Pressure Drop

Pressure difference between the current wellbore flowing pressure $\begin{array}{l}p_{wf}\end{array}$ and the wellbore flowing pressure $\begin{array}{l}p_{wf}(t_0)\end{array}$ at time moment $\begin{array}{l}t=t_0\end{array}$ when flowrate has changed :

$\begin{array}{l}\displaystyle \delta p (t) = p_{wf}(t_0) - p_{wf}(t)\end{array}$