Page 113 - Basic Well Log Analysis for Geologist

LOC SPOTL PRETATION

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carbonate with moldic (i.e. comoldic, fossil-moldic. ete.) Table 8. Bulk Volume Water as a Function of Grain Size and
porosity and low permeability. Lithology. A comparative chart.

Grain Size (millimeters) Bulk Volume Water_
Bulk Volume Water
Coarse 1.0 to0.5mm 0.02 to 0.025
The product of a formation’s water saturation (S,,) and its Medium 0.5 to0.25mm 0.025 to 0.035
porosity (d)) is the bulk volume of water (BVW). Fine 0.25 to0.125 mm 0.035 to 0.05
BVW =S,x Very Fine 0.125 to 0.062 mm 0.05 to).07
Silt (<0.0625 mm) 0.07 to 0.09
Where:
(Modified after: Fert! and Vercellino, 1978)

BVW = bulk volume water
CARBONATES*
Sy = water saturation of uninvaded zone (Archie
Vuggy 0.005 to 0.015
equation)
Vuggy and intercrystalline
co) = porosity
(intergranular) 0.015 to 0.025
If values for bulk volume water, calculated at several Intercrystalline
depths in a formation, are constant or very close to constant, (intergranular) 0.025 to 0.04
they indicate that the zone is homogeneous and at Chalky 0.05

irreducible water saturation (Sy, ;,,). When a zone Is at
*Carbonate values (for BVW) are to be used as a general guide
irreducible water saturation, water calculated in the
to different types of porosity.
uninvaded zone (S,) will not move because it is held on
grains by capillary pressure. Therefore, hydrocarbon
production from a zone at irreducible water saturation
should be water-free (Morris and Biggs, 1967). Rar = resistivity of mud filtrate at formation
A tormation nor at irreducible water saturation (S,, ;,,) temperature
will exhibit wide variations in bulk volume water values. R, = resistivity of formation water at formation
Figure 39 illustrates three crossplots of porosity (@) versus temperature
Sy irr for three wells from the Ordovician Red River B-zone,
In water zones (Sy, = 1.0):
Beaver Creek Field, North Dakota. Note, that with
increasing percentages of produced water, scattering of data Ryo X Rypand Rg = Fx Ry,
=
F
points from a constant value of BVW (hyperbolic lines)
Where:
occurs.
Rar = resistivity of mud filtrate at formation
Because the amount of water a formation can hold by
temperature
capillary pressure increases with decreasing grain size, the
R,,. = shallow resistivity
bulk volume water also increases with decreasing grain
F = formation factor (i.e. a/h™)
size. Table 8 Ulustrates the relationship of bulk volume
R, = resistivity of formation water at formation
water values to decreasing grain size and lithology.
temperature
R, = wet resistivity (i.e. resistivity of a zone 100%
Quick Look Methods
water saturated with water of acertain R,,. From
General—Quick look methods are helpful to the Chapter I, R, = R, in wet zones).
K R,, = shallow resistivity from Laterolog-8*,
geologist because they provide “flags” which point to From the above equations, the SP equation can be
possible hydrocarbon zones requiring further investigation. rewritten as:
The four quick look methods which will be discussed are:
(1) R,,/R, curve, (2) Ry, curve, (3) conductivity derived SP = ~K x log (Ryxo/Ro)
porosity curve, and (4) R, curve. Where:
R,,/R, curve-—The R,,/R, curve is presented in track # |
as an overlay to the spontancous potential curve (SP). From Microspherically Focused Log* or
Chapter IT, remember that the SP equation is: R, = wet resistivity (R, = R, when S,, = [00%)
Microlaterolog*
SP = —K ™ log (Ra/R,,)
Where: = spontaneous potential R, (R, for S,, = 100%; Rypg or Rpg) can be used to calculate
In water-bearing zones, the measured values for Ry, and
SP
= 60+ (0.133 x formation temperature)
a value for SP. This calculated value for SP should duplicate
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