well log fundamentals
TRANSCRIPT
Fundamentals of Log Interpretation - I
SPONTANEOUS POTENTIAL LOGS
GAMMA RAY LOGS
Log Characteristics
• The difference between the electric potential of a moveable electrode
in the borehole and the electric potential of a fixed surface electrode
is measured as a function of depth
• Opposite shales, the SP curve is usually a more-or-less straight line –
called a shale baseline
• Opposite permeable formations (sands), excursions from the shale
base line are observed. Opposite thick permeable beds, the
excursions reach a constant value – called a sand line
• The deflections on a SP log may be either to the left (negative) or to
the right (positive), depending on the relative salinity of the mud
filtrate and the formation waters.
if, water salinity > mud filtrate salinity, deflection to left
if, water salinity < mud filtrate salinity, right deflection
• SP log cannot be recorded in holes filled with non-conductive (oil
based) muds. If resistivities of mud filtrate and formation water is
similar, SP deflections will be minimal and featureless
Sand line Shale line
Basic Principle
• SP currents caused by electromotive forces in the formations:
− Electrochemical component
− Electrokinetic component
• Two electrochemical effects
I )
Layered clay structure and charges on the layers favor the transport
of Na+ ions from the filtrate. Movement of charged ions electric
current
The electrical potential that induces the flow of cations through shale
is called membrane potential
II) At the edge of the invaded zone:
Since formation water is more saline than mud filtrate, transport of
Cl- ions from formation water to mud filtrate, current in opposite
direction to flow of anions
Electric potential driving this current is liquid junction potential
SHALE
SAND
Saline mud
filtrate
Electric current
SHALE
SAND
Saline mud
filtrate
Electric current
More saline formation water
Basic Principles
• Electrochemical forces:
− Membrane Potential >> Liquid junction potential
− If permeable zone contains some shales or dispersed clay, total
electrochemical emf reduced since these shales/clays produce an
electrochemical membrane of opposite polarity to that of the
adjacent shale bed
• Electrokinetic Potential: Induced when an electrolyte flows through a
permeable, non-metallic porous medium
− Magnitude of electrokinetic potential depends on the differential
pressure producing the flow and the resistivity of the electrolyte
− Electrokinetic potential (Eek) is produced by the flow of mud
filtrate through the mudcake found opposite the permeable zones.
Low permeability of mudcake high differential pressure
higher Eek
− Electrokinetic potential also produced across the shale (low
permeability)
− Net electrokinetic potential contribution to the SP reading is the
difference between the contributions of the mudcake and that of
the shale
SHALE
SAND
Direction of electrokinetic current
Mud cake
SP Principles
Current direction in above figure corresponds to formation water being
more saline than mud filtrate
Potential adjacent to permeable sand bed negative compared to potential
adjacent the shale – Negative (left) deflection of SP curve
If formation water is fresh (less saline), opposite direction of current
flow – SP deflection to the right opposite permeable bed
SP currents flow through four different zones/media:
− borehole
− invaded zone
− uninvaded portion of the permeable zone
− shales
SP measurements are of the potential drop in the borehole only – a small
portion of the total potential drop
If the current loop could be prevented from being complete – potential
drop in the borehole (mud) will be equal to the total emf
SP under such idealized condition is called static SP (SSP)
SP characteristics
• Small cross-sectional area of the borehole available to flow of current
compared to the formation implies that maximum potential drop
occurs across the borehole
SP deflection opposite thick permeable sands do approach SSP
value
• SSP value can be determined from SP curve if there exist clean, thick,
water bearing beds in the given horizon. A line is drawn through the
negative maxima opposite the permeable bed. Another line is drawn
through the SP opposite intervening shale beds. The difference in mV
is the SSP
Shape of the SP curve
If formation resistivity and the mud resistivity are comparable , then the
resultant SP curves yield a much crisper definition of the bed boundaries
than when Rt = 21 * Rm. Why?
SP Baseline Shift
Baseline shift occurs whenever formation waters of different salinity are
separated by a shale bed that is not a perfect cationic membrane.
