Glossary of Terms Used in Well Logging

8/15/2019 Glossary of Terms Used in Well Logging

1/26


2/26

GLOSSARY OF TERI•S

USED IN %VELL LOGGIi•G

By

R. E. SHERIFF

Reprinted rom

GEOPHYSICS

VOL. 35, No. 6, December, 1.970

Society of Exploration Geophysicists

P.O. Box 3098

Tulsa, Oklahoma 74]35


3/26

GEOPHYSICS, VOL. 35, NO. 6 (DECEMBER 1970), P. 1116-113% 15 FIGS

GLOSSARY OF T/F •$ USED IN WELL LOGGING-t'

ROBERT E. SHERIFF*

The "Glossary of Terms Used in Geophysical

Exploration" published n Geophysics n Febru-

ary 1968 met with some justified criticism be-

causeof the emphasison seismicexploration or

oil. The Addendum published in April 1969

attempted to restore balanceas far as mineral

explorationwas concerned.But one major area of

geophysics ad sti l been slighted: that of well

logging. This present addendum is intended to

complete our coverageof geophysical xploration

terms.

Well logging echnologyhas seen apid develop-

ment in the last few years with the advent of sev-

eral types of acoustic and radioactive logging

methods and other new logging ools. Computers

are beginning o have an impact on this field by

making readily available, from combinationsof

logs, information which previously could be ex-

tracted only by tedious methods.

The author believes that a "geophysicist"

should be conversantwith aspectsperipheral to

his central concern and central application, and

that a geophysicalnterpretation ought to be con-

sistent with all available data of all kinds, not

merely with that portion of the data in which the

interpreter specializes. onic logging nformation

is vital to seismic interpretation and density

logging nformation o gravity interpretation.But

vital also are measurementsof other physical

propertieswith which an interpretationought to

be consistent, just as a seismic interpretation

ought to be consistent ith gravity and magnetic

observations.n a rapidly changing ield such as

ours, he boundaries etweenareasof specilization

must be crossed more and more often as we ad-

vance nto the future. Hence t seems ppropriate

to present a "Glossary of Terms Used in Well

Logging."

? Manuscriptreceivedby the Editor June 8, 1970.

* ChevronOil Company,Houston,Texas.

A revision of the "Glossaryof Terms Used in

Geophysical xploration" s now underway nd t

is intended hat this "Glossaryof Terms Used in

Well Logging" should be included as an integral

part of the revision. The present publication is

made, therefore, n the samesense s the previous

publications:as a semifinaldraft of the entriesac-

cessible or proofreadingby a much wider group

of knowledgeablegeophysicists han we could

otherwise reach. Comments, corrections, or dis-

agreements re invited, so that they may receive

consideration before the revision is finalized.

Readerswill note that someentriesappearboth

in the previousglossaryand in the present one

with somewhat different specialized meanings,

dependingon the area in which the word is used.

Becausesuchdifferencesnvolve vocabularyout-

side of well loggingusage, t did not seem ap-

propriate to include them here. The forthcoming

revisionwill attempt to clarify such differences.

Groundruleshave beenessentiallyhe sameas

those laid down for the precedinggeophysical

glossary:

1. An equal sign indicates hat what follows s

an equivalent. ogs ndicated s beingequiv-

alent, however,may not be identical (e.g.,

neutron ifetime log and thermal decay ime

log).

2. A colon indicates that what follows is a defi-

nition or explanation.

3. Wordsunderlinedwith a single ine are isted

themselves and should be referred to in order

to complete the defintion.

4. Wordshavebeendefined sthey areactually

used.

5. The numbering f differentmeanings oes

not indicatepreference s to usage.

6. Where several ermsare essentially quiva-

Copyright •) 1970by the Societyof ExplorationGeophysicists.ll rights eserved

1116


4/26

Glossary of Terms Used in Well Logging 1117

lent, the principal entry is that most used,

even where this gives preference to a trade

name over a generic name.

Trade names are included where they are

in general use. Such entries begin with a

lower case etter where heir usage s for the

class f devices; lthough he name or a spe-

cific logging ool may begin with a capital.

An attempt has been made to include trade-

mark registration where 17 was aware of

such. Neither inclusion nor exclusion should

imply any judgments about the merits of

specific devices.

I wish to expressmy appreciation o the com-

panies and peoplewho have helped me, including

some whose names I do not know. I want to

thank especially' Mr. G. W. Burns of Chevron

Oil Company, Jackson,Mississippi; Mr. E. E.

Finklea of Schlumberger imited, Ridgefield,Con-

necticut; Mr. Noel Frost of the Birdwell Division

of SSC, Tulsa, Oklahoma; Dr. D. W. Hilchie of

Dresser Atlas, Houston, Texas; Mr. John I.

Myung of the Birdwell Division of SSC, Tulsa,

Oklahoma; Mr. R. A. Robertson of Standard Oil

Companyof California, San Francisco;Mr. Terry

Walker of Welex, Houston, Texas; and Mr. J. E.

Walstromof StandardOil Companyof California,

San Francisco. I also want to thank the Chevron

Oil Companyand the StandardOil Company of

California for permissiono publish his glossary.

I again remind you that I do not regard this

draft as final and invite suggestions or correc-

tions,additions,or revisions.

R. E. Sheriff

Fro. 1. Idealized acoustic wave train.


5/26

1118 Sheriff

BEFORE SQUEEZE

,diP*

AFTER

SQUEEZE

,

i

o

FIc. 2. Microseismogramog usedas cementbond og. CourtesyWelex.

I,,,


6/26

G•ossary of Terms Used in Well Logging

MUDCAKE

a b

F•r:..•. (a) Schematicof compensated ensity log sonde. (b) Compensated ensity log.

Courtesy Schlumberger.

O TRUE DIP ANGLE

rn

, J

IDEPTH• I

NTERVAL

TRUE DRIFT

DIRECTION

OF HOLE

a b

Fro. 4. (•) Dipmeter log. (b) Calculateddipmeter og (tadpole plot). Courtesy Schlumberger.


7/26

1120 Sheriff

AMPLIFIER

/ AND

OSCILLATOR

/ ,/ HOUSING

RECEIVER / COIL

RECEIVER

• GROUND

AMPLIFIER LOOP

FOUCAULT

CURRENT

ION

ITRANSMITTER:R

T RANSM I TTER

OSCILLATOR•COIL

SPOHTANEOUSOTEHTIAI 1•E$ I $ T V I T Y

mllllval•,i ohms,.m'/m

a b

Fro. 5. (a) Schematicof two-coil induction sonde. (b) Dual induction--laterolog. CourtesySchlurnberger.


8/26

Glossary of Terms Used in We• Logging 1121

•? •oo ....

o-

/ FOR MATION WATER

..• )__ .......... .......................

I i iRADIAL DISTANCE

I i i

RADIAL DISTRIBUTION OF FLUIDS ABOUT BOREHOLE

I i I i

i I I I

i I I I

i I

I I

I i I 1

i I

/

Rxo .... • \•

i I I Rt

I i

Rrnc" -If I 1

I I • I

Rm I I I

i II I i I

RADIAL DISTRIBUTION OF RESISTIVITY ABOUT BOREHOLE

II I I I

I 1 i I i

I ii i I I

11L•I I I

I itz i

I i1•ø I

I 1 i

I 11 ::) I• I UNCONTAMINATED

MUD ; •-I •. •iNVADED I I ZONE

CAKE I I• ZONE• 1

I II I I I

Fro. 6. Invaded zone. Courtesy Schlumberger.


9/26

1122 Sheriff

LXTEROLOG

CONDUCTIVITY

400 200

.:_:::::.=5:::::

a b

Fro. 7. (a) Schematic f laterolog7 showing urrent low lines. b) Laterologwith hybrid scale.



10/26

GJossa•':yf Tea'ms Used in Well Logging 1123

RESISTIVITY

ohml - m•/m

io

•cro Nm.mal 2"

I0

MICROCALIPER

RESISTIVITY

ohml - m•l/m

POROSITY Per Cent

Fro. 8. Microlog and microcaliperog (on left), •nd

microlaterologon right). CourtesySchlumberger.

•ORRELATioN.... COUNTSERINUTE

CUEVE

..........

....

SIGMA

. _..

...................................

Fro. 10. Neutron lifetime log.

Courtesy Dresser Atlas.

Fzo. 9. Movable oil plot. Courtesy Schlumberger.


