Geology Reference
In-Depth Information
Determination of the Geometrical Fracture Network Parameters
Identification of Fracture Sets
Assuming that the rose diagram for the fracture orientations reflects the
strike of sub-vertical fractures only, we define two fracture sets. First one
representing sub-vertical fractures, striking to North +/- 30° and a second
set, for the rest of the population. A third set is introduced for sub-horizontal
fractures. There is much less constraint on this third set, since we only have
visual indications in dry dug-wells and indirect indications of fluid occurrence
in producing wells.
Densities of the Fracture Sets and Fracture Sizes
The determination of these parameters is highly empirical. At the scale of
the visited excavated areas, there is no evidence of single sub-horizontal
fractures that crosscut the domain. However, the sub-horizontal fractures
may arrange into relay structures, therefore resulting at a larger scale in more
continuous horizontal features. We assume that horizontal single elements
have extensions ranging in between 5 and 10 m that is the size of a dug-
well's side, with a mean value of 75 m. This in turn, indicates a mean
fracture area of about 45 m
2
. With a frequency of about 0.4 (i.e. one fracture
in 2.5 m) we deduce the density, expressed in fracture centre/unit volume of
rock, in the range of 0.4/45 = 0.0088. With a frequency of about 0.1 we end
with a fracture density of about 0.0022. As a starting value for modelling
purposes, the density of sub-horizontal fractures has been set to 0.005.
Concerning the sub-vertical structures, we arbitrarily assume a total density
of about 0.01 and smaller sizes, ranging from 2 to 5 m. As the sub-vertical
north striking fractures do not form the majority, we assume a density of
0.004 for this set. These numbers are starting numbers and will be subject
to numerical tests to evaluate how sensible the model is to any significant
variation.
The FRACAS model assumes a lognormal distribution for the fracture
radius. Two parameters are required, a mean value and a deviation . The
mean size in the real space, expressed in [m] is given by the expression
r
= exp ( + 0.5
2
). The parameters used in the next sections are tabulated
hereafter.
Table 1:
Fracture Set Parameters
Fracture set
Mean value
Deviation
Mean radius
(m)
Set 1: Sub-horizontal
1.20
0.5
3.75
Set 2: Sub-vertical, North-South
0.79
0.5
2.50
Set 3: Sub-vertical
0.79
0.5
2.50
