Transcript SUBSURFACE WELL LOGGING February 25, 2009 Purpose of logging a well Identify rock-types and correlate important rock units. Identify stratigraphy throughout thick interval. Determine thickness.

SUBSURFACE WELL LOGGING
February 25, 2009
Purpose of logging a well
Identify rock-types and correlate important rock units.
Identify stratigraphy throughout thick interval.
Determine thickness and depth of important rock units:
Isopach maps
Structure maps
Determine reservoir quality and volume.
Calculate hydrocarbon saturation and reserves.
Digital log data enables computer interpretations and workstation analysis.
Invasion Profile of Drilling Fluids
(typically used for resistivity
terminology & measurements)
Adjacent
Bed
Rm
Flushed Zone of
Zone Transition
or
Rxo
Annulus
Rmc
B ed
Adjacent
Rmf
Mudcake
Bed
Thickness
Invasion
rs
Diamete
Sxo
Uninvaded
Zone
Rt
Rw
Sw
Rt =Resistivity uninvaded zone (true resistivity)
Rw =Resistivity of formation water
Rxo = Resistivity of flushed zone
Rm = Resistivity of drilling mud
Rmc =Resistivity of mudcake (solid part of mud)
Rmf =Resistivity of mud filtrate (liquid part of mud)
Sw = water saturation
Resistivity of the zone
Sxo =water saturation in flushed zone
Resistivity of water in the zone
dh = diameter of borehole
Water saturation in the zone
hmc = thickness of mudcake
Resistivity & Density of common rock forming minerals
Mineral/Rock
Anhydrite
Halite (salt)
Coal
Shale
Calcite (limestone)
Dolomite
Quartz (sandstone)
Oil
Gas
Res
(ohms)
g/cc
(ρ)
103
2.98
5
10
2.17
~high but variable
~1.35
2-10
~2.6
7
10
2.71
8
10
2.85
10
10
2.65
8
10
~<1 (0.85 avg)
8
10
~ρ of C1-C4
Pe
5.05
4.65
0.17
~3+
5.08
3.14
1.81
NA
NA
Water density, salinity of dissolved salts in parts
per million (PPM), and formation resistivity (Rw)
PPM
Resistivity @125
degrees F (ohm-m)
~500-1,000
~6 to 3
1,000-35,000
~3 to 0.11
35,000-50,000
~0.11 to 0.08
Brine
>50,000
< 0.08
Typical subsurface
(Oklahoma)
150,000
~0.035
Classification
“Fresh” water
~Density
~1.0
Brackish water
Saline (sea) water
~1.1
Logging Tool Response for Some Common Clays
Clay & Formula
Density
g/cc
Neutron
porosity
PE
GR
(API)
Kaolinite
Al4Si4O10(OH)8
2.41
37
1.83
80-130
Chlorite
2.76
(Mg,Fe,Al)6(Si,Al)4O10(OH)8
37
6.30
180-250
Illite
KAl4(SiAl)O20(OH)4
30
3.45
250-300
Montmorillonite
2.12
44
(Ca,Na)7(Al,Mg,Fe)4(si,Al)8O20(OH)4(H2O)
2.04
150-200
2.52
Bentonite
similar to montmorillonite
(Al,Fe,Mg),Si4O10 (OH)2Na,Ca
Smectites (swelling clays)
COMMON WELL LOGS
Some of this information is condensed from Schlumberger
(also available from Reeves & other wireline service companies)
1.
GAMMA-RAY (GR)
Principal:
Measures natural radiation within well-bore. Nearly all GR
from potassium (K40) with lesser amounts from thorium and
uranium. High GR in shale, low GR in carbonates and most
sandstone (quartz).
Uses:
Lithology identification; differentiates between shale and
non-shale rock units (shale vs. sandstone or, shale vs.
limestone). Cannot distinguish between sandstone and
limestone. The GR log is the principal tool used in
determining the textural profile of sandstone intervals
including the nature of their upper and lower contacts. This
log can be used in open or cased holes, with or without borehole fluid.
