sigma log project

8/13/2019 Sigma Log Project

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Sigmalog Project Kuwait 2004


Abstract

Wellbore instability and pore pressure uncertainty contribute to more than $8 billion ofplanned and unplanned drilling costs each year (source: www.halliburton.com). In anendeavor to improve on current pore pressure estimations Halliburton Sperry Sun, iththe support of !"#, introduced the Sigmalog as a ne pore pressure model to !uait.

he ob%ective of this manuscript is to assess the performance and value of the nelyintroduced Sigmalog for pore pressure estimation in !uait. Wells from several areas(Fig.1) ere studied and information from old ells using &'eponent and ne ellsusing Sigmalog ere compared.

Fig 1. Map showing main fields of Kuwait. (Qureshi & Wahadan, 1!"#


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Introduction

$b%ecties of pore pressure ealuation are:

Have an accurate *noledge of formation pressure for effective ell control +stimation of formation fracture pressure in order to set limits of or*ing pressure eduction of the ris* of secondary ell control through the anticipation of pressuretransitions

Wellbore pressure analysis Hydraulics optimisation

Introduction to D-exponent

he &'eponent is used to predict abnormally pressured formations by giving a non'dimensional number, hich is based upon the relationship beteen the "- and

formation pressure. ($'ene'in, ))1) In a constant lithology the &'eponent shouldincrease as the depth, compaction and differential pressure across the bottom increase.pon penetration of a geopressured /one, compaction and differential pressure illincrease and ill be reflected by a decrease in the &'+ponent, therefore an increase inpore pressure. (*a+er ughes -nte 1/).his techni0ue or*s best in areas here a normal compaction trend can readily bedeveloped, here the lithology is moderately constant and here overpressure is due todise0uilibrium compaction. (0. warbric+, ))2)he e0uation as developed for 1ulf #oast conditions here drilling homogeneous shaleis the norm.-rior to 2332 the &'eponent as the preferred method of pore pressure estimation in

!uait.

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Formula forcorrected D-exponent

Formula foruncorrected D-exponent


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Fig. 3boe and below, two graphs showing nature of 45e6ponent.

7aton8s formula

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Introduction to Sigmalog

Sigmalog as developed by 1eoservices and 51I- in the mid seventies to be a morereliable overpressure prediction and evaluation techni0ue. In essence Sigmalog is aninstantaneous drilling rate 6 pore pressure prediction model based in the relationship

beteen "-, drilling parameters, roc* strength, a pseudo'differential pressure at bit andpore pressure. Whereas the &c eponent can be thought of as a 7normali/ed drill rate,the Sigmalog may be considered as representing a roc* strength parameter. (9 *circa 1!)8s)

Sigmalog as developed by 1eoservice and 51I- in the -o valley region of 9rancehere large sections of :imestone are drilled. If you compare the regions here eachmethod as developed, the -o ;alley appears to be lithologically closer to !uait ratherthan the 1ulf #oast here the &'eponent as developed.

Sigmalog as introduced to provide a chec* for &'eponent and to improve on the

limitations of the &'eponent. he main limitation of &'eponent is that it is lithologydependant due to the fact that &'+ponent re0uires a normal compaction trend to bestated and this trend is generally set in Shale as its is the most unvarying lithology. +venin Shale changes in provenance and composition may render application of a single curveuseless for prediction (0. warbric+, ))2) or suspect at least. #alcareous #laystonesill also affect the trend line. hese ill push the trend line to the right, giving theimpression of loer pressures than are actually present. (alliburton $erpressureManual, 1). In !uait for eample, the 5hmadi Shale here the


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Calculating procedure for Sigmalog

When Sigmalog as first introduced, several of the published papers containedmied units and incorrect e0uations resulting in programming and utili/ation problems.

Observed Sigmalog5 total roc* strength or ra Sigmalog, tis calculated from normali/ed drilling

parameters as in the &c eponent method. he basic e0uation is?

his ra Sigmalog is then corrected for the effects of compaction for determining porepressure at shallo depths.

Porosity and lithology functionso evaluate the Sigmalog calculations porosity and lithology functions are

established. he porosity function @nA represents the time ta*en to e0uali/e thedifferential pressure through the cutting. his depends on the lithology and particularlyits porosity.

B


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he lithology term 9C is modeled to represent the effect of lithology and porosity on thedifferential pressure at the bit. i.e. ho does the type and volume of the pores affect chiphold don or release of cuttingsD

Reference trend line

When loo*ing at a Sigmalog plot, the points to the left indicate porous orfractured formations hile those on the right indicate impermeable formations such asshale or marls. nder normal pressure and therefore, normal compaction conditions,

these points form a straight line called the reference trend line, r. he e0uation fordetermining the reference trend line is?

