20443-real-time pore-pressure evaluation from mwdlwd measure

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Real-Time Pore-Pressure EvaluationFrom MWD/LWD Measurements andDrilling-Derived Formation StrengthJ.C. Rasmus, SPE, Anadrill, and D.M.R. Gray Staphans, Schlumberger Cambridge Research

CaoqqsSum m a ry. Tr ad it i eml pore -p r es sure in t e~ re t s t ions in t e r t ia ry unde rcompact ed sha le s have been based on”empi r ica l r el a t ions hi psbemv een a pa r t icu la r measu remen t ( such as r e si st i vi t y ) and pore p res sure in pounds per gd lOI I. I t i s WeU-known , however, t ha t t h emea su rem en t s sr e r espond ing to the “excess” poros it y in the sha fe r a the rt han to thepore p res su red ir ect l y. Anew techn ique i s i ll us t r a t -e d i n w h ich a l l a v a il a bl e m ea s ur em en t sa r e f ir st ch a m ct em e d i n t er m s o f t h is e x ce ss p or os it y s nd f it h ol on w i t h measurement-responseequa t i on s . Th i s a l lows a ma tbema t i cz i minimh t ion technique to solve s imul taneously the var ious measurernent -respxtseequat ionsfor t i is por mily and l it hcdogy on a foo t -by -f cot ba si s. A cenven t ioez l compact i on poros ii y le ff ecdve-s t r es s mOde l is t hen used to det e r-mine th e a dd it i on a l pore p res su re caused by th is exces s poros it y. The resu lt i s a s ing fe pore -p r es sure e st ima t e tha t i s i ndepen dent o ft he n u m ber o f m easu remen t sand th s tha s an accm’scy t i a t impreves wi th the tot a l number o f measu rement sused in the in t erp re t sdon .The interpre ta t ioncan be performed at thew eUsi tein red t imeby useof ra teof penet ra t ion(ROP), measurementwhi le dr i Il ing (MWD),and logging wbife dr i fl ing (LWD) measurementsor af terdr i ll ingby me of thesemeasurementsin cnnj rmcdcmwith wire linemeasurements.

IntroductionThe t r ad it i ona l e mp ir i ca l r el a t ionships tha t have been develop fbetweena particularmeasurementand pressursme usuallydisplayeda s a s er ie s o f l in es or t r en ds t h a t t h e u ser p la ces ov er t h e m ea . mu e-m en t. E xcu rs ion of a m ea s ur em en t f mm t h e oor ma l h yd ros ta t ict rendis then in terpre tedas abnormal pressureand isa utomat ica l lysca l ed in t e rm s of pore p res sure in pounds per ga ll on , wfdch i s t heequivalentmud &nsi ty nwessary te balancetheformat ionpm press-u r es . Th i s me t bcd ha sbeen used sumessfuf ly in theU .S . gu lf coas tby skifledinterpreters.This methodha s ssveral shottcondngs, how-ev er . I t cm la d t o a s m a ny por e-pr es su re es ti ma t es a s t h er e a r emea.wrements to evahra t t i the empir icaf re la t ionships are locaf lycmdined anda re .notgenera l lyapplicableouts ide theU.S. gUKWJ aSGand Iitbologicsf variat ionsirt thesha lesand theireffects en the meas-urementsare not accounted for, causing addi t ionaluncer ta in t iesi r tthe pore-pressure estimates.

The us e o f ROP for pore p res sure ( t heD expanen t1, ba sp rovedd it l ku lt t o a ppl y b ecauseo f va r ia t i ons i r rMbology, b it wea r s t a t e,

su rf a ce .t e -dowrdmle we igh t t r an s fe r e f fi ci ency, and b ]t t ypes. An ew a ppr oa ch fm t cor r ect s R OP f or bit w ea r b y u se of d ow ’n hol emeasu rem en t sof we igh t and torque. ROP i s S ISOnommdized forbi t lype , dmvnbolew eight ,a nd surface revolut ion per minute.Thisproduces an apparent format ion s t rengththa t i s .a measurementoft ie mck fz ifurew. istance.to thebi t teeth .previous a l tempts tochar-ac ter izea ndqua nt i fy th is s t rengthin termsof Iabora toty-meamredrock pmperdes havebeenlargely unsuccessful becausethebi t teethd o n ot fa il t he f or ma t ion in t he s am e m a n ner a s a la bor a tor y loa dce ll . f n ou r ne w a pproach, t he fo rma t ion s t r eng th is cha rac t er i zedby t r a dki ona l i n t erp re t a t ionvolumct r i cs . A thorough ana lys is o ft i e f or m a t ion s t ren g th sh ow s t h a t i t i s a s t mr t g fu nct ion of t h e f i-‘t bolog y a n d ef fect iv e por mi~ of t h e for ma t ion . E v id en ce of t h enatureof theserela t ionshipsis feund in &ts whsre theclassic shalysand ‘‘ be ome ran g” seen on the neu t ron -dens it yc ros sp lo t i s a l soseen o n a fOn na t i On-s t r eng th/gammamy cres sp lot . Th is un iqueana lys is a l lows t he forma t ion s t r eng th to be in t erp re t ed in a man-ne r con s is t en t w i th conven t i ona l l og ana lys is whe re cl ay, ma t r i x,and effect ive paros i~ volumes are der ived. The format ion s t rengthi s a l so q u a nt it led i n t er m s o f O r ei n-s it u s tz es s s t a t eof t h e f Or m a -t ion , w h ich is l a rg el y a f un ct ion of t h e m u d p re ss ur e s n d n ea r -b itpore pressure.

