This document provides information on formation evaluation from well logs, including determining porosity, permeability, water saturation, and identifying depleted gas reservoirs. It discusses: 1. Using density and neutron logs to determine porosity in gas-filled reservoirs, and relying on density logs for porosity in oil reservoirs due to clay effects. 2. Calculating water saturation using resistivity logs and accounting for formation water resistivity based on known water saturated zones. 3. Methods for estimating permeability, including using core data, pressure decline testing, porosity-permeability plots, and resistivity and SP logs. 4. Identifying depleted gas reservoirs by looking for large separation between density and neutron porosity logs,

1. 1
Formation Evaluation
(Lecture)

2. 2
Fresh water
Salt water
Really messed-up water!
Porosity%
Ρb, Bulk Density g/cc
Formation Density Log Determination of Porosity

3. 3

4. 4

5. 5
Typical hydrocarbon/water contact
on resistivity log
Ro

6. 6
Shaliness:
Determined from Gamma-ray log. Not reliably interpreted from SP
in impervious strata or in thin beds.
Establish 0%, 50%, and 100% shale lines on the GR log. SS
and LS intervals above the 50% cutoff are shaly. Rattiness of the GR
log to the left or right of the 50% line indicates thin beds of SS & LS
respectively. These beds are too thin for accurate log resolution but
Are still very real! Shaly sands and carbonates (>40%) are generally
worthless reservoirs despite their “apparent” porosity.
Formation Factor (F) – needed for Sw calculation
Ways to calculate F:
F = Ro/Rw use only in clean, water-bearing formations having
little clay or hydrocarbons. Formula has limited value or accuracy
throughout an actual field or large area. Cannot be used when
evaluating multiple reservoirs. Note: R0 = bed resistivity 100%
water saturated and Rw = resistivity of formation water.
F = a/Φm Archie equation or modification there-of (best)
a = tortuosity constant dependent on rock texture
(grain size, sorting, cementation, etc). Use 1 for
carbonates and .81 for most sandstones in OK.
m = cementation factor (1.4 to 1.7 slightly cemented;
1.8 to1.9 moderately cemented, 2.0 to 2.2 highly
cemented). Use 2 for carbonates and sandstone in OK.
Therefore: for sandstone use F = .81/Ø2
for limestone use F = 1/Ø2
FORMATION EVALUATION FROM WELL LOGS

7. 7
Water/hydrocarbon saturation determination:
Can be interpreted from:
SP log – difficult, incorporates too many variables. All rock
factors being equal, the SP is diminished in hydrocarbon
-filled reservoirs as compared to when filled with water.
Cores – often a bad choice since permeable rocks will be
flushed during drilling thereby distorting values badly.
Formula – Best, most accurate, and easy to use in all rock
types and saturation conditions. Uses “hard” numbers
instead of guesses and extrapolated values.
Sw = FRw
Rt
solving for Rw: Rw = Sw
2
Rt
F
Using this method, find a nearby well (usually low to production)
that has good reservoir characteristics but is decidedly wet. This
zone will have the lowest resistivity for the particular reservoir of
interest and will obviously be dry. It can safely and accurately be
assumed that Sw in this interval is between 95%-100% and
obviously, even an error of a few % will not result in large error in
calculation of Rw. From the same well and zone, Rt and F can
accurately be determined from well logs to calculate Rw.
Then, use the SW formula (above) with the calculated Rw to
determine hydrocarbon saturation in the same reservoir

8. 8
Determining water/hydrocarbon saturation, continued
Alternatively, find an interval of the same rock type
stratigraphically close to the reservoir of interest that has
unusually low resistivity compared to know pay zones in
the field area. These zones may have residual
hydrocarbons but the presumed water saturation is still
high and probably in the range of 85-90% (or higher).
Using this presumed water saturation, back-calculate Rw
and then use the Sw formula to determine
water/hydrocarbon saturations.
Another useful formula but of limited value is below:
Sw = Ro
Rt
Its use should be limited to beds having similar reservoir
characteristics (texture, sorting, cementation, porosity), and
similar formation water salinity. These conditions obviously
do not often occur in nature so the formula is appropriate
only for a thick, relatively homogenous reservoir having oil
saturation above a water column.