RESISTIVITY
GR & SP
0
API
150
1
10
100
1K
4200
Limestone
SP
4300
Sandstone
GR
4400
Deep
4500
50% SS or LS
0
100
% Shale
Shallow
2. SPONTANEOUS POTENTIAL (SP)
Principal:
Very complicated! In general, this log measures electrochemical
currents that originate from ionic movement between formation electrolytes (salty
formation water) and fresh borehole mud. Specifically, two types of currents comprise
the SP log deflection or total electrochemical potential Ec. Membrane potentials Em
are due to cation (Na+) transfers from salty formation waters across a charged
membrane (bounding shale beds). Liquid junction potentials Elj arise when anions
(Cl-) migrate across the contact of salty (formation) water and fresh water (drilling
mud filtrate) during invasion. SP units are measured in millivolts (+ on the right and –
on the left). The SP is recorded in open holes having relatively fresh, but conductive
mud.
Uses: Very definitive in identifying qualitative permeability in either sandstone or
limestone. Can also be used to determine values of formation water resistivity (Rw)
and resolve the nature of formation contacts (sharp, transitional, shaly, tight, etc.).
The maximum SP that can be obtained is called the static SP (SSP). This can only
occur when the potential permeable reservoir is~>10 ft thick. Thinner beds will
diminish the SP response. Entirely shale strata define the shale baseline. Log
response is attenuated by bed thickness (<10 feet), adjacent high-resistivity beds
(limestone), the presence of hydrocarbons, and the ratio of resistivity between the
mud filtrate (Rmf) and formation water (Rw).
The nature of multiple thin permeable beds interstratified with shale cannot be
resolved by the SP log.
Fresh mud
Rmf ~1 ohm
– SP +
Shale
base-line
The SP kicks to the right in freshwater sands and to the left in saltwater sands (typical in reservoirs)
Example of the SP shale baseline.
-
Liquid
Junction
Potential
Sources of SP potential
in the subsurface
in borehole
Membrane
Potential
Fresh mud
Fresh mud in borehole
+
+
+
+
The SP cell borehole
equivalent
Shale Rs = 1
Sandstone Rt = 1
Shale Rs = 10
Sandstone Rt = 10
Shale Rs = 1
Sandstone Rt = 10
SP response relevant to current
distribution and bed resistivity
- SP +
Ideally, the SP is attenuated somewhat in a
hydrocarbon-bearing zone as compared to a water
zone. This is often the case in thick, relatively
uniform reservoirs having a water leg.
SP & SSP response
SP response in
relationship to bed
thickness
Assume Rw << Rmf
d = borehole diameter
f (usually ~8”)
Bed thickness ratios
SP response of thin
permeable beds bounded
by highly resistive beds
3. RESISTIVITY- there are many types of tools in use but the Induction
tool is by far the most common. Short-spaced, shallowly penetrating
tools and older electrical methods utilize contact electrodes: the normal,
lateral, and focused laterolog.
Formation resistivity is influenced by several factors including the rock
matrix, cementation, hydrocarbons, and formation water. The latter
probably has the greatest influence on measured rock resistivity
because saline formation water has very low resistivity. Therefore,
recorded resistivity of rocks in the subsurface is relativity small when in
fact the actual matrix grains and/or cement have almost infinity
resistivity. The small-scale log format also displays conductivity.
Principal of induction log: AC current is applied to several transmitting
coils creating a magnetic field around the wellbore. This creates an
induced current that is measured at several receiver coils higher on the
tool. Depending on the spacing between the transmitting coils and
receivers, three types of resistivity measurements can be made that
reflect different electrical paths into the rock (i.e., depths of
investigations):
Shallow (focused) log (SFL)
Medium induction log (IML)
Deep induction log (ILD)
~10” depth of investigation.
~30” depth of investigation.
~60” depth of investigation.
Induction resistivity logs can only be recorded in open-holes (no pipe in
the ground). The medium and deep measurements can be run in holes
filled with air and/or gas whereas the shallow recording device requires
bore-hole fluid.