Com b@ the fonnaden -. skeng thmeaswementwi th e therMWDand/or LWD measurementsaf lows theporepressure ,Mbology, ef-f ect i ve p or os it y, a n d s a tu r a ti on of t h e f or m a t ion t o b e com pu t edin r ea l t im e d ur in g d ri llin g a n d a llow s d ecis ion s t o b e m a de t h atp r emot e s a f er and more e ff ici en t d r il li ng .

copyright {99 edd y of P etroleumwln wr s

2.54

TechniqueMerprdationMotfeL A f or m a t ion ca n t e d es cr ii b y t h ev ol um ess how n i n F ig . 1, w h ich r ep rr s en t t h e m a jor iq ’ of t h e con st it u en t sof s ed im en t a l r eck s. Th e v ol um es d et er m in ed b y t h e i nt er pr et a -

t ion progmm descr ibed in th ispaper are i lfustmtedon ther ight s ideof F ]g . 1. When the in t erp re t a t ionprog ram has det e rmined t ha t as ha l e is p res en t , m a t ri x (u su a ll y q u a rt z), w e t cl a y, ov er pr es su r epor es iv, a n d ef fe ct i ve p xos i~ v ol um es a r e s ol ved f or. Th e s a tu -r a t ion i s s et eq u a l t o u n i ty i n s ha l es . w %en t h e pr og r a m h a s d e t er -m in ed &a t p or ou s s a nd s a r e p res en t , m a t r ix , w e t cl a y, ef fect iv epor os it y, a n d w a t e r s a lu t a ti on a r e s ol ved f or. Th e p or e p res su r ecem pu t ed i n t h e s h a le a b ov e t h e s a nd is m n si der ed t o a pp ly t o t h esand in te rva l a l so.

O ver P -e P O res it y t o P r e35r rr eC fr a ra ct er i2s fi 0n . Th e ef fec-t i ve s t r es s2 and equ iva l en t dep th3 concept s a r e i ll us t r a t ed in F %2. k a norma lp res sureenv ironmen t (r i gh t s i deo f F ig. 2), t h e r. xk- ~ mmp-a nd w a ter is expild a s the overburdenst ressincresses.The wa tercontainedwithinshakesconsistsof wa terbaud

t o t h e cla y s a n d n on bou nd or “ fr ee. ” w a t er con t a in ed w it hin t h epor e spa ce. Th e w a ter expelled du rin g com pa ct ion con sist sp r edominan t l y of the t i es wa t e r. Conven t i ona l l og in t erpr e t a t ionnomencl a tu r edef ie s an e ff ec t ive poros it y cons is t ing o f t h is f r eew a t e r a n d a n y h y dr oca r bon s. Al th ou gh “ ef fe ct i ve p or os it y ” m a yn ot be t he best t er m t o descr ibe sh ale por osit y, it U N be usedthoughout the folfowing discus3i0n3in keeping with normal mage.As the f r ee wa te r e scapes , t hee ffect i ve pore si t y decrea se s sn d th ee ff ect i ve s t r es son the g ra in s incr ea se s. Th is s t r es s i s m odel ed b yN ur 2 a s

P ~gr=po,b —crpw. . . . . . . . . . . . . . . . . . . . . . . . .. (1)

Tbe B i ot con st a n t i s g iv em a s

a=l.o–(Kdf/Kgr). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..(3

The va fue o f Kg, can be estimated from the I i tbology. BecauseKq d ep en d s o n t h e p or os it y a n d g ra i n ce men t a t ion of t h e r o ck , i twi l f changeas theeffec t ive s t ress(or depth) increases .Ter t ia rydel -t a i c s edimen t swi ll genera l ly havea K@ K r Tiwrefom, the Biot

t%on st a nt is g en er zl ly t a ken t o be u nit y in es e en vi ron men t s.D a t a pu blis hed by M ag a ra 4 s how t ha t in n or ma lly pr es su red

s+w f imen t she poroshy decrea$ecaused by the incr ea s i ng P, ~ 8 , i s@~ by

D = c1O+ + . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...@)

Th e ov er pr es su re cs se is s how n on t h e l ef t s id e o f F ig . 2. Th es bz fe h a s b een “s ea led ” a t 5,O OOft , m ea n in g t ha t n o a d dit ion dwa te r i s 2U0wed to w+ capeupon fur ther b rmizf .Th is im pl ie s th a ttbe shafeeffec t ive porosi ty remainsconstant wi th depth . Thus , thes bz le con t a in s a por os it y ca f le d a n “ ex ce ss ” ef fect iv e ~ r os h y t h a t

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is g r ea t e r t h a n t h e p or os it y of a n or m a ll y com pa ct e d s ha l e. w h ht i e w a t e r t r a pp ed , t h e s ~ e ca n n ot com pa c t a n d t h e ef fe ct i ve g r a in -t o-g m in s t re ss d ce s n ot i ncr ea s e w i t h d ep t h. Th e w a t e r b ea r s t i ei ncr ea s in g ov er b ur d en s t re ss , r es ul ~ g in a p or e p re ss u re g r e a t erthan hydros ta t ic pressure.

E x pr es si ng d ep t h i n t er m s o f e ff ect i ve s t re ss in E q . 3 a n d com -bin in g it w it h E q , I r es uft in a n expr es sion for por e pr es su re inpounds per square inch , PW, asa function of ovcrpressureporosi-CJ , ‘$Op:

&p= (l/-b)bJ g[(po,b -p~ )/(p.,b-pm)]. “.. (4)

lle interpretation p ;og r a m d et e r m in es t h e q u a n t i t y + O Pf r om a l lMe avai lab lemeasurementsand convem tlds to pressurewi th F.q .4.

