Geoscience Reference

In-Depth Information

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