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MODULE 5 5.0 WELL LOG ANALYSIS 5.1 Wireline Geophysical Well
Log
continuous recording of a geophysical parameter along a
borehole. Table 5-1. Common wireline geophysical well measurements
(Rider, 1996)
Measurement Log Type Parameter Measured Mechanical Caliper Hole
diameter Spontaneous Temperature Borehole temperature
Self-Potential (SP) Spontaneous electrical currents Gamma Ray (GR)
Natural radioactivity Induced Resistivity Resistance to electric
current Induction Conductivity of electric current Sonic Velocity
of sound propagation Density Reaction to gamma ray bombardment
Photoelectric Reaction to gamma ray bombardment Neutron Reaction to
neutron bombardment
Table 5-2. Principal uses of wireline logs (modified after
Rider, 1996)
Tem
pera
ture
Cal
iper
SP
Res
istiv
ity
Gam
ma
Ray
Spe
ctra
l GR
Son
ic
Den
sity
Pho
toel
ectri
c
Neu
tron
Dip
met
er
Imag
e lo
gs
Lithology -- general - - - + + + + - General Geology
Unusual lithology: Volcanics - - - - - Evaporites - + + - - - -
Mineral identification - + - + - - Correlation: stratigraphy - - -
- - - - Facies, dep. environment - - - - - - - -
Fracture identification - - + + +Reservoir Geology
Over-pressure identification - + + - Geochemistry Source rock
identification + + + + + - Maturity + + Petrophysics Porosity + C C
C Permeability - + - + Shale volume + + - Formation water salinity
C - Hydrocarbon saturation C + Gas identification - - - - - Seismic
Interval velocity C Acoustic impedance C C
dip dip
LogUses
Legend: (-) essentially qualitative; (+) qualitative and
semi-quantitative; (C) strictly quantitative
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5.1.1 Log Presentation The values of the parameter measured are
plotted continuously against depth in the well. Hard copies of well
logs are in standard API (American Petroleum Institute) log format.
The overall log width is 8.25 in., with three tracks of 2.5 in.
wide each. A column 0.75 in. wide separates tracks 1 and 2 where
the depths are indicated. Track 1 is always linear, with ten
division of 0.25 in. while tracks 2 and 3 may have a linear scale
similar to track1, a 4-cycle logarithmic scale, or a combination of
logarithmic scale in track 2 and linear scale in track 3. For most
well logs, the common vertical scales used are l:200 and 1:500 but
for image logs (microresistivity) it is usually 1:20 and 1:40.
Every log is preceded by a header. It shows pertinent information
for proper interpretation of the log and in addition, some details
of the well and the log run. 5.1.2 The Logging Environment
Pressure
Formation pressure the pressure under which the subsurface
formation fluids and gases are confined.
Hydrostatic pressure the pressure exerted by a column of fluid.
In the borehole, it is due to the column of drilling mud and is: Ph
(psi) = 0.052 x height of fluid column (ft.) x density (ppg)
Overpressure any pressure above the hydrostatic (or normal)
pressure Temperature Geothermal gradient
DTsTfG /)(100 =
Formation temperature Tf = Ts + G(D/100)
G = geothermal gradient, F/100 ft. Tf = formation temperature, F
Ts = surface temperature (80F) D = depth of formation, ft.
Graphical solution of formation temperature is provided by
Schlumberger Gen-6 chart.
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Borehole Geometry From caliper Gauged hole diameter of hole is
about equal to the bit size Increased borehole diameter
Washout general drilling wear, esp. in shaly zones and dipping
beds, both caliper larger than bit size, considerable vertical
extent
Keyseat asymmetric oval holes, formed by wear against the drill
string at points where the borehole inclination changes
(doglegs)
Breakout similar to keyseat but not due to doglegs, small
brittle fractures (spalling) due to existing stress regime of the
country rock
Decreased borehole diameter - generally due to formation of mud
cake
Mud cake thickness = (bit size diameter caliper diameter
reading)/2 - mud cake formation indicates permeability and involves
loss of mud
filtrate into a permeable formation invasion. Invasion Profile
Figure 5-1 (Gen-3, Schlumberger Charts) shows invasion by mud
filtrate of a permeable bed in a borehole. Also shown are the
nomenclature of the corresponding resistivities and saturations in
each zone.