Measurement CharacterizationR es is t iv it y C h a r a ct e ri za t i on . f n s a nd s, t h e r es is t iv it y i s ch a m c-ter izedin termsof sa tura t ion ,wet clay volume, and effect iveporos-i t y w it i t h e f ol low i ng m od if ie d S i ma n d ou x5 &q m .t iou

1 Yc,sw Sg+pc m e a — –—+—.

R. . . . . .. . . . . .

mea, R., alfw,. (5)

f n n or m a l ly p r es s ur e d s h a le s, t h e r e si st i vi t y r em a i n s r e la t i v el yconsta ntover thousandsoff&t, 3 b u t w i t h v m ia t i on s ca u s ed b y I o-ca l ch a n ge s i n w e t cl a y v s. s il t v ol um es . ‘Th es e va r ia t i on s a r e d e-s cr ib ed b y t h e Vcl/Rc[term in E q . 6. Wh en a n ov er p re s su r ed s h a l ei s e nco un t e r ed , t h e m e a s ur e d r es is t i ve ly d e cr e a s es b eca u s e o f t h eov er p r es s vr e p or os it y, + O p, J n s h a le s, t h e r ef or e , t h e r e si st i vi t y i sch a r a c t er i ze d b y t h e f ol fow i n g e q ua t i on

c em =+=~+~. . . . . . . . . ... .... p or e p re ss ur e) a n d t h eh ig h er t h e m u d w e i gh t , t h e s l ow e r t h e R O P. I n t h e f ie ld , m u d w e i g hta n d m ud p res su re a r e d epen den t va r ia b les a n d it is d ifficu lt t osepara tethe t wo effects , To circumvent th is , labora torys tudieswereco nd u ct e d i n d r if li ng m a ch in es w h e r e t h e a n m iu s p r es s u re couldbe mainta inedby a choke ig there turn l ine , therebykeeping i t in&-pen den t of t h e m ud w eig ht . f?a r ly s tu dies w e re pe rfor me d w it hmicrobi t s (1 .25-in . d iameter )and did not sca leup accura te ly to fuLs iz e b it s .

C h ea t i a m 8 i ll us t r a t e dt h e I a b or a t m y d et e r m in e d e ff ect s of m u dw e ig h t a n d p r es s u re o n R O P f or a WI -s iz e 6%-i n. -d i a m et e r r o ll erb it d rillin g M a ncm s ha le. Th er e w e re n o a t t em pt s t o m on it or ort o con t r ol t h e p or e p re ss ur e o f t h e s h a le d w i ” g d ri ll ii g. Th e s ha l ew a s n ot f il ly s a t ur a t ed a nd , t h er ef or e, in a d r a in ed s t a t e, t h e p or ep r es s ur e w a s a s s u me d t o b e a t m o sp h er ic. To d et e r m in e t h e i nf lu -e n ce of e le va t e d s h a le p or e p r es s ur e s on d r il li ng r e sp on s e, a s im f ed r il li ng p r og r a m w a s u n d er t a k e na t S ch l um b erg er C a m b r id g e R e -s ea r ch . A h a r d s ha l e w a s d r il le d w i t h 8. 5-i n. r ol le r< on e a n d P D Cb it s i n a f df -s ca l e d r il fi m gm a c hi ne , a n d t w o h a r d a n d o n e s of t s h a le

S P E D r i ff in gE @&r i r, g , D e a a n b er 1991 265


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16

~ 14

? 12J_ 10

-i-S8ES6a

?402

0-1000 0 1000 2000 3000 4000 5000

Pm- Pw (PSI)

Tg.3—Mud weight and preszurs effesfs on the measursd for-natkm strsrmth.

were ri l ledwith 3.5-in. oller-conebits in a small dril lingma chine.P m bi nm y r es uf t si nd ica t e th a t ti e l a bm a t o~ R OP i n t h e s of t s ha l eis a function o f t i e d if fe rent i a l p r es su reo r the d i ff er ence betweenth emu d and forma t ion pore P re ;su re s. Th i s r esponsewou ld be ex-p ect ed w he n young sod lo r undercompact ed sha le s, wh ich a re thes ub ject of t h is p a pe r, a r e d ri ll ed .

Th e R OP, t or q ue , s nd WO B f rom t h e r o ll er -con e e xp er im en t sw e r e u sed t o com pu t e t h e f or m a t ion s t ren g th s w it h I @ 7’. Th es ef orma t ion s t r eng th swere then .o rmd i?ed to the stm@ at 9 lbmlga l

266

a nd zer o differ en tia l pr essu re. F ig. 3 sh ow s t he r esult s of t hef lea tbam &ta, where theFOrepressureis assumedtobe ahnospher-ic b eca u se of t h e d min ed s ta t e of t h e s h a le. Th e &m f or t ie s ot lsha lesfa l l a long the ssmetrendswhen different ia lprsssore is used .

The fol lowing equat ion descr ibes theseenvi ronmenta leff tcts onthe meazu red forma t ion s t r eng th .x

0-= (O~W)g,0[2.CQ(W ’.0)l.@ .l[0.COZ(P~ -P W)+ l.0].

. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . ...(8)

Th e s econ d t er m in E q . 8 a ccou nk f or t h e ef fect of d ecr ea s in gR OP, w i t h i ncr ea s in g m ud w e ig ht s a n d w a s d er iv ed f rom r ol le r-con e a i d P D C l a bor a t or y d a t a . Th e t h ir d t er m a ccou n ts f or t h e ef -f ec t o f decr ea sing ROP wi th incr ea s ing d if fe re nt i a l p r e ss u re a i dwas der ived t i om rol le r -cgne l abo ra to~ da t a . b it i a l ob se rv a t i cm sf rom t i eld da t aha ve shown tha t t h i s t e rmappl ie s to P DC b it s a l so.

Li .%Iogy Effects. The measured apparentformationstrengthhasb ee n f ou n d t o b e a f un ct ion of p or e, cl a y, s nd m a @x v ol um es . ~ sis best i . lusmatexly comparingth eformat ionstrengthres+ mnsewitht he w ir elioe n eu tr on den sit y r ss pon se. F fg . 4 sh ow s ? cla ssicneu tmndens it y boomerang p lot t h rougha sect i on o f s h sl e a n d ba rsa nds fr om offsh or e t he U .S . gu lf coa st . Tbe w et &d d ry cla y,wa t e r, sod ma t r i xpoin t s a s shown on thec ros sp lo t. Th e v ery s il t ys t ies (t h e h a r d s ha l e p oi nt ) cor r es pon d t o t h e b ot t om s ha l y p or -t ion of t h e b a r s an ds , w h er e t h e cl ay s h a ve a t en den cy t o be. d is -~m ed with in theformat ioo. Channelsandsdonot haveth iscoherentpat te rnbecause of the ir more laminated sandkhde s t ructure .Ffg . .5 s how s a (.S m eJ 9 ,0 v s. g am ma r a y plot ov er t h e s a m e i nt efv a l.The s il t y shs le t r end i s s eenon th isp lot a l so. Th is a l lows t hem ea6 -u r ed f or m a t ion s t re ng t h to b e w r i tt en a s a f un ct ion of m a t r ix m dclay volumes . The effect ive porosi ty a lso influences t he format ion

WATER

7J?~

E

2~3In@

POROUSE

% a=a)c

~

Y3

QUARTZ

D~YCUY3

Neutron-porosity measurement (W)

Fig. 4—TypicaI neutron-density boomerang In a shalelsand Interval.

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3

O JAR=

~

Q

mo

z

z[ ~~~

Gzw

u

~“

5.

5

P

<

g

g

oGAMMA RAY (countslsec)

80

‘ig. 5—The shalelsand boomerang allowsthe formation strength to ba characterizedin term>ftraditional volumetric.

s &en gt h i n a m a n ner s im il a r t o cl a y b ut t o a m u ch g r ea t er d eg r ee. The neut ron measu remen t i s cha rac t er i zed asTh e i ncr ea s inge ffect i ve poros it y t r end i s s eena s a t r endo f poin t st o t he s ou th w est on F ig. 5. The cha rac t er i za t i ouof forma t ion (4N)mw=V, tdNcI + Vidm +v2 N2+ o’ e~Nmf

s t re ng t h in t o por os it y, w ct cl a y, a n dm a t ri i v ol um es i sa cb iev ed + (l-sxJ @e+ ~ + dop + Nw . . . . ... . . . . . . . . . . . . .. . .. .. (12)wi t h th e f ol lowing response equa t i on ’der ived fmm the cros sp lo ttrends:

Vc z 4. o m m , )g J= Om – O m —–o m — –om~

vc~)exl A J e x t +ext

, . . .. . . . . . . . ... . . . . . . . . . . . . . . . . . . . . ...(9)

F iel d ex per ien ce h a s s how n t ha t E q s. 7 a n d 9 a pply t o P D C bit sw h en t h ey h a v e t h es a me d ri ll ii g ch a r a ct er is t ics a s a r ol ler -con ebit , s uch a s a n i ncr ea se in R OP w it h a n in cr ea s e i n WO B , R OT,fon i um i on poros it y, o r forma t ion cl ay cont ent .

Gamma Ray CharacterizationTh e g ammara y measu remen tmay be ob t a ined f rom LWD, MWD,or w i r el ine measu remen t sand i s cha rac t er i zed as

Sonic.Velocity Characterization

Whensonic ransit t imesa rea vailablefrom wirelinemeasurements,t heycan be incorpora t edw i th&e LWD/MWD measu re ment san dused in the p rog ram. The son i c measu remen t i s cha ra c t er i ze db yth r ee r e sponse equa t i ons for t heuser t o choose f rom . Th e WyU i eequa t i on i s

tm = VdGl tc I+ IZ*+ v2 t~+ + , +v +@p)tm f Fq . (13)

The s impl if ied Hunt-Raymer-Ga rdner equat ion is

1 ‘1 (’$, + ‘bw++op)—= —— . . . . . . . . . . . . . . . . . . ..(14)%a .$ Lna 0.625tm

t w is com pn ted w it h

7mas=vc/7cl+v171 +v272. . . . . . . . . . . . .. . . . .. . . . ..(10) V dc /t c l v ~q + v ~q~m =

Bulk-Density and Neutron-Porosity. . . .... .’ ... . . . . . . . . . . . . .(15)

V*J+ v, + V2CharacterizationThe b ti -d em it v m ea s ur em en t i s ch a r a ct er iz ed a s The full Hunt-Raymer-Ga rdnerequat ion is

Pmas = v c f Pc t + V I P1 + v 2 Pz +s . P~ 1 [1 –(&[email protected])120 + (~c+vbw+ ~) ,,, ~1~—.+(l–sx& epfi+&ppw. . . . . . . . . . . . . . . . . . . . . . . .. (11) k, GM hf

SPE DriJlingEn@eering, December 1991 267


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—~g. 6-Pt’Og~ computationsthrough ap overpressured shale and hydrocarbon-bearingsand made withMWD msistlvity,gamma‘8Y,and formation-stmngth measurements.