11/26

1124 Sheriff

NEUTRON

160

GENERATOR METER

GENERATOR ME TE R

A

CING

M

N

a b


12/26

Glossaryof Terms Used n We• Logging 1425

14'iC;.l. (a) Neutron log. (b) Sidexvall eutron og. CourtesySchlumberger.

Ri

R

LOWERRANSMITTE•/

1/////////

CALIPER

HOLE DIAM.

iNCHES

6

BHC SONIC LOG

2 • SPAN

At •u SEC/FT.

oo 7o 40

•.

a b

3. (a) Schema'it BHC sonic log sonde. (b) Sonic log. Courtesy Sclllumberger.

12. Basicelectric1o• arrangements'a) Normal configuration.b) Lateral configuration.



13/26

1126 Sheriff

S.R

•o rnv

--• I•+

SAND-LINE

RESISTIVITY

0 SHORT NORMAL 0

0 LONG NORMAL I0

J SHALE

?AS,ELINEI

,

i '

i

'

?

•,,

I

F'm. 14. SP log in a sand shale serieswith

fresh mud in borehole. Courtesy Schlum-

berger.


14/26

G•ossa•y of Terms [Jsed in We• Logging 1127

MICROSECOND----.>

1000

1500

BOUNDARY.WAVE '

SHEAR WAVE

Fro. 15. Three-dimensional elocity log. CourtesyBirdwell Division of SSC.


15/26

1128 Sheriff

•a•9.•sti•al.l_o.o_: (1) a generic term for well logs

which display an aspect of acoustic wave propa-

gation. In some acoustical logs (soni•, con

tinuous velocity log), the traveltime of the

compressional wave between two points is mea-

sured. In others (amp_l.tud.e... .,o•.), the ampli-

tude of part of the wave train is measured.

Still other acoustical logs (character log, three

• l•, VDLog,microseismogramog, ignature

• display most of the v&ave rain in wiggle

or variable density form. Still others (cement

bond log, fracture log) are characterized by

the objective of the measurements rather than

their form. Borehole televiewer is an acous-

tical log also•--•p•ically, a sonic log_,

••t• '• : elastic wave : seismic wave - sonic

wave: Specifically, the wave train which results

from the emission of an acoustical pulse by a

sonic logging sonde. This wave train is detected

by one or more nearby detectors and various as-

pects of it are measured (see acoustical io_9).

The wave train is a composite of various modes

of energy transfer. The first arrival usually

results from compressional (P or longitudinal)

waves travelling in the formation; •h• inverse of

its velocity is measured by the sonic 1o•. A

second arrival is sometimes iden•"•Ffied as shear

(S) wave travel in the formation, sometimes as

Rayleigh arrivals (not Rayleigh waves as used

in seismology). Compressional waves travelling

through t13e mud usually have relatively high

frequency content. One or more modes of high

amplitude, low frequency tube waves or Stonetey

waves are usually a very distinct arrival. See

Figure t.

•CtJv_o__t]o•.n_9•: Awe.l lo•qingechniqeUen

•E5 •-•ormation is irrad ated with n utrons

that transmute some nuclei into radioisotopes

whose radiation is measured after a time lag.

• : acoustic velocity log = sonic log.

•(t_G_•_JY•j=•JLelectrochemical): The relative tendency

of a substance to enter into a reaction. When

the shales adjacent to a reservoir are perfect

cationic membranes and the permeable bed is clean.

the electrochemical SP can be found from the ac-

tivities (equiValent concentrations of dissolved

salts) of the formation water and mud fittrate.

•j=•p •: Theatioof p•.s_eudostaticP o static

S__•P.

•_.•D.._l_••._.__9•.q,:_boreholeo• of theamplitudef

portion of t e acoustic w ve used in acoustic

logging, See cement bond log and fracture log,

.,_•_•: •l• Variationf a phYnsicalroperty

epending •o he orientation ale g whic it is

measured. (2) "Microscopic anisotropy" is a

variation of a property of a formation measured

perpendicular to the bedding from that measured

parallel to the bedding because plate-like min-

eral grains and interstices tend to orient them-

selves parallel to the sedimentation. (3) "Macro-

scopic anisotropy" is a variation due to the vol-

ume over which a measurement is made, including

.thin beds whose properties differ appreciably.

(4) The resistivity anisotropy coefficient is the

square root of the ratio of the resistivity mea-

sured perpendicular to the budd•n

sured parallel to the bedding.

a__nn_o_U]_u•.•s:1) That space between a drill ;•i•:•,• and

the formations through which the returr,]ng o•l-

ling fluid (mud) returns to the surf•cu. •2) Tne

space between tubing and casing or between casing

and formation. (3) A low resistance ring seine-

times produced by invasion processes in hydro-

carbon-bearing beds. Because of their greater

mobility, hydrocarbons are displaced farther

beyond the invaded zone than conductive forma-

tion water. See Figure 6.

•I=•=UJ•.___.._..t,:l) A unit of counting rate for the

g9•m_•9-r•. The difference between t))e h•gh

and low radioactivity sections in the API cali-

bration pit is defined as 200 API units. (2) The

reading in the Indiana limestone portion of the

API calibration pit (which has 19 percent poros-

ity and is saturated wit• fresh water) is defined

as lOO0 API units.

••L•S•,...•.•i]•: The resistivity recurd•d by

an electrical log, which differs from the true

resistivity of the formation because of the pres-

ence of mud column, invaded zone, influence of

adjacent beds, etc.

A•• fo rm• 1ai: Empirical relationships

tween the formation resistivity factor F (some-

times written FR), porosity •, water saturation

Sw and resistivities in clean granular rocks.

F : R0/R = a c-m,

F•

R0/R : Sw ,

where m = porosity exponent or cementation fac-

tor, which varies between 1.5 and 3,

a = proportionality constant varying from

0.6 to 1.5,

R0 : resistivity of the formation when 100

percent saturated with formation water,

Rw = formation water resistivity,

R : true formation resistivity,

n : saturation exponent; often n = 2.

"ArchiJ'sLaw" ssumeshat m : 2 anda = 1

The Humble formula is the special case where m =

2.15 and a = 0.65.

•j •lJa bowpringr everonnectedoa

g'n sonde which presses against the ore-

hole wall to centralize the tool, to push the

tool to the opposite side of the borehole, or

to hold a sensor pad to the borehole wall. (2)

To prepare a perforating gun for firing.

.azro:•::plo• : •_a_d•ole plot: A display of dipmeter

or drift data.

• = lO • cm•: A unit for measuring c•-

- ture cross-section.


16/26

G•ossary of Terms Used in We•t Logging 1t29

ease-line silift: A change i.• the i¸cat_or. it the

shale base line on the SP curve. A $hkft

occur when waters of differerst s•llnities are

separated by shale beds %.,'hlch do not act as per-

fect cationic membranes, when the formation

water salinity changes ',vithin a permeable bed,

or when the resistivity of the mud changes ,½zith

depth in the borehole.

• : borehole compensated sonic log. See com-

pensated lo•. BHC s a Schlumberger

BHC Acoustilog a Dresser Atlas traderlame.

B..•..H_=•bottom hole temperature.

B__H._T_]•borehole televiewer.

.b._ ..]_n•__•.z..A?•: portion of a formation in which a

logging tool response is too low; also called

"shadow zone." A blind zone occurs because of

the finite size or configuration of tha logging

tool. For the lateral curve, a blind zone (ab-

normally low reading) is recorded when a bed

which is highly resistive compared to the over-

lying and underlying formations is present be-

tween current and measuring electrodes.

b_½_f_eh•f_e_6_ii: A distortion of a well log be-

cause of the size and influence of the bore-

hole or sometimes the invaded zone.

borehole 1o__.9_•wel 1 1og.

b.ou%e_b_9_]e;..e]_?__v_i_ewecseisviewer' A well log sys-

tem wherein a pulsed, narrow acoustic (sonar)

beam scans the borenole wall in a tight helix as

the tool moves up the borehole. A display of the

amplitude of the reflected wave on a cathode ray

tube (television screen) is photographed yield-

ing a picture of the borehole wall, revealing

fractures, rugs, etc. BHTV is a Mobil Oil trade-

mark.

b_o._•.n•.d.at_e/_: Water absorbed in or chemically com-

bined with shales, gypsum, or other material and

which is not free to flow under natural condi-

tions.

b_r_i_d/•: The insulation-covered lower portion of

the cable to which the logging tool is con-

nected.

