Page 182 - Hydrocarbon

Reservoir Description 169

permeability ‘log’ based upon the assumption that reservoirs with smaller pores have
a lower permeability. This type of measurement is usually calibrated with core
measurements.
The methods discussed above only give an indication of permeability near the
wellbore. Reservoir permeability is usually estimated from production tests and is
described in Section 10.4, Chapter 10.

6.4.7. Quick look evaluation

Using the logs and processes described in Sections 6.4.3–6.4.5, a quick look
evaluation of the reservoir can be performed on an individual well fairly quickly.
An example of a ‘quick look interpretation’ employing the logs discussed so far
is shown in Figure 6.57.

1. The lithologies (sand, shale, etc.) are picked using the GR/sonic logs. Where GR
is greater than the ‘shale line’ indicates a shale. Otherwise, the lithology is a
sandstone. The high sonic layers (dotted shading) indicate the sandstone is calcite
cemented, and are not net sand. The ‘net’ thickness is recorded in the results
column.
2. The density log is subdivided into smaller intervals within the sandstone and
an average value assigned. These values of RHOB (bulk density) are used to
calculate the average porosity over the interval and recorded in the results
column.
3. The average values of the deep induction log over the sand intervals are assigned
and are taken to represent R t . These values along with the calculated porosity are
input into the Archie equation (Section 6.4.5) to calculate saturation.
4. Water is interpreted in the sand at 13,950 ft and oil is still present at 13,900 ft,
albeit at a low saturation. The OWC is somewhere between the two, but cannot
be observed in this well due to poor reservoir quality.
5. A small gas cap is interpreted at the very top of the reservoir where the neutron
log has a very low value.
6. In the oil leg the separation between the deep resistivity and shallow
indicates that the shallow resistivity is measuring conductive mud filtrate
which has invaded the formation. This suggests the reservoir has a good
permeability. In the waterleg there is no separation, which only indicates the
deep resistivity is now reading conductive water, but does not indicate low
permeability.

6.4.8. Integration of core and logs

After the quick look evaluation has been undertaken further, data such as core
analysis and image logs will become available which can be used to build a
geological interpretation and refine the evaluation.