Well Log Interpretation

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Well Log Interpretation
Earth Environmental Science University of
Texas at Arlington
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Interpretation
The primary goal of well log interpretation is to
determine whether there is petroleum, and if so,
how much can be recovered and how fast it will
flow. Well log interpretation is used to
determine the economic viability of the well How
profitable it will be and how soon the drilling
costs can be recovered.
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Interpretation
Unless the stratigraphy and reservoirs are well
known, the first step is to scan the well log for
likely reservoirs. The well site geologist will
have information about the location of petroleum
shows
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Interpretation
Which logs would be used for each of these steps?
What would you look for in those logs?
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Quick-Look Methods Rxo/Rt
One of the reconnaissance methods is the
relationship between the SP curve and the
resistivity ratio Rxo/Rt
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Quick-Look Methods Rxo/Rt
This technique works because the SP log is based
on differences in salinity which in turn are
related to differences in resistivity
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Quick-Look Methods Rxo/Rt
When the rock contains only water, Rxo/Rt will
differ from the SP only by a constant. If
petroleum is present. Rt increases so the two
curves deflect away from each other.
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Quick-Look Methods Rwa
Apparent Water Resistivity (Rwa) compares the
deep resistivity of various zones in the well
bore. The lowest Rwa is assumed to be water, so
high Rwa must contain petroleum.
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Quick-Look Methods Rwa
This works because
Need info about lithology and porosity
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Quick-Look Methods Rwa
If the lowest Rwa reading reflects only water in
the pores, then the apparent water saturation
(Swa) can be estimated by
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Quick-Look Methods Rwa
This Swa assumes that the zones being compared
have the same lithology and porosity.
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Quick-Look Methods Resistivity porosity
This method calculates a porosity from
resistivity data using the Archie Equation, and
assuming Sw 1 In zones that are water filled, F
is high and equal to the true porosity. In zones
that have petroleum, Rt is high and F is lower
than the true value. FR is plotted with porosity
logs and knowledge of the lithology is assumed.
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Quick-Look Methods Wet Resistivity (Ro)
Ro is the actual resistivity of the formation and
fluids. Rt is the measured value. Ro can be
estimated from the formation factor (a, m F),
and Rw. Assuming a value for Rw and F, then Ro is
the estimate for the resistivity of a water
saturated zone.
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Quick-Look Methods Wet Resistivity (Ro)
When the calculated Ro is plotted with Rt, the
deep measurement by the log, the two traces
should overlay if there is no petroleum.
Otherwise, the two curves will diverge.
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Detailed Log Analysis
Once prospective hydrocarbon zones have been
identified, then calculations of the desired
parameters for economic evaluation are made.
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Detailed Log Analysis Water Saturation
Water saturation in the flushed zone and the
uninvaded zone are calculated using the Archie
Equation.
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Detailed Log Analysis Water Saturation
Instead of calculating Sw and Sxo separately, it
is useful to calculate their ratio, because the
lithology factors are eliminated.
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Detailed Log Analysis Water Saturation
Sw/Sxo is the Moveable Hydrocarbon Index. If
Sw/Sxo 1, no hydrocarbons were moved. If it is
less than 0.7 for ss, or less then 0.6 for carbs,
then petroleum will move.
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Detailed Log Analysis Water Saturation
Instead of calculating Sw using the Archie
equation where lithology parameters must be
known, water saturation can also be estimated
using the ratio method without knowing the
lithology parameters.
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Detailed Log Analysis Water Saturation
The saturation ratio can be determined using only
resistivity data (above). If petroleum is
present, then
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Detailed Log Analysis Water Saturation
Substituting Sxo gives Swr (water saturation
ratio method).
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Detailed Log Analysis Water Saturation
Swr can be used as a check on Sw computed using
the Archie equation
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Detailed Log Analysis Irreducible Water
Saturation
Water saturation, Sw, includes water that is
bound to particle surfaces, and water that will
not move because of capillary pressure. This is
called irreducible water saturation, Swirr. If Sw
Swirr, then no water will be produced, which is
important to know in making an economic
evaluation of the well.
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Detailed Log Analysis Bulk Volume Water
Bulk water volume (BVW) Sw F. Table 7.1
shows estimates of BWV at irreducible water
saturations, so calculation of BVW can show
whether the reservoir will produce water along
with petroleum
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Detailed Log Analysis Bulk Volume Water
Buckles plots are a way of determining whether
the reservoir is at Swirr. (The ordinate should
be Sw, not Swirr).
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Detailed Log Analysis Bulk Volume Water
Plots of F against Sw will follow the hyperbolic
curves of BVW if the reservoir is at Swirr
(left). Otherwise, both petroleum water
