Page 49 - Well Logging and Formation Evaluation

Quicklook Log Interpretation 39

• The porosities calculated in the well are incorrect.
• The zone may not in fact be 100% water bearing as assumed.
• The value of m needs to be adjusted.
• The regional value is not applicable in this well.

Reasons why the regional value may not apply are:

• The salinity may be different in this well.
• The chart books assume that the conductivity of the brine is caused only
by the presence of NaCl. If other chlorides are present (e.g., MgCl), the
R w calculated from the chart book will be wrong.
• The samples from which the salinity has been measured in other wells
may be contaminated or affected by salt dropout when the samples were
recovered at surface.
• If shale effects are predominant, the conductivity arising from clay-
bound water may have a different salinity from that produced in a well.
Typically, clay-bound water will be fresher than free water.
• The water zone may have originally been oil bearing but became
flushed by injection water of a different salinity (this is common off-
shore, where seawater is often used for injection).

In theory, the spontaneous potential (SP) curve may be used in some
instances to derive a measurement of R w, although I have never had much
success with it. The procedure for doing this is as follows:

1. On the SP log, draw a shale baseline, which is a line defining an
average of the SP readings in 100% shale zones.
2. Determine the maximum SP deflection (in mV) from the baseline to
the reading observed in thick, porous parts of the reservoir.
3. Using the appropriate chart as supplied by the contractor, convert the
maximum SP deflection to a static spontaneous potential (SSP) value.
This corrects for invasion, borehole, and bed effects.
4. From the appropriate chart, determine kinetic energy (E k), mc (the
mudcake contribution).
5. Calculate E k, shale using E k, shale =DP(bar)/6.9, where DP is the
pressure difference between the mud pressure and formation pressure.
6. Calculate the Eckert number (Ec) of the bottomhole temperature
(BHT) in °C:

Ec BHT) = SSP + E k , mc - E ksh .
(