Skin-factor (mechanical)

Normalised dimensionless difference between the sandface bottomhole pressure (BHP) $\begin{array}{l}p_{wf}(t)\end{array}$ and the sandface reservoir pressure $\begin{array}{l}\displaystyle p({\bf r}, t) |_{{\bf r} \in \Gamma_s}\end{array}$ at the boundary $\begin{array}{l}\Gamma_s\end{array}$ of damaged reservoir zone $\begin{array}{l}A_s\end{array}$ :

$\begin{array}{l}\displaystyle S = \frac{2 \pi \sigma}{q_t} \cdot \left[ p_{wf}(t) - p({\bf r}, t) |_{{\bf r} \in \Gamma_s} \right]\end{array}$

where

$\begin{array}{l}q_t\end{array}$	total sandface rate
$\begin{array}{l}\sigma\end{array}$	formation transmissibility at the boundary $\begin{array}{l}\Gamma_s\end{array}$ of the damaged reservoir zone $\begin{array}{l}A_s\end{array}$
$\begin{array}{l}A_s\end{array}$	damaged reservoir zone
$\begin{array}{l}\Gamma_s\end{array}$	boundary of damaged reservoir zone

It quantitatively 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 the well it can be estimated from Hawkins equation:

(1)	$\begin{array}{l}\displaystyle S_M = \left ( \frac{k}{k_s} - 1 \right ) \ \ln \left ( \frac{r_s}{r_w} \right )\end{array}$

where

$\begin{array}{l}r_w\end{array}$	well radius from drilling
$\begin{array}{l}r_s\end{array}$	damaged reservoir ( $\begin{array}{l}k_s \neq k\end{array}$ ) radius: $\begin{array}{l}r_s > r_w\end{array}$ ( the most typical range is: $\begin{array}{l}r_w < r_s < 1\end{array}$ m )
$\begin{array}{l}k\end{array}$	absolute formation permeability in the undamaged reservoir zone away from well location
$\begin{array}{l}k_s\end{array}$	absolute formation permeability in the damaged near-well reservoir zone

The definition

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suggests that

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

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

Formula

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provides a good example how mechanical skin-factor affects pressure dynamics.

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