Calculating pore pressure

he pore pressure gradient (atmE3m) at a depth of interest is calculated using thefolloing procedure.he lithology term 9C is calculated by?

9C F r E tand this term is utili/ed in the final pressure gradient formula?1p F df 6 G23('9C)EnC:C9(2'9C)J

he !true Sigmalog"

he true Sigmalog ocan be formulated by ad%usting tith the lithologyfunction, 9C.

K


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his calculated function, the 7true Sigmalog, is related to tto establish the porosityof the formation. his porosity is utili/ed to calculate interval bul* density andoverburden.

Sigmalog behavior5s a general trend values of oalays increases ith depth through shales,

indicating the presence of normally compacted formations. Hoever, hen the Sigmalogcurve deviates to the left from the reference trend line (li*e the &c eponent) anoverpressured /one is indicated.

he Sigmalog curve often dramatically shifts, brea*ing the curve into manysegments. o compensate the reference trend line has to be shifted to avoid incorrectvalues of pore pressure. he factors, hich cause these abrupt shifts, can be?

(i) hose hich affect all other drilling models, e.g. drilling parameters suchas rapidly varying drilling parameters, coring, mud drilling, bit typechanges and hole si/e changes.

(ii) hose that are not normali/ed by the model, such as ne bits, geologicalfactors of faults, unconformities, lithology changes etc and hydrauliceffects on the drill rate.

Sigmalog modeling #trend line shifting$

he ob%ect of trend line shifting is to avoid the influences of spurious data.herefore, as ith all models, the or*er must be careful to ensure that the reasons forma*ing such shifts are perfectly understood. here are no fied rules concerningreference trend line shifts, but before interpreting the Sigmalog curve it is very importantto be in possession of all the pertinent data such as bit records, lithologies and casingdepths. his data should be considered for the effects that may have been imposed on the

Sigmalog.

Having determined here the shifts in the reference trend line are re0uired they can beshifted by (i) a mathematical or (ii) a graphical solution.

(i) 5 ratio method can be utili/ed hich divides the last Sigmalog valuebefore the shift by the first Sigmalog value after the shift, and multipliesthe resultant by the last trend line value before the shift.

rne F rold C (o ne E oold)

(ii) 5s the hori/ontal scale for Sigmalog is linear, it is possible to measure the

distance beteen old and ne Sigmalog values and shift the referencetrend line by the same amount.

%ield procedure for calculating pore pressure

he calculations are made by hatever softare pac*age is available. Hoeverthe ma%ority of such pac*ages re0uire the input of the appropriate b value to compute the

correct rat each depth interval. he value of can be found by using parallel rulers on

L


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the graphical plot to produce the trend line bac* to the surface and establish the intercept

value.5lternatively the e0uation belo can the used to provide a mathematical solution.

5ssuming the value of rat K333m is 3.LM, hat is the value of D

giving?3.LM ' (3.88CK333E333)F3.LM'3.B4 F 3.2K F

Zero intersect 0.838332

SR Depth M Depth Ft

1.1 2!0.122 !"2

With this data the computer can provide or plot oand rproducing the calculatedpore pressure, hich should be plotted alongside mud eight.

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+cel Spreadsheet


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M

Fig 2. howing the nature of igmalog, note the man' brea+s in the cure.


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Discussion

Without actual pore pressure data from 1eo'ap -W&, =& or 9 tools it issomehat difficult to assess ho accurate the above methods of pore pressure predictionactually are. 5n accurate value of pore pressure can be ta*en from *ic* data, hoever a

*ic* in essence demonstrates a failure of pore pressure estimation. In order to assess theeffectiveness of Sigmalog information from several ells has been studied and specificeamples from each ell have been highlighted.

he old style of pore pressure estimation (pre 2332) has large %umps or steps, an eamplebeing &9'3 in the 1otnia. 5t B33A the pore pressure %umps from >.K to K.M ppg andat 8B3A the pore pressure %umps from K.M to 8.L ppg. 5lthough these steps seemlarge, this very strict method of using only shale points is theoretically the most correctdue to the fact that &'+ponent is lithology dependant. Sigmalog in comparison gives asmooth natural loo*ing curve on hich each point could be valid as Sigmalog is notlithology dependant. 5nother observation about Sigmalog is that in most cases the value

of Sigmalog pore pressure appears to be slightly higher than that of &'+ponent.

&'-()*NH'328 began drilling in late 2334 in the Nahrah field of


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9olloing the umaila, the curve is less variable for both &'eponent and Sigmalog anda good pore pressure estimate can be confidently chosen (fig. /#.5t this stage Sigmalogand &'eponent follo each other closely, if not precisely. his is probably due to thelithology being composed mainly of shale.

