Normalised dimensionless difference between the actual drawdown wellbore pressure p_{wf}(t) and a model of a full-entry vertical well with homogeneous reservoir and non-damaged near-reservoir zone p*_{wf}(t):
| S = \frac{2 \pi \sigma}{q_t} (p_{wf}(t) - p*_{wf}(t)) |
where
q_t – total multiphase rate
\sigma – formation transmissibility
By definition the skin-factor is a pressure adjustment at the well wall and does not affect pressure distribution in reservoir away from wellbore r > r_s.
The total skin is usually decomposed to geometrical skin S_G, related to deviation of the well-reservoir contact from the simplest model and mechanical skin S_M, related to pressure drop caused by the near-reservoir zone formation damage.
Dimensionless value characterising permeability change in a thin layer around the well cased by stimulations or deteriorations, represented by Hawkins' equation:
| (1) | S = \left ( \frac{k}{k_s} - 1 \right ) \ \ln \left ( \frac{r_s}{r_w} \right ) |