Normalised Normalized dimensionless difference between the drawdown survey wellbore pressure the sandface bottomhole pressure (BHP)
and a
reservoir pressure of a reference model of a full-entry vertical well with homogeneous
reservoir and non-damaged
near-well reservoir zone estimated at wellbore radius :
| LaTeX Math Block |
|---|
|
S = \frac{p_{\rm ref}(t, r_w) - p_{wf}(t)}{ \left[ r \cdot \frac{2\partial p_{\pirm \sigma}{q_t} \cdotref}}{\partial r} \right]_{r=r_w} } |
It can be interpreted as the dimensionless ratio of linear-average pressure gradient between wellbore axis and wellbore radius to the actual pressure gradient at wellbore radius:
| LaTeX Math Block |
|---|
|
S = \left[ \frac{p_{wf\rm ref}(t, r_w) - p^*p_{wf}(t)}{ r_w } \right] |
where
– total sandface rate...
\Big/
{ \left[ \frac{ \partial p_{\rm ref}}{\partial r} \right]_{r=r_w}} |
By definition the skin-factor is a pressure adjustment at the well-reservoir contact and does not affect pressure distribution in reservoir away from wellbore
.This means that skin-based pressure calculations in the damaged or in non-homogenous and non-radial-flow area around a well may become a bit inaccurate.
Nevertheless the size of a damaged area is usually much smaller than a drainage area during the well testing () the skin-factor concept works just fine for the most practical well tests applications.
The total skin is usually decomposed into a sum of two components:
| LaTeX Math Block |
|---|
|
S_T = S_G + \frac{A_w}{A_{wrc}} \cdot S_M |
where
Based on definition the wellbore pressure dynamics
of the well with
skin-factor can be writen as:
| LaTeX Math Block |
|---|
|
p_{wf}(t) = - \frac{q_t}{2 \pi \sigma} \, S + p^*p_{wf\rm ref}(t,r_w) |
...
where
...
...
...
| a reference model of a full-entry vertical well with homogeneous reservoir and non-damaged near-reservoir zone |
...
See Also
...
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Well Testing / Pressure Testing
[ Well & Reservoir Surveillance ]
[ Skin-factor (geometrical) ][ Skin-factor (mechanical) ]