_b_J_ktc• •_e_]_E.6_t_r_•_4_½_••Lu_ad el ec trodes.

b_u]]•_t: (t) A device for obtaining sidewall

cores. (2) A device for e•.r__ati_•.

butl;or[: The small circular electrode on a micro-

resistivity sonde.

•_81 b•_ati•: Calibration test records run

before and/or after a log run and •attached to the

logs.

cal•.j•_ero•: Awell ogwhicheasuresoledi-

.......=m'e•"e, -open hoe caliper logging too s some

times have 1, 2, 3, or 4 arms. Also called

section gage. See Figures 8 and 13. Tools

for studying the corrosion of casing or tubing

use many "fingers."

,G.,a•Q$_U_.••._C,•Q•_S__•s_6_•]•0•.: ( 1 ) "A t om c c a p u r e c r o s s -

-•'•-•-• for neutrons is the effective area with-

in which a neutron has to pass in order to be

captured by an atomic nucleus. It is a proba-

bilistic value dependent on the nature and energy

of the particle as well as the nature of the

capturing nucleus. Atomic capture cross-section

is often measured in Darns (1 barn : 10-•cm•),

(2) "Macroscopic capture cross-section" • is the

effective cross-sectional area per unit volume

of material for capture of neutrons; hence, it

depends on the number of atoms present as well

as their atomic capture cross-sections. The

unit of measure for 7] •s cme/cm or reciprocal

cm (cm'•); :C is often measured in "capture units"

•r "sigmanits."1 c.u. ]O'•cmM •3)The

ate of absorption of therma neutrons •ith a

velocity v is tqus

•=•F•ture un•.• = c.u. = s.u. = sigma unit = lO'•cm -• .

A unit of measure of macroscopic capture cross-

section.

laterolog, microlaterolog, sonic, density, and

sidewall neutron logs which includes a secondarz

•]'9]lt•F•ndex curve and a movable oil •1ot.

.9=•,,,,0_•,._C•]•- ..•r•.• a t_•.r•: A m g ne t i c o r sc r a t c he r

device for locating casing collars, whict• are

used for correlating collars with other logs and

½•hic. often serve as reference depths in subse-

quent completion operations.

c..a_t_i.=•_nJ,,,•...,m•_,m•b•.•e•: me mb r a n • wh i c h p e r m t s t h e

passage of cations but not of artions. Shale

ac:s as such a membrane, allowing 5odium ions

Co pass but not chloride ions. Important in

generating the electrochemical SP.

,•.% : cement bond lo•.

• = continuous d•meter.

•r•t•i•_•a._G•.,.E.: The exponent m in the •rchie

formula.

•nt= bo_n_d.]._•: A wel I log of the amol i rude

of the acoustic wave which indi.cates tne degree

of bonding of the cement to the pipe. If the

casing is poorly cemented, energy travel through

the casing at the fast speed of acoustic waves

in steel is strong and little energy travels in

the formation; if the casinq is well cemented,

the casing signal nearly disappears and the

formation signal is strong. The log may con-

sist of (1) an •l•tude_l• (CBL) which repre-

sents the amplitude of a portion of the longi-

tudinal acoustic wave train or (2) a display

of the acoustic wave train such as the charac-

ter log, three-D, microseismogram, VDL, or

acoustic signature log. See Figure 2.

c•.b•a•.•• : signature log: A display of the

acoustic wave train in wiggle form, as opposed

to the similar sort of display in variable

density form in the three-D lo•, microseismo-

gram, or VDL log. See also sonic 1o•, fracture

1•, and cement bond l o•.

.G.hl_oni_ne: A log based on the counting rate of

capture gamma rays produced by capture of thermal

neutrons by chlorine in the formation. By limit-

ing the count to a certain energy range, the tool


17/26

• • 30 Sheriff

is made more sensitive to chlorine and rela-

tively insensitive to formation porosity. The

chlorine log is now essentially replaced by the

neutron lifetime loq and thermal decay time log.

cb•ro•_at•: See ar•ion gas chromatograph.

•J_e•t_•: Containing no appreciable amount of clay

or shale. Applied to sandstones and carbonates.

Compa e •_• _t•j .

•;_O_l]_•dc__]_D•:c•collar locator.

.c_O_•l_p_a_•j_on_•,•_r_e_½.L•JD_•:n empirical correction ap-

plied to porosity derived from the sonic log in

uncompacted formations such as sands at shallow

depths or formations under abnormally high pres-

sures. Undercompaction is indicated by abnor-

mally low sonic velocities in adjacent shales.

.•c•m•_.•__n•_•_s•_e•: Awel 1ogmadteith sonde

esigned to correct out unwan ed effects. The

compensated density log (FDC) uses the signal

from a secondary detector to correct for the

effect of mud cake and small irregularities in

the borehole wall. The compensated sonic log

(BHC) uses a special arrangement of the trans-

ducers to correct for irregularities in bore-

hole size and sonde tilt.

•QJ]_cL•c_t_i_v_i•: The ability of a material to con-

duct electrical current. See m. o per meter.

Conductivity is the reciprocal of resistivity.

•,•ojl_n_•__te•_•_%_ej•:1) Water trapped in sediments at

the time of deposition. (2) Formation water.

•_e•c_t•.•O•L: A microloE in which the sonde is held

against the borehole wall. Welex tradename.

•n•_g9_u_s• ._•e _•z•J_•_•_y._• : so n c l og. CVL i s a

Bi rdwel 1 trademark.

œ_Q•..e_n•_a•_l•: Cores from boreholes are analyzed

for porosity, permeability, fluid content

and identification, lithology, and structure

(fractures, cross bedding, etc.). Results are

often illustrated on a log or coregraph on which

porosity, permeability, water saturation, oil

saturation, and lithology are graphed against

depth.

.•_r_•: See core analysis.

c.q.•_=•__5_l.,._i_g_ez:devicesing iamondt-edged•ades

hich cuts a triangular core abou one inc on

a side and up to three feet long from the side

of a smooth borehole.

c r_r•: (1) The determination of equivalence

in stratigraphic position of formations in dif-

ferent wells. Similarities in the char. acter of

well togs and the occurrence of distinctive fea-

tures which serve as markers from one well to

the next are used. (2) The matching of different

wel 1 1 ogs and other wel I data either in the same

well or in different wells.

.•ros•splotz: A graph to determine the relationship

between two different measurements. A crossplot

of porosity measured from a sonic log against

porosity from a density log, for example, is used

to show secondary porosity effects.

cr__c9_•S•sec•Li_O_•: See c•pture cross-section.

continuous velocity log = sonic long_.

cvcle•skip.: In acoustic or sonic logging the firs1

arrival is sometimes strong enough to trigger

the receiver closest to the transmitter but not

the farthest receiver, which may then be trig-

gered by a later cycle resulting in an errone-

ously high transit time. This situation is

called "cycle skipping." Its onset is charac-

terized by an abrupt deflection corresponding

to an added cycle of travel between receivers.

"Short cycle skipping" where the near receiver

is triggered a cycle too late can also occur,

resulting in an abnormally short traveltime.

•: A unit of permeability; the permeability

which will allow a flow of one milliliter per

second of fluid of one centipoise viscosity

through one square centimeter under a pressure

gradient of one atmosphere per centimeter. The

commonly used unit is the millidarcy or 1/1000

darcy.

_DarcJL_•saw' A relationship for the fluid flow

rate q.

kA

q -- • •--•x

where k : permeability, A : cross-sectional area,

• = viscosity, •,p = pressure differential across

the thickness, x.

.d9...•si•]_• = densi•. Densilog is a Dresser

Atl as tradename.

.d•e•i_...t._..••: A well log which records the forma-

tiondensity.h• oggingool onsistsf a

amma-ray source e.g., Cs ?) and a detector

so shielded that it records backscattered gamma

rays from the formation. This secondary radi-

ation depends on the density of electrons,

which is roughly proportional to the bulk den-

sity. The source and detector are on a skid

which is pressed against the borehole wall. The

compensated density logging tool (FDC) includes

a secondary detector which responds more to the

mud cake and small borehole irregularities; the

response of the second detector is used to cor-

rect the readings of the main detector. Density

is related to porosity by the equation

q•= Pma Plog

Pma- Pf

wherePmas the densityof the rock matrix, Plog

is the density recordedon the log, and pf is the

density of the f•prmation fluid. Sometimes called

gamma-gammaq•g. Compare nuclear cement log and

hotS. See Figure 3.