268 SPE Drilling F.r@&ing, December 1991


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000

100

200

300,

H“,”) ~

-.-—

HYDROCARBONS (V/Vi

WATERVN)

[01

—

ETEo

[OHI

-La-- –mvlry

20,00

REs s

M.M)

——

,,”,,,

20,00

).,0

—

—

.=.—

—

—

—

—

—

—

IJ

QAMMA RAY

m) ,00,0

Q 4, ,. W. E7 ED . AM lh

>,00 RAY [CPS) 100.0

MEASURED FORMATION

STRENOTH (KP.S)

>,00 400.0

----------

NTERF’BE, ED FORMATIOA

,,00 S TR EN GT H , 00 ,0

..--- ——— —— -

—

—

—

—

—

—

—

—

—

=““,

.

1

UNCWTNNIYOF M%

PORE PRESSURE

D, ILL,W3 FLUD DENSITY

,00 {LW.3 AL) a m. ---—--—-— . VERAQW [INTERPRETED

PORE PRESSURE ILWQ

.00 18,00

MWD PO RE P RES SU FI E +

,00 SIGMALStOAL)8,00————

M WD P ORE P fl ESS UR E -

,00 SIQMALWGAL)8,00

,“. me ,O,AL . QAS (%}6,00 0,00

19.7—Pm9mm cOmPtiations showing underbalanced drllllngthrough an overpressured shale and hydrocarbon-beating sand ~mde with MWD reslstivity, gamma ray, and formation-strength measurements.

SP E Drill ingEngimaing, December1991 269


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.,

‘lg. S—Progrim coniputations made with MWD formation-strength, LWD resistivlty, gamma ray, neutron, and density meae-wements.

270 SPEDrill ingEngineering,December1991


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t3LGBAL Interpretation Process

A program hasbeen developedto solvethe abovemeasnrement-respcmse“equationsbased on tie GLOBAL9 technique. TheGLOBAL t echniquerequi res tha t the measurement-responseequa-t ion s b e w r it t en a s f un ct ion s of t h e u n m ov m v ol um es (I lg . 1) a n dt h e k a s s oci a t ed r e sp on s e p a r a m e t er s , a s s h ow n i n &I s . 5 t h r ou g h16 (r e sp on s e p a r a m e t er s a r e t h e m e a s u re m en t ’s r e ~ on s e t o a par-t icu la r v ol um e w h en on fy t h a t v ol t ie i s p ~ es en t ). T&$ G L O B AL~ c bn iq u e d et e r mi ne s t h e v ol um e s t h a t r e pr e se nt t h e b es t s ol ut i ont o t h e m ea s u re me nt -r es pon se e q ua t i on s. ~ a ccom pl is h t h is , t h ep rog r a m m in im iz es a n i ncoh er en ce f un m on g iv en a s

n[Czi-fi x )]zIs.,= z ~ + ~ 8k(x~2- . . . . . . . . . . . . ..(17)-.. i=* atew: k= 1 r :

A u ni qu e f ea t u r e o f t i e G L O B AL t ech ni qu e i s t h e com pu t a t ionof theore t ica lmeasurements , i(x), by use of thecomputedvolumesand & user-suppliedresponsepara meters.The volumes tha tsatisfit he s et of eq ua don s a s a g rou p m a y n ot be t he bes t solu tion fora par t icu lar individual measurement : responseequat ion . The fus tt er m in E q . 17 is a m ea s ur e of h ow w ell t h e t h eor et ica l m e a su re-ment resembles thea ctua l measurement . I ffhe volumes solved forsatisfy the individualmea surement-responseequationsand descniet he m a j@y of t he for ma t ion ’s lit holog y a nd por osit y, a nd t heresponsepara metersare wel l chosen , thenthe t heore t ica lmeasure-m e nt s w i ll ov er l a y t h e i n pu t m e a su r em e nt s . Wh en t h e f it i s g cod ,t h e com pu t ed t ot a l i ncoh w en ce is s ma i l. Th e u se r r el ie s on t h es e

t w o ob se r va t i on s to d et e rm in e t h e q u a fi ty of t h e a n s w er.

Data PresentationFig. 6 i ll us t ra t es a com pu t a t ion m a d e w i t h t h e M WD f or m a t ionstrength,gamma ray, and resisdvilythr oughan overpressuredshale.Tr a c k 1 d kp la y s t h e v ol um e t ri c com p ut a t i on s f r om l ef t t o r i gh t o fVcl, VI (taken t o b e q w q ’t z), C $e,, op, a n d @w. Tr a ck 2 d is pla y st h e m e a s u re d a n d t h e or et i ca l r es is t m m ie s, a n d Tr a c k 3 s h ow s t h em ea s u re d a n d O ie om t ica f g a mm a r a y a n d f or m a t ion s t ren g th . Afis ht a il P D C b it w a s u sed ov er t h e in ter va l ilfu st ra t @ Th e h ig ha c t iv it y o f t h e f or m a t i on s t r.m g t h. is cm . w d m a k d y b y R O P f lu ct u -a t i on s t h a t a r e a r es u lt of i Om l og ica l a n d p or os i t y v a r ia t i on s . ‘f h ecom p ut e d p or e p r es s ur e is d is pl a y ed i n Tr a ck 4 a l on g w i t h t h e m u dw e ig h t a n d i ncoh er en ce . Th e s ha d in g en com pa s si ng t h e p or e-p re ss ur e cu w e r ep re se nt st ie u nce ti t y of t i e com pu t ed p r es su r e.I t d ecr ea s es a s t h e n um ber of i npu t m ea s ur em en t s i ncr ea s es , t h ev ol um e of ov er p rcs s u re p or os it y i ncr ea s e s, a n d t h e i n coh e re ncedecreases.

