
Publication History:
This article is based on
Chapter 8 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell Books 1986 Updated 2004.
This
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
permission. 
Moveable Hydrocarbon Saturation
Since water saturation is the natural result of the previous calculations,
it is often reported by the log analyst as one of "the answers".
However, it is the amount of oil or gas, not water, that is wanted.
This information flows from the following equations:
Total
hydrocarbon saturation:
1: Shy = 1.00  Sw
Residual
hydrocarbon saturation:
2: Shr = 1.00  Sxo
CAUTION: The invasion profile has to be "jsut right" for this
calculation. If invasion is not deep enough for the shallow
resistivity tool, Sxo will be too high. In a project in Kuwait,
invasion was so shallo that Sxo = Sw so moveable oil was zero.
We know this is not true.
Compare Sor from core to Shr from logs and adjust Sxo
parameters to obtain a good match. In depleted oil zones, Sor
from core should match (1  SW) from logs.
Moveable
hydrocarbon saturation:
3: Smo = Shy  Shr
OR 3A: Smo = Sxo  Sw
Moveable oil is indicated when Sor from core is less than (1 
SW) from log analysis. Unfortunately residual gas in a core is
meaningless, so this test cannot be made in gas zones.
Recovery
Factor:
4: RF = Shr / Shy
Where:
Sor = residual oil in core analysis listings (fractional)
Sxo = water saturation in invaded zone (fractional)
Sw = water saturation in uninvaded zone (fractional)
Shy = hydrocarbon saturation in uninvaded zone (fractional)
Shr = residual hydrocarbon saturation in invaded zone (fractional)
Smo = moveable hydrocarbon saturation (fractional)
COMMENTS:
These saturations are needed in order to find hydrocarbon volume
which could be produced from the well.
There
are many situations where Smo cannot be calculated, and many silly
results have been presented to clients over the years. If invasion
is very shallow, Smo may be zero when in fact the zone is an excellent
producer. Likewise deep invasion gives useless results. If your
computer program insists on calculating Smo, you will have to
fiddle with RMF to get rational results, or set RMF > 10 to
make Sxo = 1.0 always.
Fluid Volume Calculations
For reserves calculations, it is sometimes convenient to convert
saturations to volume of fluid per unit volume of rock. These
volumes are often called bulk volume and the "per unit rock
volume" is unstated. These volumes are also needed when using
computers for plotting log analysis results versus depth.
Water
volume:
5: Vwatr = PHIe * Sw
Total
hydrocarbon volume:
6: Vhydt = PHIe * Shy
Moveable
hydrocarbon volume:
7: Vhydm = PHIe * Smo
Residual
hydrocarbon volume:
8: Vhydr = PHIe * Shr
Rock
matrix volume:
9: Vrock = 1.00  PHIe  Vsh
Where:
PHIe = porosity from any method (fractional)
Shr = residual hydrocarbon saturation in invaded zone (fractional)
Shy = hydrocarbon saturation in uninvaded zone (fractional)
Smo = moveable hydrocarbon saturation (fractional)
Sw = water saturation in uninvaded zone (fractional)
Vrock = volume of matrix (fractional)
Vsh = volume of shale (shale content) (fractional)
Vhydt = bulk volume hydrocarbon (fractional)
Vhydm = bulk volume moveable (fractional)
Vhydr = bulk volume residual (fractional)
Vwatr = bulk volume water (fractional)
COMMENTS:
This data is used to calculate hydrocarbons in place and recoverable
reserves.
When
making depth plots of log analysis results, the usual approach
is to plot the following curves on a scale of zero to one:
Value to Plot Result
Vwatr water volume
Vwatr + Vhydm moved hydrocarbon
Vwatr + Vhydm + Vhydr residual hydrocarbon
Vwatr + Vhydm + Vhydr + Vrock matrix rock plus porosity
The
remaining distance to the border of the track will be Vsh, the
shale volume. If bound water volume is desired on the plot, it
can be presented as part of the shale volume by plotting;
Vwatr + Vhydm + Vhydr + Vrock + (Vsh * BVWSH)
If
the rock volume is broken into two or more constituents then these additional curves should be plotted
and shaded appropriately.
Moveable Hydrocarbon EXAMPLE
This example shows a comparison of residual oil from core in a
depleted zone (M1 interval) and in a bypassed zone (M3 interval).
The Sor from core equals (1  SW) from log analysis, so there is no
moveable oil in the M1. In the M2 Sor from core is less than (1 
SW) from logs, so there is moveable oil here.
Computed results for carbonate example. Note higher water saturation
on M1 compared to M3. M3 is bypassed pay. M1 is depleted oil. Dots
are core data. Note that residual oil on core in M1 matches
calculated
Sor = (1  SW). In M3, Sor on core is less than (1  SW) from log
analysis, so there is moveable oil in
M3 interval. Calibration to core permeability needs more work to get a
decent match.