Fig. " *oth igmalog and 45e6ponent reading the same in 0umaila, 3hmadi, Wara

formations.

hrough the Oubair both &'eponent and Sigmalog are erratic due to the very variablenature of the formation. (Fig. /#

Fig. / *oth igmalog and 45e6ponent are erratic in


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Fig. ! igmalog and 45e6ponent drift apart in 0atawi imestone.

:arge shale beds indicate the base of the =a*hul and as a result &'eponent moves bac*toard the Sigmalog ith pore pressure estimates less than ppg difference (fig. ).

Fig. 45e6ponent moes closer to igmalog in Ma+hul(hale beds#.

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5t the top of the Hith both models indicate an increase in pore pressure (fig. 1)#,Sigmalog shos the greatest change (from .B ppg to K.3 ppg), hilst &'eponent risesfrom 3.B ppg to 2.K ppg. he value indicated by Sigmalog of K.3 ppg appears to be alittle high, as the mud eight is only B.B ppg, it is probably due to this over estimate thatthe &'eponent figure as used. he mud eight as increased to L. ppg at the base

of the Hith.

Fig. 1) >ore pressure estimate increase at top of ith.

5t the top of the 1otnia the &'eponent shos a value of K.M ppg and the Sigmalogshos a value of 8 ppg. 5t the base of the first Salt the &'eponent shos L. ppg andthe Sigmalog shos 8.B ppg (fig. 11). he mud eight is 8.K ppg. he &'eponentvalue is preferred due to the overestimation of Sigmalog further up the hole, even thoughany shale points in the salt could be suspect.

Fig. 11 >ore pressure estimate increases in first alt.

5s the first 5nyhdrite is penetrated the &'eponent is pushed to the right toard the trendline due to the change in lithology, Sigmalog is shifted for this change and reads 8.B ppgto M.3 ppg (fig. 1a&b). 5t this point a *ic* occurs during the circulation of bottoms upith L2B psi SI&-- and 22B3 psi SI#-, giving a pore pressure calculation of M.K ppg.he values of Sigmalog reflect closer the actualpore pressure than &'eponent. hehigh'pressure estimate shon previously by Sigmalog as probably due to the trend lineat 4343A being shifted too far to the right. 5 slight ad%ustment to the left ould haveprobably not caused the overestimation further up the hole and ould have still given anaccurate pore pressure estimate in the 1otnia that could be confidently used.

4

K.3ppg

2.Kppg

L.ppg

8.B

ppg

8.3ppg

K.Mppg


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Fig. 1a Kic+ in first 3nh'drite.

Fig. 1b Kic+ in first 3nh'drite.

he pore pressure as *ept at M.K ppg until the


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5t >33A an increase in pore pressure estimate is indicated by both models, Sigmalogshos B.8 ppg and &'eponent shos 4.B ppg (fig.1").

Fig. 1" igmalog and 45e6ponent show increase in pore pressure at 1;1)).

he =iddle =arrat illustrates ho detailed Sigmalog can be and ho, if Sigmalog isstudied closely and accurately, trends of pore pressure can be interpreted and don hole

problems may be prevented. Sigmalog pore pressure estimates ere revieed andad%usted telve times in a K33A section folloing increasing and decreasing trends ithinthe formation and using connection and bac*ground gas for verification (fig. 1/).

Fig. 1/ igmalog showing increasing and decreasing trends in detail oer a smallsection in Middle Marrat, note connection gas leels in relation to pore pressureestimates.

5t & Sigmalog and &'eponent ere less than ppg apart, ith Sigmalog readingslightly higher.

B

B.8ppg

B.Bppg

B.KppgB.Lppg

B.Kppg

B.Bppg

B.>ppg


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he ell &9'2 as drilled in 233> in the &harif field. +ngineers used Sigmalog and &'eponent models for pore pressure estimation, using the offset ell &9'3 and &9'comparisons can be made in the effectiveness of both models.

D%-+( , D%-+)

&9'3 as drilled in the summer of 233 in the &harif field, estern !uait. he & ofthe ell as >33A -ore pressure estimates ranged from M ppg in the atai Shale to8.L ppg in the 1otnia. he most significant event regarding pore pressure in &9'3 asat 4L2MA (=iddle =arrat) here ithout any indication of increasing pore pressure asudden flo as observed, the ell as shut in ith M33 psi SI&-- and 8>B psi SI#-and a B bbls gain. (ee Fig. 1=) he pore pressure estimate as originally 4.3 ppg basedon a &'+ponent at 2M33 ft. he ell as *illed ith >.8 ppg mud and losses of BKbbls. If e compare this to &9'2 at around the same depth as the *ic* (fig. 1!) porepressure estimates are around B.3 ppg and at the depth here the &'eponent asestimated (2M33ft) the Sigmalog shos about >.3 ppg, because Sigmalog is notlithology dependant the gradation can be seen throughout this section. "ne point to note

about the &'eponent of &9'2 is that it had not been used in the traditional ay herethe trend line is reset for only bit runs and hole si/e changes. he &'eponent trend linehas also been set for lithology changes, hydraulics changes and "- changes, foreample at the depths, M4B3A, MK>3A, 3433A, 23BA, 4L3A, BL3A, K23A, 83MA,8BBA, 4M4BA. 5lthough this seems to give the &'eponent a smoother appearance andestimates that match or are close to Sigmalog, it does brea* the rule of strict &'eponentprocedure. he different lithologies in particular ould have a different compactiontrend to that of the


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Fig. 1! -llustrating euialent depth +ic+ in Marrat, 4F51.