•a_•_tu•r• curve: Graphs which allow one to cor-

rect for measuring conditions which differ from

standard. Such curves, for example, might

correct for differences in temperature, hole di-

ameter, mud type, adjacent beds, invasion, etc.

de?th•of •..nvasio...n.: See invaded zone.

•ep.th of investJ•.gation: The radius about a logging


18/26

G•ossa•y of Te•ms Used •n We•l Logging •13•

sonde within which material contributes sig-

nificantly to the readings from the sonde.

._.d_e_t.i•: A borehole log plotted at a scale

larger than conventional (1 inch per 100 ft);

specifically, an electric log at a scale of 5

inches per 100 ft.

e•L•i•: Departure of a borehole from vertical.

See drift, directional survey_and rectify.

_d_iP_•.og dieter. Diplog is a Dresser Atlas

tradename.

.c[• '._p____..•:1) A well log from which formation dip

magnitude and azimuth can be determined. The

resistivity dipmeter includes three or four

microresistivity readings made using sensors

distributed in azimuth about the logging sonde

and a reading of the azimuth of one of these;

a reading of the hole deviation or drift angle

and its bearing; and one or two caliper mea-

surements. The microresistivity curves are

correlated to determine the differences in

depth of bedding markers on.different sides of

the hole. (2) Other types of dipmeter use

three SP curves, three wall scratchers, etc.

(3) A log showing the formation dips calcu-

lated from the above, such as a tadpole plot or

stick plot. See Figure 4 and HDT.

dire_•ti_o•]• sur_•e•: Measurements of drift, the

azimuth and inclination of a borehole with the

vertical. A directional survey is often made

as part of a dipmeter survey or sometimes as a

continuous log with a poteclinometer. Some-

times measurements are made at discrete levels

with a photoclinometer.

dir_f•w = shaly: Containing appreciable amount of

shale dispersed in the interstices. Such shale

lowers the permeability and effective porosity

and affects the readings of many types of logs.

down. ole_____ground•: long electrode often attached

to the logging cable some distance (perhaps 100

ft) above an electrical logging sonde or hung

just below the casing, used as the reference

electrode instead of a reference electrode at

the surface, Used in the case of bad SP inter-

ference from electrical surface facilities or

tell uric currents.

•_•_.f•.t_: The attitude of a borehole. The drif•

angle or hole deviation is the angle between

the borehole axis and the vertical; the drift

azimuth is the angle between a vertical plane

through the borehole and north.

•?i]]•i•ng___b•ak: An increase in the penetration

speed of the drill bit caused by a change in

formation, often indicative of penetration into

a porous zone.

• ]...ling %jm•]_O•: A record of •he time to drill

a unit thickness of formation.

d•__i._]__l_p_i_p_eo_•:Awell ogw•ichs obtainedrom

logging instrument w i h as a self-contained

recording mechanism. The log consists of an SP

and short and long normals. The tool is lowered

through the drill pipe and the flexible elec-

trode assembly is pumped through a port in the

bit. The log is recorded by a tape recorder

within the tool during the process of coming out

of the hole. The tape is played back to obtain

the log. Welex tradename.

dual inducti•' An induction log consisting

of two induction curves with different deoths of

investigation. Usually run with a focused resis-

tivity device with a shallow depth of investi-

gation, such as a shallow laterolog.

• = deep well thermometer: A sonde for recording

temperature logs,

•_5•?c.n_t.•e'._•: A device which presses a sonde

against the borehole wall.

elastic w__a_v_•e.acoustic wave = seismic wave =

sonic wave.

•e ]_e•_l;•ric_al_0•: A generic term including all elec-

trical logs. Compare electrical surve_.•.

•e]_e_c]ric 1•%= : electrolog: A borehole log

which usually consists of SP and two or more

resistivity logs, such as •ort and long nor•ma•l

and long lateral resistivity logs. Electrolog

is a Dresser Atlas tradename.

e_]eC•roch•e•m•: The component of the SP com-

prised of the sum of the liquid-junction poten-

tial and the shale potential, both of which are

determined by the ratio of the activits. of the

formation water to that of the mud filtrate.

The liquid junction potential is produced in

the formation at the contact between the inva-

sion filtrate and the formation water as a re-

sult of the differences in ion diffusion rates

from the more concentrated to the more dilute

solution ("concentration cell"). The nega-

tively charged chloride ions have greater

mobility than the positive sodium ions and an

excess negative charge tends to cross the

boundary, resulting in an emf. The shale po-

tential results because the shale bed acts as

a cationic membrane permitting the sodium

cations to flow through it but not the chloride

anions. The liquid junction potential and

shale potential are additive. See also SS_____•.P.

elect__r_••_c__ ?._Lte__.ni•j..•.a_..]•streaming potential =

---•-•ectrofiltration potential: A component of the

SP produced as a result of movement of the inva-

•-fon fluid through the mud cake. Variations in

ion concentration produced by the processes which

generate the electrochemical SP tend to be swept

along by the movement of the invading fluid and

this flow of charge results i n the el ectro-

kinetic potential. The electrokinetic poten-

ti al i s usual 1 y cons i dered smal 1 compared to

the electrochemical SP and is often neglected

when estimating formation water resistivity from

the SP.

elec•r•on.__den•i•t•: See density 1og•.

•mp•:•_h_o_],•l:A borehole which is filled with air

or gas.

e__•: A s ne• 1og.

e=•oo_•i•t•h•e_r_m1.•n•: See neutron loq.

E..•S electrical survey.


19/26

1132 Sheriff

F__&_]•formation anal sis lo .

F_D_C. compensated formation densi.•,• loci.

F• : formation densi•.

F_F__•=free fluid index: See also nuclear magnetism

if••_•,•?, = mud cake.

•]_a_t-•__t_o_•j=•n=•q=:maximumbeyond which values are

not recorded, Loss of sensitivity due to satura-

1]ion of some part of the measuring system.

•]•.•O•].•e_•.•r• A device which measures the flow of

fluid. Sometimes the flowmeter is lowered through

the flow stream in a borehole; sometimes it is

set in one spot with a packer. See •inner

s u v•.•. .

• i••m•Z,: See format ion tester.

•l•_ •d ,.[•_•: See invaded zone.

•_•_e•__....•.,].,•.•: Often refers to la[erolo• or guard

log. Sometimes refers [o induction lo•.

•"•'•-•-• •-•'•'•stivity an app rent poro-

sity based on induction log and either sonic or

density log d•ta. Also called Rwa nalysis log.

•_•r$_•_J.•,a•_v• t i,_o•: T h e a na 1y s i s a n d i n e r -

pretation of v•'"'•i-'i--logdata, drillstem tests, etc.

in terms of the nature of the formations and

their fluid content. The objectives of forma-

tion evaluation are to ascertain if commerci-

ally productive hydrocarbons are present and

the best means for their recovery and to derive

lithology and other information on formation

characteristics for use in further exploration.

•_o••0•••_ = F: The ratio of the resis-

tivity of a formation to the resistivity of

the water with which it is saturated. While

formation factor is, strictly speaking, de-

fined only for clean brine-saturated sands (for

which it is nearly independent of the fluid

resistivity), it is often used without this

restriction. Apparent formation factor is a

function of porosity, pore structure and size

distribution, and clay content. See Archie's

formu 1 a.

•.[m•im•o_$_•_•Le•_: A tool run on a wireline to

obtain samples of formation fluid. Hydrostatic,

flow, and shut-in pressures are recorded,

•_•: See microlaterolo•, Welex tradename.

•_e•]o•: A well log of the cumulative ampli-

tude of the wave arrivals from a sonic logging

tool during a certain gate time. A fracture

zone attenuaCes the acoustic energy and delays

the arrival of the shear wave.

.•C•e•,.fluid i•d• = FFI: The percent of the bulk

volume occupied by fluids which are free to flow

as measured by the nu___clear_ neti•. Gas

gives a low FFI.

•_f_•_•%h_:ery low in dissolved salts. Sometimes

,used comparatively with respect to normal sea

water (which is 35,000 parts of dissolved salts

per million), sometimes used comparing formation

water with mud flitrate. "Fresh water" has less

than 2000 ppm dissolved salts.

•q:•N•N•=•=•=•__,•: See de ns__i_t•y•.