Uses:
Defining bed boundaries, especially the SFL
Stratigraphic correlations and a good shale “finder”
Qualitative determination of permeability
Calculation of hydrocarbon saturation
RESISTIVITY
GR & SP
0
API
150
1
10
100
1K
4200
Limestone
SP
4300
Sandstone
GR
4400
Deep
4500
50% SS or LS
0
100
% Shale
Shallow
GR & SP
0
API
RESISTIVITY
150
1
10
100
1K
Note negligible
separation in tight
and impermeable
strata; i.e., little or
no invasion
0
100
% Shale
4.
POROSITY LOGS (Sonic, Microlog, Density, Neutron)
There are several logging tools that quantifiably determine porosity although only 2 are
commonly run in most wells. This practice simplifies the interpretations of porosity though
it should be noted there are many caveats in their use that can cause incorrect porosity
determinations. Because of the very shallow depth of investigation for all porosity tools,
considerable error can occur in rough holes.
SONIC LOG:
Not discussed. Seldom included in log suites.
MICROLOG: This is a very shallowly penetrating resistivity log that is extremely sensitive
to minute bedding changes.
Principal:
The logging tool has 3 contact electrodes each spaced 1” apart
vertically. Therefore, resistivity measurements can be made across 1” and 2” intervals
simultaneously; the log displays are called 1” microinverse and 2” micronormal,
respectively. The 1” recording essentially measures the resistivity of mudcake built up
adjacent to permeable zones as filtrate invades permeable strata and is not reflective of
formation resistivity at all. This value is usually very small and in the range of only a few
ohms. The 2” log is has slightly deeper penetration and records formation resistivity
within the proximal flushed zone just beneath the mudcake. Therefore, the 2” log is
influenced by both the formation and filtrate. This resistivity measurement is almost
always slightly greater than the 1” resistivity value (when drilling with fresh water mud).
When the 2” resistivity is greater than the 1” resistivity, the display is called “positive” log
separation. It is very definitive of both permeability and porosity. Tables are available to
quantify actual porosity based on the values from the 1” and 2” recordings.
Uses:
Excellent for determining bedding/lithology boundaries and also for
determining general values for porosity.
Hypothetical Sonic Log Response
SH
SS
SH
SS
SH
LS
SH
DOL
SH
4.
POROSITY LOGS (Sonic, Microlog, Density, Neutron)
There are several logging tools that quantifiably determine porosity although only 2 are
commonly run in most wells. This practice simplifies the interpretations of porosity though
it should be noted there are many caveats in their use that can cause incorrect porosity
determinations. Because of the very shallow depth of investigation for all porosity tools,
considerable error can occur in rough holes.
SONIC LOG:
Not discussed. Seldom included in log suites.
MICROLOG: This is a very shallowly penetrating resistivity log that is extremely sensitive
to minute bedding changes.
Principal:
The logging tool has 3 contact electrodes each spaced 1” apart
vertically. Therefore, resistivity measurements can be made across 1” and 2” intervals
simultaneously; the log displays are called 1” microinverse and 2” micronormal,
respectively. The 1” recording essentially measures the resistivity of mudcake built up
adjacent to permeable zones as filtrate invades permeable strata and is not reflective of
formation resistivity at all. This value is usually very small and in the range of only a few
ohms. The 2” log is has slightly deeper penetration and records formation resistivity
within the proximal flushed zone just beneath the mudcake. Therefore, the 2” log is
influenced by both the formation and filtrate. This resistivity measurement is almost
always slightly greater than the 1” resistivity value (when drilling with fresh water mud).
When the 2” resistivity is greater than the 1” resistivity, the display is called “positive” log
separation. It is very definitive of both permeability and porosity. Tables are available to
quantify actual porosity based on the values from the 1” and 2” recordings.
Uses:
Excellent for determining bedding/lithology boundaries and also for
determining general values for porosity.
6
0
CAL &
GR
Inches
16
API
150
Micro-resistivity
0
10
20
30
Borehole
caving
Positive
separation
Mudcake
buildup
Positive separation between the 1” and 2” micrologs and formation of
mudcake in the borehole
DENSITY LOG:
Probably the most useful single porosity log mainly because
it is not appreciably affected by small amounts of interstitial or interbedded clay
(apparent log density of shale is similar to that of common sandstone). Whereas
small amounts of clay will make the neutron and sonic log go ballistic!
