Quick look

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QUICK-LOOK METHODS
Quick-look methods are helpful to the geologist because they
provide flags, or indicators, that point to possible hydrocarbon
zones requiring further investigation. The four quick-look
methods discussed here are:
1. Rxo/Rt
2. Apparent water resistivity (Rwa)
3. Wet resistivity (Ro)
4. Conductivity-derived porosity

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1. Rxo/Rt METHODS
The Rxo/Rt technique relies on the comparison of the resistivity
ratio, plotted as a curve on the log display, to the SP curve.
Procedure
1. Calculate the Rxo/Rt ratio from the deep and shallow reading
resistivity measurements;
2. Plot the ratio in the same track as the SP, and look for the
following patterns.
Patterns to observe
1. In a shale, the ratio is relatively constant, like the SP, and usually
close to 1.
2. In a water-bearing zone, the Rxo/Rt ratio tracks the SP.
3. In a hydrocarbon-bearing zone, the Rxo/Rt ratio moves away from
the SP [a deflection to the right for the case of a normal (negative)
SP].

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1. Rxo/Rt METHODS

Sw = Sxo = 100%, two equation become:

Combine equation

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1. Rxo/Rt METHODS
The equation relating the SP to fluid resistivities is:

Replace

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2. Apparent water resistivity (Rwa)
The Rwa technique relies on the comparison of calculated values of
water resistivity between intervals in a well. This comparison can be
made between different zones or within the same zone if a water-
hydrocarbon contact is suspected in that zone. The assumption is that
this lowest value of Rwa is the closest approximation to the true
formation water resistivity (Rw) and that values of Rwa greater than
the minimum value are indicative of the presence of hydrocarbons. A
water saturation can also be calculated from the values of Rwa.

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2. Apparent water resistivity (Rwa)
Procedure
1. Calculate an apparent water resistivity (Rwa) from the porosity and
uninvaded zone resistivity measurements.
2. Look for the lowest value of Rwa in a porous and permeable zone and
compare it to the values of Rwa calculated in the other zones.
3. If desired, an Archie water saturation can be calculated from the Rwa
values in the compared zones.
Patterns to observe
1. The zone with the lowest value of Rwa is the most likely to be water-
bearing, and the value of Rwa is closest to the actual value of Rw in the
formation.
2. Zones with values of Rwa greater than the minimum observed are likely
to have some hydrocarbon saturation.
Interpretation pitfalls
The Rw values in the zones that are compared are assumed to be the same.
In low-porosity zones (less than about 10% porosity), the Rwa value is
lower than the actual Rw value.
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2. Apparent water resistivity (Rwa)

Rt/F calculate by 2 equation

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2. Apparent water resistivity (Rwa)
The following values are used for simplicity: a = 1.0, m = 2.0

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3. Wet resistivity (Ro)
The wet-resistivity curve (Ro) is one of the oldest quick-
look techniques. Unlike the other curves, which tend to be
compared to the SP curve, it is plotted as an overlay on the
resistivity curve.
Procedure
1. Calculate Ro from the porosity and an estimate of
formation water resistivity (Rw).
2. Plot Ro as a curve at the same scale as the resistivity
curves and compare the values to the value of the deep-
resistivity measurement.
3. If desired, an Archie water saturation can be calculated
in any zone from the values of Ro and the deep resistivity
measurement.
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3. Wet resistivity (Ro)
Archie equation

Zone Sw = 100% => Ro = Rt

Patterns to observe
1. In water-bearing zones, Ro and the deep resistivity should
overlay.
2. In hydrocarbon-bearing zones, the deep resistivity is higher
than Ro, with the separation increasing with increasing
hydrocarbon saturation

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4. Conductivity-derived porosity (also referred to as resistivity-
porosity)
Although this technique originated with the use of the
conductivity curve (Dresser Atlas, 1975), the conductivity
curve is now rarely presented on log displays, and the
technique is described here using the resistivity
measurement.
This technique calculates a porosity from Archie’s
equation, using the form of the equation for waterbearing
zones (Sw = 1). The porosity values are generated as a
curve and are normally displayed in the same track as the
SP, scaled from high porosity values on the left to low
porosity values on the right.

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4. Conductivity-derived porosity (also referred
to as resistivity-porosity)
Procedure
1. Calculate the porosity from Archie’s equation (assuming
Sw = 1.0), using the uninvaded zone resistivity
measurement.
2. Compare the curve values and the relative position of
the curve in porous and permeable zones.
3. If desired, an Archie water saturation can be calculated
by comparing the conductivity-derived porosity to porosity
from one of the porosity measurements (sonic, density, or
neutron).

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4. Conductivity-derived porosity (also referred
to as resistivity-porosity)

Patterns to observe
1. In water-bearing zones, the conductivity-derived porosity is high
and approximately equal to the true formation porosity.
2. In zones that contain hydrocarbons, the conductivity- derived
porosity is low, lower than the true formation porosity.
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Ratio method
Water saturation of a formation’s flushed zone (Sxo) is also based on
the Archie equation, but two variables are changed: mud filtrate
resistivity (Rmf) in place of formation water resistivity (Rw) and
flushed zone resistivity (Rxo) in place of uninvaded zone resistivity
(Rt).