•m•.•_=•.•r•__]•g•: A well log uhich records the natural

radioactivity, (1) In sediments the log mainly

reflects shale content because minerals contain-

ing radioactive isotopes (the most common of

which is potassium) tend to concentrate in clays

and shales. Volcanic ash, qranite wash, and

some salt deposits may also give significant

gamma-ray readings. The log often functions

as a substitute for the SP for correlation

purposes in cased holes, in nonconductive muds

in open holes, and for thick carbonate intervals.

See Figures 3, 7, and ll, (2) Used in explora-

tion for radioactive minerals.

•9•,.g$__e_t_tF_i_r.•._aj•f_•,G$_o_r_:he geometry dependent weight-

ing factor for determining how the conductivities

of each medium in the vicinity of a sonde affect

the apparent conductivity measurement, especi-

ally used with the induction log.

$r.•] j__•qz.a•_•q•et•n: A device for determining the

density of the wellbore fluid by measuring the

pressure on two bellows and therefore the ver-

tical pressure gradient.

•8_,nj•s_l_•m: A combination of logs or a computa-

tion procedure for calculating the depth of

invasion and the resistivity of both invaded

and uncontaminated zones, based on a dual in-

duction laterolog and a proximity log or micro-

laterolog.

•,.q,•cJ.__e.]•_c•o.J...e• bucking electrodes: Electrodes

from which current flows so that the current

flow from a measuring electrode is focused into

the formation. See laterolog.

...•d__j_=..• : laterolo• : focused log: Log made with

•uard electrodes.

•Z = high resolution dipmeter: A dipmeter which

records four high resolution micrOresirstivity

curves and has an additional electrode on one pad

which yields another curve at displaced depth.

The displaced depth curve is used to correct for

variations in sonde velocity.

h,i.•b _r•eso]_u=tioo];_b_•_m•_•e•: A small-diameter fast-

response thermometer for logging open or cased

boreholes with a temperature resolution of 0.5øF.

_h_otwi•]•_e_•: A device used to detect hydro-

car•øn gases returned to the surface by the dril-

ling mud. Basically a Wheatstone bridge, two


20/26

Glossary of Terms Used in Wel• Logging 1133

arms of which are kept at a high temperature.

Hydrocarbon gases become oxidized as they pass

over one arm, which increases its temperature,

changes its resistance, and unbalances the bridge.

The hot wire analyzer response is sometimes

plotted as a well log.

•Humbl_e•.•fo_rmu•la•: special form of Archio's formula,

applicable in granular (sucrosic)-•-6'•s:

F = 0.62

where F = formation factor and • = porosity.

.•,•b•_•_ri•__•....•]_e•: scale used with laterclogs which

is linear with resistivity for 1o,; resistivities

and linear with conductivity (the reciprocal of

resistivity) for high resistivities. See Fig-

ure 7.

=h=¾=•=r•o••: Hydrogen per unit volume com-

pared with that in fresh water. Neutron log

response depends mainly on hydrogen index.

•_•L.d_..•.r...,.0_.•_..a_r__b_0n__•Ltura•i_o_9_:raction of the pore vol-

ume filled with hydrocarbons.

•S•. : IEL = induction electrical survey: A bore-

hole log which usually includes SP, 16 inch nor____-_

m 1, and deep-investigation induction loq._•.s.

]LL_d• deep i nvesti gati on i nducti on 1 oq..

I_L_nl medium investigation induction loq•.

in•cl•in_or•_t_e_.r.: A device for measuring hole inclina-

tion and azimuth. See directional survez.

.•n•__•_G_t•i_•n__: An lectricalconductivity/resis-

'i-•-•-•il log based on e ectromagnetic induc

tion principles. A high-frequency alternating

current of constant intensity induces current

flow in the formation. This "Foucault current"

flowingn the ormationgeround•o•" auses

n alternating magnetic fi ld whic reduces a

current in a receiving coil. The receiving coil

current is nearly proportional to the conduc-

tivity of the formation. Induction sondes may

have several transmitting and receiving coils

to produce a highly focused log. An induction

log can be recorded where the borehole fluid is

conductive or nonconductive, as in oil base

muds or gas. See Figure 5.

?]_L•,..s,_t_j.•._tJ_a.l__•,L•.,L•_:ater i n the i ntersti ces or

pore spaces in a formation.

in;•_[•?•j__..t•_si••: The traveltime of a com-

pressional sonic •-•eismic) wave over a unit dis-

tance, hence proportional to the reciprocal of

P-wave velocity. Measured in the sonic log,

usually in microseconds per foot.

i n r i ns•_r m 1 n__e__u_t_•Q_n_•_d_e_C_•_•_.t•e__: The t r ue

decay time of the formation as opposed to the

measured decay time which is sub3ect to hole and

diffusion effects. See neutron lifetime lo•q.

inv•a.ded zone: The portion about a wellbore into

which drilling fluid has penetrated, displacing

some of the formation fluids. This •nvasion

takes place in porous permeable zones because

the pressure of the mud is kept great,'•r •.han

that of the formation fluids. A "rilud c•ke"

builds on the formatinn wall, limitin d furIF•r

flow of mud fluid (Flitrate) into th• furma-

rion. Directly behind •:he mud ca•'e is

"flushed zone" i•Fom •,•lcn al,,•o•t all ,:f

formation water and most of the h'/drecarbc, ns

have been displaced by fil•ate. ThL, inva-

sion process alters the distribution of resis-

tivities and other pro,,Jerties and tensefluently

the value which logs read. The "depth of

vasion" is the equivalent depth in an •de•l•zed

mod•l rather than tt]e maximum depth reached by

filtr•te. In oil-bearing intervals, the fli-

trate may [•ush a bank of f


21/26

1134 Sheriff

l•ttle s_la• = small slam.

]_j.']_h_Q]•: A log showing lithology as a func-

tion of depth in a borehole. Sometimes a strip__

lo__0_9.ased on samples, sometimes interpreted from

other borehole logs,

See we l log.

lo.•j•.• normal: A normal resistivity log made with

the A and M electrodes in the sonde (usually)

64 inches apart.

•__•: Subscript used with log terms to indicate

values appropriate to the rock matrix.

_.r_o•ic .•i•_Q_•=: Resistivity perpendicular

to the bedding which differs froin resistivity

parallel to the bedding because of the inclusion

of interbeds of markedly different resistivity.

•_•_= millidarcy = 1/1000 darcy.

•: Subscript used with log terms to indicate

values for the mud filtrate.

•ll.r.hrOr:r,,,pe•r•.•:•r•.•t_•=_r- lOO0 mmho per meter: A unit of

conductivity, being the conductivity of a

material a meter c

rance of one ohm t

pos ire faces. Rec

•.j•,c. o i n e ' See

........ )] n v..e_•. m

microlateroloe. : min

t--•• •.' A mi

aterolog type wit

monitor electrodes

on a pad which is

so that the curren

gradually flaring

ube of which offers a resis-

o current flow between op-

iprocal of ohm-meter.

icrolo•.

ifocused log = FoRxo =

croresistivity log of the

h a bucking electrode and two

arranged concentrically

pressed against the formation

t flow is concentrated into a

tube. Since the spacing is

small, the log responds to the resistivity of

a small volume of formation in front of the pad.

For the proximity log the electrodes are mounted

on a wider pad and the focusing is such that

thicker mud cakes have less effect. See Figure

8. Micro aterolog is a Schlumberger trade-

mark.

•icrolo_• : contact log : minilog: A type of micro-

resistivity log using three button electrodes

spaced in a line one inch apart and located on

a pad which is pressed against the borehole

wall. The lower electrode is the A current

electrode. The potential of the upper electrode

with respect to a reference electrode on the

surface gives a 2 inch micronormal and the dif-

ference between the two upper electrodes gives

a 1« inch microinverse (lateral type measure-

ment). Because the mud cake usual y has appre-

ciably smaller resistivity than the formation,

the microinverse will read less than the micro-

normal when mud cake is present. This dif-

ference (called "separation") indicates a per-

meable formation. A caliper log is usually re-

corded at the same time. Microlog is a

Schlumberger trademark. See Figure 8.

.m_•cronormal: See microlog.

micm.resisti.vitvo•): Awell og esignedo

-easur6 he-•F•s•'ivity of the f ushed zone

about a borehole, recorded with electrodes on a

pad pressed against the borehole wall. See

microlo.g. nd microlaterolo 9.

microse__•_i•m_o_cLr_amq•L : variable density log :

three-D lo•: See Figure 2.

m..ini]•.: See microlog_. Dresser Atlas tradename.