Shale base line does
not return to the
earlier base line
Gamma Ray Logs
• Measure of natural radioactivity of the formations
• Gamma rays are bursts of high-energy electromagnetic
waves spontaneously emitted by radioactive material
(potassium, uranium and thorium) in rocks
• Radioactive material predominantly in shales. Why?
o The lattice structure of clay materials has holes (gaps)
that are occupied by radioactive material
o Shales have very low permeabilities – little possibility
of radioactive material getting washed out
o Clays have igneous origin – more likely to be
radioactive
o Sandstones have quartz origin that have very tight
lattice and cannot accommodate radioactive material
• What comprises a gamma-ray logging device?
o A detector to measure gamma ray radiation emanating
from near the borehole
o Gamma ray propagation through porous media is a
statistical phenomena - measurement may fluctuate
over time
o Average gamma ray intensity over a time window
(‘time constant’ of the tool) is recorded
• The propagation of γ-rays through reservoir rocks is
controlled by the density of the rock – higher the density of
rock, lower will be the measured γ-ray count
Gamma Ray Characteristics
• Resolution of the gamma-ray tool is generally:
o 6 inches to 2 ft. vertically
o 6 inches to 1 ft. horizontally
• Resolution is governed by:
o the time constant of the tool : if time constant is too
large, the γ-ray emitted by thinner shale features in the
reservoir will be averaged out
o Logging speed : slower speed more time to establish
meaningful statistical counts – good record
• Gamma-ray useful for establishing shale beds:
o Can be used in cased/uncased holes
o Can be used in holes with any type of drilling fluid
• Bed boundary picked midway between maximum and
minimum deflection opposite a shale feature
• Gamma-ray can also be used to compute volume proportion
of shale at any vertical location along the well:
o Calculate γ-ray index: sandcleanshale
sandclean
GRGR
GRGRGRI
−
−=
o Read volume of shale Vsh from chart
o 3 curves in the chart: 45o line – upper bound regardless
of formation,
line 2 – older (pre-tertiary rocks – more dense), line 3 –
younger (tertiary rocks – less dense)
Vsh correlation (empirical – Dresser Atlas)
POROSITY LOGS
Neutron Log
• Delineates porous formation and determines their porosity
• Respond primarily to the amount of hydrogen in the
formation (predominantly in water and liquid
hydrocarbons) reflects the amount of liquid-filled φ
Tool cannot distinguish between oil and water
• Gas zones are primarily detected by comparing neutron
logs to other logs e.g. density logs
• Neutrons (neutral particles – mass nearly equal to
Hydrogen atom) emitted into formation collide with
nuclei of formation material reduction in energy
• Maximum reduction in energy when neutron collides with
particle of same mass (hydrogen nucleus)
• After few collisions, velocity of neutrons slow sufficiently
to diffuse randomly in the media and are captured by nuclei
of atoms such as Cl, H or Si
• Capturing nucleus is excited and emits a burst of high-
energy γ-ray of capture : tool measures γ-ray and/or counts
the neutrons impinging a detector
• When H-conc. in area around well bore is high (high
hydrogen index), most neutrons captured in immediate
vicinity of well neutron count rate decreases
Neutron Logging Tools
• SNP – Sidewall Neutron Porosity Log
� Neutron source and detector mounted on a skid that is
pressed against the hole wall
� Detector is shielded - only electrons with energies above
certain (epithermal) threshold are detected
� minimizes spurious effects due to strong thermal neutron
absorbers (e.g. Cl and Bo) in formation waters
� Provides good measurement in open holes – liquid-filled or
empty
• CNL – Compensated neutron Log
� Mandrel-type tool – tool in conjunction with other types of
logging tools
� Dual- thermal neutron detectors – ratio of recordings at the
two detectors used to compute neutron porosity index
� Long source-detectors spacing gives greater depth of
investigation
� Effect of borehole parameters reduced by taking ratio of
two readings
� Can be used in liquid –filled cased or uncased holes but not
air-filled holes
� Since thermal-neutrons are captured – tools affected by
presence of elements having thermal neutron capture
properties such as shales
� Neutron tools tend to read high porosity in shales – due to
bonded-water in clay. If gas present in shale, neutron count
is high – offset by low count due to shale neutron capture effect of gas is masked on the log readings
Neutron-Log Corrections
• Correction for varying borehole radius
• Salinity effects (mud and formation fluid) : Chlorine an
excellent neutron absorber – reduction in neutron count –
higher φ
• High apparent porosity in shale due to bonded water
• Mud and mudcake have high hydrogen count – apparent high
neutron porosity
• Porosity reading affected by lithology – SNP/CNL scaled for
limestone matrix – correction for other matrix using figure
Density Log
• Gamma rays emitted by a source in the tool. These
γ-rays interact with atoms in formation material and
dislodge an electron – Compton scattering
• G-ray diminishes to lower energy level and is
recorded by a detector
• Extent of Compton-scattering (hence of energy of
impinging g-ray) related to electron density in
formation
• Electron density is directly related to material bulk
density (i.e. combined density of rock matrix, fluids
in pore spaces)
Skid-mounted tool
Density tool corrections
1. Mud cake correction – Ideally count rate in both short
and long-spaced detectors should be same in absence
of mud-cake - however mud-cake invariably
present
formation density constant, but
thicknessmud cake varied
Density log corrections
Mud-cake correction charts
mud cake density constant
but thicknessmudcake and
formation density varied
mud-cake density and
thickness varied, formation
density constant
2. Lost pad contact: Results in density reading to be
affected by borehole fluids. If problem is severe:
significant deviations from average readings in a zone.