This in ter v~ w a s d rilled a s a $d et sa ck t o a w ell w h er e a k ickb a d occu r r ed a t t h e e q u iv ~ e n t d ep t h of 475 f t i n t h i s s i d et r a c kw e fl .Th e k ick r es u lt e d i n t h e b ot t o mh ol e a s s em b ly (B H A ) b ei m gl os t i nt h e w e fL Th e or ig in a l h ol e w a s d r if kd w i t h 13. 2 I bm /g a l m ud a n dt h e k i ck w a s m e a su r ed a t 16 l b m /g a 3. Th e a n a l y si s o f t h e s i d et r a c kshows tha t theor igina lw df was being dr i lfed sf igb lyunderba lancedju s t b ef or e t h e k ick w a s t a k e n. Th e s id et r a c k a n a l y si s s h ow s p or ep re ss u r es a s h ig h a s 15. 8 I b m/g a f f r om 460 t o 620 f t a n d t h a t s u ff lci en t ov er b a la n ce w a s b ei ng m a i n t a in ed . Tr a c k 4 a f s o c m. a im f h em ea s u re d m ud g a s, w h ich a t a d ep t h of 610 f t cor r el a t es w e fl w i t ht h e h y dr oca r b on s how ca k uf a t ed b y t h e p rog r a m.

Elg. 7 il lustrat esan overpressuredshalebeing dril ledwith apdkdtooth bit s l igh t lyunderbab .needfrom 000 to 105 f t . ‘fbe mid weight

w a s 13.5 l bt ig a l a n d t h e com pu t ed p or e p re ss ur e i s 14 I bm /g a l.A hydrocarbon-bear ing sand is d ri fled a t 105 ft , a n d a s ma ll k ickr es ul t ed in t h e m ud w e ig h t b e in g r a is ed m 14.2 i bm /@. N ot e a l sot he m ud/g as sh ow in Tr a ck 4. Aft er d rillin g t o 150 ft , a bit t ripw a s m a de a n d a fis ht a il P D C b it a n d m ud m ot or w e re u sed t o d r illt h e r e m a it ig i nt er va l s how n . Th e a p pa r en t f or m a t ion s t re ng t h ism uch low er for ~ s bit r un b eca u se of t h e g rea t er R OP for a g iv enWO B com pa r ed w i t h t h e m i ll ed t oot h b it a n d i s a ccou nt e d f or b yu se of a low er Om pa r am et er i n E q. 9. Th e m ud w eig ht r em a inss l ight fybelow thepore pressureand is subsequent lyra isedin incre-ments in respons to thehydrocarbon-bear ingsanddr i l led t lom 275t o 300 f t . S u ff ici en t ov er ba l a n ce is m a in t a in ed f or t h e r em a in de rof t he interval.

SP B Drill ingEngineering,December1991

F ig . 8 i ll us t ra t e st h e com pu t a t ion w i t h M WD a n d LWD m ea s -u r em e nt s d is pl a y ed w i t h o t he r i n t er p re t a t i on sa n d m ea s u r em e nt sa t t h e w e f ls it e i n r ea l t im e d ur in g d ri ll in g. Tr a ck 1 d is pl a ys B H AMct ionaf fac tors10computed from thedownhole-measuredweightand torque. Not ice the s l id ing fr ict ion increasesunt f l ,the pipe wasworkeda t 325 t l , a f te rwbich i t s tab i lizes .Track2 conta ins theBHAw i t h t h e’LWD t ool s a n d f be ir m e a su r e p oi nt s d is pl zy e d i ns id e t h ecof la r s . Th e r es is d vi t y ig a m m a r a y i s t h e l ow e r m os t p oi nt a n d t h euPP eI’mostiS the ne~tmndensi ty measurepoin t . Track 3 COn,f&ISt h e I i t bol of l, p or b su y, a n d s a t u r a t m na n a l y se s f r om t h is p r og r a m ,w i t h Tr a ck 4 s how i ng t h e r es ul ti ng p or e p re ss u re i n p ou nd s p erg a llon . Tr a ck 4 a ls o con ta in s t i e m ud g a s a n d t h e d if fer en ce b e-

t w e e n t h e t o t a l m u d f low i n a n d ou t . Tr a c k 5 con t a i ns t h e m e a s ur e-m en ts of g am ma r ay , for ma t ion st m@b, a nd R OP.F rom t h e bit d ept h t o t h e LWfJ r es is tiv i~ /g a mm a -r a y s en sor

