm pressure } \, \, {\rm drawdown}

as the initial bottom-hole pressure value represents formation pressure :

p_{wf}(t_0) = p_e

See Also