Pressure difference between the current wellbore flowing pressure p_{wf} and the wellbore flowing pressure p_{wf}(t_0) at time moment t=t_0 when flowrate has changed :
| \delta p (t) = p_{wf}(t_0) - p_{wf}(t) |
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| Fig. 1. A sample of typical DTR (pressure drop P in blue and log-derivative P' in red) presented in log-log coordinates for PTA type-library analysis |
In particular case when the well has been shut=ij for a long time and then opened for flow the pressure drop will be equal to pressure drawdown:
| \Delta p(t) = p_e - p_{wf}(t) = p_{wf}(t_0) - p_{wf}(t) = \delta p (t) |
as the initial wellbore pressure point represent formation pressure:
| p_{wf}(t_0) = p_e |