Porosity from Density Logs
• Reading from density log affected by matrix + fluids
(volume of pore spaces)
i.e. ρdensity=ρfluid * φ + ρmatrix * (1-φ)
or fluidmatrix
densitymatrix
ρρ
ρρφ
−
−=
ρmatrix commonly:
2.65 - for sands and limestones
2.68 - “limey” sands or “sandy limes”
2.71 - limestones
2.87 – dolomites
ρfluid commonly:
1.1 gm/cc – highly saline water
1.0 gm/cc – fresh water
>1.0 gm/cc – oil based mud
<1.0 gm/cc – in flushed oil/gas zones
• Shaly formations
o If laminated shale present:
shaleshaledensitycorr V*φφφ −=
where: fluidmatrix
shalematrixshale
ρρ
ρρφ
−
−= in nearby shale
bed
Vshale is proportion of shale (a fraction)
o Dispersed shale:
shaledensitycorr V−= φφ
φcorr due to dispersed shale < φcorr laminated shale
Density logs
• ρshale formation specific and depends on overburden
stress etc. – best read from log adjacent to a shale bed
When shale forms the structural framework of the rock
i.e. is the matrix material:
densitycorr φφ =
• When residual fluid saturations are high: low recorded
density φdensity> φtrue
Empirical corrections:
Oil zones : densitycorr φφ ×= 9.0
Gas zones : densitycorr φφ ×= 7.0
POROSITY CROSSPLOTS
Presence of Gas
• ρb (bulk density) read to be too low φdens too high
• φneutron reads low (provided no shale present)
“Football” effect seen on the log
Detection of gas zone difficult in shale zones
Lithology determination
Neutron- density cross plot for freshwater mud/formation
water (ρwater = 1.0 gm/cc)
Note: The porosity on both axes is limestone porosity. If
φdensity is computed in sandstone, calculate:
( )fluidmatrixsanddensitymatrixsandbulk ρρφρρ −⋅−= −−
Calculate: fluidmatrix
bulkmatrix
ρρ
ρρφ
−
−=
−
−−
lime
limedensitylime
Lithology determination
Lithology Determination
Lithology Determination
Mixture of matrix material (sandstone + limestone,
limestone + dolomite or sandstone + dolomite)
Presence of Gas
• Low neutron porosity reading and high density
porosity reading
• Connect equal porosity tie lines on sandstone and
limestone curves and extend to porosity read value –
gives approximate φ for the formation
Evidence of shale
• Shale point established using readings on nearby shale
bed
• Plotting points corresponding to values within a
formation of interest, we might observe a trend or
trajectory:
o If trajectory resembles A – structural shale
o Trajectory B – laminated shale
o Trajectory C – dispersed shale
o Recall- structural shale implies little correction to
φdens, dispersed shale – maximum correction,
laminated – less correction
RESISTIVITY LOGS
Basic Concepts
• Ability of formation to conduct current is directly related
to the amount of water in formation
• Rock grain material have very low conductivity, hence
measured conductivity (resistivity) is a function of water
saturation and porosity
• Resistivity is related to resistance through:
L
ArR ×= R – resistivity ohm-m
• Formation resistivities range from 0.2 – 1000 ohm-m
• Current flow in formation through water made
conductive by salts (Na+, Cl- ions)
• Resistance due to a cube full of water:
A
LRr ww ×=
Replacing water with porous material 100% saturated
with water, resistance measured is:
'
'
A
LRr wo ×= (neglecting resistivity of rock material)
LL >' due to tortuosity, AA <' reduced by the effective
pore volume available for current flow.