L

B.ppg

B.Bppg


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D%-++ , D% -+)

&9' as drilled in 5ugust 2332, %ust before the introduction of the Sigmalog and ithighlights very ell ho effective the Sigmalog can be. Whilst drilling the =a*hul aflo as observed at 3K28A, the mud eight as 4.2 ppg and the pore pressureestimate as 2.> ppg (set K33A from *ic* depth) (Fig. 1) he ell as *illed ith B.3

ppg and then increased to B.L ppg due to high bac*ground gases. he pore pressure asthen recalculated to B.3 ppg on the basis of MB3 psi SI&-- and then B.> ppg based onthe high bac*ground gas and &'eponent. When e compare &9'2 Sigmalog (Fig.))for the same section e see the pore pressure estimated at 2.B ppg (K3ft abovee0uivalent *ic* depth) rising gradually to 4.3 ppg 33A from e0uivalent *ic* /one. Itthen rapidly increases to B.3 ppg, the same as the calculated pressure from the *ic*..2 ppg and thepore pressure as estimated to be >. ppg based on the high gas level of 3B3 nits.(Fig.1) In the same /one of &9'2 (Fig. ), the Sigmalog shos pressures rangingfrom >.K ppg to B.2 ppg, higher values than both &9'3 and &9', the +#& asB.Bppg ith no losses.

Fig.1 -llustrating +ic+ in Ma+hul of 4F511

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Fig ). -llustrates euialent depth in 4F51

?radual increase in pore pressure estimation.

M


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Fig.1 -llustrates +ic+ in Marrat of 4F511

Fig.. -llustrates the euialent depth in 4F51 Marrat.

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Conclusions

he introduction of Sigmalog has also brought about a greater understanding of &'

eponent, its limitations and strengths. It is advisable that both models be run onevery ell in order to verify one another and give increased confidence in porepressure estimation. Hoever, neither Sigmalog nor &'eponent can give definitepore pressureP they are estimatesand should be treated as such.

"ther factors can lead to inaccurate estimates of pore pressure including lateral transferinflating the pore pressure at the crest of tilted reservoir relative to the overlyingshales, hich their pressure prediction is made. :ithological variation and shallooverpressure create difficulty in defining the appropriate


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sing only a handful of ells and a fe carefully selected sections cannot fully assess theeffectiveness of one model in comparison to another. Hoever, the above eamplesdo highlight the limitations of &'eponent in variable lithologies and the increasedaccuracy of pore pressure predictions by Sigmalog based on *ic* data. he

Sigmalog has the great advantage of having continuous data points therefore slightchanges in pore pressure trends can be folloed closely.

&espite being labour intensive and re0uiring a high level of operator s*ill Sigmalog hasproven to be the highly effective model it as hoped it ould be. It has addedincreased confidence to pore pressure estimates and has verified estimates fromprevious ells.

he opportunity for error during shift changes ill be greatly reduced as engineersbecome increasingly familiar ith the model. he accuracy of the estimates togetherith the increase of valid data points ill ensure pore pressure trends are identified

0uic*ly.

In the challenging pressure environment of !uait the combination of specialist s*ill and*noledge of engineers in the field of pore pressure estimation could ma*e thedifference in drilling a successful ell.

References

5bdullah, 9.H.5P 5 -reliminary +valuation of Qurrasic Source oc* -otential in !uait,7arth and 7nironmental ciences 4epartment, Kuwait @niersit', 2332

Na*er Hughes Inte0P 9ormation -ressure +valuation echni0ues,Formation >ressure7aluation 0eference ?uide, Qanuary MMK

Halliburton +nergy ServicesP Ruantitative -ressure +valuation from the dc +ponent,$erpressure Manual,MMM

ressure Manual, - >>'B32333

Sarbric*, ichard +.P #hallenges of -orosity'Nased -ore -ressure -rediction,B7? 0ecorder, September 2332

"yeneyin, = NabsP 9ormation -ressure, =Sc "il and 1as +ploration +ngineering,

0obert ?ordon @niersti', 233Rureshi WahadanP 5nalysis of formation pore pressure in !uait oil fields. perr'5

un manuscript, M8B

C.?.>ar+in, Kuwait ));

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