Normalised dimensionless difference between the drawdown survey wellbore pressure p_{wf}(t) and a model with non-damaged near-well reservoir zone p^*_{wf}(t):
| S = \frac{2 \pi \sigma}{q_t} \cdot \left[ p_{wf}(t) - p^*_{wf}(t) \right] |
where
q_t – total sandface rate
\sigma – formation transmissibility
It 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 a well it can be represented by Hawkins equation:
| (1) | S_M = \left ( \frac{k}{k_s} - 1 \right ) \ \ln \left ( \frac{r_s}{r_w} \right ) |
where
r_w | well radius from drilling |
|---|---|
r_s | damaged reservoir radius r_s > r_w |
k | absolute formation permeability in the undamaged reservoir zone away from well location |
k_s | absolute formation permeability in the damaged near-well reservoir zone |
Definition suggest that
- deteriorated near-well reservoir zone
k_s < k is characterized by a positive skin-factor
S>0,
- improved near-well reservoir zone k_s > k is characterized by a negative skin-factor S<0.
The most popular practical range of skin-factor variation is -5 < S < 8.