17155521-Well-Logging-Basics

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)

T
e
m
p
e
r
a
t
u
r
e

C
a
l
i
p
e
r

S
P

R
e
s
i
s
t
i
v
i
t
y

G
a
m
m
a

R
a
y

S
p
e
c
t
r
a
l

G
R

S
o
n
i
c

D
e
n
s
i
t
y

P
h
o
t
o
e
l
e
c
t
r
i
c

N
e
u
t
r
o
n

D
i
p
m
e
t
e
r

I
m
a
g
e

l
o
g
s

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
Log
Uses
Legend: (-) essentially qualitative; (+) qualitative and semi-quantitative; (C) strictly quantitative
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:

P
h
(psi) = 0.052 x height of fluid column (ft.) x density (ppg)

Overpressure – any pressure above the hydrostatic (or normal)
pressure

Temperature

Geothermal gradient
D Ts Tf G / ) ( 100 ÷ =

Formation temperature
Tf = Ts + G(D/100)

G = geothermal gradient, °F/100 ft.
Tf = formation temperature, °F
Ts = surface temperature (80°F)
D = depth of formation, ft.

Graphical solution of formation temperature is provided by Schlumberger
Gen-6 chart.

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.
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 (ĭ).
- 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-meter
2
/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 m
2
/m)
ty conductivi
1000 1×
= (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 (ij) [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
































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.





















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.

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 =
e Rw
e Rmf
K
) (
) (
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 T°F)
K = 65 + (0.24 x T°C)

Shale Volume

100 ) 0 . 1 ( (%) × ÷ =
SSP
PSP
Vsh

PSP (Pseudo-static SP) – the SP value in the water–bearing 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.












5.3.2 Gamma Ray





























Figure 5-4. Idealized GR and SGR log.


Volume of Shale from GR

Vsh = 0.33 [2
(2 x I
GR
)
- 1.0]

Vsh = 0.083 [2
(3.7 x I
GR
)
- 1.0]

min max
min log
GR GR
GR GR
I
GR
÷
÷
=






5.4 Porosity Calculations – sonic, density, and neutron logs

5.4.1 Sonic




























Figure 5-5. Idealized Sonic log.

Wyllie’s Time Average Equation

ǻt = ĭ ǻt
f
+ (1- ĭ) ǻt
ma

ĭ = porosity
ǻt = log reading in microseconds/foot (µs/ft.)
ǻt
f
= transit time for the liquid filling the pore (usually 189 µs/ft.)
ǻt
ma
= transit time for the rock type (matrix) comprising the formation

ĭ =
ma f
ma
t t
t t
A ÷ A
A ÷ A





5.4.2 Density































Figure 5-6. Idealized Density log.


ȡ
b
= ĭ ȡ
f
+ (1- ĭ) ȡ
ma

ĭ = porosity
ȡ = log reading in microseconds/foot (µs/ft.)
ȡ
f
= transit time for the liquid filling the pore (usually 189 µs/ft.)
ȡ
ma
= transit time for the rock type (matrix) comprising the formation

ĭ =
f ma
b ma
p p
p p
÷
÷





5.4.3 Neutron






























Figure 5-7. Idealized Neutron log.



Read directly from logs

May need matrix correction



ĭ =
2
N D
o o +
if no light hydrocarbons


ĭ =
2
N D
o o +
if light hydrocarbons as present

5.5 Water Saturation (Sw) Calculations

Archie’s 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 =
m
a
o


ĭ =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 . 2
62 . 0
o
best average for sands (Humble)
F =
2
81 . 0
o
simplified Humble
F =
2
1
o
compacted formations

Sw
n
= Ro/Rt

Sw = water saturation
Rt = resistivity of rock when Sw < 1

Combining the above equations gives Archie’s equation, the most
fundamental equation in well logging.

Sw
n
=
Rt
aRw
m
o
= F
Rt
Rw


Practical average Archie’s Equation – general equation for finding water
saturation.

Sw =
Rt
Rw
×
×
15 . 2
62 . 0
o


Symbol Character Derived from
ĭ Porosity Porosity logs (sonic, neutron,
density), cross-plots, etc.
15 . 2
62 . 0
o

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
water Sw
ns hydrocarbo Sw
% 100
=
Rt
Ro



Sw Calculations

Conventional
Quick look
Rwa
“F” overlay
SP Quick Look


Clean Formation
Shaly