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

total sandface rate

formation transmissibility at the boundary of the damaged reservoir zone 

damaged reservoir zone


The  can be re-wrriten as:

p_{wf}(t) = p^o_{wf}(t)| - \frac{q_t}{2 \pi  \sigma} \ S_M

with the meaning that near-reservoir damage is resulting in additional pressure drop quantified by the value of mechanical skin-factor 


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 

well radius from drilling

damaged reservoir () radius: ( the most typical range is: m )

absolute formation permeability in the undamaged reservoir zone away from well location

absolute formation permeability in the damaged near-well reservoir zone


The definition of  in  suggests that:
 


The most popular practical range of skin-factor variation is  with upper limit may sometimes extend further up.

For the negative skin-factor values there is a natural limitation from below caused by the Mechanical Skin concept itself.

The Mechanical Skin concept is trying to approximate the true inhomogeneity of the near and far reservoir zones with homogenous far reservoir model and additional pressure drop at the well wall.

In case of high permeability

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} \rightarrow \infty \, \mbox{  when  } S_M \rightarrow -5


In other words, the highly negative skin-factor  should be modelled as composite area around near-reservoir zones rather than using the concept of Mechanical Skin.


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

Well & Reservoir Surveillance ][ Skin-factor (total) ]Skin-factor (geometrical) ]




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