depth , the I i thology is d et e r m in ed f r om t h e d ow n h ol e w e i gh t a n dtorque% simpfy sand or shafe.11The in terpre ta t ion progmm cOm-putesa pore pressurewi th theformat ions t rengthmeasurementwi thMS . 4,8, a nd 9, a sim nin g a con st a nt v olu me of w et cla y for t heM hol og y. U s in g on ly on e m ea s u rem en t a n d a s su m in g a con st a n tf it lolo~ account for theI mgeuncer ta in tya ssoc ia tedwi th th ispOre-pressure computa t ion . At the LWD res is t ivi ty /gamma-ray sensordepth, the program incorporatesthesea dditiond measurementsandcomputes the volume of wet clay, quar tz , e ffect ive porosi ty, wa ter-.f dl ed p or os i, ~ , a n d ov er pr es su r e p or os it y, a s s how n in Tr a ck 3.Th e w e t cl a y v ol um e i s s h a d ed w i t h t h e M h ol og y d et e r mi n ed f r oma p re de fm ed d a t a b a se. 12 Th is s eg m en t a t ion h el ps p rov id e t h eIithologica.1boundar iesfor the assignment of cutt ingsdescriptions.At the LWD neutron-densi ty-measurementsensor depth , the pro-g r a m r ecom p ut e s a n d r ed is pl a y s t h e a n s w er s , i n cor p or a t i n g t l es ea d d it i on a l m e a su r em e nt s . Th e I i t hol og y s pi ke s i n Tr a c k 3 r e pr e -sent th inbedsdeterminedfrom a curvdmpe anafys isof theueumn-d en s i~ m e a su r em e nt s , TI E d ~ p la y w r ol ls u pw i n d a s d r il fi ng con -t i nu es , p r ov id in g a f oot -b y -f oot a n a l y si s a n d p ict u r e o f t h e f or m a -I ion charac ter i st ics and the i r effects on dr i ll s t r ing behavior. Thispmvidcs a c lear indica t ionof when to ch a n g e t h e m u d w e ig h t , m a k ew i pe r h ip s, a n d i ni t ia t e ot h e r d r il li ig d eci si on s .

c0nclu810ns

An interpretationprofgam tha tUW”tbe GLOB ALtechniquebas beendevefoped for analyz ingdownhole dr i ll ing , ROP, MWD, and LWOm e a su r em e nt s i n t e r ms of p or e p r es s ur e, M I I O1O S Y,or os it y, a n dsa tura t ion .This e liminates the need for a t rend-typepore-pressureanalysis for undercompactedshalesand providesa fcot-by-foot eval-u a t i on d u r in g d r if li ng . K n ow i n g t h e se f or m a t i on a t h i bu t es d u ri n gd ri ll in g h el ps d et e rm in e t h e, m ud w e ig h t n ece ss a r y f or s a fe , a n dyet eff ic ien t , dr i ll ing .

Nomenclature~ = fom ~ tion fa @or ~ ~ ~ ~ ~ t

a i = Mea surem ent iA, = gouging component of bit t or qu e

b = constant~ = ~ om~ t , ft

c m = measured conduct iv ity, Sdb = bit dia met er, in.D = d ep t h , f t

Ed = bk e ff ici en cy b a se d on t oot h w e a rfi(x) = Response E qua tion i a s a fu nct ion of xFq = sonic compact ion fac tor

g .&x ) = C On @a i nt E q u a t ion N um ber k as a f un ct io n of x10 = incoherence functionK df = bu fk m od ulu s’ of d ry fr am eKg, = b ul k m od ul us of g ra i ns

m = cem en ta t ion ex pon en tm + = ce me n?t i on e xp on en t of ov er pr es su r e p or os it y

n = s a tu ra t ion e xpon en tOm = f or m a t ion s q en gl b of 100% M in er a l 1 a n d for

M in er a l 20 mea l = m e a s w ed f or m a t i on s t r en g t h , p s i,

271


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John G Rasmus 1 a n e ng in ee ri ngspecialist with AnadrOl in Sugarland, TX,

worldng on MWD/LWD volumetric and

pora-pre.ssure interpretation. He has held

a variety of positions, including appllca-fions development engineer and productdevelopment manager with Schlumber-ger. He h ol ds a S S d eg r ee i n m e ch a n ica lengineer ingfrom IowaSt a te U. Photo andblogfaphlcal information on D.M.R. tlraySteiahens are unavailable.

(Om a ,)9,0 = m ea s ur ed f or ma t ion s tr en gt h a t 9 ibm /g al a n dO psi

pm = m ud pres su re, p siPo,b = overburden pressure, psiP m = pr essur e of por e w a ter in n or ma l pr essur ed

sha le , p siP ~ = w at er pressure in “ tie pores, psi

P i ng , = n or ma l ef fect iv e pr es su re on g ra in s, psiRcl = r es is t iv i~ of ” 1 00% w e t cl a y, Wm

“Rm=, = m ea s ur ed r es is ti vi ~ , QR.,. = r es is t iv it v of f r ee w a t e r. Q /m

R.+ = r es is t iv it y of w a t e r con t a in ed i n t h e ov er pr es su r eporosi ty, Wm

SW= saturation of e ff ec t ive poros it yS.. 4,) = volum e of m ud f dh ’a t et .h h J 2 = son ic r espon ses t o 10% of t he r espect iv e,

n n nis

b = son t r a ns it t im e of 100% M in er a l 1 a n d /orMineral 2

‘ = s on ic t r a ns it t im e m ea s ur em en tVP = ROP, ft /brv. = ROT. revlmin. .

bw vc~r&)c* Vt i )d = volum e of bound wafer in 100 w et clay,

fractionv ~ volu me of w et cla y

Vcl,VI,V2 = v ol um e s ol ve d f or

(Vc& = ext r apol a t ed volume of cl ay whe re (OmW)g ,O=OVdcl = v olu me of d ry cla y= Vd – VbW

Wi = user supp li ed unce rt a in tyo f Measu rem en t iWb = downboIe WOB , I bmWm = mu d we igh t , i bm/ga l

Z = vect or of solut iona = B iOt ~ On~ ~ t

a n = B i ot , con st a nt f or n or ma l pr es su red s ha fea op = B lot con st a nt for ov er pm ssm ed siza le

YcbY I ,72 = g-a ra Y r~ ponse tO 100% Of f ie leipe~ Ycmineral

7= , = g-a ra y m ea surementM,P I A = buk -dens i t y response to 100% of the r e sp ect i v e

mineral

PA = b u~-d en si t y response to 100% hydrocarbonp.= = b ufkdens iw measu rementP ti = b~ k-densiq response t o 100% m“d ~ ~ ~p. = b ul l-d en si ty r es pon s e t G l KI % s ha l e w a t e rU i = in ter na lly com pu ted u ncet it y of Mea mr cm en t irk = uncctity on constmint equ2tion@ = poros ity

= ex~po~ ~ Ovewewre Pwsiw where(O_)9,0 = 0, fr act ion

4~ = hyd-bon volume, fra ct ion4Nh = neuEon-pOrO~@re -wense to 100% h y d r oca r bon s

(’ N)rraas = n@[email protected]~@ m~r.ment

4N~f = ne@wwrOsiW r=pODse w 100% mud ffl~ate.

6Nw = neU*owemsitY response tb NO% shale “ iwwer40P = ex ces s ef fect iv e por os it y in z n ov er pr es w red s ha le

@w = fr ee w a t er volu me, fr a ct ion

AcknowledgmentsWe thank Anad r il l/Sch lumberger for permis sion t o pub li sh t h ispa per a n d t h e v a riou s oil com pa n ies f or pr ov id in g t h e d a ta .

References1, Jorden, J.R. and Shirley, O.J.: “Applicationof DrillingPerform an ce

Data to Overpressure Detection,” JPT (Nov. 1966) 1387-94.2. Nur, A. a n dB y er ke, I .D .: “b B x a g E f f4v e S t res sL a w F or E l a s-

tic Defor ma tionof R eck Wi th Fl ui ds, ” J. Geoplz.wicalesearch(Sept.1971).

3. Ham,H.H.: “A Methcd of Esdmadng FormationPressures Fmm GulfCoast Well Logs,” Trans., Gulf Coast Assn. of Geological Societies(1966) 16.

4. Magam, K.: “Compacdon andFluid M,@on,>, Ekevier science FUb-Iishers (1978).

5. Simmdoux, P.: “Mesures di&[email protected] en IDik” POEUX, applicadm3.la mesure dcssaturationsneau, 6h dedu coniportwnent des massifsargileux,” Revue de 1‘Inst.Frm@s du P&role,SupplementmyIssue(1963).

6, B&e&, T.M, andtiO, W.G.: “MeasuringheWearof MilledTc.xhBits by Use of MWD Torque and Weight-on-flit,’, paper SPE 13475presented at the 1985SPElfADC Drilling Conference, New Orleans,March 5-S.

7. white, D.B., Cuny, D.A., and Gavignet, A.G.: “Effects ofNozzIeConfiguration on Ro .ler-Cone-Bit Performance> paper SPE 171SSPmsated at the 19g8L4DCJSPEDrillingConference.Dallas. Feb. 2g-M?.rch 2.

g. Chwtlmm, C.A.: “Effects of Selected Mnd Properties on Kate ofPenetm.tionin FulMkale . & Drilling Sirmdations,’xpaper WE 13465Preserted at the 1985 SP EI L4DCDrillingConference,New Odeam,March 5-8.

9. Mayer, C. and Sibbit, A.: “GI.OBC A New Approachto Cm@er-Processd Log Interpretation,” paper SPE9341presentedtthe19S0SPE AMIMITe&icaI Conferenceand Ex&ibidon,Dallas, Sept. 21-2A.

10. J.&sage,M., Fakoner,I .G., andW,&, C,: “Ew kadng Drill ingpracticein Dev ia tedWellsW& Tomuean dWeis%Da fa .33 PEDE(Sem. 19g8)24S-52.

11. Burgess,T. , Falconer,I .G. , andSheppird ,M. : ‘‘Sepaat ingBi ta ndLith ologyEffectsFromD rillingMechanicsDa ta ,,,pa perSPE 17191presen tedt th e19SSIADC /SP EDr ilfingCon feren ce, Dal&.,Feb.28-Ma,ch 2.

12. DeUiner,P.C., P eyret,O., andSerra ,O.: “AutomaticDeterminat ionof L it h @y F r om Wel l L a g s ,, , SPEFE Sept . 19S7)303-10.

S1Metric Conversion Factors

Cycleskc x 1.0 E+OO = Hzft x 3.04S* E –01 = m

in , X2.54*

E+I3O = cmIbmfga l X 1.19S 264 E + 02 = kg/m3psi x 6.894157 E + OO = kpa

W4nvaslon factor s exact, SPEDEOriglnd SPE mm,crl@ reca[vedor wl~ SsPt, 23,1990, PaPer ,cce@ed far ~“blica.110”w.t. 24,?991,Revisedm”umriPtecslvedS,PI, 17,1S91PaQ,r(SPE2C443)itstPresenteda: Ihe 1990WE Annual Techni o? Con f wen.e and ExMbl lon hel d I n NewOrmm. Sept. 22-28.

172 SP E Dri J ingEn@eering, December1991

20443-real-time pore-pressure evaluation from mwdlwd measure

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