Normalised dimensionless difference between the sandface bottomhole pressure (BHP) and the sandface reservoir pressure at the boundary of damaged reservoir zone :
S_M = \frac{2 \pi \sigma}{q_t} \cdot \left[ p_{wf}(t) - p({\bf r}, t) |_{{\bf r} \in \Gamma_s} \right] |
where
total sandface rate | |
formation transmissibility at the boundary of the damaged reservoir zone | |
damaged reservoir zone | |
boundary of damaged reservoir zone |
The can be re-wrriten as:
p_{wf}(t) = p^o_{wf}(t)| - \frac{q_t}{2 \pi \sigma} \ S_M |
with the meaning that near-reservoir damage is resulting in additional pressure drop quantified by the value of mechanical skin-factor
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:
S_M = \left ( \frac{k}{k_s} - 1 \right ) \ \ln \left ( \frac{r_s}{r_w} \right ) |
where
well radius from drilling | |
damaged reservoir () radius: ( the most typical range is: m ) | |
absolute formation permeability in the undamaged reservoir zone away from well location | |
absolute formation permeability in the damaged near-well reservoir zone |
The definition of in suggests that:
The most popular practical range of skin-factor variation is with upper limit may sometimes extend further up.
For the negative skin-factor values there is a natural limitation from below caused by the Mechanical Skin concept itself.
The Mechanical Skin concept is trying to approximate the true inhomogeneity of the near and far reservoir zones with homogenous far reservoir model and additional pressure drop at the well wall.
In case of high permeability
The values of 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 below:
k_s = k \cdot \left[ 1+\frac{S_M}{ \ln \frac{r_s}{r_w}} \right]^{-1} \rightarrow \infty \, \mbox{ when } S_M \rightarrow -5 |
In other words, the highly negative skin-factor should be modelled as composite area around near-reservoir zones rather than using the concept of Mechanical Skin.
For horizontal wells the lower practical limit when Mechanical Skin concept can be applied is even lower and usually assumed as 0.
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Well Testing / Pressure Testing
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