9. 9
Porosity Determination:
In gas-filled reservoirs: use standard density and neutron cross-plot
values when available. Splitting the difference between the two log
traces will yield sufficiently accurate values in LS strata. In sandstone
reservoirs the cross plot porosity may have to be reduced a few
porosity units to mimic true porosity as determined from core data.
In oil reservoirs: there will be little or no density-neutron porosity cross
over. If the reservoir is clean (little shale/clay, you probably can proceed
as described above. If there is appreciable clay/shale (as determined by
the GR log) I would ignore the neutron porosity and rely solely upon
density values since they are not nearly as affected by clay. If necessary,
reduce the density porosity a few units to match expected core data.
Remember that porosity logs are usually calibrated to a limestone matrix.
Therefore, theoretical porosity in sandstone may be too high on both the
density and neutron logs and too low in dolomite. Porosity values are also
influenced by matrix material between grains, cementation, pore fluids,
among other things. Log corrections are NOT done by the service
companies nor represented on the log. YOU must do it if necessary.
Locally, no corrections are necessary when core porosity mimics log
porosity. If not, you may need to subtract 2-3 porosity units from that
recorded on the density log in sandstone strata.
Gas seriously affect values on all porosity logs. If you have only one
log, say density porosity, you must decrease its value appropriately to
diminish this gas effect. This can be done by documenting the gas
effect in other wells having both density & neutron log suites. The cross
plot porosity in the later will indicate how many porosity units are
causing the gas effect and a ratio (usually ~.65 to .7) can be applied to
the well having only Density porosity.
Personally, I would not want to use strictly neutron porosity because of its
sensitivities to both clay and formation gas.

10. 10
Detecting a Depleted Gas Reservoir
When cross-plot porosity exceeds 10-12 porosity units,
pressure depletion can be presumed
GR & SP % Porosity
30 20 10 0
14-16
porosity units
of separation
= pressure
depletion!
8-10
porosity
units
separation
= normal
pressured
reservoir
(gas effect)

11. 11
Determination of Permeability (K)
1. From cores – usually the best method but cores are few and far-
between!
Permeability measurements are expressed in millidarcys - md (one
thousandth of a darcy). It is affected by many formation attributes such as
pressure, rock texture, and fluid content. For convenience, It is measured in
the lab by passing inert gas such as helium or nitrogen through samples.
The resulting flow is converted to values relevant to common air (KA).
Because this data is often unrealistic, it is frequently converted into units
that more accurately relate to liquid permeability (pure water). Note that the
viscosity of water is similar to that of many oils. This “liquid” permeability is
then called “Klinkenberg” permeability or KK. KA is generally quite
inaccurate in tight reservoirs but closely approximates KK in reservoirs
having > a few hundred md.
2. Pressure decline testing (cannot do on a well-by-well basis or for
multiple reservoirs) conveniently.
3. Porosity vs. Permeability Plot. Very good – see examples
provided. Need to get only one or two cores in the nearby area
having reliable density-neutron log suites. You can then input any
porosity value into the plot to get a good value of permeability.
4. Interpreted quantitatively from micrologs, conventional resistivity
logs (noting separation between the shallow and deep
measurements), and from the caliper log (which measures
mudcake buildup that is a function of permeability).
5. From porosity and Swi (irreducible water saturation) This method is
not easy to complete accurately using standard log suites. Swi is
very, very sensitive to porosity, reservoir texture, oil viscosity, and
just having a bad day in the office! I personally have not used it
successfully and do not recommend its use.
6. From SP logs. A very good qualitative method of estimating
reservoir permeability. Limitations include bed thickness, fluid
content, and resistive bounding strata.

12. 12
Porosity vs. Permeability Plots from core. This is perhaps the
best way to determine permeability in the same formation in
nearby wells having a reliable porosity suite

13. 13

14. 14
ppg = pounds per gallon
Common
normal
subsurface
pressure
gradient
pcf = pounds per cubic ft
Over-
pressured

15. 15