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 sandface 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 into sum of two components:
| S_T = S_G + S_M |
where
S_G | Geometrical skin, related to deviation of the well-reservoir contact from the simplest model |
S_M | Mechanical skin , related to pressure drop caused by the near-reservoir zone formation damage |