Geoscience Reference
In-Depth Information
Oil reservoirs under depletion are sensitive to
2 orders of magnitude of permeability varia-
tion per porosity class;
Heavy oil reservoirs, or lighter crudes under
secondary or tertiary recovery, tend to be sen-
sitive to 1 order of magnitude of permeability
variation.
This simple rule of thumb, which has become
known as 'Flora's Rule' (after an influential res-
ervoir engineering colleague of one of the
authors), has its foundation in the viscosity term
in the Darcy flow equation:
Rubble
layer
Basalt
layer
Lava flow Direction
Rubble
Rubble
Basalt
Basalt
¼
k
μ
∇
u
ðÞ
P
ð
2
:
1
Þ
Rubble
Rubble
Basalt
where:
u
Basalt
¼
fluid velocity
¼
Fig. 2.12
Exotic elements: reservoir breakdown for a
bimodal-permeability gas-bearing volcanic reservoir in
which model elements are driven by cooling behaviour in a
set of stacked lava flows (Image courtesy of Jenny Earnham)
k
permeability
μ
¼
fluid viscosity
∇
pressure gradient
Because the constant of proportionality
between flow velocity and the pressure gradient
is k/
P
¼
2. The interpretation of the architectural
arrangement of those elements represented in
a simple sketch - the 'concept sketch';
3. The reservoir quality contrasts between the
elements, addressed for example by looking at
permeability/porosity contrasts between each;
4. The fluid type (gas, light oil, heavy oil);
5. The production mechanism.
The first steps are illustrated in Fig.
2.13
in
which six potential elements have been identified
from core and log data (step 1), placed in an
analogue context (step 2) and their rock property
contrasts compared (step 3). The six candidate
elements seem to cluster into three, but is it right
to lump these together? How great does a con-
trast have to be to be 'significant'? Here we can
invoke some useful guidance.
, low viscosity results in a weaker depen-
dence of flow on the pressure gradient whereas
higher viscosities give increasingly higher depen-
dence of flow on the pressure gradient. Combine
this with a consideration of the mobility ratio in a
two-phase flow system, and the increased sensi-
tivity of secondary and tertiary recovery to perme-
ability heterogeneity becomes clear.
Using these criteria, some candidate elements
which contrast geologically in core may begin to
appear rather similar - others will clearly stand out.
The same heterogeneities that are shown to have an
important effect on an oilfield waterflood may have
absolutely no effect in a gas reservoir under deple-
tion. The importance of some 'borderline'
heterogeneities may be unclear - and these could
be included on a 'just in case' basis. Alternatively, a
quick static/dynamic sensitivity run may be enough
to demonstrate that a specific candidate element
can be dropped or included with confidence.
Petrophysically similar reservoir elements
may still need to be incorporated if they have
different 3D shapes (the geometric aspect) if,
for example, one occurs in ribbon shapes and
another in sheets. The reservoir architecture is
influenced by the geometric stacking of such
elements.
μ
2.4.4.1 Handy Rule of Thumb
A simple way of combining the factors above is
to consider what level of permeability contrast
would generate significant flow heterogeneities
for a given fluid type and production mechanism.
The handy rule of thumb is as follows (Fig.
2.14
):
Gas reservoirs are sensitive to 3 orders of
magnitude of permeability variation per
porosity class;
