The Property Model - Reservoir Model Design - page 100

and k h throughout the procedure, and also gives a
sound basis for estimation of upscaled k h and k v .
The upscaled k h (170 mD for the whole interval)
is significantly lower than the arithmetic average
k h because of the effects of sandstone connectiv-
ity and the presence of shales and mudstone
layers. A degree of validation that we have
derived a “reasonable estimate” for k h is found
in the observation that k h lies in the range
k geometric to k arithmetic (Fig. 3.36b ).
The main challenges of the Total Property
Modelling approach are:
1. The approach requires some form of explicit
upscaling, and upscaling always has some
associated errors;
2. Where only log data are available (i.e. in the
absence of fine-scale core data) some form
of indirect estimate of the fine-scale sand/
mud ratios and rock properties is needed,
and this inevitably introduces additional ran-
dom error in the estimation of N/G res .
However, for challenging, heterogeneous or
low-permeability reservoirs, these (generally
minor) errors are preferable to the errors
associated with the inappropriate simplifications
of the N/G approach.
In summary then, the widely used N/G
approach is simpler to apply and can be justified
for relatively good-quality reservoirs or
situations where quick estimates are warranted.
The method tends to embed errors in the process
of re-scaling from well data to reservoir model,

and care should be taken to minimise and record
these errors. The TPM approach is generally
more demanding but aims to minimize the
(inherent) upscaling errors by making estimates
of the effective flow properties of the rock units
concerned. N/G ratios can be calculated at any
stage in the TPM modelling workflow.

3.6 Vertical Permeability

and Barriers

3.6.1 Introduction to k v /k h

The ratio of vertical to horizontal permeability,
k v /k h , is an important, but often neglected, reser-
voir modelling property. Too often, especially
when using the net-sand modelling method, a
value for the k v /k h ratio is assumed at the last
minute with little basis in reality. Figure 3.37
captures a typical “history” for this parameter;
neglected or assumed 1 in the early stages

then rapidly drops after unexpected barriers are
encountered and finally rises again to a more
plausible value late in the field life.
The problem of vertical permeability is also
further confounded because it is very difficult to
measure. Routine core plug analysis usually
gives some estimate of core-plug scale k v /k h but
these data can be misleading due to severe under
sampling or biased sampling (discussed by
Corbett and Jensen 1992 ).

¼

1

0.1

?

0.01

0.001

0.0001

0.00001

0.000001

Fig. 3.37 Typical
“history” of the k v /k h ratio