Sxo = water saturation of the flushed zone

Rmf = resistivity of the mud filtrate at formation temperature
Rxo = shallow resistivity from a very shallow reading device, such as
laterolog-8, microspherically focused log, or microlaterolog

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Ratio method
The ratio method identifies hydrocarbons from the difference
between water saturations in the flushed zone (Sxo) and the
uninvaded zone (Sw). When the uninvaded zone form of Archie’s
equation is divided by the flushed zone

The moveable hydrocarbon index by the ratio method is:

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Ratio method

If the ratio Sw/Sxo is equal to or greater than 1.0, then hydrocarbons

were not moved during invasion. This is true regardless of whether
or not a formation contains hydrocarbons. Whenever the ratio Sw/Sxo
is less than 0.7 for sandstones or less than 0.6 for carbonates,
moveable hydrocarbons are indicated.

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Ratio method
In the flushed zone of formations with moderate invasion and
average residual hydrocarbon saturation, the following relationship
normally works well:
Sxo = water saturation of the flushed zone
Sw = water saturation of the uninvaded zone

Swr = moveable hydrocarbon index

Rt = true formation resistivity (i.e., deep induction
or deep laterolog corrected for invasion)
Rmf = resistivity of mud filtrate at formation temperature
Rw = resistivity of formation water at formation
temperature
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Ratio method
1. If Swa ~ Swr, the assumption of a step-contact invasion profile is
indicated to be correct, and all values determined (Sw, Rt, Rxo, and
di) are correct.
2. If Swa > Swr, then the value for Rxo/Rt is too low. Rxo is too low
because invasion is very shallow, or Rt is too high because invasion
is very deep. Also, a transition- type invasion profile might be
indicated and Swa is considered a good value for the zone’s actual
water saturation.
3. If Swa < Swr, then the value for Rxo/Rt is too high. Rxo is too
high because of the effect of adjacent, high-resistivity beds, or Rt
estimated from the deep resistivity measurement is too low because
Rxo is less than Rt.
4. If Swa < Swr, the reservoir might be a carbonate with moldic (i.e.,
oomoldic, fossil-moldic, etc.) porosity and low permeability.

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Ratio method
Determine moveable hydrocarbon index
Hydrocarbon in filtrate zone

Hydrocarbon in uninvaded zone

Hydrocarbon moveable is

IPH: Index production Hydrocarbon

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Ratio method
Recovery factor is

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Crossplot porosity and resistivity
Comparison porosity or porosity data with resistivity we can
determine:
 Resistivity of water
 Factor m
 Properties of matrix
Two crossplot porosity and resistivity applied is
1. Hingle Crossplot
2. Pickett Crossplot

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1. Hingle Crossplot
The first cross plot technique to be considered is the Hingle plot. In this
case, assuming that a porosity measurement is available, even if the
matrix values are unknown, a plot can be constructed which will give
porosity and water saturation directly.

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1. Hingle Crossplot
It is clear that the 100%
water-saturated points will
fall on a straight line of
maximum slope. Less-
saturated points, at any fixed
porosity, must have a larger
resistivity and thus fall
below this line. Once these
points have been identified
and ignored, the line
corresponding to Sw = 100%
can be drawn, as shown in.

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1. Hingle Crossplot
Advantages
Water saturation can be predicted without prior knowledge of Rw, ρma
or Δtma, ρma or Δt ma can be directly predicted from the intercept of
the water-bearing line at a conductivity of zero (a resistivity of
infinity).
Disadvantages
Values for tortousity factor (a) and cementation exponent (m)
must be assumed. To use the plot, the resistivity parameter has to be
calculated or a special graph paper used.

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2. Pickett Crossplot
The second useful graphical technique is the result of work by Pickett.
A knowledge of porosity is required, but the values of m, Rw, and Sw
can be obtained. In this method, the power law expression for
saturation is exploited by plotting on log–log scales. Starting with the
general saturation expression:

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2. Pickett Crossplot

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2. Pickett Crossplot
which represents the line of 100% water saturation. In this case the intercept at the
100% porosity point gives the value of Rw directly. For values of Sw less than 100%,
the relationship between φ and Rt will be represented by lines parallel to the 100%
saturation case but displaced to the right.

A log–log representation of resistivity and porosity attributed to Pickett. It is useful

for determining the cementation exponent that best describes a given formation.
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2. Pickett Crossplot
Advantages
Water saturation can be predicted without prior knowledge
of Rw, a, or m. Rw is directly predicted (if tortuosity factor
(a) is known or estimated) from the intercept of the water-
bearing line at a porosity of 1 (φ = 100%). Cementation
exponent (m) is directly predicted from the slope of the
water-bearing line. When using the plot by hand, the graph
paper to be used (full logarithmic) is readily available.
Disadvantages
Values for ρma and Δt ma must be assumed (although in
cases with a wide range of porosities in the water-bearing
zone, the matrix values can be estimated from the water-
bearing line).
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 Zone Depth ρb φD Rt
number (ft) (g/cm3) (decimal) (ohm-m)
1 4400 2.38 0.163 1.7
2 4402 2.44 0.127 2.1
3 4410 2.35 0.181 1.3
4 4414 2.42 0.139 1.6
5 4426 2.42 0.139 1.8
6 4430 2.33 0.194 1
7 4438 2.3 0.212 0.9
8 4536 2.3 0.212 40
9 4540 2.3 0.212 45
10 4546 2.3 0.212 40

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