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S T A N D A R D S
During our years of Petrophysics, we have seen many different models
used to evaluate hydrocarbon volumes. Unfortunately, such diversity
makes it impossible to compare data from different wells, fields and
regions. Since Operators want to be able to use the results from one
well to guide them in decision-making with another, we advocate a
consistent log evaluation methodology.
Core Calibrated Log Evaluation
The ideal well evaluation uses parameters determined from measurements
on appropriate core corrected to in-situ conditions. Where such
measurements are available they should be used.
Where core data is absent, the parameters should be derived from
similar Formations elsewhere in the area or Basin where core data is
available. If such data is not available, experience must be used to
select the most appropriate log evaluation parameters.
Consistency
The most important part of this approach is to maintain consistency.
Even if the methodology is not perfect, consistency allows inferences
drawn in one well to be transferred to others both in the basin and
elsewhere.
WellEval.com uses a database of core-based log evaluation parameters
together with the same hydrocarbon saturation model (Waxman-Smits) to
ensure a consistent approach for all petrophysical evaluations. If
core-based parameters are not available, the Waxman-Smits model allows
for in-situ calibrations that can be correlated with core later on. Comparing interpretations from WellEval.com is a legitimate thing to do.
Note that it is very difficult to maintain consistency on a widespread
basis unless your core material has been analysed using a total
porosity approach. There are so many divergent ways of measuring
"effective" porosity, that it is better to measure "total" porosity and
calculate "effective" porosities later, if required (more on porosity here).
Uncertainty
Most Petrophysics make only a cursory attempt at quantifying
uncertainty, if they bother at all. We believe that it is important to
know how wrong you could be, to aid with decision-making. If results
falling anywhere in your uncertainty range make no difference to the
decisions you make, then that's great.
Unfortunately, in many cases, the uncertainty range suggests something
else is possible. In these circumstances, steps should be taken to
resolve the uncertainty. It is important to know that you are in this
situation or you may commit or waste large sums of money carrying out
inappropriate activities!
Examples Showing Value of Uncertainty
Should you test that dodgey interval in your exploration well? It'll
only cost $0.8 million or so find out what it is? Nevertheless it's a
lot cheaper to get your Petrophysicist to calculate the possible range
of water saturations. If none of these exceed a reasonable threshold
(typically Sw<65%) and the system isn't fractured, then testing is a
waste of money!
That offshore oilfield you are developing has reserves of 160 million
bbls of oil and you're sizing the production platform to suit. What's
the uncertainty on your volumes in place? Is the Petrophysical
uncertainty well characterised? If you've got the Petrophysics wrong
and the volume in-place is 30% too high, you'll spend too much money on
your facilities. If the volume in-place is 30% too low, you won't size
your facilties to get the optimum return on investment!
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