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EAS 430: Petroleum Geology

Lab 2:

Interpreting Geophysical Well Logs

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Historical Aspect-Schlumberger brothers, Conrad and arcel, are credited !ith

in"enting electrical !ell-logs#

- $n September %, &'2(, the )irst *!ell-log+ !as created in a

small "illage named echelbroon in rance#

- In &'.&, the )irst S /spontaneous potential0 log !as

recorded# 1isco"ered !hen the gal"anometer began *!iggling+

e"en though no current !as being applied#

-he S e))ect !as produced naturally by the borehole mud at

the boundaries o) permeable beds# 3y simultaneously

recording S and resisti"ity, loggers could distinguish bet!een

permeable oil-bearing beds and impermeable nonproducing

beds#

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ypes o) Logs

a0 Gamma 4ay

b0 S /spontaneous potential0

c0 4esisti"ity /Induction0

d0 Sonic

e0 1ensity56eutron)0 Caliper

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a0 Gamma 4ay

7 he gamma ray measures the natural radioacti"ityo) the roc8s, and does not measure anyhydrocarbon or !ater present !ithin the roc8s#

7 Shales:radioactive potassium is a commoncomponent, and because of their cation exchangecapacity, uranium and thorium are often absorbedas well.

•Therefore, very often shales will display highgamma ray responses, while sandstones andlimestone will typically show lower responses.

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7 he scale )or G4 is in AI /American

etroleum Institute0 and runs )rom 9-&2%

units# here are o)ten &9 di"isions in a G4

log, so each di"ision represents &2#% units#

7 ypical distinction bet!een bet!een a

sandstone5limestone and shale occurs

bet!een %9-9 units#7 $)ten, "ery clean sandstones or carbonates

!ill display "alues !ithin the 29 units range#

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b0 S /Spontaneous otential0

7 he S log records the electric potentialbet!een an electrode pulled up a hole and are)erence electrode at the sur)ace#

7 his potenital e;ists because o) theelectrochemical di))erences bet!een the !aters!ithin the )ormation and the drilling mud#

7 he potenital is measured in milli"olts on arelati"e scale only since the absolute "aluedepends on the properties o) the drilling mud#

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7 In shaly sections, the ma;imum S response to theright can be used to de)ine a *shale line+#

71e)lections o) the S log )rom this line indicates

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c0 4esisti"ity /Induction0

7 4esisti"ity logs record the resistance o) interstitial)luids to the )lo! o) an electric current, eithertransmitted directly to the roc8 through an

electrode, or magnetically induced deeper into the)ormation )rom the hole#

7 here)ore, the measure the ability o) roc8s toconduct electrical currents and are scaled in units

o) ohm-meters#7 $n most modern logs, there !ill be three cur"es,

each measuring the resistance o) section to the)lo! o) electricity#

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7 orous )ormations )illed !ith salt !ater /!hich is "erycommon0 ha"e "ery lo! resisti"ities /o)ten onlyranging )rom &-&9 ohms-meter0#

7 ormations that contain oil5gas generally ha"e muchhigher resisiti"ities /o)ten ranging )rom &9-%99 ohms-meter0#

7 With regards to the three lines, the one !e are most

interested in is the one mar8ed *deep+# his isbecause this cur"e loo8s into the )ormation at a deptho) si; meters /or greater0, thereby representing theportion o) the )ormation most unli8ely undisturbed bythe drilling process#

7 $ne must be care)ul o) *e;tremely+ high "alues, asthey !ill o)ten represent

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QuickTime™ and a decompressor

are needed to see this picture.

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QuickTime™ and a decompressorare needed to see this picture.

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d0 Sonic

7 Sonic logs /or acoustic0 measure the porosity o)the roc8# Hence, they measure the tra"el time o) anelastic !a"e through a )ormation /measured in =-

microseconds per meter0#7 Inter"als containing greater pore space !ill result in

greater tra"el time and "ice "ersa )or non-poroussections#

7 ust be used in combination !ith other logs,particularly gamma rays and resisti"ity, therebyallo!ing one to better understand the reser"oirpetrophysics#

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e0 1ensity56eutron

7 1ensity logs measure the bul8 electron density o) the)ormation, and is measured in 8ilograms per cubic meter/gm5cm. or 8g5m.0#

7 hus, the density tool emits gamma radiation !hich isscattered bac8 to a detector in amounts proportional to theelectron density o) the )ormation# he higher the gamma rayre)lected, the greater the porosity o) the roc8#

7 >lectron density is directly related to the density o) the

)ormation /e;cept in e"aporates0 and amount o) density o)interstitial )luids#

7 Help)ul in distinguishing lithologies, especially bet!eendolomite /2#?% 8g5m.0 and limestone /2#(& 8g5m.0#

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7 6eutron Logs measure the amounts o)

hydrogen present in the !ater atoms o) a

roc8, and can be used to measure porosity#

his is done by bombarding the the )ormation

!ith neutrons, and determing ho! many

become *captured+ by the hydrogen nuclei#

7 3ecause shales ha"e high amounts o) !ater,the neutron log !ill read @uite high porosities-

thus it must be used in conunction !ith G4

logs#

7 Ho!e"er, porosities recorded in shale-)reesections are a reasonable estimate o) the

pore spaces that could produce !ater#

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7 It is "ery common to see both neutronand density logs recorded on the same

section, and are o)ten sho!n as ano"erlay on a common scale /calibrated)or either sandstones or limestoneBs0#

7 his o"erlay allo!s )or better

opportunity o) distinguishing lithologiesand ma8ing better estimates o) the trueporosity#

When natural gas is present, therebecomes a big spread /or crossing0 o)the t!o logs, 8no!n as the *gas e))ect+#

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>;ample o) dolomite o"erlying limestone, as

distinguished by the neutron5density#

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)0 Caliper

7 Caliper Logs record the diameter o) the hole# Itis "ery use)ul in relaying in)ormation about the@uality o) the hole and hence reliability o) theother logs#

7 An e;ample includes a large hole !heredissolution, ca"ing or )alling o) the roc8 !alloccurred, leading to errors in other logresponses#

7 ost caliper logs are run !ith G4 logs andtypically !ill remain constant throughout#

eas 430 lab2

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