Basic Concepts (cont’d)
• The ratio w
o
r
r is a measure of the formation
characteristics
• Taking L = 1 m and A = 1 m2, w
o
w
o
R
R
r
r= and formation
resistivity factor , w
o
R
RF =
• Since Ro is different from Rw because of tortuosity
(related to cementation and rock texture) and reduction
in area (due to porosity), formation resistivity factor F:
m
aF
φ= (Archie’s
formula)
a – rock texture; m – cementation index
• Calibration results:
2
81.0
φ=F in sands
2
1
φ=F in carbonates
• Humble formula:
15.2
62.0
φ=F sandstone
Saturation determination
• In formation containing oil or gas (insulators), resistivity
is a function of F, Rw and water saturation Sw (fraction of
pore space occupied by water):
t
ow
R
RS =
2 Archie’s formula
• Ro, resistivity corresponding to formation 100%
saturated with water is rarely measured directly.
Knowing porosity from sonic and/or neutron log:
t
wm
t
ww
R
Ra
R
RFS
⋅⋅=
⋅=
−φ2
Rt measured by resistivity log, Rw from oil field water
catalog / water compositional analysis / SP log
• The Rwa log is computed as
F
RR t
wa =
where Rt is from a deep-investigation resistivity log and
F is calculated from a porosity log reading.
For clean water bearing zones, Rt = Ro = FRw, which
implies that Rwa = Rw
Resistivity of formation
If a consistent low value is observed in the Rwa log for
several potential reservoir zones, then that low value of
Rwa is probably the formation water resistivity Rw.
Water resistivity from SP logs
Recall SP is natural potential induced due to salt
concentration difference between formation water and mud
filtrate:
mf
w
a
aKSP log⋅=
aw is the chemical activation potential of formation water,
amf of filtrate and K is a solution constant (function of
Tformation)
Expressed in resistivity terms:
mfe
we
R
RTSP log)133.060( ⋅+−=
T – formation temperature in oF
Formation waters rarely composed of pure NaCl, other ions
such as Ca+ and Mg+ are present. Similarly, mud filtrate
may sometimes contain potassium, calcium or magnesium
Rmf measured must be converted to Rmfe using chart in
following page
e
e
mf
w
R
RKSP log⋅−=
K = -(60 + 0.133T)
K
SP
mfw
e
e
RR
−
=
10
Formation water resistivity
Read SP corresponding to clean water bearing layer.
Calculate Rwe, calculate Rw from chart or from:
)24.069.0(
1058.0−
+−= weRwR if Rwe @ 75oF > 0.12
ohm-m
we
wew
R
RR
377146
577
−
+= otherwise
Resistivity Tools - Normal Device
• Current of constant intensity between electrodes A & B.
• Equipotential lines due to current are spheres
• Difference in potential between M and N (located an
infinite distance away) measured
• Potential recorded is related to resistivity of formation
• Distance AM – spacing of tool (16 in., 64 in etc.)
• Deepest point where measurement is made corresponds
to point O (zero point - midway between A and M)
Resistivity Tools - Lateral device
• Constant current between A and B
• Potential difference between two points M and N on two
concentric spherical surfaces centered on A measured
• Zero point is at O midway between M and N
• Spacing of tool is AO (18 ft. 8in lateral etc.)
• In general for both normal and lateral tool, longer the
spacing, deeper the radius of investigation
Normal and Lateral Curves
• Normal curve opposite resistive formation – apparent
bed thickness less than true by distance equal to tool
spacing – thick bed Rt equal to true resistivity (no
invasion)
• Thin resistive bed – curve reversed:
Rt-app <Rsh when Rt-true > Rsh.
