Geology Reference
In-Depth Information
where a is the shape factor parameter depending on fracture and matrix
geometry and
r
w
is well radius.
Equivalent Porous Medium Model
This is also known as equivalent continuum model. Here it is assumed that
flow in a large volume of fractured medium can be similar to that of a
porous medium especially when fracture density is high. In such cases
conventional pumping test methods can be used.
Discrete Fracture Network Models
Network models use fracture characteristics and heterogeneity of rocks mass
based on field data. The hydraulic behaviour of a discrete fracture aperture,
length, density, orientation, connectivity of fractures and fracture filling
material is discussed later.
Two-dimensional and three-dimensional fracture network models have
been suggested by Lee and Farmer (1993). These models evaluate flow in
fractures or fracture sets with synthetic distributions of apertures, orientations,
spacings and dimensions and take into account various surface roughness,
flow channelling, and mixing phenomena at fracture intersections. The
application of these theoretical models to natural systems has been limited.
These models are useful where the area of interest is small—as in the study
of the effect of radionuclide transport through fractures as a result of nuclear
waste storage in geological formations over a long period of time. The
disadvantages of the discrete modelling are that statistical information about
fracture characteristics may be difficult to obtain.
HYDRAULIC CONDUCTIVITY OF FRACTURED MEDIA
In fractured rocks, a distinction can be made between hydraulic conductivity
of fracture,
K
f
and of intergranular (matrix) material,
K
m
. As fractures form
the main passage for the flow of water, the hydraulic conductivity of fractured
rocks mainly depends on the fracture characteristics, e.g. aperture, spacing,
stress, infilling (skin), connectivity etc. as discussed below.
Relationship of Hydraulic Conductivity with Fracture
Aperture and Spacing
The relationship between hydraulic conductivity (
K
f
) of a single plane fracture
with aperture (
a
) is given by equation:
2
a
K
f
=
(10)
12
The equivalent hydraulic conductivity of a rock mass, (
K
s
) with one parallel
set of fractures is expressed by:
