. . ... .... . . sFE--f?23-

FORMATION

EVALUATrON

r

Using Compressional and Shear Acoustic Amplitudes for~The Location of Fractures

‘.

R. 1. MORRISMEMBER AIME

D. R. GRINE

T. E. ARKFELD,

Abstract

i:irld results huve shown that fr(tcturecl ZWeS tIICiYbe1,)’at<d hy their a(tetld~nt reduction of acoustic amplitude.Laboratory and theoretical investigations confirm this tech-)Iiql(epbut itlterpretatiotz of amplitude logs is complicatedly the many variable factors eticountered in actual logging()p’.rations.

The acoustic a));p[itlde ittvesligatiolls covered by this[wper were lnade by Cotltinllous ttle&metttettts of the peakaniplitudes Of single, and ~ve[l defined compressionai andvllvur.wave apri~a[s. A S~mul@~eously recorded tneasure-II[mt of interval tr&rft -time or total travel titne, in eachcase, itu!icated ,vjr~tjler or Ilot there had been a continuouswplitude tneasuttnzent of the saint wave arrival.

[nvestigatiotlshave Sijowtz thai the angle at which a!racture platze crosses a boreitole afidcts the attenuation OfWN[srie signals. Theoretically,’ horizontal ftuCtllreS [thePerpend@dar to the axis of the borehole) should causelitl~e or no ittemiation Cf the contpressioilal wave; this is~Otlfirn&d by field e.~at@es. Shear-velocity wa W’S, On tflCother /lalt/f, are Significantly attenuated by horizontal frae-rures.

lVh)le obliqm fractures cause c reduction o} compres-$iottal-}vave atnp[itude, silear-wave antpiittide nleawre-InettIs in’ slich cues ttlay not be GS definitive. Since theMy contpressionai arrivals are not subject tointerferencecntnplicatiotu, as are shear arrivals, both tneasurements~hm!d fse ,nade and Ilsec[ to wtnp!etnent each otiler.

Introduction “,

An hportzk problem in formation cv;lutttion is the‘°Cation of fractllred zones. Because of the rehttively low‘atioof fracture void to bulk volume, down-hole measure-“ttt$of formation resistivitv. acoustic velocity. ordensitk

A Wssible ~oiufion to the probl&uwas noted on early.4‘ sonic [.ogs ~,u~ ifi’hard fornlatfons,’ Cyc!e-skipl>ing, hdi-

- .

j,,~vrly~nd-fi’[lnuSF&~~~c,.e(v~li~-fj&Fety”O~Pet-ml&i Emritwrs dice... ---- . . . .,

SCHLUMBERGER WELL SURVEYING CORP. ,HOUSrON, TEX.

RIDGEFIELD, CONN..1

ENID, OKLA..,

cttting weak compressional arrivals, occurred opposite sus-pected fractured intervals.

z

subsequent developments have permitted recording ,!acoustic amplitude as well as studying scope pictures of Ithe wave train. Several papers have shown th~d,qfefulness ~of these methods to locate fractures, $-:

The process of fracture detection’froms ignalamplitude ‘variation is complex. Laboratory and field studies haveshown that the effects of a number of variables must be iconsidered in the interpretation of amplitude changes.

It is the purpose of this paper toexplorethe use of theacoustic signal to detect fractures, taking into considera-tion the cfiect of these other variables.

Propagation of The Acoustic Signal

A simplified representation of the sonic waves formed :by a source of sotmd in. a liquid-filled borehole is given inFig. 1. Capital letters designate the wave when it is trav- :cling in the mud, subscripted letters when it travels in theformation. A theoretical description of these waves for theanalogous problem of wave propagation in a liquid layerover a solid ‘half space was given by Strick~ The waveslabeled P and St are the direct and Stoneley waves, respec- ‘ ‘tively. The theory of their propagation in a borehole hasbeen developed by Biot’ and they have been observed byPickett? The other waves in the borehole labeled PPP andP,P, are the refracted compressional arriva[ and the shearvelocity arrival. The R wave will be mentioned later. ~

The Refraded @npre83imsd WaveIf compressional velocity in the formation Vp, is enough

higher than mud ve!ocity C the first arrival in the sonicwa~form is a wave that has traveled from the transmitterto the formation’ as a compressional wave in the mud,has -been wfraded- at. the “tioiehde -wall,-+md-has-traveiedalong the wall at the compressional-wave. velocity in theformation. In an ‘inllnite ‘medium, each small pafiicle af-

1

!

;, .

fected by a compre~sional wave osci!kttes onl~ in the di- “twctioh of wave propagation as the compressions and rarc-fisctions trravel ~ast. h.. When a. borehole-. exist%- Panicle .-,motion is more “complex. II

Referrin? to. Fig. - ], as. [he P, wav~ mche; dmsg ~hc .. . . . :. ... . I

-. . . . . . . . .. . . .

,

,

/

,t--

-borehole wall produchg an axiid particle motion, theformation also movcs out into the morp compressible mudm the rock ~s compressed, Luxi LIwtsyfrom the mud asit is rarefied. This radial particle motion generates II shearwiwc h-s the formation P,,. and, more importantly, gen-cmtes a compressiontsl wave in [he mud, PI,P, This las[is the conical mud wave which is the first arrival @ soniclogging if cornpressional vklocity in the formation ishigher “ihan in fhe mud,

Fig, 2a shows the ‘ray ptith of the first comprcssionid-wave arrival, P,,P just described. Part’ of [he energy of the

‘mud wave P is refracted at level r forming P,,, whichemerges at level I as P,,P, the first arrival at the reccivcrR.

Mos~ of the @ergy of the mud witvc P k, howqver.retlected from the hole wall at the critical angle as PP(Fig. 2b). Again, pa~t of the energy of PP is refracted atlevel s as ray PP,, which emerges fronl the formation Wlevel t as PP,,P, which reaches the receiver later than P,,Pby the “first reverberation delay titne~’ w shown in thefigure.

Another compressiontrl-veloci!y arrival having the samedelay time k shown in Fig. 2c. In this case, part of theenergy of Pp emerges at level u, forming P,,P, which. inturn, is reflected at level t, as P,,PF’, to the receiver.

The arrivals of Fig. 2b and 2C are the main contribu-tors to the “first reverberated arrival.:’ Attenuation of thesound in the mud, and distortion of the signal in reflec-tion from the borehole wall and passing through the sonde,will lower the amp]itude of these artiva]s. The arrivalsin Figs. 2b and 2C wifl tsereforebc individually less inamplitude {than the first arrival (Fig. 2a). However, sincethe ‘arrivals in 2b and 2C are simultaneous, they are addi-tive and their sum may be even linger than the firstarrival.

Borehole *G (j

‘c

/c is mud velocity -,

Vp is formation compressionol velocity#

ys is formotian shear velocity

P“ is the direct mud wove

Pp is the, refrocted “compressianal wove

PS is the refr-crcted shear wove

R is the pseudo Royl&gh wove

St is the Stoneley wavti ‘.. . . . .. . . . . . . . .,. . .

The rcvcrbcratcd arrival by caCh possible poth ~dn ~vthollght of us rc,$!fed to iI f\~CllOLI T of the fjrst ~r.riviil. Since lhc )1 rcvcrbcrat!,on has, (J1 ~ ] ~ possiblepwhs, the amplitude of the “n the rcvcrberatlon is ~PproXinliltCly (11+ 1) T“. The fo{mula does not quite... .apply smc~ the ChStOrt:On m t@CCllOrJm from the borehole~,all, and m transmwslon, does Stoi give ~ simple coc~. ~‘cicnt 2“. However, ~hc formula ~ocs roughly describe therevcrbcrat ion LImphtudes. Multlp],e Conlpresslonalj rCvcr.hcrations frequently fo:nl the rilLijOr pOrllOt’t of Ihc no~~cbirckground for the ~mvill of the rcfractcd, shear ~,tlvc. ,

The !hmr Velocity Arrival]<eferring again ,to Fig. 1, if the shc:r velocity in IIlc.

fornlation V, is hlghcr Ih:n ~~ud veloclt y C the Sccol)tiiIIlporfant arrival. IS a comhl nat Ion of ref~~cted shear ~Vlil~and Stricvs pseudo-R ayleIgh w’avc. This combinat ic>llifgenerptcd as a confprcssional wave at [he tran\nli[lur,and trii\/els up the borehole wall at hearlY the vclocily ‘}fi shear waye in the formation. The .refriscjcd shcw U;t,cproduces a’ particle motion ~hich M radtal in bor~}lt,ic

gcomelry. The pseudo-lbybgir W~VC }Ias out-of -ph:i~vradial and axial particle motions in h formation, “l’hc,ctivo waves have such simiiar velocities that they urriic

too close to one another to be distinguished. In this pilllt.l,they are hereafter sunply cailcd the shear-vc]ocily ~1,.rival. The strong radid Pmlicle motion of lhc SIW,U.velocity arri vaI produces a compressiorxd wave in I II(.

mud, designated on Fig 1 x l’.~. 11 cm be dctcckl lUthe receiver, although this is oflen di~cuh bcc;llI~L,,,arrives along with the kter comprcssionirl arrivals.

TJte Use’ Of Compressional AmplitudeIn Sonic-Arnplifude Lagging

An cimly piiper on sonic logging dischssed tiw indit.,tion of amplitude reduction opposite fractured inlcr~.tl.Fig. 3 is an example of”a Sonic Log run through linv.~t(uwCycle skipping, recognized as due to weak comprusi~m,,tarrivals, occurs between 10,10(I ami 10,200, ;Ind i! i\.l\assumed that the skipping was caused. by frac[urc~.

“k\

—D—

/PpP

R

(al

.

r

‘P ~

PPp

t t,.

f

/

.—D

PP

“(b] ‘

x

)“P

f

—D’

f’P

u

PpP

PpPP

\R

./

.’

4 “,

(c-) j

1. .,.. . . .

Pps is the formation shear wave dragged”by Pp-? .---” .,-.:’>..- ,. :’-.

. ,1/2

PpP is ihe mud wo;e drogged by JPp”

,11

,.:, ‘“’, “

P5P is the mud wove dragged by ps,0 FIRST REVERBERATION DELAY TIME= D ;2 ;Z

P.,

. . . . . . . . . . . . . . . . ------ . . . . .. . :.. .,- . .... .. . . . . .. ... ....-. .-:. .s-..

,

,._-. .

,“- ”-” 1,,

. .,. . . ..1 . . . . . ..,” -- “---– -

I !]< dcvck~pmcnt ot’ amplitndc mcnsnrements for the{ ~illctlt Ilond I.0~ hd ;11s0‘Io the invcstig~tio[l Of ampli.!:!LICrtcording in open hole.. The open-hote Sm-ric-Anlpli-!I:tlc I.og rccorck the ilfllpIltLldC d an cady significant,.,un[m’ssimxd arrival. i.e., the second or E,, p~ak.j:

(XWLsf the first successful itpplicatirms of the Iog \\,asIII Ilw location of fractured intervals in the shallow chalk.L. rib pf north Lollisiantr. Fig, 4 is an exiunple of 1!%,S,,[liu AI, :InLI Sohic-Anlplitudc Logs reccmdcd opposite uL’hdlk i[lt~rvid. Reductions in illllplilllde ift-c noted over/,qIcs A and B. [n the same intcrvds. the tmnsit time is

I,w ~ indicating little. porosity,, The zones wire interpreted.,. cimtainirrg fractures and were perforated, resulting int(ti[hl production flow of 252 BOPD on a 12/64-in, choke.Ihc low wnplitude between 3,2!30 and ;3,300 also indicatesri)~~iblcf~lctllrcs, hilt the interval was not tested.

Smaller tunplitmie changes w-e interpreted’ irs irnonmlies‘ J thin forrnatiot~ discontinuitics. Comparison of the am-

I, IIIWICvtmiations with the transit tirnc and/or resistiv&.,.“urvc’$is USCd to dtfferenticde between the effect of fra&.-

IIircs i[nd [he ctfect of other formation variables. A de-crtmc in amplitude corresponding to it IOWsonic travelfme m high resistivity generally indicates tfvz fritctured/,1[11-.\.

:\ [Llcil[ study made ef the kfississippi limestone inwwhcrn Oklahonla compared core descriptions and pro-(Iuction results with compressiona[-a nlplitude changes ob-~crv;d over the fornlation. It was established empirically,u itb,in a given area, that amp]itttde reduction to 5 mv- orIcw indicated (he presence of a productive fracture system.()[hcr mnplitlide ~changes were found to be indicative of

‘ln;i[ri~ changes, Iithologic discontinltities, or minor non-[’rOLlllCtiVe fracturing.--—‘(1 f:: ),c~:k is the sccomt half-cycle uf I%Soni. ~rri,ql: E.F,, ,L~OmP,=,-,<m:llk.zLWnk k sem .as the. fi r.st positi>.e Imik on thL. W:w’ernlnls of

I ‘:<. s.

‘SP I 1’ SPAN ‘3, SpAN’

75 *S I

B

---3%m-:YCLE SKIPPIA dENTtJATE

s—~

?

s

,.,

/

2

. ..4

,,’

40 mv.

-;

. ..__-,... ___ _.

.Y.....

iiwtir{~sJinwwtotle:. (A). I. ft.sFn,tr-jft ~p~j~ with iof(,nticfif~tlly il~-.. ,.—.. .... . ,., ctmtnjttc.1 cy&.9kipping.:W .-...... ;.?;:!l;%~. ~wg:+:,;‘-: . . .“- >:- .:M..-. -- ●. .-. ..-.. -..,>, L. -,--- -. .. ..-. .--:. . -..-...-’:. .. . . .. .. ...- :.. ,.—.- ...-. .--: V.-

A lug from this arco is shown in Fig. 5, The sonic-ilmplittldc and singie-reccivcr trtivcl-tinlc curves were‘recorded simu Itaneous[y, The : iattcr recording serves ttsindicate iithology chirnges and also monitors tool center-ing. The sonic transit time irnd gamma my curves areused to recognize and discount the effects of iithologicnnomdlies other than fractures on the amplitude curve.Zone A is described hy cores as cherty iimestone withextensive, pirrtially open. steeply inclined fracturing andstreaks of porosity up to 15 per cent. The cwnp:essiondiamplitude is greatly reduced by such a fracture system.The transit time indicrttes the presence of porosi~y, andthe gamma ray shows the zone to be clean. Other mnpli-tude variiitions follow iithology changes.

These examples show, very briefly, how the Sonic-An~pIitude Log has been successfully interpreted and usedto Iocat,e fractures in some areas.

It had been hoped that, the cornprcssionai arrival corddbe used quantitatively and universally kt fracture Iota’tion,primarily because this arrival is the cleanest and can bevery accurately recorded, both in time and amplitude.However, investigation has shown that” while the. conl-

T pressionai-ampiitude measurement is necessary, it alone isnot sufficient for location of ai i fractured zones.

m! IND.RES.1

‘o Qt3ms. ICOMPRESSIONAL AMPLITUDE

SP ~.- m“ NJ 40-------------------------- ISONIC At

+ I50mv

1

IOQ

1

us 50

3200, ;

A ?zx=?

T-

%

, .

.

~3300~

~g. *-% exumpfe sitowing indttctiorl rcsistivity, S.P.,Sonic-At, and SOnic-Ampli!nde fags ill Frurtwresi %ratognchaik, north Imuisirma. ‘I%e WCIIis prmftlcin~~ from zone~

A and B where lower trrnplitndcs indicaie frnr[ure zones.

— ,. . . . . .. _—..._ _ . ,._._-,__ ..._____ _ ____.—

SINGLE RECEIVERTRAVEL TIME COMP AMR - SONICAf

‘1i.,.-.,...=-—I

1

,. q,. ...

[ ).I .. .. ...i.“” .,, --- ,.~.,

<z

) ,, / ,,

-. --

,,/’ !’

T

./-- ‘ -’!:”---- --;, ,,.~.I%sclurcs

“,.f%cturcs in the kh-mtion reduce (1IC conlprcssional-

r&d shear-velocity arrivals in three ways, First, they aclas thin beds filled with a substance with a strong impe-dance mismatch to the rock. However, interference of re-flec~ions from the two, surfaces produces good transmis-sion of a compressionral wave normal to the fracture ifit is thin compared with the wavelength of the signalin the filling fluid, For exsunple, experiments have shown[hat a 1/16-in, oil-filled crack reduced the amplilude by10 per cent for a 25 kc signal in aluminum.

Second, fractures can affect the’ rugosit.y of the bore-hole wall since chips may be broken from the wall whenii rock drill penetrates rt frac[ured zone. Coupling the bore-hole signal to the forriation is reduced by rugosity becausesignal compon~nts rea,ching ,the boreho!e wall at differentazimuths will arrive &tt-of-phase at the receiver. In addi-tion, each c~pped area in the borehoje wall prodtices a‘diffracted wave which interferes with the signal. Chipswith depths only one-tenth of a wavelength of the signalin wa}er (or %r in. for a 25 kc signal) have been ob-served to alTect signal ampIittide.

‘The third way in which fractures tiect signal ampli-tude is by permitting movement of one side relative to theother in the plane of the fracture. Knopoff, et al.’ calcu-lated curves of transmission coefficients for plane com-pressional and shear waves across an infinitely thin lubri-cated crack in an infinite medium. Fig, 6‘ is taken froinKnopoff’s curves for a medium with a Poisson’s ratio of0.3, which is a typical value for hard carbonate forma-tions. The refracted compressional wave in borehole gco:metry should be similar to a plane compres~onal wave.

‘ The effect of a frhctdre on it should be similar to Knop-“off’s calculated effect of Fig. 6.

The shear velocity arrival should “show a transmissioncoefficient which waries with fracture dip but may ,be quite

,

.

Lo --b

.9 k

COMPRESSthNA> \ ‘,

/

1 , “,6

\

.7DIP POLARIZE D \ f

SHEAR- 9.6 -

\k ,/.5

.4

\\----- — .. ......

I1’

.1 ... /

‘o..:,() .- .+(). ~~ . ~fj., - ~~. so . Go.. ?0 -: so gl-j ~~

DIP OF FRACTURE ( degrees )

different from the curve for dip-p~larizcd shear shown!in Fig. 6. The refracted shear wave m borehole geometry

is radially polarized. This polarization correspond to cir- ,cular polarization. ‘of A shear wave in an infinite medium,sd that half of its energy could be regarded as dip-polwiz.cd and half as strike-polarized, Knopoff et al. show thata strike-polarized shear wave would be totallY reflectedfor all fracture dips. The refracted shear component ofthe shear velocity’ arrjval would therefore have a lowertriuysmission coefllcient than that for a dip-pol?rwed shearof Fig, 6.

The pheu~o-Rayleigh component has u different p?u~clcnlotion thah the refracted shear wave. ~ts transmissioncurve is ribt expected to agree with either curve of Hg, ~6. However, the shear velocity arrival, because it has astrong radial component, was expected to’ have a trans-mission across fractures of the same general shape as thatshown for dip-polarized shear, It should be poorly trans.mitted by nearly horizontal fractures,

Therefore it is indicated tha{the amplitude of the shear. ,velocity arrival should be more diagnostic in fracturelocation when the fractures are at very low or very highdip. Conversely, the amplitude of the compressional at.rival should be diagnostic when the fracture dil arcbetween 33 and 78’. The presence of exactly vert,iidfractures should not directly increase the attenuation ofeither the shear ~or compkessional arrivals. If the verticalfractures lead to rugosity of the borehole wall, the sigrt:dcould be reduced by coupling effects. i

,.~”Model Slndim on Effects of.Fractures on the Sonic Signal .,

\Laboratory experiments were made using borehole

models wi(h” cuts or “fractures” intersecting the ~oreholc !

horizontally,. vertically, at 45”, and at 60°. These CX. 1

periments were made to check the attenuation caused, byrklative motion of fracture faces, This &rtechanisnl \~ii\ {.regarded as particularly important because it producc~[he ‘only uniq”ue effect of fractures on- the sonic sign;lt,A thin bed tNled with a s~~bstancewith a strong ,inlpctkim-cmismatcl,t may be a silale stringer, and borehole rougbnmmay be. caused by ‘features othep than. fractures, “

Small blocks of Indiana limestone, 5 x 5 x I z in., will)9. ‘per cent, porosity and a Poisson’s ratio of 0.3 WNrvacuum saturated with’ water. Anlplitud~s of the COMpIU.

sional-’ and shear-velqcity arrivals were measured by pli\<-ing the receiver at various distances frc}nl a fixed sourwlocated in the bottom of a J~ -in; or 1-in. hole drilled nlon;’

the center’ of” the 5 X 5-in. faces. The bIocks were Ihm

Ctttat various angles to the ho]es; the matin~ su[fiic~~ Illthese artificial fractures were ground; and th~ blocks (vtrc

resaturated and as~emb]ed with the fractures shinrmdopen 0.004 in. The anlplitude runs were then rcpcaic(l i!)

_ ..th~ .’ffracture& nlodels, The 20ft.kc.peak of .thc spcclr!l!i:.of the signal used gav~’ wavelengths of 1 in. in the r(lck

A 5 X 5-ft cross-section Jndian~ limestone hlocli Milll..a horizontal 0.(J4-in/ fracture intersecting its 5 in. di~lfll.,eter borehole gave lhe same result as the +nud I INm.zontal fractured mode]s. Therefore, the results fron] Il},”

~small hodels should scale to fll]l-size boreholcs.Tw~ ‘@&~tS ~,ere .o~~erv~~- ~-n’~ttenuatio~ of -t])&J’I ~~

rivals, varying with’ djfferent fracture angies; and an ill 11.1

i

..1

.

,

( Poisson’s ‘Rdio = .3r) ) ference -at th~ transducers evidently pr~duced hy a w,.1}~r diffracted from the intersection of any fracture with 1111”

Fig,. 6—Calcul@ed curves of transmission voe~lcients fnrplane compression+l and plane -djp-polnrized -shear yay,es... . borehole. W.herf the amplitude runs were rcpc~tcd ii11(”:

... . .at%ws nrr irifMieIy Ihfn Iuh-ficated crack ili .hii”-fb finite .“

7teariti~ dOwti --and ‘reassembling the. pieces of w krd II‘1>: .- . ~-‘

,, medium (Knopofl, et al,). model, the htterference “effect was’ d ~ffIcu1t: to r~prt~~lll~~ ““’,--’

,.. !. . . . .-. .— - . . . .. :..,. ____ 1-! ... ..- . . . ,. ..__ . . . . . ---‘,1 ‘ ,. --’ --~ “ : /

IJ I])&!Inolkw crc ilSS!Xlblt2d’ “with ~arc, the fr~cturcs

,Il;ldc thin, ,mf the borehole smooth. [he interference],~~;inlcS,nmll.

., i ~..= “I”}wrcductiott in ill)lplitLtdc iS&rossufructure Wasmcrrs-,,,r,.,1 hv commsrinr? wm[itude runs acrOss a fractUre (withi

t)

f’

I‘-

.,

i

,..--. -,—..I!Iall interference; wi~h those made before the’cut wasJ)EKICi:~ the rock. This comparison crmcels out any ef-.-—. .lccl~ of inhomogene[ttes m the originat model or of change,11IIN absolote”signal level,

!$L)llle of the experimental results are shown in theIwst three figures. Fig. 7 shows the waveforms from ulrmsmitter-receiver pair, spaced 2 ft iipart, as’ they weremiwcd pm.t a 0.04-in. horizontal fracture in a large lime-O{UICmmlci. ‘The shear-velocity rrrrival is seen starting.([ three divisions ,in the top arid bottom pictures, when[Iw fracture was not between the transducers. It cannotIW seen in the center picture; taken when the fractureJ\as between the so-uu~~ and ~ecgiver. Fig. 8 shows theclrwt on the early “compressional arrival of a 0.004-in.Ir;wture dipping ut 45° m the’ receiver was moved pastL[in one of the smaller models. The first two half-cycles\\crc reduced by 40 per cent. Fig. 9 shows plots of com-prmsionii[-arrivttl amplitude vs distance on repeated runspmt a small-n!odel section.before rrnd after cutting verti-UJ1 (90°) fractures. In this series Of experiments noulticiure~bk effect was seen. The section of the modelwith vertical fractures was bounded by horizontal cuts\\l]ich attenuated the shear-velocity arrival so that theMm of the vertical fracture on this arrival could notIw determined. The attenuation of each’ arrival, -for their;wxLlre dips investigated, is listed in Table 1, Npte thatLIWSQresults show that fracture dip has~an effect on theIr:lnsmission coefficients of the conlpressional “and shear\dccity ar,rivals, Neither arrival agrees with either curve..

..

ABOVE FRACTURE

t

of Fig. 6, ksut thecrimprpssiontil transmission coefliciwrtis Pairly close. The shear velocity arrival sho~~s “lowertransmission for lower &lps.... .

Field Studies of SiguUl Arrivnl ,inqdi~udc~The above observations led to frcld studies of both ~

compressional- and shear-velocity arrival amplitudes, Thesestudies have been made in wells \vhich \vere cored and in\vhich core _analyses included descriptions of fracture ,orientation to”the borehole. Fracture orientation isdescrib- ~ed as either horizontal or kertical. There seems to be noclear-cut agreement as to the magnitudes of the rrngles ofinclination in classifying fractures as horizontal or, verticalfrom cores. However, fractures inclined at angles greater -than about 60” would most Iikely be classed rts “vertical”:T,hus,, many of the fractures classified as “VertiCtLl”, underthe discussion of field examples, would still be within’the range (Table 1) where the. compressional amplitudewould be reduced.

The examples include amplitude readings of both thecompressional and shear-velocity mrivals. The compres- “siortal amplitude log records the amplitude of the secondhalf-cycle of an early compressional arrival (seen in Fig.S as the first positive peak in the waveform~. The shearamplitude log records the amplitude oi the second half- “cycle of an early shear velocity arrival (seen in Fig. 7 asthe positive half-cycle at three and a half divisions), Asimultaneously recorded trave;-time ci.uve, in each case, ~measures the total time from T:, to detection of the nleas-ured signal. .,.

The interval transit-time or tota[ travel-time curves ~shown in the examples record compressiorml arrival time.’ ‘These curves are helpful -in estimating bcd thickness. ,’ ~

In addition, \vhile formation acoustic imper-krnce @ a ,func[ion of both velocity and density, ‘it has been observedthati the time curve often permits a rertsonable evaluationof the impeduuce contr&t from one fqrr!mtio~ :o. another ----~

..

when a density measurement is not a’vailuble. With littl~or,. no change id velocity, hence in travel time, througha recorded interval, one may expect little change %acoustic impedance through the formation, or across theformation boundaries. Therefore, the possibility of”ampli-

.

RECEIVER

;.:

AT

,<

I2cm.

CRACK AT 11.5 cm. -,

RECEIVER AT’’’/iCm.

... .. . . .,--- ----

12 #s/cm. AND 2mv.)crn.’ -.

Fig. 8--E~perimeutui rewth sItuwiiIg tlh~ effert ‘on the’ &II. -plilndo Of+lhc Cwnlprc!!siolmi urrivul of is.0.00Lin. frl@U~{:

$ d@iug..Ut. @’’:.. ..,. “ ,:. ,.., .. ....+ ,..- ..: . ...m. / :.-’ ..-. .

. .. .. ,,.. .&-.. ... .-. . ;!, .

.,/. . . .’ -.. .

f,.

,.4

TABLE I—ATTENUATION OF COMPRESSIONAL AND St4EAR ACOUSTIC ARRIVALS

., Fracture DIP CompvMonal Arrival Shear ValocltY Arrival.-- —-. -.—. —

Atlenuatlon “’—--”-T&@@wlTten$ml;siori - “ Afi”Cn-UatIDn(db) . $Q~~;!!q_ — ~) Cootndont

o 0.0 ::s4.5 0.6 8

%“ 0.4 2 0:890., ::: I.0 ,_

lode chunges tfLIc 10 such variables is reduced. If VelOCitydoes vary, then amplitudes could also be affected by theindicated Iithological changgs. depending on the velocitycontrast from one hcd to another, the bed thickness indi-cated, and the tsbruptncis of the velocity chwsgc.

In. each of the following examples, the time curveindicts[cs ]i~t]e,velocity change. Thus,. any 8rOSSanlplitudevariations should be more a function of fracture effectIhwr of the previou..ly. mentioned variables.

Fig. 10 shows both cornpressional- and shear-velocityamplitudes recorded over an interval of Mississippi lime-stone, The single-receiver travel-time curve, recorded inthe left track, shows very Iitile change in velocity through-out the zone, discounting the possibil~ty of formation orboundary effects’ on amplitude. Cores: fron~ Zones ~ aredescribed as containing long, single, “’vertical” fractures,mostly healed. There is a small compressiordl-amplitudereduclion in these zones, but no pronounced reduction inshear amplitude. Zones B are described. as containingpartially-open horizontal fractures. The shear anlplitodeis reduced to half the inaximum value in both ‘ktervals.With no “vertical” fracturing present, the compressionalamplitude increases in value. Thus [he fracture orientation(from core description ) a~ects the arrffi]itudes in the,manner expected.

A short cored int’erval shown in Fig. 11 was describedas limestone, clean to silty and shaly. The transit-timecurve shows a change in velocity between the upper andIower half of the cored zone. The change is gradual, how-ever, and-rather thick bedsore indicated. Only small, ifany, effects of formation changes wouId be expected onthe amplitude curves. A fi-ft section of core was describ-ed ‘a.wconlaining natural, partially-open, vertical fractures.The compressional amplitude is reduced opposi[e the frac-tured zone hut the shem’an~plitlide khows no reduction.

In Fig. 12 the transit-time curve intiicates uniformvelocity from 6,4S0 106.520, whereas variations in velocityfrom 6,520 to 6,540 indicate thut formation effects ma}’appe,ar’on the mnplilUde curves over this in[crval. A ,Gam-.ma Ray-Neutron, not shown, confirms that the velocity

._r >

34

1

,-

.30

11:”-“”’~BEFORE CUTTING ‘+

$26 AFTEU. CUTTING A A-

AFTER CUTTING o 0,

;2,’ AFTER CUTTING K x

k*Q

.,if~la .8 ,

** ‘ZREARC?SfkPY ~ Q...?~ 14“ %.. >

\/

C]ltlllgcS frt>lll 6,520 to 6,S40 ft arc ~LLc 10 chunges jnt’onllillion porusity. A 46-ft cOrc f~oIM $4~~ kt 6,S34 fl\V;IS descrihcd as contuihing ,prlrnardy hormmtid fractur~,~nd a plot of the nu,mber of fractures Ip,foot appeunin the ]eft track of Fjg, ]2. The. cmnp,remwtd and Shearunlplitudes can be coenparcd wlt~ this plot. .There is ~marked reduction fronl the maximum values m shear.velocity tunplitude which corrclat~s falrlY well with frac.turc density. These fractures do not affect the COUlprC$.sional-tmlplitmlc curve. :

Fig. 13 shows thc”comprcssional- ‘and Sheitr-velocilvamplitudes rccorde~ ‘thrOUgh a portion of the,,osage n~cll,.

SINGLE’ RECEIVERTRAVEL TIME

00 ‘ ’20c~

‘)

B’

A

.—B

)’ mv. 500------ ------ -

.s

.l;ig. 3O—Cojnp~essionsd- aud kkk-vela~i~~ ywlit~l+ i’;fractured Mississippi Iinmstonc. ‘ZOIICSA cont:un IOIW,WIgle, %ertiml’? fractures and Zones B eontxin I,[,rin,ltto I

fractures. .,

: COMPRESSIGNAL AMPLITuDE “1

o 40’

SONIC At —~— 1SHEAR VELO:&~V AMPLITUDE>70 JIS” 40 0 Rrv.----- --------- ----- ------%.

. .

:/ T ‘<:--- - ‘ f

---●- -

..- -,

--,-. --

4 ~ ~...---, -

-“=-a--: ----- --

- -t,..--r - -. - ..- ..- . .- ,-, —“; % .’

Cored ; .!.

,.. 1!

;

,,.;

,!

.-..,.1

. ..-.

f.

,’

-- .’. ..-:---

. .

.

—.. ....,..

IWr d the Mississippi lin~estone. The sonic trnp~it- time TABLE 2—SHEAR AND COMPRESSIONAL AMPLITUDES COMPARED WITHFRACTURES (EXAMPLE OF FIG. 13) I

,~( the compressional arrival is in the left tritck; it irtal- fracture Amplitudes

C:itcs thick beds with little velocity contmst in euch bed inlerval ~Or[enfOtlOn Shear Cc.mwesslonnl

LIIKI l:lrue COn@@ between bc~s. There +OUld4 be someA horizontal 00 low ,highB fe; s%~;dd high high

change ~rt”the amplitude levels from one formation to, an-‘Jthcr. However, even in thk case large variations in the

:, H and V I*W lawvertical (VI hish 19W

VIII pli[ude should be due primarily to fracturing, iexcept - none . M&: . high‘F

in the thick Woodford shale, section where the shea~,many verticol Vwy iaw 7

velocity i~rrival practically disappears. The shear and corn- ~.orcssioniil, amplitudes are conlpared with fracture desCrip- may have significant effect. on amp[itude are briefly dis- ‘,..ti~)n from co;es in Table 2, T“hese examples tend to cog- cussed.Iirm a selective effe$t of fracture orientation on the corn- ‘prcssional and shear amplitudes.