Two spurs observed – distance between spurs equal to
true bed thickness + tool spacing
• Thick bed less resistive than surrounding formation –
apparent thickness greater than true thickness by amount
equal to tool spacing
Effect of bed thickness on lateral device
Formation more resistive than
surrounding
Formation less resistive than
surrounding
• A minimum bed thickness is needed to obtain plateau
reading uninfluenced by surrounding formation
• In very thin beds – strong peak corresponding to resistive
bed, “blind zone” below the bed and a spurious reflection
peak
• Curves in all cases not symmetrical • When formation less resistive than surrounding, anomaly extends
below bed for distance > tool spacing
Resistivity Tools
Focussed devices (Laterologs, SFL)
Useful when:
− Rt/Rm ratios are large
− Beds show large resistivity contrasts and/or
are thin
− Drilling muds are salty and conductive
• Comprises of a center electrode A0, three pairs of
electrodes: M1M2, M1’M2
’ and A1A2
• Each electrode pair symmetrically located with
respect to A0 and connected to each other (short-
circuited)
• Constant current emitted from A0. Adjustable
bucking current emitted in A1A2. Current intensity
adjusted until same potential measured at the
monitoring pairs M1M2 and M1’M2
’
• M1-M2 and M1’- M2
’ are at same potential (since they
are shorted). M1-M1’ are at same potential. No
current flowing in hole between monitoring pairs
current from A0 flows as a sheet into the formation
• Potential drop between M1M2 (or M1’M2
’) and ground
electrode recorded
Effect of bed thickness
• Thickness of current sheet is approx. 32 in when
length A1A2 is 80 in. (Laterolog 7)
• If bed thickness is greater than 32 in. – adjacent bed
effects eliminated
• If bed thickness < 32 in. current divided between bed
and adjacent formation – apparent resistivity reading
increased if Rsh > Rbed and lowered if Rsh < Rbed
• In general, even if Rt/Rm > 5000, beds can be clearly
delineated
Induction Tools
• Measures conductance (inverse of resistance) of the
formation
• An alternating current is applied to the insulated
transmitter coil, produces an alternating
electromagnetic field
• Magnetic field penetrates formation and induces
current
• Formation current induces secondary magnetic field
around receiver coil
• Secondary field converted to current whose intensity
is proportional to conductivity of formation
• Tool response can be visualized as sum of all
formation loops (mud + invaded zone + virgin zone +
surrounding formation) Total induced current on
receiver coil can be written as:
ssttxoxommI CGCGCGCGC +++=
G’s are geometric factors
and 1=+++ stxom GGGG
Induction tools
• Volume of space (mud, invaded zone etc..) defined
only by its geometry relative to the tool and this can
be used to prepare correction charts for invasion,
mud etc..
• In dual induction tool– a shallow curve measures
flushed zone resistivity, a medium zone measures
invaded zone and a deep zone reflects Rt
• Induction log ideal for air-drilled holes or holes
drilled with non-conducting mud
Induction log corrections
Borehole diameter correction
Induction log corrections
Bed thickness correction
Porosity- Resistivity Crossplot (Pickett plot)
tm
w
t
w
t
ow
n
R
Ra
R
RF
R
RS
⋅
⋅=
⋅==
φ
Taking logarithm of above expression:
wwt SnRamR log)log(loglog ⋅−⋅+−= φ
If we have a formation with constant lithology (texture)
index “a” and Rw, then in regions with constant Sw :
.loglog ConstmRt +−= φ
i.e. a log-log plot of Rt versus φφφφ (or vice-versa) will be a
straight line, with slope = m− (cementation factor).
If Sw = 100%, then )log(loglog wt RamR ⋅+−= φ
Another straight line, parallel to other Sw lines. At φ φ φ φ =
100%, )log(log wt RaR ⋅=
Therefore:
Log Rt
Log φ
Slope = m Constant Sw
Log Rt
Log φ
Sw=100%
φ=100%
aRw
Resistivity-Porosity Cross plots
When it is not known that the resistivity values
correspond to 100% water saturation, but aRw is
known, using estimate for m and plotting a point (Rt =
aRw, φφφφ = 100%), draw the line for Sw=100%
Log Rt
Log φ
Points with
unknown Sw
φ=100%
Sw=100%
Assumed m
(Rt=aRw, φ=100%)
Adjusting log values for presence of shale
ShNshNcorrN
ShDshDcorrD
V
V
φφφ
φφφ
⋅−=
⋅−=
Compute corrected porosity as:
2
corrcorr ND φφφ
+=
Then calculate the corrected saturation as:
sh
wShArchiesww
R
RVSS
⋅⋅
⋅−=
φ4.0
This is called Fertl’s correction
Log reading in a shale zone
Log reading in a shale zone