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5.1.3 Process of Interpretation
Identify potential reservoir intervals; distinguish
non-permeable, non-reservoir intervals from porous potential
intervals.
Estimate thickness of the potential reservoirs. Determine
lithology (rock type) of the potential reservoirs. Calculate
porosity (). Determine resistivity of formation water (Rw).
Calculate water saturations (Sw, Sxo) using resistivity (Rt, Rxo).
Estimate in-place and movable hydrocarbons.
Figure here (Flow chart for log interpretation, Asquith,
p.104-5) 5.2 Resistivity Logs Resistance is the opposition offered
by a substance to the passage of electric current. Resistivity is
the resistance measured between opposite faces of a unit cube of
the substance at specified temperature. Resistivity is measured in
ohm-meter2/meter, more commonly shortened to just ohm-meter.
Resistivity logs do not always measure resistivity directly. Some
resistivity logs (actually induction logs) measures conductivity
instead which is the reciprocal of resistivity.
resistivity (ohms m2/m) tyconductivi
10001= (millimhos/m) Induction logs are used in wells drilled
with a relatively fresh-water mud (low salinity) to obtain more
accurate value of true resistivity. Table 5-3. Principal uses of
the resistivity and induction logs Used for Knowing Quantitative
Fluid saturation:
Formation Invaded zone (detect hydrocarbons)
Formation water resistivity (Rw) Mud filtrate resistivity (Rmf)
Porosity () [and F] Temperature
Texture Calibration with cores Lithology Mineral resistivities
Correlation Facies, bedding characteristics
Gross lithologies
Compaction, overpressure and shale porosity
Normal pressure trends
Semi-quantitative and qualitative
Source rock identification Source rock maturation
Sonic and density log values Formation temperature
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Figure 5-2. Idealized resistivity log. 5.3 Spontaneous Potential
and Gamma Ray The SP and GR logs measures naturally occurring
physical phenomena in in-situ rocks. 5.3.1 Spontaneous Potential
The SP log is a measurement of the natural potential difference or
self potential between an electrode in the borehole and a reference
electrode at the surface (problem with offshore wells, no ground).
No artificial currents are applied. Three factors are necessary to
produce an SP current:
1. a conductive fluid in the borehole, 2. a porous and permeable
bed surrounded by an impermeable
formation, and 3. a difference in salinity (or pressure) between
the borehole fluid
and the formation fluid.
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Figure 5-3. Idealized SP log. Table 5-3. Principal uses of the
SP log Used for Knowing Quantitative Formation-water resistivity
Mud filtrate resistivity and
formation temperature Shale volume SSP (static SP) and shale
line Qualitative Permeability indicator Shale line Facies
(shaliness) Clay/Grain size relationships Correlation Bed Boundary
Definition and Bed Resolution Sharpness of a bed boundary depends
on the shape and extent of the SPO current patterns. When there is
considerable difference between mud and formation water
resistivity, currents will spread widely and the SP will deflect
slowly: definition is poor. When the resistivities are similar,
boundaries are sharper. In general, SP should not be used to
determine bed boundaries. If it has to be used, place the bed
boundary at the point of maximum curve slope. (GR defines bed
boundaries better.) Shale Baseline and SSP SP has no absolute
values and thus treated quantitatively and qualitatively in terms
of deflection, which is the amount the curve moves to the left or
to the right of a defined zero. The definition of the SP zero,
called shale baseline, is made on thick shale intervals where the
SP curve does not move. All values are related to the shale
baseline.
