Page 112 - Well Logging and Formation Evaluation

102 Well Logging and Formation Evaluation

complicated and will affect different tools and different depths of
investigation differently. Luckily, most of the world’s oil is stored in quite
thick sands/carbonates. Particularly with OBM, invasion is not much of
1
an issue, and objective hole sections are usually drilled with 8 / 2 -in. hole,
for which the borehole corrections are very small.
For quicklook evaluations, except in rare circumstances, all the
necessary operational decisions, working sums, and averages can be made
without the use of borehole corrections other than those automatically
applied during the logging process by the contractor. For STOIIP deter-
mination, as I have stated earlier, I believe the only sensible way to go is
to derive a saturation/height function. If calibrated from logs, then I have
presented a means whereby the effects of invasion and thin beds can be
corrected for.
Invasion as a phenomenon can actually be very useful when applied
to time-lapse logging (i.e., relogging the same zone some hours or days
later). For a start, the presence of invasion, as observed from a change of
resistivity with time, indicates that permeability must be present. In some
cases the change in properties can enable one to make conclusions regard-
ing the nature of the formation fluid.
One area in which correction for invasion may be very important con-
cerns modeling of acoustic impedance for seismic modeling, which will
be discussed in the next chapter. In this situation, it is essential to re-create
the virgin-zone sonic and density log responses from the log data, which,
because of the shallow reading nature of these tools, will not be correct
without proper modeling.