Page 457 - Petrophysics

LINEAR FLOW THROUGH FRACTURES AND CHANNELS 425

Figure 7.2. Poiseuille’s flow system for straight capillaries.

Substituting A = nR2 (R is the radius of total cross-sectional area) in
Equation 7.28 yields:

N
1
k = - 4 (7.29)
8R2 .
J=1
If the radii rj are the same for all tubes, this equation becomes:

nr4
k=- (7.30)
8R2

The dimension of permeability is, therefore, L2 (length squared). Thus,
if L is in cm, k = cm2. This measure is, however, too large to use with
porous media, and the units of Darcy, or mD, are preferred by the oil and
gas industry.
This approach is, of course, an oversimplification of fluid flow in
porous media, as the pore spaces within rocks seldom resemble straight,
smooth-walled capillary tubes of constant diameter.

LINEAR FLOW THROUGH FRACTURES AND CHANNELS

Oil reservoirs with fracture-matrix porosity also contain solution
channels. The matrix (intergranular porosity) is usually of low permea-
bility and contains most of the oil (96%-99%). Whereas these fractures
and solution channels may not contain a significant volume of oil,
generally less than 4% of the total oil in a reservoir, they are
very important to the attainment of economic production rates [6].