(1)	$\begin{array}{l}\displaystyle t_D=\frac{\chi \cdot t}{r_e^2}\end{array}$

where

$\begin{array}{l}t\end{array}$	transition time
$\begin{array}{l}\chi\end{array}$	pressure diffusivity
$\begin{array}{l}r_e\end{array}$	characteristic radius of reservoir flow inflow or outflow area (usually a well radius or half-size of oil reservoir)

(2)	$\begin{array}{l}\displaystyle t_D= 6.328 \cdot 10^{-3} \cdot \frac{\chi \cdot t}{r_e^2}\end{array}$

where

$\begin{array}{l}t\end{array}$	$\begin{array}{l}r_e\end{array}$	$\begin{array}{l}\chi\end{array}$
days	ft	(md·psi)/cp

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