_mmho mil imho: A unit of electrical conduction,

1/1000 of a mho, the reciprocal of ohm.

I•zo=v_o_b•l•e_•l.• : MOP: A well log calculated from

other logs on which three "porosity" curves are

plotted: "total porosity," q•t, such as is derived

from the sonic log; "apparent water-filled poro-

sity" ½• derived from a deep investigation resis-

tivity •eviceSUChs the a•erolog;nd ap-

zparentaterialled orosityf the lusheid

ne" •xo from shallow investigation res s-

tivity device such as the microlaterolog. The

separation between the first two curves inci-

cares the volume fraction of hydrocarbons in

the noninvaded zone and the separation between

the last two curves indicates the volume frac-

tion of movable oil. The remainder represents

residual hydrocarbons left in the invaded zone.

See Figure 9.

m__u_d___c_a_k•e•filter cake: The residue deposited on

the borehole wall as the mud loses filtrate

into porous, permeable formations. The mud cake

generally has very low permeability and hence

tends to retard further loss of fluid to the

formation. See invaded zone.

•mu•d•._.cu•: A container used to measure mud resis-

tivity.

ELUJL.f._i_]_•C2Lt•_:luid which enters permeable forma-

tions from the mud, leaving a mud cake on the

borehole wall. See also invaded zone.

•=•: A 1og made with a mi crol oc) sonde with

the arms collapsed so that the measuring pad

is not pressed against the borehole wall. Mea-

sures resistivity of the mud at in-hole con-

ditions.

•j•: Analysis of samples of the drilling

fluid (or mud) and cuttings to detect signs of

fluids which have entered the mud from the forma-

tions. Hydrocarbons may be evidenced by fluo-

rescence, by chromatographic analysis, gas, and

other ways. The mud is also monitored for

salinity and viscosity to indicate water loss

or cut. Plots of such data often include a

sample log and a drill time curve.

•):•j•p.]• . ...... _t.o•ol•: A device run in a borehole

to measure the direction of the borehole at

several levels, See directional survey.

neutro.Q lifetime og•: Similar in use to the decay

time log. A well log of thermal neutron capture

cross-section. A Van de Graaff neutron genera-

tor in the sonde periodically releases a burst

of neutrons which enter the formation and begin

to lose energy in collisions. At two discrete

time intervals after a neutron burst, measure-

ments are made of the gamma rays which result

from the capture of neutrons by nuclei in the

thermal decay time log, whereas measurements are


22/26

G•ossa•y of Te•ms •Jsed •n We• Logging •35

made of the thermal neutrons in the neutron

lifetime log. The quantity plotted is sometimes

the reciprocal of the percentage which decay per

unit of time, called the thermal decay time •;

sometimes, the time for the thermal neutron popu-

lation to fall to half value, called the neutron

I i fetime L; sometimes the macroscopic cap_ture

cross-section ., which s derivable fro• t-h fore-

going (2 = 4.55/• : 3. 5/L). Thermal neutrons are

captured mainly by the chlorine present and hence

this log responds to the amount of salt in forma-

tion waters. Hydrocarbons result in longer decay

times than salt water. Log readings are porosity

dependent and sensitive to clay content and per-

meability changes. This log is used in cased

holes where resistivity logs cannot be run or to

monitor reservoir changes to optimize production.

It resembles a resistivity log with which it is

generally corre atable. Dresser Atlas trademark.

See Figure 10.

neutron lost: A well log of hydrogen density. Fast

ne•emitted by a source in the tool are

slowed to thermal speed by collisions with (main-

ly) hydrogen atoms. The thermal neutrons are then

captured by atomic nuclei of the surrounding mate-

rial (mainly chlorine atoms) at which time a

characteristic gamma ray of capture is given off.

The neutron log detector may record the capture

gamma ays (n -y), thermal neutrons (n - n), or

epithermal neutrons (those just above thermal

speed). A low hydrogen density indicates low

liquid-filled porosity. Porosity calculated from

the neutron log is affected somewhat by the forma-

tion matrix and by the presence of gas. Neutron

logs are used in crossplots• to detect gas and de-

termi ne 1 thol ogy. Neutron 1 ogs are sometimes

scaled in API units, sometimes in porosity units

assuming a limestone matrix. See also sidewall

neutron log. The neutron log can be recorded in

cased hol es See Figure 11.

NML = nuclear magnetism log.

[Lp_r_lsa.L:A resistivity well log in which a constant

current is passed between a current electrode in

the sonde and one at the surface (electrodes A

and B) while the potential difference is measured

between another electrode in the sonde and a

reference electrode at the surface (electrodes M

and N). The "spacing" is the distance between the

A and M electrodes for the normal. A spacing of

about 16 inches is used for the short normal and

64 inches for the medium or long normal. See

Figures 12 and 14.

)l_•..qlar cemen% oR: A wel 1 1 og of scattered gamma

rays, differing from the density loll in that the

gamma-ray source and detector are so spaced as to

be sensitive to the density of material in the an-

nulus. Used for distinguishing between cement and

fluids behind casing. Can be run in empty hole.

•Ur_½_l_•ar•oj•_ejLi•m].O•.q NML - free fluid log' A

'•V-611i6•-'•-hi•h•S dependentn the alignmentf

the magnetic momentof protons (hydrogen nuclei)

with an impressed magnetic fi el d. Protons tend

to align themselves with the magnetic field and

when it i s removed they precess i n the earth ' s

magnetic field and gradually return to their

original state. The proton precession produces

a radiofrequency signal whose amplitude is

measured as the free fluid index (FFI). The

rate of decay of the precession signal depends

on interactions with neighboring atoms and hence

on the nature of the molecule of which the proton

is a part. The s•gnal from the uorehole fluid

decays very rapidly because of disseminated iron

(from steel worn from drill pipe and bits); by

slightly delaying the time of measuring• ti•e

hole signal is minimized. Fluids bound to sur-

faces (as the water in shales) do not give appre-

ciable response. Thus the FFI indicates the free

fluid (the hydrogen in free fluid hydrocarbons

and water). Gas gives a low reading because of

its low hydrogen density. Sometimes thermal re-

laxation time, the rate of polarization buildup

as a function of polarizing time, is measured

to distinguish between water and oil.

T='•meter, being th• •i• vity of a meter

cube which offers a resistance of one ohm to the

flow of current between opposite faces. Recip-

rocal of mho per meter.

[a__d_ sidewall pad: A footing on the end of an

arm which presses against the borehole wall.

ß p•t.j• ?_.•_.n___•q•a•s__c.__r_o_m_a_t9_•q=r_ra•h_: A de v i ce f o r q ua n t i -

tative analysis of hydrocarboq constituents.

A fixed quantity of sample is carried •,;ith a

stream of sweep gas through a partition column

packed with an inert sol i d coated with a non-

volatile organic liquid. The lighter fractions

traverse the column faster than the heavier

fractions so that the components aF,?ear separ-

ately at the column exit, where their amounts

can be measured.

_.•.•__fo_cat •: Opening holes through casing and into

a formation so that fluids can flow from the

formation into the borehole.

r•: A measure of the ea3e with which a

formation permits a fluid to flow ti•rough it.

Measured in millidarcy (1/1000 darcy) units.

ph_h_O_t_oclin•.me•__•Jze.?_: well oggin•evicehich

-p-h'o-togr'aphiCally records the ngle and azimuth

of borehole deviation from the vertical. Com-

pare potecIinometer.

...o_t_o•____9•: A wel 1 1og oi scattered gamma rays,

differing from a densit•1o in that tile sonde

is not pressed against the borehole wall and

hence is especially sensitive to changes in

hole diameter or density of the fluid in the

borehole.

•__•: Poreolumeeruni• olumefformation.

ften indicated y th symbo ½. Porosity is de-

termined from sonic loqs using the [,]zllie rela-

tionship, from densit•---•-i-og3_,rom n•tltron lo9_[,

o•resi sti•{ -i'"6-g"S--byssumi• •-l-q-{i n-

ship such as Archie's formula. See movable oil

lp..•. "Primaryporosity" refers to the porosity

remaining after the sedimen'•s have been com-

pacted but without considering changesbecause

of subsequen: chemical action or flow of waters

through the sediments. "Secondary orosity"

is the additional porosity created by such

chemical changes, especially fissures, frac-


23/26

tures, solution vugs, and porosity created

by dolomitization. "Effective porosity" is

the porosity available to free fluids, excluding

unconnected porosity and space occupied by

bouncJ •:ate•' and disseminated shale.

