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THE SPE I MAGE LI BRARY SPE 27488

SPE 27488

Por e Pr essur e Est i mat i on From Vel oci t y Dat a: Account i ng f orOverpressure Mechani sms Besi des Under compact i on

Gl enn L. Bowers, * Exxon Product i on Resear ch Co.

*Now wi t h Appl i ed Mechani cs Technol ogi es.

Copyr i ght 1995 Soci et y of Pet r ol eum Engi neer s

Or i gi nal SPE manuscr i pt r ecei ved f or r evi ew Mar ch 10, 1994. Revi sedmanuscr i pt r ecei ved Nov. 30, 1994. Paper accept ed f or publ i cat i on Feb.27, 1995. Paper ( SPE 27488) f i r st pr esent ed at t he 1994 I ADC/ SPEDr i l l i ng Conf er ence hel d i n Dal l as, Feb. 15- 18. SPE Dr i l l i ng &Compl et i on, J une 1995.

Summar y

A new met hod f or est i mat i ng pore pr essure f r om f or mat i on soni c

vel oci t y dat a i s pr esent ed. Unl i ke pr evi ous t echni ques, t hi s met hodaccount s f or excess pr essur e generated by both undercompact i on, andf l ui d expansi on mechani sms such as aquathermal pr essur i ng, hydr ocar bonmat ur at i on, cl ay di agenesi s, and char gi ng f r om ot her zones.

The met hod i s an ef f ect i ve st r ess appr oach; t he ef f ect i ve st r ess i scomput ed f r om t he vel oci t y, and t he r esul t i s subt r act ed f r om t heover bur den st r ess t o obt ai n por e pr essur e. To i ncl ude mul t i pl e sour cesof over pr essur e, a pai r of vel oci t y- vs. - ef f ect i ve- st r ess rel at i ons ar ei nt r oduced. One r el at i on account s f or nor mal pr essure and over pr essurecaused by under compact i on. The second i s appl i ed i nsi de vel oci t yr eversal zones caused by f l ui d expansi on mechani sms.

Exampl e appl i cat i ons of t he met hod ar e pr esent ed f r om t he U. S. gul f coast , t he Gul f of Mexi co, and t he Cent r al Nor t h Sea. Some ot her por epr essur e est i mat i on appr oaches ar e al so exami ned t o demonst r ate howt hese t echni ques have unknowi ngl y account ed f or over pr essur emechani sms ot her t han under compact i on.

I t i s al so expl ai ned how vel oci t y- vs. - ef f ect i ve- st r ess dat a can beused t o i dent i f y t he gener al cause of over pr essur e i n an ar ea. Fori nst ance, t he empi r i cal cor r el at i on of Hot t man and J ohnson i ndi cat est hat over pr essure al ong t he U. S. gul f coast cannot be due onl y t o

under compact i on.

I nt r oducti on

Numerous methods have been devel oped f or est i mat i ng pore f l ui dpr essur e f r om geophysi cal dat a, and t he l i st cont i nues t o gr ow.Empi r i cal appr oaches equat e depar t ur es f r om t he nor mal t r end l i ne of some porosi t y- dependent measur ement t o an equi val ent pore pr essur egr adi ent . Recent methods have f ol l owed t he more f undament al ef f ect i vest r ess approach pi oneer ed by Fost er and Whal en, Ham, and Eat on.

Al l cur r ent por e pr essur e est i mat i on met hods f ai l t o f or mal l y t ake

i nt o account t he cause of over pr essur e. I t wi l l be demonst r at ed t hat

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THE SPE I MAGE LI BRARY SPE 27488

t hi s can l ead t o si gni f i cant er r or s. For a gi ven vel oci t y at a gi vendept h, t he pore pr essur e can var y by 4 l bm/ gal or more, dependi ng uponhow t he excess pressure was generat ed.

Thi s paper pr esent s a met hod f or est i mat i ng por e pr essure f r om soni cvel oci t y dat a t hat syst emat i cal l y account s f or t he cause of pr essur e.When appl i ed t o wi r el i ne soni c l ogs, i t i s pr ef er abl e t o onl y useshal e dat a t o mi ni mi ze the ef f ect s of l i t hol ogy changes. However , t he

met hod i s al so appl i cabl e f or pr e- dr i l l pr edi cti ons f r om sei smi ci nt er val vel oci t i es.

The paper begi ns wi t h a revi ew of f undament al aspect s of shal ecompact i on behavi or t hat f or m t he f oundat i on of t hi s met hod. Thi s i sf ol l owed by a di scussi on of how di f f er ent causes of over pr essur eaf f ect t he soni c vel oci t y. Some cur r ent por e pr essur e est i mat i onmet hods ar e t hen exami ned i n l i ght of t hese concept s. The new met hodi s t hen descr i bed, and exampl e appl i cat i ons ar e pr esent ed anddi scussed.

Compact i on Behavi or

Non- Decr easi ng Ef f ect i ve St r ess St at es. Under i ncr easi ng ef f ect i vepr essure, sedi ment s compact , and t hei r soni c vel oci t y goes up. At t hel i mi t , t hei r por osi t y appr oaches zer o, and t hei r soni c vel oci t yappr oaches t he val ue f or t he sedi ment gr ai ns. Bor r owi ng t er mi nol ogyf r om soi l mechani cs, t he vel oci t y- ef f ect i ve st r ess r el at i on f ornon- decreasi ng ef f ect i ve st r ess st at es wi l l be r ef er r ed t o as t hevi r gi n cur ve. Fi g. 1a pl ot s shal e vi r gi n cur ve dat a der i ved f r om wel ll og and RFT measur ement s f r om t he Gul f of Mexi co sl ope, and shows anest i mat e of t he compl et e vi r gi n cur ve.