J

,,

Other Factttrs Affecting Sonic Amplitudes

The cornpressional- ,arid shear-velocity arrivals are af-fected by many factors other than fractures. These in-chidc tool ckmtralization, instrumentation, mtt~ properties, ,,hole size, geometrical spfeoding, formation-to-mud im-pcdwke snatch, borehole rugosity, attenuation of sound:velocity gradients and bed boun~ries. Tbe factqrs which

HORIZONTAL GOMF! AM?FRACTURES/ Ft. o m v. 2’2.5 ‘

SHEAR VEI..AMF! ,SONIC At

8 6 4 ,2,0 g mv. 40----------/’ )

,

.-

(<:..

655 0!- .-3 ?

‘1 .--- . ,,., -----. . (

k’i~.12—ES~mp[e Sl,oWingtllecoqrela:ionof shenr-~eloeity

:lfttplituderedllctio~ with nunlberof horizontal frmtures perfoot ill lim&t~ne.~*e fr*~ture~ donotresince thecOmpres-

‘! siomsl ampli:ude%

.. ,,,

COMPR5SS10~AL AMPLITUOE ;g l-w. 20 40

SONIC A?SHEAR VCLOt31TY AMPLITuDE I

70 ~= 40

)A ~-----r

.,

I

. . .- . ___.,

------------ . . .‘. .

( j“ )

Poor tool centraliziition reduces” amplitude because com-ponents of the signal leaving the transmitter at different ~azimhths have different travel times to the receiver, These ‘components ,then add out-of-phase with one another to ‘reduce amplitude. Fig. 14 shows a plot of compressional-arrivaI amplitude vs the distance eccentered in. inches.A 50 per “cent reduction im amplitude occurs when the ~ j’tool is eccentared by % in. Hole elfiPicitY maY Produce 1a similar effect. 1 .,’

Different muds change coupling of the signal from ‘mud to formation and back if Ihey have different acoustic. !impedances (i.e.’, different products of density times sound,velocity). A more important effect is the attenuation ofthe sigrfal by a gas-cut mud, when the gas is presertt inbtJbbles. The low amplitude caused by the bubbles willprobably not repeat because the bubble distribution usual- , I

Iy changes with time.Hole size affects amplitude in two ways: (1) the longer ,,, ~

the path, through the mud, the greater the Ioss of ampli- ,tude, and (2) changes in curvature of the mud-formation ,interface affect the focusing of the signal. .

As Iithology ch~pges, so do the formation-to-mud im- . ! .pedauces and fhe transmission coefficients. “Since these iaffect the compressional- atid shear-velocity arrivals acrossthe mud-formation interface, any Iithology variations mustbe considered..

Coupling of the borehole sigrtqb to the formation is’reduced by rugosity of the borehole wal[. This effect hasbeen mentioned previously.’

Radial gradients of velocity are caused by the” drilling .process and by inviision of the’ formation by mud filtrate,Velocity gradients affect amplitude by refracting sound ~

-either toward ot away from the borehole. For example,invasion” of a gas zone by mud t?ltrate would raise the

.: .+..

. .

_LY-“L ‘“,.

r ...-. —.. - ----- ...4 : l“ ,, Cl l“‘ ;C:--: -.

. . .

DIST&NCE .ECCWERED.

.“! .’- Fig.. 14+lmi~i~&i[}ikd nrri,voi t@]imdc vs dle distmuw! itl--, iuches by. whicfi a iuol R eci%witctied in flpmr;lloleo --- -. . .,

f

,,., ,. ,.- ... .... .. .. . . . .. . . .. . . . . .. . . . .. ....’.? .

comprcssirxtal velocity ncrrri the hole. The vhcily wuuld

grirduidly drop Jo the vck?city in virgin formation outsidethe invtrsion zone. Such a velocity grtrdicnt will rcfrisctcompressional energy awrty from the borehole resulting inan apparent high atteinration of the cornprcssionrd arrival.

Bcd boundaries lower the qmplitttdes of the compTcs-siorml- itnd shear-velocity arrivals by reflcc[ing part of thesigrial. Both acoustic impedance and hcd thickness arcimportrmt in detern~ining the amount of signal transmitted.The impedance contrast across the bounckwy determinesthe transmission coetTrcient. Trsrnsmiision across a thinbed, such as i shale stringer, is also affected by interfer-ence of reflections from’ the two. interfaces and thus bythe thickness of the bed.

,.

Assuming that there is good tool centralization andthat, the borehole remains’ at bit size, the effects of bedthickness and changes in acoustic impedance across bedboundaries arc the factors which are of primary inlport-ance in amplitude studies, / /.

Further Fichl Examples of “Litholbgic and Fracture Effects

Fig. 15, represents a complex picture of both lithologic’and fracture effects on the amplitudes. The transit-timecurve shows time changes of 55 to 58 microsec. Sharp.changes between the two values occur at short intervals,suggesting considerable interbeddii-rg with velocity contrastand thin beds. These conditions .shotiid effect some re-duc[ion of the amplitude. The curv,$ shows less cent rastfrom 7,740 to 7,760 and 7,820 to,,$,860.

Assuming the effect of fracture orientation” on the ampli-tudes, the shear-velocity amplitude indicates horizontalfractures in the intervals” from 7,?50 to 7,784, 7,792 to7,802, and 7,814 to 7,82$. Sorn’c “vertical” frrtcturingshould be expected as shown by the lower cornpressionalamplitudes between 7,?80 to 7,818 an-d 7,826 to 7,860, Asindicated by the smoother time curve, less formation efl jetis expected in the latter interval, ..’-

In an analysis of the logs it would appear that bothhorizontal and “vertical” fractures are indicated by theamplitudes, with the fracture effect superimposed on) theeffects of other formation discontinuitiei. Core -desciip-‘tion verifies this interpretation to an acceptable degree.Considerable interbedding is indicated from 7,760 to 7,82o.

‘CCW AMR ‘ PARTIALLY0 mv.

SONIC A?O PEN FRACTURES :-’

SHEAR VEL. AMi? per Foot

E:2:470 .lrS 40 O-.---?K---9QQ f,g’o

--f-.. ------------. ----; --

-.:---.<.-.-

*/ ---’:-

--: ‘ ,7800 -------- ,-

-——... > -.------ ,.

“-m ~“.;.

Sihsionc predominates. will] )hin hcds Of Sihy, shalyIimc, shale, and cherty sihstonc. The, Intctvid f~onl 7,@to 7,860 is dcscribcd m sihstonc ~lih some, ltnlc inclLl-sions, A comparison .belween [rttcture descnp:ion fronl,core analysis irnd the mtcrprctutlors of the relat]vc amPli_ ,,tudes is shown in Table 3.