Principal: The logging tool emits gamma-rays into the formation. They collide
with electrons in rock formations and lose energy with each subsequent
electron collision. The amount of gamma-ray energy reaching the detector is
proportional to the electron density (# electrons per cc) of the rock and is an
indication of formation density. Therefore, strata having high density will
attenuate gamma-rays reaching the detector. The opposite is true of low
density rocks. The electron density in turn is related to the true bulk density
(gms/cc) and depends on the combined rock matrix and cementation density,
formation porosity, and the density of the pore fluids and/or gas. The depth of
investigation of the density log is only about 4” and for most practical purposes,
can only be run in uncased holes.
Uses:
Porosity determination
Strata determination:
Limestone vs. sandstone
Diagnostic of coal, certain evaporates (anhydrite & halite), & dolomite
Evaluating shaly sandstone reservoirs
Gas detection and/or depletion (when used with Neutron log)
Density porosity
Hypothetical
Neutron porosity
Lithology
0%
SH
SS
SH
SS
SH
LM
SH
DOL
NO Φ
SH
Hypothetical porosity based on limestone matrix 2.71 g/cc
NEUTRON LOG:
By itself, this log is generally unreliable for determining
lithology (other than shale) and porosity in both sandstone and carbonate
reservoirs. This occurs because the log is very sensitivity to clay and interstitial
gas. In clean rock formations having no gas components, this log may yield
satisfactory porosity determinations. However, these conditions must first be
ascertained using core data or additional logs.
Principal: The neutron logging tool emits high-energy neutrons (electrically
neutral particles) into the formation. They collide with nuclei of formation
material and with each collision, lose energy. The amount of energy lost per
collision depends on the relative mass of the nucleus and is greatest when a
neutron strikes a nucleus of nearly equal mass, i.e., a hydrogen nucleus.
Collisions with heavy nuclei do not slow neutrons very much. Thus, hydrogen is
the primary impediment to neutron movement; accordingly the counting rate
increases when hydrogen concentration decreases and vice versa. Neutron
logging tools record the actual amount of neutrons reaching the detector, or in
some instances, the intensity of gamma-rays produced as a result of neutron
collisions. The depth of investigation of the neutron log is only about 10 inches.
Because of the nature of neutrons, this logging technique can be accomplished
in both cased and uncased holes.
Uses and Drawbacks:
Shale and clay indicator (“sees” bound water in clays)
With Density log, it can help define gas-filled or depleted reservoirs
Erroneously high porosity in dirty sandstone or limestone
Erroneously low porosity in clean, gas-filled reservoir
Very diagnostic of coal
Density porosity
Hypothetical
Neutron porosity
Lithology
0%
SH
SS
SH
SS
SH
LM
SH
DOL
NO Φ
SH
Hypothetical porosity based on limestone matrix 2.71 g/cc
5.
OTHER LOGS
Pe LOG – photoelectric absorption index (value range of 0-10).
Principal: Responds primarily to rock matrix rather than porosity and pore
fluids. Other details are not important here.
Uses:
Commonly run with density or density & neutron combo logs.
Great at delineating sandstone (values ~2 to 3) from limestone (values
~4 to 5).This distinction may be problematic using other log suites. Also
good for distinguishing between limestone (4-5) vs. dolomite (3).
CALIPER LOG – usually run with porosity log suites. This log has a 10-inch scale
most often in the range from 6-16 inches
Principal:
Spring-loaded arms on logging tool measure borehole
diameter in inches. This log is usually included in the left track on porosity
logs because they (porosity logs) are very sensitive to irregular boreholes
and some sort of compensation is attempted.
Uses:
Identify irregular borehole that may affect other logs suites.
Identify mudcake buildup–an indicator of permeability and porosity
GR & SP
RESISTIVITY
0 API
1
150
10
100
GR & CAL
1K
6
(in.)
Den & Neutron Porosity
30
16 0
20
PE
10
0%
10 (hypothetical)
4200
CAL
SP
GR
4300
Neutron
(dashed)
GR
4400
Gas
effect
Deep
Shallow
Density
(solid)
4500
0
100
% Shale
Sandstone
Limestone