Page 111 - Well Logging and Formation Evaluation

Advanced Log Interpretation Techniques 101

In many field static models currently being developed, all the net sand,
porosity, and permeability are input from each well in the form of logs
and geostatistics applied to determine the fieldwide averages, sometimes
also using stochastic approaches (e.g., regarding the distribution of sand
bodies). In this case the sampling part of the above becomes redundant.
Such models will typically be upscaled for reservoir simulation. The
saturations will then be initialized using a saturation/height function
(supplied by the petrophysicist).
Sometimes it is the case that the whole model is completely wrong; for
instance, based on one sample, the reservoir is assumed to be oil filled
when in fact there is a gas column occupying most of the reservoir. In this
case the “uncertainties” presented are obviously meaningless. It is rec-
ommended that these eventualities be considered up front and, if neces-
sary, completely separate scenarios built up, within which the theories
presented above can still be applied.

Exercise 5.5. Error Analysis

1. Copy the GR, density, and deep resistivity values from the top of the
log to the OWC (oil/water contact) into a spreadsheet. For the purposes
of this exercise, treat this as a single zone.
2. Use Monte Carlo analyses to determine the error in net/gross, poros-
ity, and saturation for the averages derived in this well.

5.9 BOREHOLE CORRECTIONS

I do not wish to cover this topic in any great detail. All the contractors
provide borehole, shoulder-bed, and invasion correction charts for their
tools, which can be applied as appropriate.
I would like to make a few remarks about resistivity tools. Modeling
of resistivity tools using analytical approaches or finite element modeling
is extremely complicated. In fact no one has yet successfully modeled the
combined effects of the borehole, invasion, and multiple dipping beds on
the induction tool.
Chart books treat each of these effects separately. Hence there is one
chart for the borehole size/salinity, another for invasion, and another for
(horizontal) shoulder-bed effects. Some software is available for handling
dipping beds, but these programs usually assume no borehole or invasion.
In reality, all these effects combine to produce a response that is extremely