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Normalised dimensionless difference between the sandface bottomhole pressure (BHP)
and the
sandface r
eservoir pressure | LaTeX Math Inline |
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| body | \displaystyle p({\bf r}, t) |_{{\bf r} \in \Gamma_s} |
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at the boundary
of
damaged reservoir zone :
| LaTeX Math Block |
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S_M = \frac{2 \pi \sigma}{q_t} \cdot \left[ p_{wf}(t) - p({\bf r}, t) |_{{\bf r} \in \Gamma_s} \right] |
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The
| LaTeX Math Block Reference |
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can be re-wrriten as:| LaTeX Math Block |
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| anchor | P_wf_skin |
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| alignment | left |
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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.
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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
seesee | LaTeX Math Block Reference |
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below:| LaTeX Math Block |
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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.
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