Page 560 - Petrophysics

PETROPHYSICAL PROPERTIES 527

Assuming wfhf >> wf, Equation 8.56 further reduces to (with less than
5% error):

3
SVP = wf (8.57)

It is clear from the above equations that the value of the facture shape
factor K,f changes with fracture shape.

HYDRAULIC RADIUS OF FRACTURES

The effective radius of a fracture (rw) can be obtained by representing
the fracture as a capillary tube. Equating Equations 3.10, which is valid for
a capillary tube system, and 8.3 la, which accounts for fracture geometry,
yields:

(8.58a)

Solving for the radius results [32] :

(8.58b)

Equation 8.58b is very important equation in a sense that it interprets the
fracture geometry in terms of equivalent hydraulic radius and thus can
be incorporated in any tube model (Figure 8.20).

Wf
Figure 8.20. Fracture hydraulic radius in fractured and unfractured rocks. A rock
sample with one fracture of hydraulic radius equal to I m is equivalent to a rock
sample with one solution channel of radius I m.