Modeling Sonic and Density From Neutron and Gamma Ray
Logs
One log that is relatively unaffected by noise and bad hole
effects is the neutron log. It is a good source of total
porosity (PHIt) and can be used in the time average equation to
generate a sonic log:
1: DTCsyn = DTCMA + (CTCW
- DTCMA) * PHIN
This
can be rewritten in its more usual form as:
2: DTCsyn = DTCMA *
(1 - PHIN) + DTCW * PHIN
Neutron logs can be run through casing and many are available in
well files where no sonic or a poor sonic is present. Because
neutron and sonic logs respond similarly to shale, no special
shale compensation is needed with this method.
The
density log is not as strongly affected by shale, so it requires
more attention to detail:
3: Vshg = (GR - GR0) /
(GR100 - GR0)
4: Vshs = (SP - SP0) /
(SP100 - SP0)
5: Vsh = Min (Vshg,
Vshs)
6: PHIe = PHIN - (Vsh *
PHINSH)
7: DENSsyn = (1 - Vsh -
PHIe) * DENSMA + DENSW * PHIe + Vsh * DENSSH
If there is no neutron log to use as a guide, the
following will give reasonable results for seismic purposes:
8: Vsh = (GR - GR0) /
(GR100 -GR0)
9: DELTmod = DELTMA *
(1 - Vsh - PHIMAX) + DELTW * PHIMAX + DELTSH * Vsh
10: DENSmod = DENSMA * (1
- Vsh - PHIMAX) + DENSW * PHIMAX + DENSSH * Vsh
Where:
PHIMAX is determined from offset well data (zoned according to
lithology).
PHIN
is too low in gas zones, giving DELTmod too low and DENSmod too
high.
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