Dimensionless value characterising permeability change in a thin layer around the well or fracture plane caused by stimulation or deterioration.
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:Normalised dimensionless difference between the sandface bottomhole pressure (BHP)
and the sandface reservoir 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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| anchor | TI84WSMdef |
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S_M = \left ( \frac{k}{k_s} - 1 \right ) \ \ln \left ( \frac{r_s}{r_w} \right ) |
Motivation
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При бурении и освоении скважины призабойная зона пласта подвергается поражению в зоне
, где – радиус скважины по долоту, – радиус поражения призабойной зоны, где произошли изменения в пласте,В зависимости от типа коллектора и технологии бурения и освоения скважины проницаемость призабойной зоны
может оказаться как лучше так и хуже удаленной зоны пласта : .В общем случае, для учета этого явления при расчете динамики давления пласта необходимо применять радиально-композитную фильтрационную модель: внутреннее кольцо пораженного пласта + внешнее кольцо невредимого пласта.
Однако, если кольцевая зона поражения пласта намного меньше радиуса дренирования
| LaTeX Math Block |
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| anchor | rsrwre |
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r_s - r_w \ll r_e \rm \, , |
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frac{2 \pi \sigma}{q_t} \cdot \left[ p_{wf}(t) - p({\bf r}, t) |_{{\bf r} \in \Gamma_s} \right] |
where
The
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can be re-wrriten as...
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| anchor | P_wf_skin |
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p_{wf}(t) = p^o_{wf}(t)| - \frac{q_t}{2 \pi \sigma} \ Sгде _M |
with the meaning that near-reservoir damage is resulting in additional pressure drop quantified by the value of mechanical skin-factor
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– гидропроводность дальней зоны пласта,...
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:
| LaTeX Math Block |
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| Skin |
S_M = \ biggleft ( \frac{k}{k_s} - 1 \biggright ) \ \ln \ bigleft ( \frac{r_s}{r_w} \bigright ) Из определения видно, что
ухудшенная призабойная зона |
where
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The definition of
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in | LaTeX Math Block Reference |
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suggests that: - deteriorated permeability of the near-well reservoir zone
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- is characterised by a positive skin-factor
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- improved permeability of the near-well reservoir zone
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- is characterised by a negative skin-factor .
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The most popular practical range of skin-factor variation is
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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
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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 | LaTeX Math Block Reference |
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below:| LaTeX Math Block |
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| 1J03S |
p_{wf}(t)k_s = p(t,r_w) = p_i + k \cdot \left[ 1+\frac{qS_ tM}{4 \ piln \ sigmafrac{r_s}{r_w}} \,right]^{-1} \bigg[rightarrow -\infty 2S +\, \mbox{ when {\rm Ei} \bigg( - \frac{r_w^2}{4 \chi t} \bigg) \bigg]Заметим, что значение скин-фактора оказывает влияния на поведение давления только в самой скважине, и не влияет на распределение давления за пределами зоны поражения пласта 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.
See Also
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Petroleum Industry / Upstream / Subsurface E&P Disciplines / Well Testing / Pressure Testing
[ Well & Reservoir Surveillance ][ Skin-factor (total) ][ Skin-factor (geometrical) ]
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| r > r_s | (сравните с | LaTeX Math Block Reference |
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anchor | | displaytext | 1DR pressure diffusion of low-compressibility fluid |
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p_F Line Source Solution (LSS) @model| pressure diffusion of low-compressibility fluid |
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provides a good example how mechanical skin-factor affects pressure dynamics
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