Pressure difference between the current bottom-hole pressure p_{wf}(t) and the bottom-hole 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) |
Pressure drop is:
positive | \delta p (t) >0 | when production rate is increased or injection rate is decreased |
|---|---|---|
negative | \delta p (t) <0 | when production rate is decreased or injection rate is increased |
On log-log plots the pressure drop is always pictured as positive.
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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 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 t_0 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 p_{wf}(t_0) represents formation pressure p_e:
| p_{wf}(t_0) = p_e |