Page 117 - Basic Well Log Analysis for Geologist

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LOG INTER DRE VATION

1 to 7) were used to establish the R, line (Sy = 1.0). The adjusting by constants for the effect hydrocarbon density
hydrocarbon-bearing Morrow sand from 4.536 to 4.546 ft has on permeability (Wyllie and Rose, 1950; formulas). The
(points 8 to 10) plot below the 20% water saturation (S, = following data are required to calculate permeability by the
0.2) line indicating the sand ts productive. Coates and Dumanoir (1973) formula.
The limitation imposed by evaluating a log from a Ry, | = formation water resistivity at formation
crossplot is thata relatively large range of porosity values in temperature
water zones is required to properly define the R, line (Fig. Rie = true formation resistivity from a formation at
43) and determine resistivity of formation water (R,,). Also, irreducible water saturation
the lithology and mud filtrate must stay fairly constant in the py = hydrocarbon density in gm/ce
interval being evaluated. b = porosity
estimating permeability in formations at irreducible water permeability formula is calculation of values for two
A first step in the Coates and Dumanoir (1973)
Permeability From Logs
Log-derived permeability formulas are only valid for
constants: C and W.
C= 23 + 465p, — 188py2
saturation (S, j73 Schlumberger, 1977). When a geologist
evaluates a formation by using log-derived permeability Where:
formulas, the permeability values, if possible, should be Q tI constant in Coates and Dumanoir (1973)
compared with values of nearby producing wells from the permeability formula
same formation. You can make productivity estimates based Py = hydrocarbon density in gm/cc
on log derived permeabilities if the formation evaluated is
2 = (3.75-—¢)+ {Hoel t 22E
compared with both good and poor production histories in
2.0
these nearby wells. By using comparisons of log-derived
permeabilities from several wells, a geologist ts not using Where:
an absolute value for log derived permeability. W ~~ = constant in Coates and Dumanoir (1973)
Two methods for calculating log-derived permeability are permeability formula
discussed here: the Wyllie and Rose (1950) formulas and the ob = porosity
Coates and Dumanotr (1973) tormula. Before these R, = formation water resistivity at formation
formulas can be applied, a geologist must first determine temperature
whether or not a formation is at irreducible water saturation. Ric = deep resistivity from a zone at irreducible water
Whether or not a formation 1s at irreducible water saturation (Sy in)
saturation depends upon bulk volume water (BVW = S,, x
Once determined, the constants C and W can be used to
cb) values. When a formation’s bulk volume water values
calculate permeability (Coates and Dumanoir, 1973).
are constant (Fig. 39). a zone is at irreducible water
Cx gw
saturation. If the values are not constant, a zone is not at
Kl2 = re
irreducible water saturation (Fig. 39). W4 & (RY/Ri in)
The Wyllie and Rose (1950) method for determining
Where:
permeability utilizes a chart (Fig. 44), or the following two
K!/2, = square root of permeability; therefore: K equals
formulas:
permeability in millidarcies (md)
Kl? = 250 X /Sy in (medium gravity oils) Cc = constant based on hydrocarbon density
Kl? = 79 & B/S, in (dry gas) W = constant
b = porosity
Where:
Rin = deep resistivity from a zone at irreducible water
Kl? = square root of permeability; therefore: K is Ry, = formation water resistivity at formation
saturation (Sy ir)
hydrocarbon density is put into the equation, instead of Shaly Sand Analysis
equal to permeability in millidarcies
cb
= porosity
temperature
Swir = water saturation (S,) of a zone at irreducible
water saturation
A more modern, but also more complex, method for
calculating permeability is the Coates and Dumanoir (1973)
The presence of shale (i.e. clay minerals) in a reservoir
formula. Unlike the Wyllie and Rose (1950) formulas,
derived from logs. These erroneous values are not limited to
can cause erroneous values for water saturation and porosity