These corrclitt ions of interprctatiorr with core descri~ ,tion indicate thiil the conclusions. misde frotp log exan}i- Ination arc essentially corr,ect. It WIIIhe no!ed that no Cc,n. “, gclllsions are nlade regarding fmcturc dcns!ty and possibleproductivity of the fractured begs. There IS no p~rti:ulilrcorrelation bet wcen shear amphtude a:d -the .itumbcr “fopen fractures per foot from core description m the horj.zontdlly fractured intervals. The sam~ remark holds forthe compressifonal ampli!ukie. @ the “verhcal~y” frilc[tlredzones, The superimposed effects of other” formation di~.continuities and possible coupling changes are. no do~tht,factors in this lack of correlation. ‘

fikewise, and poisibly for “the wfrmereason, the a,,,~t;...........tude values cann~t be related to ‘possible productivity. The ,,.interval shown in Fig. 15 and a lower zone were u3~.~.forated and treated. E~en with the relatively extensive ~r~i. ~ ‘turing indicated by the logs, only a small amount of ‘)ilwas recovered on swab, and the well was finally CO1ll.pleted in another formation. .1

Fig. 16 shows, in another. well, the same fornmtit}llin~ervsd..shown in Fig. 15: The transit-time curve ;ig;Li,,indicates considerable interbedd ing. but with more v~loc.ity contrast. Cycle-skipping occurs from 7.850 to 7,S.$7.From these observations. one should expect cfi’ccts frolll ‘both formation-impedance changes and thin-bed rctl,.c. ,,t ions. Horizontal fracturing is suggested by lower ~llraramplitude from ‘7,7S4 “to 7,860. Scattered “\-ertical”” fr;[c.tures are suggested by intervals of reduced cpmprcssioa:ilamplitude between 7,770 to 7,832. More extensive ‘“vcrti.cal” fracturing is indicated by very. low comprcssiofl;, I

amplitudes from 7,846 to 7.85S and from 7&60 to 7.x11(I.Lithology is described by core analysis as h:lvin: ;I

Ihigh degree of iriterbedding. Limestone predon~ina[m. 1$iIII ~thin beds of shaly, silty limestone, siltstone, SIXIIC;IINIchert.

Fracture description is tabulated against ‘IOg an;t[}.i.in Table 4.

This comparison of interpretation wilh tori dcscription indicates that conclusions obtained from the CIVN-.

pressional amplitude are essentially correct and th:lt c{m-’chtsions drawn from [hc shear “ump]itode arc SOIIWNhJIoptih~istic. Again, there is no”corre!~tion” between t}]c 1111111.ber Of open fractures per foot and shear or con]prc~~i(}l[,llamplitudes except at 7.846 10 7,85S. ]t appears th;l[. Nill)the exception “nf this latter zone, the anlp]itudcs Urc Nl-fected not only by fractures but also by other forma[ i(fndis~ontinuities, ancj possible coupling changes.

The interval was perforated as shown, and ]ofvcr’ZO(IC. : . - )were also perforated. The WCI[’ is p~odtlcin~ hot th~ pr,l-. ,.

f ‘{ . . ,’. .

TABLE 3—COMPARISON OF 10G If WERPRETATION AND ‘FR=URIDESCRIPTION (EXAMPLE OF FIG. 13)

: Demh ‘Interval LO(3

(ft) Inferpretdian---1. ~.==..~,-=v > .--F!@’”’” D“’’’’’””” --- ---775G.?7S4 , horizontor

,=_____wimtiril$ honzunlrl., f,aaure$ bleeding oil or gas

7792 .78;2 horizontoi pritiorily horizonfol,

7E14;~<6-

ffacfures bleeding eil or s?shori<onfol vdrficoi fmctvtes ond v.:< fro.,f,ocfvres 731A:20 ond harizo..fc.l !I-,,QO,

at 7S22.30

,,.”.,.,. > ,,”,,, ,“”. ..-

,,,. ..! ,,.”! fe~, Drimorily .$,1: <61fl.Cl.,’,.- .,s<l”,e%

,, -.. :.

.. . . . ,..—. . . -. . . . ..... . . --- ---- . . .. .. . . .------------

j,\f)~E 4: FRAC1URE OESCR(PIICN TABULATED AGAINST LOG”ANALYSIS[EXAMPLE OF FIG. 16}

D..~,h la>(11) Inteterelotion ‘Fracture Description. . —-. ..—.. —

~In4.7n50 ‘h~rl=nt~l scotletod horizontal frn<turesfmchl:os from 7764.86, ?802.04, 7810.23,

7fJf0..t2, few $cattered showsbleeding oil or gas

j770.7832 Ica’ tared ve;tical scottered (verfitol frahre$fractures from 7770-90, 7795.7800, 7808,

7818.7W18

78 A6.78>8 extensive ve;ticnl many hairline vertical fracturesfra<tures accompanied br aerodly and

~cod show of OH cnd gas

7360 7W6 vertical fractures hairline vertical fractureswith no show

_.. —,,

duuI ion ctml ribu! ion of the sev’eml perforated intervalsis no[ known. Empiricts! studies in the area have shownflm[ a compressiomkuuplitude reduction to S mv orIVWindicfitcs a probable productive zone. This. observa-tion, plus the core description,, suggests that the better

. producing zone is at 7,846 to 7,!3S8 and that the otherin!ervats contribute little, if any, production.

Tbc presence of fractures” is indicated by the amplitudesin thcs~ examples. Howevek, it is also evident that acautious approach to interpretation of the amplitude v,ah.tesi\ necessary when the formation is composed of thin bedsu i[h impedance confmsts.

it has been shown that small changes in 1oo1 centering[lyiy have a large effect on the signal amplitude. Suchcwrtering changes should be detected by comparing the[ivo-receiver transit time with the single-receiver totallnLve\ tiqle which. is recorded simultaneously with amp]i-(ULIC.Fig. 17 shows a schematic .Iog comparing sonic[ransit [ime with total travel ~ime. If the, tool remainscentered, the total travel time should change proportion-Mcly with the transit-time changes as at A and B. If theloot becomes eccentric to the borehole axis, the total trav-CI [ime should show a lower tiine compared to transitIiflwas’ at C. The signal amplitude at C wou]d qlso” beIcss according to the degree of eccentricity. If eccentri-city is so indicated by the logs, repeat runs through thezpne may show differences on both the total time and~mplitude curves. ,.

The greatest care i$ taken in assuring good centraliza~[ion. Even SO, field observations have indicated that a

.’. ”,’I

COMP AMP 1PARTIALLY~ mv 22 OPEN FRACTURES

i SONIC A! SHEAR VEL. AMP I per Foot .

la:;$5:-erfs. <:

-— . .~-:’.:

--— —- —-—#.=-,.

;-. .-.-=-

7s00 --:---.’., =.;:”:-

.~.: _= .... —._.2.

. ---= =— —.- -—. —

-----;-:-- ----

-- -L- .- .-. ..-=”~--

, . . . . . . ‘ ‘– *-->- —.—. *. —’—- .—-..=.