•,•:-o.qi• overlay: ?orosity values based on dif-

ferent logs plotted on top of each other.

•.?•tectinn•eter: A device for measuring the angle

and direc'•ion of borehole deviation continuously

during a log run. A pendulum moves a variable

resistor arm so t•at the resistance is a measure

cf the angle with the vertical and a compass

needle moves another arm so that another resis-

tanc• is a measure of t•e a2imuth, Often run

•½i'• dipmeters. Con,pare •otoclinometer.

J•.• • = par•s per million.

•.howing ,'Lffective porosity and the percent q of

the total porosity occupied by clay. Low clay

content and high fluid-filled porosity suggest

good permeability.

•]LO....dJ•t.j...,9_•]•]•,: A well log run in a production or

injection well. Small diameter sondes are

used so that they can be lowered through 2 inch

ID tubing. Devices include continuous flow-

meter, packer flot,•meter, gradiomanometer, mano-

meter, densimeter, watercutmeter, thermometer,

radioactive tracer tools, through-tubing cali-

per, casing collar locator, and fluid sampler.

••j•::••=: A microresistivity log similar to

the microIaterolo• but less sensitive to mud

cake t•icknesses. "Proximity" is a Schlumberger

trademark.

•gn=•j)o.?_•:ometE=•_•__,•j•ja]ac?_•,_•O•:A coefficient used for

estimating the response of a resistivity mea-

sur•zmentR at different invasion depths:

R : RxoJ Rt(1 - j),

N,•here Rxo = flushed zone resistivity, Rt : un-

contaminated zone resistivity, and J = pseudo-

geometrical factor, a function of invasion depth.

•e..Ed•O•S]_O:•.],_c_.• PSP: The "static SP"of a shaly

sand. Static SP refers to a clean sand.

•.• : porosity unit = 1 percent porosity.

:q=: The fraction of total porosity occupied by dis-

persed shale. See •_rodu__cibil•i_t.•y_.i•nde___Zx•.

•_a.•j_O•...t]_V•.•__t•n.a•c._e_r•__],•_•:1og i nv o 1v ng t he de -

tection of radioactive materials dissolved in

water or oil to detect movement of fluids. A

slug of radioactive material might be injected

into the fluid and movement of the slug moni-

tored to detect casing lea';


24/26

G•ossary of Terms Used in Well• Logging 1137

(sodium, potassium, chloride, sulfate, etc.)

_$_a•.,dount: (1) The total effective thickness of

---permeable section excluding shale streaks or

other impermeable zones. Often determined from

electrical logs (SP or microlog). (2) The number

of separate permeable sands separated by imper-

meabl e zones.

sandine:(1) A inethatcan e rawnhrough

-th•imum deflections for thic , clean sands

on an SP log in a section where the formation

water is of constant salinity. See SP and

Figure 14. (2) A wire line on a driTTing rig

often used to run or recover tools inside the

drill pipe.

saturation exponent: See Archie's formula.

sawtooth SR: When a very permeable salt water

sand containing a shaly streak is invaded by

fresh mud filtrate, the filtrate tends to ac-

cumulate just below the shaly streak, setting

up an electrochemical cell which causes the SP

to develop a sawtooth appearance.

5_C]3_m•.d••' A polar plot where the angle

indicates dip or drift direction and the dis-

tance from the origin indicates the dip or

drift magnitude. In the "modified Schmidt di-

agram" used for plotting low dips, zero dip is

on the outside and dips become larger toward

the diagram center.

5_e.c..ondary_orosity.: Porosity resulting from the

alteration of the formation such as by frac-

tures, vugs, solution channels, dolomitiza-

ti on, etc.

secondary po•rosit_v 'n]_o_d_e•SPI: A measure of the

secondary porosity, calculated from sonic log

values in conjunction with either density or

neutronogvalues.f q•Ds the orosityal-

u ated from a densit (or neutron log and

•sonic is the porosity calculated from a sonic

log, SPI is sometimes efined as (•D - •sonic),

usually as (1 -•sonic/d•D).

.%e_•_•.O• aLL_q9. .•.a. per 1og.

Sei•vie• : borehole televiewer. Birdwell

trademark.

self-.Dotentia_l_ : spontaneous potential = SP.

e•_•B_•a.rati•QQ:1) A difference in reading between

two curves on a log. (2) The difference in

resistivity readings from two tools which have

different depths of investigation (see Figures

5 and 8). Low resistivity mud cake, which

becomes thicker where formations are more per-

meable, cause the apparent resistivity of a

shallow measurement to be lower than that of

a deeper measurement, a situation called

"positive separation." See also movable oil

plot_.

shadow zone: See blind zone.

_shale base line_' (1) A line drawn through the

minimum d•ctions characteristic of imper-

meable shales on an SP log (see Figure 14), which

is used as the reference in making measurements

to determine the characteristics of sands and

their formation waters. (2) The characteristic

of thick shales on the gamma ray log.

shale.p..ot_en._ti.Ll: A part of the electrochemical SP.

h_s_b_a]_ines__._ •: The content of shale (or clay) in a

dominantly nonshale formation. The degree to

which ion exchange processes contribute to

resistivity measurements because electrical

conduction in shales is an ion exchange pro-

cess whereby electrons move between exchange

sites on the surface of clay particles. See

dirt•/.

_s_b_e_•rwave: See acoustic wave.

_s.ho.r];•o_•mal: A n_.orm_.9_•atresistivity log made

with the A and M electrodes in the sonde about

16 inches apart.

shoulder bed effect : adjacent bed effect: Effect

of adjacent beds on a log reading. For example,

high resistivity beds adjacent to a low resis-

tivity bed may result in more current flowing

in the low resistivity bed than if the high

resistivity bed were not present, thus changing

the apparent resistivity of the low resistivity

bed.

&•dde•all_•: A formation sample obtained with

a wire line tool from which a hollow cylin-

drical bullet is fired into the formation and

retrieved by cable attached to the bullet. The

type of bullet and size of charge is varied to

optimize recovery in different formations.

$.i.dewall_ neutron lo• - SNP' An epithermal neutron

lo__9_.•ade itha s•i• whichs pressed

the borehole wal 1 and may cut into the mud

cake to minimize borehole effects. SNP is a

Schl urnberger trademark. See Figure 11.

sidertall pa•d: A measuring device which is pressed

against the side of a borehole, such as used

with microresistivity logs, density logs, many

radioactivity logs, etc.

•idewall., samoler: A wireline device for taking

•'i dbwal•l c• •e'•

siema uni• : capture unit: A unit of measure of

capture cross-secti on.

_.

single.•hotool: A deviceo obtaln nemeasure-

en= of the d""f'rection of a boreho e at a par-

ticular depth. See directional survey.

skin e..ffec...t.: (1) The tendency of alternating

currents to flow near the surface of a con-

ductor. (2) The propagation of an electro-

magnetic wave through a conductive formation

results in a phase shift which shows up as a

reduction in apparent conductivity. In induc-

tion logging in high conductivity formations,

a correction is made for this effect.

o: See cycle s:kip.

m l•__sl•__5.]_.•: log combination and computation

procedure for calculating the resistivity of

the invaded zones, the true resistivity, the

diameter of invasion, and the water saturation

based on a dual induction-laterolog and a

porosity log of some type, often a sonic log.

SNP = sidewall neutron (porosity) lo_9...


25/26

1138 She•,iff

•: A poorly consolidated sand-shale

sequence.

•,t•_iC_t__•9_•_ksedimentary rock: Used o distinguish

between mining (hardrock) and petroleum (soft-

rock) objectives.

&Jzz]_d_•:A logging tool such as is lowered into a

•ehole to record resistivity, sonic, radio-

acti vi ty, or other types of 1ogs.

• = acoustic velocity log = continuous

velocity log: A well log of the travel,time

(transit time) for acoustic waves over .a unit

distance, and hence-"•h-e•'r•e-ciprocal f the

longitudinal wave (P-wave)velocity. Usually

measured i n microseconds per foot. Especial ly

usedorhPOrosityeterminationy •e W_.•

elations ip. The interval transit ime is

integrated down the borehole to give the total

traveltime. For the compensated onic log,

two transmitters are pulsed alternately;

averaging the measurements tends to cancel

errors due to sonde tilt or changes in hole

size. See also c cl•, three-D 1•,

character lOcL, ementbond o..q., nd¾-racture oq.

