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 (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:
(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 of
S_M in
(1) suggests that:
- deteriorated permeability of the near-well reservoir zone
k_s < k is characterised by a positive skin-factor
S>0,
- improved permeability of the near-well reservoir zone k_s > k is characterised by a negative skin-factor S<0.
The most popular practical range of skin-factor variation is -5 < S < 8.
The values of S < -5 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 (2) below:
(2) | k_s = \frac{k}{ 1 + \frac{S_M}{ \ln \frac{r_s}{r_w} }} |
For horizontal wells the lower practical limit when Mechanical Skin concept can be applied is even lower.
See Also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Well Testing / Pressure Testing
[ Well & Reservoir Surveillance ][ Skin-factor (total) ][ Skin-factor (geometrical) ]