Normalised dimensionless difference between the sandface bottomhole pressure (BHP) p_{wf}(t) and the sandface reservoir pressure \displaystyle p({\bf r}, t) |_{{\bf r} \in \Gamma_s} at the boundary \Gamma_s of damaged reservoir zone A_s:
S = \frac{2 \pi \sigma}{q_t} \cdot \left[ p_{wf}(t) - p({\bf r}, t) |_{{\bf r} \in \Gamma_s} \right] |
where
q_t | total sandface rate |
\sigma | formation transmissibility at the boundary \Gamma_s of the damaged reservoir zone A_s |
A_s | damaged reservoir zone |
\Gamma_s | boundary of damaged reservoir zone |
It quantitatively 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 the well it can be estimated from 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 ( k_s \neq k) radius: r_s > r_w ( the most typical range is: r_w < r_s < 1 m ) |
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 |
The definition
(?) suggests 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.
(2) | k_s = \frac{k}{ 1 + \frac{S_M}{ \ln \frac{r_s}{r_w} }} |
See Also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Well Testing / Pressure Testing
[ Well & Reservoir Surveillance ][ Skin-factor (total) ][ Skin-factor (geometrical) ]