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The theoretical maximum deflection of the SP opposite permeable
beds is called the static SP or SSP. It represents the SP value
that would be measured in an ideal case with the permeable bed
isolated electrically. It is the maximum possible SP opposite a
permeable, water-bearing formation with no shale. The SSP is used
to calculate formation-water resistivity (Rw). Formation-water
Resistivity (Rw)
(S)SP = eRweRmfK
)()(log
S(SP) = SP value: this should be the SSP (Rmf)e = equivalent mud
filtrate resistivity: closely related to Rmf (Rw)e = equivalent
formation water resistivity: closely related to Rw K =
temperature-dependent coefficient K = 61+ (0.133 x TF) K = 65 +
(0.24 x TC) Shale Volume
100)0.1((%) =SSPPSPVsh
PSP (Pseudo-static SP) the SP value in the waterbearing shaly
sand zone read from the SP log. SSP (Static SP) the maximum SP
value in a clean sand zone. The formula simply assumes that the SP
deflection between the shale base line (100% shale) and the static
SP in a clean sand (0% shale) is proportional to the shale volume.
This is qualitatively true but quantitatively there is no
theoretical basis. Shale content from SP is subject to
complications due to SP noise, Rw/Rmf contrast, HC content, and
high salinity drilling fluids.
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5.3.2 Gamma Ray
Figure 5-4. Idealized GR and SGR log. Volume of Shale from
GR
Vsh = 0.33 [2(2 x IGR) - 1.0]
Vsh = 0.083 [2(3.7 x IGR) - 1.0]
minmax
minlog
GRGRGRGR
IGR =
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5.4 Porosity Calculations sonic, density, and neutron logs 5.4.1
Sonic
Figure 5-5. Idealized Sonic log. Wyllies Time Average
Equation
t = tf + (1- ) tma = porosity t = log reading in
microseconds/foot (s/ft.) tf = transit time for the liquid filling
the pore (usually 189 s/ft.) tma = transit time for the rock type
(matrix) comprising the formation
= maf
ma
tttt
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5.4.2 Density
Figure 5-6. Idealized Density log.
b = f + (1- ) ma = porosity = log reading in microseconds/foot
(s/ft.) f = transit time for the liquid filling the pore (usually
189 s/ft.) ma = transit time for the rock type (matrix) comprising
the formation
= fma
bma
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5.4.3 Neutron
Figure 5-7. Idealized Neutron log. Read directly from logs May
need matrix correction
= 2
ND + if no light hydrocarbons
= 2
ND + if light hydrocarbons as present
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5.5 Water Saturation (Sw) Calculations Archies Equation
F = Ro/Rw
F = formation resistivity factor or simply formation factor Ro =
resistivity of rock when water saturation is 1
(100% saturated) Rw = resistivity of saturating water
F = ma
=porosity a = cementation factor m = cementation exponent
Figure 5-8. Schematic illustration of three formations with same
porosity but different values of F (formation factor). Formation
factor equations have been approximated through the years by
various workers and the following are the commonly used.
F = 15.262.0
best average for sands (Humble)
F = 281.0
simplified Humble
F = 21
compacted formations
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Swn = Ro/Rt
Sw = water saturation Rt = resistivity of rock when Sw <
1
Combining the above equations gives Archies equation, the most
fundamental equation in well logging.
Swn = Rt
aRwm = F Rt
Rw
Practical average Archies Equation general equation for finding
water saturation.
Sw = RtRw
15.2
62.0
Symbol Character Derived from Porosity Porosity logs (sonic,
neutron,
density), cross-plots, etc.
15.262.0
F (formation factor) Calculated using empirical formulae
(e.g. Humble formula) and porosity as above
Rw Formation water resistivity SP or laboratory measurements of
resistivities of formation water samples
Ro Rock resistivity saturated 100% with water
Ro = F x Rw (can only be calculated, cannot be measured with
logs)
Rt True formation resistivity Induction Logs and Laterologs
(deep resistivity)
Sw Water saturation of pores waterSw
nshydrocarboSw%100
= RtRo
Sw Calculations Conventional Quick look Rwa F overlay SP Quick
Look Clean Formation Shaly
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WELL LOG ANALYSISWireline Geophysical Well LogLog
PresentationThe Logging EnvironmentProcess of Interpretation
Resistivity LogsSpontaneous Potential and Gamma RaySpontaneous
PotentialGamma Ray
Porosity Calculations sonic, density, and
neutrSonicDensityNeutron
Water Saturation (Sw) Calculations