Normalised dimensionless difference between the sandface bottomhole pressure (BHP)  and the sandface reservoir pressure  at the boundary  of damaged reservoir zone : 

S_M = \frac{2 \pi \sigma}{q_t} \cdot \left[ p_{wf}(t) - p({\bf r}, t) |_{{\bf r} \in \Gamma_s} \right]

where

It quantitatively characterises permeability change in a thin layer (usually < 1 m) around the well or around the fracture plane, caused by stimulation or deterioration during the reservoir invasion under drilling or well intervention or under routine production or injection.

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

For the radial-symmetric permeability change around the well it can be estimated by means of Hawkins equation:

S_M = \left (  \frac{k}{k_s} - 1   \right ) \ \ln \left (  \frac{r_s}{r_w}   \right )

where 

The definition of  in  suggests that:
 

• deteriorated permeability of the near-well reservoir zone  is characterised by a positive skin-factor ,
   
• improved permeability of the near-well reservoir zone   is characterised by a negative skin-factor .

The most popular practical range of skin-factor variation is .

The values of  are usually not supported by the majority of commercial simulators as these values assume almost infinite permeability in the 10 m area around the well see below:

k_s = k \cdot \left[ 1+\frac{S_M}{ \ln \frac{r_s}{r_w}} \right]^{-1}

For horizontal wells the lower practical limit when Mechanical Skin concept can be applied is even lower and usually assumed as 0.

See Also

Petroleum Industry / Upstream / Subsurface E&P Disciplines / Well Testing / Pressure Testing

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