Ef f ect i ve St r ess Reduct i ons. Much ( but not al l ) of t he por osi t yl oss/ vel oci t y gai n t hat occur s dur i ng compact i on i s per manent . As ar esul t , t he soni c vel oci t y wi l l not go down t he vi r gi n cur ve when t heef f ecti ve st r ess i s r educed ( unl oadi ng) . The vel oci t y wi l l t r ack adi f f er ent , f aster vel oci t y- vs. - ef f ect i ve- str ess r el at i on t hat wi l l becal l ed t he unl oadi ng cur ve. I f t he ef f ect i ve st r ess i s subsequent l yi ncr eased, t he vel oci t y wi l l f ol l ow t he unl oadi ng cur ve back t o t hevi r gi n cur ve.

Fi g. 1b i l l ust r at es unl oadi ng behavi or wi t h l abor at or yvel oci t y- ef f ect i ve st r ess dat a f or Cot t on Val l ey shal e ( Tosaya) . Thevel oci t i es measur ed at ef f ect i ve st r esses bel ow t he maxi mum i n- si t u

st r ess st at e must be on an unl oadi ng cur ve. For compar i son, t he vi r gi ncur ve f or t he Gul f of Mexi co sedi ment s i s r epl ot t ed i n Fi g. 1b.

Over pr essure Causes/ Ef f ect s

Nor mal Pr essur e. Dur i ng bur i al under nor mal pr essur e condi t i ons, t heef f ect i ve st r ess cont i nual l y i ncr eases wi t h dept h. Consequent l y,nor mal t r end vel oci t y- vs. - ef f ect i ve- st r ess dat a f ol l ow a vi r gi n cur ve.

Under compact i on. Overpr essure most commonl y occurs when por e f l ui dt r apped by l ow per meabi l i t y i s squeezed by t he wei ght of newl ydeposi t ed sedi ment s. Thi s over pr essur i ng pr ocess i s r ef er r ed t o as

under compact i on or compact i on di sequi l i br i um.

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THE SPE I MAGE LI BRARY SPE 27488

Under compact i on cannot cause t he ef f ect i ve st r ess t o decrease.Ther ef or e, t he vi r gi n cur ve al so appl i es f or f or mat i ons over pr essur edby undercompact i on. The most undercompact i on can do i s " f r eeze" t heef f ect i ve st r ess i n t i me, whi ch woul d cause t he vel oci t y t o becomef i xed on t he vi r gi n cur ve. On a vel oci t y/ dept h pl ot , t hi s woul d appearas a vel oci t y pl at eau.

Fi g. 2 i l l ust r at es under compact i on over pr essur e i n t he Gul f of Mexi co.Por e pr essur e, soni c vel oci t y, and st r ess i nf or mat i on ar e shown i nFi gs. 2a, 2b, and 2c, r espect i vel y. Fi g. 2d pl ot svel oci t y- vs. - ef f ect i ve- st r ess dat a det er mi ned at RFT l ocat i ons. I tcan be seen t hat al l of dat a i n Fi g. 2d appear t o l i e on a vi r gi ncur ve, and t hat t he poi nt s bel ow 7200 f t ar e appr oachi ng a f i xedpoi nt .

Fl ui d Expansi on. Over pr essur e can al so be gener at ed by f l ui d expansi onmechani sms such as heat i ng, hydr ocarbon mat ur at i on, chargi ng f r omot her zones, and expul si on/ expansi on of i nt er gr anul ar wat er dur i ngcl ay di agenesi s. Her e, excess pr essur e r esul t s f r om t he r ock mat r i x

const r ai ni ng t he por e f l ui d as t he f l ui d t r i es t o i ncrease i n vol ume.

Unl i ke under compact i on, f l ui d expansi on can cause t he por e pr essur e t oi ncr ease at a f ast er r at e t han t he over bur den st r ess. Thi s f or ces t heef f ect i ve st r ess t o decr ease as bur i al cont i nues, whi ch pr oduces avel oci t y r ever sal ( see Fi g. 3b) . Vel oci t i es i nsi de t he r ever sal wi l lt r ack an unl oadi ng cur ve( s) , whi l e vel oci t i es out si de t he r ever salwi l l r emai n on a vi r gi n cur ve.

Fi g. 3 i l l ust r at es t hi s wi t h dat a f r om an I ndonesi an wel l . Por epr essur e, vel oci t y, and st r ess dat a ar e di spl ayed i n Fi g. 3a, 3b, and3c, r espect i vel y, whi l e Fi g. 3d compar es vel oci t y- vs- ef f ect i ve- st r essdat a f r om i nsi de ( open ci r cl es) and out si de ( sol i d ci r cl es) t he

vel oci t y r ever sal . The st ar t of t he r ever sal coi nci des wi t h t he t op of over pr essure at appr oxi mat el y 6350 f t .

I t can be seen i n Fi g. 3d t hat t he vel oci t i es f r om i nsi de t he r ever salt r ack a much f ast er t r end.

P. 89

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