See Figure 13.

• : spontaneous potential : self potential: A

well log of the difference between he potential

of a movable electrode in the borehole and a

fixed reference electrode at the surface. The

SP results from electrochemical SP and electro-

kinetic potentia• which' a'•e--pr•n t at the

inter;face between permeable beds and adjacent

to shale. In impermeable shales, the SP is

fairly constant at the "shale base line" value

(se• Fiqure 14). In •ermeable formations

the deflection depends on the contrast between

the ion content of the formation water and the

drilling fluid, the clay content, the bed

thickness, invasion and bed boundary effects,

etc. In thick, permeable, clean nons hale

formations, the SP has the fairly constant

"sand line" value, which will change if the

formationateralinityhanle•ns•nd•ontaining disseminated clay s le , t e P

will not reach the sand line and a pseudo-

static SP value will be recorded. The SP is

posi•'•ve with respect to the shale base line in

sands filled with fluids fresher than the bore-

hole fluid. See also SSP.

•J;L•: The separation of certain electrodes

or sensors on logging sondes. In nuclear

devices usually the distance from the source

to the detector. See Figures 3 and 12. See

a 1so

_&J3•: The separation of certain sensors on log-

ging sondes. On the sonic sonde, span is the

distance between two receivers of a pair where-

as spacing is the distance from transmitter to

the •n•dP6-•'-ntf the correspondingeceiver pair.

See Figure 13.

•s_D_and.iustment:A alculationfa logwhich

Wø•'¾'d•ha'•e•sul ed row heuseof a • dif-

ferent from the one actually used.

sp_inn•e•r.urvej•: A log of the rate of flow of

fluid in the wellbore, casing, or tubing at

specified levels in the wellbore. See flowmeter.

s•oontan.e.ou.s.otential - self potential = SP.

;P ed.cti n a_ctor

The ratio of actual SP to

j• = •tatic self p

would be recorded

position well ins

permeable, clean

thick shale• The

el ectroki neti c SP

approximately giv

SSP = -K log

otential: The maximum SP that

when the sonde passes •-Fom a

idea very thick, porous,

sand to a point well within a

electrochemical SSP (the

is often neglected) is

en by:

lO aw/amf]

• are heactivity.f the orma-

here• andmmfu . Be-

ion wa:er an d filtra{ e, •eSpectively

cause of the inverse relationship between activity

and equivalent resistivity, this equation can be

written:

SSP-K OgloRmfe/Rwe).

For NaCl muds hat are not too saline, Rmf : Rmf;

for other muds an activity correction should be

made. In these equations, K = 60 + 0.133 T, where

T = Fahrenheit temperature.

the-wall the borehole. (2) device or pro-

ducing this separation.

s•atic SP = SS_.__•Pstatic self potential.

st•.ck•plQ•: A presentation of dipmeter results

where the well bore is represented by a line

according to the projection of the wel 1 onto

a vertical plane and the components of dip in

this plane are indicated by short line segments.

$:t.o•e]e_v..•a•.•.: See acoustic wave.

s_•t•e_am•ng otentia.l: See electrokinetic potential.

str p:]•:O•: A record (often in colors and symbols)

of the lithology penetrated by a borehole, indi-

cating also shows and tests of oil, gas, etc..

• = sigma unit = .c..apture unit: A unit for

measuring capture cross-section.

vnet: A combination of data elements such

that more information is apparent from the com-

bination than from the elements treated inde-

pendently. Synergetic Log is a Schlumberger

trademark.

J;Jl•d.p•.].e•::.p __o.•tarrow plot: A plot of dipmeter or

drift results where the dip angle is plotted

versus depth as the displacement of a dot.

A line segment points from the dot in the direc-

tion of dip using the usual map convention of

north being up. See Figure 4.

tel, eviewe•r: See borehole televiewer.

.._•kp_•_Cjl_t•ue •l og: A w

"'md e w '•'h a-•fe s i s t a

Used for locating t

casing (because the

thermic and hence r

val s which are prod

expansion of gas as

ell logof temperature,ften

ce th rmometer (thermistor).

he top of cement behind

setting of cement i s exo-

aises temperature), inter-

uci ng gas (because the

i t enters the borehole


26/26

Glossary of Terms Used in Well Logging 1139

lowers the temperature), and the location of

fluid flows (particularly behind casing). The

differential temperature log records the dif-

ference between two thermometers usually about

6 ft apart and is especially sensitive to

very small changes in temperature gradient.

.%enl; old: Sudden deflections of a sonic log

resulting from c•_..__skip_ or tool sticking.

thmer•.].•Oeu•_o_•: A neutron whose motion energy

corresponds to ambient temperatures; neutrons

with mean energies of the order of 0.025 elec-

tron volts.

_il]•jlt.]... deca •_...•: See neutron lifetime 1o•.

•DT is a Schlumberger trademark.

_ii_h•m•lr•_••: See nuclear magnetism

log.

•hjxpt•ro_pi•__c•: property of gels which allows

them to become liquid when agitated. Drilling

muds are often thixotropic.

1;hree-D loQ = microseismogram log = variable

density log' A display of the seismic or

acoustic wave train received a short distance

•-3 to • fl•) from a sonic wave transmitter. See

also character log., cement bond lo•, and sonic

lo__•.•. Birdwell tradename. See Figure 15.

•: Having very 1 ow permeabi 1 i ty.

•ime con_st•LD_iL: A time over which readings are

averaged to remove statistical fluctuations,

as with nuclear log readings.

sonde.

transit time: The traveltime of a compressional

acoustic wave over one-ft distance. See sonic

1•.

•um?.e[•: A microlateroloq, in which the guard

electrodes are concentric about the current

electrode so that the current flow is concen-

trated in a tube which gradually flares out.

•ltralq..•g•.•l•=p••••]•,•,•].9 • = ULSEL: A modified

long normal borehole log mounted on a 5000-ft

bridle. The A to M spacings can be made 75, 150,

600, or 1000 ft. Differences between the mea-

sured resistivities and anticipated resistivi-

ties calculated from conventional resistivity

logs indicate nearby resistivity anomalies.

Used to define the distance to a salt dome

flank.

.v_•riab e. de.nsit...v loe : microseismogram log = three-

• log"' V'D 1 g,"

VDL lo:• = three-D log: Schlumberger trademark.

¾•l.ocimeter_: A device which measures fluid flow;

a fl owmeter.

.vi__..[scome•t_e•zi:device for measuring visc.o..sity.

Also called viscosimeter.

y_._is:go•si.i•__:esistancefa fluid o low;stressi

rate of shear .

water saturation = Sw: Fraction of the pore volume

filled with formation water. See Archie's

formulas.

water cut: The volume fraction of water produced

'' from a well.

_well___• = borehole log: A record of one or more

physical measurements as a function of depth

in a borehole. Distinction is sometimes made

between a log as an entire record which may

contain curves showing several measurements and

the individual curves themselves, which are also

called logs. (1) Wireline logs are recorded

by means of sondes carrying sensors which are

lowered into the hole by a cable. Examples

include logs which record electrical measure-

ments (SP, resistivity, etc.), acoustic mea-

surements (sonic, three-D, etc.), nuclear mea-

surements (natural radioactivity, neutron

logs, etc.), and miscellaneous measurements

(hole size, temperature, etc.). (2) Other

types of well logs are made of data collected

at the surface; examples are core logs, drilling

time logs, etc. (3) Still other logs show

quantities calculated from other measurements;

examples are movable oil plots, synergetic

logs, etc.

wi_r__elin•: Well log recorded by a sonde which

is lowered into the hole by a cable.

W l=• Ree]Jtt_j•o.n•h•: An empirical expression for

calculating porosity qb rom sonic log transit

times:

At - Atma

qb &tf - Z•tma'

where•t : observednterval transit ti•e,

5tf = transit time in the pore fluid, and

Atma= transit time in the rock matrix This

relation workswell in clean consolidated

formations with uniformly distributed pores.

In vuggy formations, the sonic log may not

reflect the secondary porosity; and in uncon-

solidated formations, this relationship may

overestimate porosity. In such cases the

formula may be empirically modified to give

better values.

x•__•o_:ubscript used with log terms to indicate

values appropriate to the flushed zone adja-

cent to the borehole.

Date post:	05-Jul-2018
Category:	Documents
Upload:	seyyed81
View:	214 times
Download:	0 times

Glossary of Terms Used in Well Logging

Documents