production are likely.
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Detailed Log Analysis Assignment
On your spreadsheet from the previous resistivity
assignment, add columns to calculate water
saturation using the ratio method (Swr), Moveable
Hydrocarbon Index (MHI), and Bulk Volume Water
(BVW). Make a Buckles plot of Sw and F to
determine whether the zones are at Swirr. For
each of the zones you have analyzed, describe and
explain the potential to recover hydrocarbons
economically.
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Detailed Log Analysis Saturation Crossplots
With the advent of computers, graphical solutions
to the Archie equation arent so necessary any
more. However, there are two that are sometimes
used to get a visual picture of the productive
zone saturation.
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Detailed Log Analysis Pickett Crossplot
The logarithmic form of the Archie equation can
be written in a couple of ways
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Detailed Log Analysis Pickett Crossplot
The form below is the one traditionally used for
the Pickett crossplot. (Note equation 7.26 in
text and the description in Fig. 7.4 is wrong).
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Detailed Log Analysis Pickett Crossplot
When F is plotted with Rt on log-log graph paper,
the slope of the line is -1/m and the intercept,
when Rt1, is (aRw)1/m.
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Detailed Log Analysis Pickett Crossplot
Note that this plot requires Sw 1.0. If enough
points can be plotted, a value of m can be
determined. a can be calculated if Rw is known
(or vice versa).
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Detailed Log Analysis Pickett Crossplot
This plot also requires that the lithology (a)
and Rw be the same in all zones plotted.
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Detailed Log Analysis Pickett Crossplot
Lines for Sw lt 1 can be drawn parallel to the
Sw1 line using the factors in table 7.2. Find Rt
for Sw1 at any arbitrary F, and multiply that Rt
by 1.56 to get the Rt at Sw0.8 for that F. Draw
the line parallel to Sw1.
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Detailed Log Analysis Hingle Crossplot
Hingle crossplots are strange and based on this
form of the Archie equation
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Detailed Log Analysis Hingle Crossplot
F, or any proxy such as the density or sonic logs
is plotted on a linear scale at the bottom. The
ordinate is 1/Rt (or conductivity) and has to be
scaled for particular values of a and m.
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Detailed Log Analysis Hingle Crossplot
The scaling of the ordinate must be designed so
that values of Rt and F plot as a straight line
for constant Sw.
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Detailed Log Analysis Hingle Crossplot
While a and m must be assumed to design a
Hingle plot to get a straight line, the data
plotted on the Sw1 line can be used to calculate
Rw. Swlt1 can also be estimated once the Sw1 line
is established.
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Detailed Log Analysis Permeability
Permeability can be estimated from porosity,
resistivity, Sw and hydrocarbon density data.
However, Sw must equal Swirr, the irreducible
water saturation. Bulk Volume Water (BVW) must be
calculated and plotted in advance to made sure
the zone of interest is at Swirr.
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Detailed Log Analysis Permeability
There are two simple formulas for medium gravity
oil and dry gas (i.e. hydrocarbon density is
assumed. For medium gravity oil
For dry gas
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Detailed Log Analysis Permeability
The equations can be solved graphically. Each
hydrocarbon density requires a separate graph.
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Detailed Log Analysis Permeability
A more complicated formula that includes
variables for hydrocarbon density is
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Detailed Log Analysis Permeability
The most reliable permeability comes from well
testing and direct measurements of discharge and
hydrocarbon density. If cores are available,
permeability can be measured in the lab.
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Detailed Log Analysis Shale/Clay Analysis
Shale and clay in in rock directly affects
resistivity and porosity measurements and all of
the parameters derived from them, especially
Sw. Phyllosilicates do not all affect resistivity
the same way. It is the cation exchange capacity
of the layer silicate that is critical
Kaolinite, chlorite muscovite and biotite with
low capacities do not affect the resistivities as
much as the smectites. Logging tools can not make
those distinctions so clay content in rocks is a
significant problem.
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Detailed Log Analysis Shale/Clay Analysis
Some knowledge of the resistivity of the
phyllosilicate component is important, so the
usual assumption is that Rt of nearby shale zones
is the same as Rsh in the reservoir. This is
often a bad assumption leading to erroneous Sw.
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Detailed Log Analysis Shale/Clay Analysis

• The usual procedure is
• Calculate a volume of shale (Vsh) using the gamma
  ray log the SP log, or a lithology crossplot.
• Use the Vsh to correct porosities calculated by
  the sonic, density and/or neutron logs.
• Measure a bound water resistivity (Rwb) from
  zones with 100 shale and a free water
  resistivity Rt from a shale free zone.

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Detailed Log Analysis Shale/Clay Analysis
4. Calculate a water-bound saturation Swb for the
100 shale zone. 5. Find the apparent
resistivity Rwa of the reservoir using the
weighted average of Rt and Rwb knowing Vsh. 6.
Calculate a total, shale corrected, water
saturation (Swt) for the reservoir.
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Detailed Log Analysis Shale/Clay Analysis
7. Then the effective water saturation for the
reservoir (Swe) is