1; 1>”” -””-”-’1 %&Z=- ---=%

.,—-. .—.. ,/‘“short’” total travel time is often associated with kriown “’vertically fractured intervals and low compressional tut\pli-tude. Repeat runs through the zone of interest generallyshow no chaqce i~ total time or in amplitude. It seems ,that in many ~nstances the short total travel time, whenrepeautblei is a ‘result of phenomena ot her thrm centeringeffects. No valid explanation for, the anomaly has yetbeen folmd. , , .,,

Amplitude And Attenuirti,onof the Acoustic Signal :

All of the exsanples showing amplitudes of the com-pressiofial- and shear-velocity arrivals have been takenfrom wells in northern Oklahoma.” The selective effect offracture orientation on the amplitudes appears to be welldefined in these cases. Further investigation of the pheno-mena in tither wells and other areas is necessary. -

Also under study is’ the ratio of amplitudes logged attwo receivers it different distances from the transmitter.Signal attenuation in decibels can be obtained using theexpression, (20/x) log A ,/A,, where x is the. distance infeet between receivers, A, is the amplihlde at the nearreceiver and A, is the amplitude at the far receiver. Usingamplitude-ratio or attenuation measurements, the. ,effects’0{’transmitter signal coupling to formation of tool center-ing should be cancelled. However, our field recordings ofsuch measurements have not yet produced any positiveresults. ,.

{

J“ Conclusionsr-The Sonic-Amplitude Log, measuring the amplitude of

the compressional arrival,, has successfully located fracture.s~stems. Howeve~, experience has shown that this measure-.ment is not universally applicable.

Thcoretica[ and laboratory work, ‘“followed by fieldstudies in cored wells, show that fracture orientation mayhave an i~porta,nt preferefi[ial effect on the amplitude ofthe’ cornpressional- and i.hear-velocity arrivals. Conse-quently, better fracture location should be pfovided byhaving an amplitude meaiiyrement of each type of arrival.

Other borehole and foriuation. parameters aho affect‘the signal a~plitude, so that judicious ‘interpretation ,ap-proaqhes are necessary. Some of these other variables may

..I I I 1

,,.-,

TOTAL TRAVEL SONIC AtTIME

400 #us 200 70 As 40

—— .—

<

—. —. —---- . . ... . . . .. -.- ..- .-.

Fig. “17—Ctmtpttr”iwns {}iiill~lc.rec.eivt.r tottil Iisniiwitll ‘twn.rcreivcr trunsit time fm. tgonitnring nf hol cqstering

(Awnmtic).

,9i

.

bc climimttcd by mcusuring signal Mtcnuaiion. However,our field experience with attenuation measurements !0date has no{ yielded any conclusive results regarding apossible advant~ge c)fthis’ method over straight amplitudelogging. Further studies may prove the measurement tobe of significant value.

Efforts to provide improved methods”of fracture loca-tion may lead to improved methods of.a?c~!atelY m=sur-irtg and recording Such data as shear-veloclty tr~vel time”Continued investigation will, no doubt, extract other usefulinformation from the acotrstic signal.

Acksrowledgmcnts

The authors wish to acknowledge the assistance andguidance given by F. P. Kokesh, J. E. Chapman andW. ‘P. Biggs in the preparation of this paper.

In, particular, the authors e%pfess their appreciation tOthe oil companies for permission to pubhsh the fieldexamples, and to J-eon Knopoff for .Permission to useFig. 6. We also wish to thank J. DennisJ-orenOf theShell Development Co. for several helpful comments onthis paper.

References

L Tixier, N. p., Ahz% R~ p. and Doh, C. A-: “sonic ~@@’*.Iam. pet. Tech, (May,1959) XI, NIJ.5,106.. .

2. Pickett, G. R. :-’’Aco&tic Character Logs and Their Applications ,in Formation Evaluation;’ Jour. Pet. Tech. (June, 1963) 659.

3. Anderson,-W. L.”and Walker, Terry: “Application of Open HoJeAcoustic Amplitude Measurement%”Paper SPJLI?2, presentedat 36th Annual Fall ~feetihg, Society Petroleum En~meers; Dallas(1961 ).

4. Walker, Terry: “Progress Report on Acoustic Amplitude Loggingfor Formation E\,aluatiorr~ Paper ,SPE.451, presented at 37thAnnual Fall Meeting, Society Petroleum Engineers, Lcs Angeles.(1962).

5. Wslker, Terry and Riddle, Georee: “Field Investigation of Full

.,

~.—, . ...—.

.,.

.-...... . ... . ,i-....-.. ,.,.=.,... e. .,..-.. ..,, .!

t

WW.WAmm& Wuw ibm-ding,” ljrlw:]li~d at k’ourth AIIIIUU]Logging Symposinm, SJ’~’J.A, Oklahonlu fitY (1963).

6, Strick, E.: “Propa~alion :f !iIastk, Waw hlotion from uri I,n.

~ressufe llesponse, 1S1‘rlIo I’~curio.Rny}ci~hwa~e, TIa;s,, ~QyU].qiveSource a]ong a k luIlf/ SaJld Interface, l!, ~l]ewetical

Sot. London (1959) 2514 No. 1~~~,7. Biot,M. A,: “ProPagatissn of Illustic Waves in a Cylindrical Bore

Containing a FluId;’ Jour. Ap,u. RLYS. (MM 23, No, $s,8. Knopoff, L., et al: 2nd AnwaL R~Port, ~rnic Scattering Proj.’

cct, ]nsthrte of Gcophysie4 UCLA (Arrrd, 1957) Chap. 12.9. Wyllie, M. R. ~., ‘Gardner, G. H. F. and Grwory, A. R.: “Stu&~,

of Elastic Wave Attenuation in Porous ~ledia~ ‘GeoP@.~;cs(1962) 27, No. 5. **

R.; L. MORRIS (right), is the regional. rfevebvnent etlgi-neer in the FieId lnterpt’etation Section for SchhlbergcrWell Surveying Corp. in Houstou. A graduate. Of the U. O!~alifornia, where he received a BS degree, he ioimdSchlwnberger in 1937 as a field engineer. D. R. GUNW(center) is a member of Schhonherger’s Sonics ResearchSection. He received BS, MS and PJID degrees jrom MITin geophysics, and has wor!ied for Geophysical Service. ~inc. and done explosives resea?ch at Stanford V. T. E.ARmtm? (left)~graduated fronf Oklahoma State U. witlia BS” in electrical engineering “and joined -Schhonbergcr it]1952 os a fieid engineer, He is presently slationfc! atEnid, Okla., as a sales engineer,

..,.

, ..-

<,-,-

,.

,,,.

_. },

. .

.. . .

i. ,/, ..

.,

. . .2 ‘., ,

. . ...>

. .,. .