Normalised dimensionless difference between the drawdown survey wellbore pressure  $//<![CDATA[ \begin{array}{l}p_{wf}(t)\end{array} //]]>$ and a model with non-damaged near-well reservoir zone  $//<![CDATA[ \begin{array}{l}p^*_{wf}(t)\end{array} //]]>$:

where

$//<![CDATA[ \begin{array}{l}q_t\end{array} //]]>$ – total sandface rate

$//<![CDATA[ \begin{array}{l}\sigma\end{array} //]]>$ – formation transmissibility

It characterises permeability change in a thin layer around the well or fracture plane caused by stimulation or deterioration during production, injection or well intervention.

It contributes to the total skin estimated in transient well testing. 

For the radial-symmetric permeability change around a well it can be represented by Hawkins equation:

Definition suggest that
 

• deteriorated near-well reservoir zone  $//<![CDATA[ \begin{array}{l}k_s < k\end{array} //]]>$ is characterized by a positive skin-factor  $//<![CDATA[ \begin{array}{l}S>0\end{array} //]]>$,
   
• improved near-well reservoir zone   $//<![CDATA[ \begin{array}{l}k_s > k\end{array} //]]>$ is characterized by a negative skin-factor  $//<![CDATA[ \begin{array}{l}S< 0\end{array} //]]>$.

The most popular practical range of skin-factor variation is  $//<![CDATA[ \begin{array}{l}-5 < S < 8\end{array} //]]>$.

Motivation