SUBSURFACE WELL LOGGING

1
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 work- station analysis.

2
(No Transcript)
3
Resistivity Density of common rock forming
minerals Res g/cc Pe Mineral/Rock
(ohms) (?) Anhydrite 103 2.98
5.05 Halite (salt) 105 2.17 4.65 Coal
high but variable 1.35 0.17
Shale 2-10 2.6 3 Calcite
(limestone) 107 2.71 5.08 Dolomite 108
2.85 3.14 Quartz (sandstone) 1010 2.65
1.81 Oil 108 lt1 (0.85 avg) NA
Gas 108 ? of C1-C4 NA
Water density, salinity of dissolved salts in
parts per million (PPM), and formation
resistivity (Rw) Resistivity _at_125
Classification Density PPM
degrees F (ohm-m) Fresh water 1.0
500-1,000 6 to 3 Brackish
water 1,000-35,000 3 to 0.11 Saline (sea)
water 1.1 35,000-50,000 0.11 to 0.08
Brine gt50,000 lt 0.08 Typical
subsurface 150,000 0.035
(Oklahoma)
4
(No Transcript)
5
Logging Tool Response for Some Common Clays

• Clay Formula Density Neutron PE GR
• g/cc porosity (API)
• Kaolinite 2.41 37 1.83
  80-130
• Al4Si4O10(OH)8
• Chlorite 2.76 37 6.30
  180-250
• (Mg,Fe,Al)6(Si,Al)4O10(OH)8
• Illite 2.52 30 3.45
  250-300
• KAl4(SiAl)O20(OH)4
• Montmorillonite 2.12 44
  2.04 150-200
• (Ca,Na)7(Al,Mg,Fe)4(si,Al)8O20(OH)4(H2O)
• Bentonite similar to montmorillonite
• (Al,Fe,Mg),Si4O10 (OH)2Na,Ca

Smectites (swelling clays)
6
COMMON WELL LOGS Some of this information is
condensed from Schlumberger (also available from
Reeves other wireline service
companies) 1. GAMMA-RAY (GR) Principal Measur
es 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 bore- hole fluid.
7
RESISTIVITY
GR SP
1 10 100 1K
0 API 150
4200
Limestone
SP
Sandstone
4300
GR
4400
Deep
4500
Shallow
50 SS or LS
0 100 Shale
8

• 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
  isgt10 ft thick. Thinner beds will diminish the
  SP response. Entirely shale strata define the
  shale baseline. Log response is attenuated by bed
  thickness (lt10 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.

9
SP
Fresh mud Rmf 1 ohm
Shale base-line
The SP kicks to the right in fresh-water sands
and to the left in salt-water sands (typical in
reservoirs)
Example of the SP shale baseline.
10

Membrane Potential
Fresh mud in borehole
Fresh mud in borehole
- - - -
Liquid Junction Potential
Sources of SP potential in the subsurface
The SP cell borehole equivalent
11
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
12
- 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.
13
SP SSP response Bed thickness ratios
SP response in relationship to bed thickness
Assume Rw ltlt Rmf
d borehole diameter f (usually 8)
14
SP response of thin permeable beds bounded by
highly resistive beds
15

• 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)

16
Shallow (focused) log (SFL) 10 depth of
investigation. Medium induction log (IML) 30
depth of investigation. Deep induction log
(ILD) 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 Qualitat
ive determination of permeability Calculation
of hydrocarbon saturation
17
RESISTIVITY
GR SP
1 10 100 1K
0 API 150
4200
Limestone
SP
Sandstone
4300
GR
4400
Deep
4500
Shallow
50 SS or LS
0 100 Shale
18
GR SP RESISTIVITY
0 API 150 1
10 100 1K
Note negligible separation in tight and
impermeable strata i.e., little or no invasion
0 100
Shale
19
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.
20
Hypothetical Sonic Log Response
SH
SS
SH
SS
SH
LS
SH
DOL
SH
21
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.
22
CAL GR
Micro-resistivity 6 Inches
16 0 API 150 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
23
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 Diagnosti
c of coal, certain evaporates (anhydrite
halite), dolomite Evaluating shaly sandstone
reservoirs Gas detection and/or depletion (when
used with Neutron log)
24
Density porosity Hypothetical
Neutron porosity
Lithology
0
SH
SS
SH
SS
SH
LM
SH
DOL
NO F
SH
Hypothetical porosity based on limestone matrix
2.71 g/cc
25
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
26
Density porosity Hypothetical
Neutron porosity
Lithology
0
SH
SS
SH
SS
SH
LM
SH
DOL
NO F
SH
Hypothetical porosity based on limestone matrix
2.71 g/cc
27
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 buildupan
indicator of permeability and porosity
28
Den Neutron Porosity
GR SP
RESISTIVITY
GR CAL
30 20 10 0
0 API 150
0 PE 10 (hypothetical)
1 10 100 1K
6 (in.) 16
4200
CAL
SP
GR
4300
Neutron (dashed)
GR
4400
Gas effect
Deep
Shallow
Density (solid)
4500
0 100 Shale
Limestone
Sandstone
29
(No Transcript)