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Dynamic Data Analysis – v5.12.01 - © KAPPA 1988-2017
Chapte
r 6 – W ell models -p185/743
When we generate a high conductivity fracture model with no specific damage or stimulation
other than the fracture itself, and then perform a IARF straight line analysis, we will get a
negative skin. This negative skin is not due to any pressure loss or gain at the sandface, it is
just due to the geometry of the well. This is what we call the
Geometrical Skin
S
G
. We can
also link this geometrical skin to the equivalent wellbore radius. This is the radius of a
theoretical vertical well that would have the same productivity as the fracture. The geometrical
skin and equivalent radius depend on our choice of fracture model:
f
f
weq
w
f
G
X
e
X
r
r
X
S
Flux
Uniform
%37
718 .2
ln
:
f
f
weq
w
f
G
X
X
r
r
X
S
ty
Conductivi
Infinite
%50
015 .2
015 .2
ln
:
Another way to present this is that an infinite conductivity fracture with a half length of 200 ft
will have the same productivity of a theoretical vertical well with a radius of 100 ft. If the well
radius is 0.3 ft, the geometrical skin
S
G
will be -ln(333)= -5.8.
The figure below shows the equivalent wellbore radius for an Infinite Conductivity Fracture
(blue) and a Uniform Flux Fracture (orange).
Fig. 6.D.11 – Equivalent wellbore radius
6.D.7.b
Model Skin and Total Equivalent Skin
In complement there may be a pressure loss or gain at the sandface, which is quantified with
the
Model Skin
S
M
. At this stage one has to be careful about the convention of how the skin is
defined. There are two ways to define the Skin factor in a fractured well model, and these are
accessible in the Saphir settings page, Interpretation option, Skin tab:
Fig. 6.D.12 – Skin convention dialog in Saphir