Paper of Saturation

.,

SPESPE 20095

Saturation Evaluation of Secondary Recovered ReservoirsMN. Hashem, Stanford U.

SPE Member

Cepyfeht lSW, Soclatyof PetmfeumEnginssraho.

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ASSTRACT over the zone and from one well to another for thesame

Consider the case of a eand atone reservoir thatzone, depending .n tha position of the well

has been under a water or steam flooding program,from the flood profile ) .

that was not efficient.A lot of effort is cur- This fact was realizedby the industryand ratherrently being done to evaluate and recover the oilthat was left in the ground, As the flood flutd

expensivemethods are followed,such as extensive

has now been introducedto the Reservoir element,coring,NuclearMagnetism tests, and EPT logs, etc.

it becomes very difficult to find the value of thejust to detarmt~e the oil saturation over thoseunswept zones.

water saturation as the salinity of the floodwater is different frorsthe original formationwater, and involvesa lot of coring and other ‘rhispaper.offersa solution to this problem thatexpensive operations to come up with the value of could save substantialexpenses to answer thatSw; water saturation. particular queetion. The results of this studyThe technique suggested in this paper finds the were compared to other saturationvalues obtainedvalue of the maturationusing logs, both old and by core analysis, EPT and GST logs. The resultsnew, that are available for that field ,the oldbeing the original resistivitylogs and the new

were agreeable,and will be discussed later.

being the logs of the new well drilled and record- The real data tested is a case in the Wilmingtoned after the flood. field,CA.This techniqueagrees very well with the satura-

~is fiald is a typical case of a longproducer that underwent waterflooding and the

tion obtained by other methods including otherlogs

chances of bypassed oil are very much there.such as the EPT log, and the core analysis Fig.(1) shows what the problem would look like on

results, within a toleranceof ~ 7%. a resistivity log,where a new water zone hasformed within or above the oil zone. That watim

INTRODUCTION zone has the characteristicthat it’s SP curve

In many fields that use water or steam flooding,full deflectiondoes not reach the SSP value of

the problem of fingeringand bypassing some of thethe clean thick adjacentwet zone that was notmixed with the flood water. Yet, the resistivity

oil zones frequentlyhappen. As a rasult, programs log shows low resistanceindicatingwater in theare made to recover the oil that ia left behind in formation.’fhisindicatesthat those two waters arathese secondary recoveredreservoir, The questionbecomes;” Howmuch oil is left behind ? “.

different;the newer being fraaher.

The problam now is that the pay zone is partly The solution suggested in this paper will utilizeflooded, that leaves the old oil zone split by a the conventional Archie’s (Pickett) crossplot,new water zone or any other shape that might formdue to tha new presence of the flood water. The

with some importantmodificationsto suit the newsituation. From there we will arrive at a new

new water zone, however, does not have the same defined 100Q water line ( Ro-Line )which containssalinity as the flood watar or the original format-ion water, but rather somte salinity inbatweeq,

the new water regardlessof ites mixture, and that

that makes the saturationevaluationfrom resis-will be our base line for evaluatingthe satura-

tivity curvas a cotspltcatedproblem. (Rwwill varytions of the adjacent oil zones, ( in this casecould be above or below it, as shown in the

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2 SATURATION EVALUATION OF S~~ARy R~~~~.RED R~=Rvo~s SPE 2(X)95.-. .-. —.. -— -- .-. — -—- —--.-— — —.—Statementof TheorY and Definitions 1

where there is compaction ,or steam flooda that ~

Our fundamentalequation %s Arcnie’s eq,:-could desolve some of the grains of the formation,the value of ‘m’ will change and a solution to

Swn - Ro/Rt-~w/Rt . . . . . . ...(1)

that is also proposed in this study.Shell proposed for ‘m’ an equationwhich dependa

m on porosity :Where F- FormationFactor = a/!l . . . . (2) m- 1.87 + 0.019/fJ . . . .(8)

aleo F - Ro/RW . . . (3) B/ ‘a’ : FormationFactor Constant :-

substituting(2) in (3) we getm

1- Surface conductanceand ionic mobflity in wateti

Ro-aRw/fl . . . . . ...(4)films: * Cations Exchange Capacity (CEC).

that leads to* @sntity of water absorbed to clayparticles.

logRo - log (sRw) - m log(~) . . . . . (5)2- Salinity of the formationwater.3- Wettabtlityrelationsbetween particularsolid

l’his is a straight line on a leg log plot ofsurfaces and the Hydrocarbon.

Resistivity (Ro) vs Porosity , with a slope (-m)4- Presence and distributionof electrical

and intersects $-1 at (eRw).conductivesolid minerals.

Ro points are the lowast NE values of the RtHere we can see that ‘a’ could possibly change and

points in the original formationwater setup.for that reason this paper works with the value of‘SRw’. If an accuratevalue of Rw-new could be

In the case whera there is no sufficientvariationdetermined,then the value of ‘a’ could be calcu-lated. Values of ‘a’ vary bemieen [0.6 - 2.0]

of porosity to construct such a plot, then eq. (5) mostly considered-1.0could be rewritten as:

LogRo- -mLog (RHO -RHO )+Log(aRw) . .(6)C/ ‘n’: Saturationexponent

ma log 1- Volume percentageof oil.or

Log Ro = -mLog (Dt - Dt )+1.og(aRw) . . .(7)2- Distributionof oil.

log ma3- nettability to 011,4- Degree of interconnectedbodies through pores.

Conventionally this wouldbe a way to determine5- Interracialtensionbetween brine and oil.

the values of the Cementation Factor (m),6- Degree of electrical isolationdue to wetting

and by Oi~.knowing Rw, then the constant (a) couldbe deter-mined. ‘n’ values normally range between 1.2 - 2.2 taken

This Ro-line would represents100% Sw line, satu-mostly as 2, however this method will allow forvarying the value of ‘n’.

ration of any point could be calculatedsimply by A widely used formula isreading it’s resistivityRt value, and project it 2.15vertically (constant~) to the Ro line to get Ro, F-O.62/$fthen the water saturationof this point is the 2value givan by equation (1). or F- .81/~

The limitationsof such techniqueare:-called Humble equation for sands.

Other aquationsused;2

1- For use in clean reservoirs rock, as shaly F- l/@ for compacted formationsandrocks will cause sw to be predicted too high. chalky rocks,

2- Formationmust have the same mineralogy, for The presence of oil and gas produces ‘n’ valueacomplex ltthology ‘m’ will vary from one type ofrock and porosity to another.

which vary with saturationand nettability. Inwater wet systems containingwater and oil, when

Figure (2) shows the conventionalArchie’s cross-oil saturationfs below critical,oil exists inInsular globules.

plot.“Theapherity of the globules

Factors Effecting the Values ‘m’,’a’,’n’will depend on it’s size with respect to the size

of the pore, and the interracialtension betwaen

A/ ‘m’:Cementation,Tortuousity,or Shape Factor:- the reservoirwater and the crude oil. Aa spheritydecreases, the saturationexponent ‘n’ will in-

1. pore Geo~try: ~ surface area, vohlme Of ~raill- crease. ‘n’ increaseefurthet when oil saturation

angularity- spherity. is above critical ●nd insular globules of oil* ce~nt.ation- compaction. become intcrconnactedthroughouttha porous meclia.

* ~ifomity of mtneral mixture. As oil saturationincreases,the electricalfnter-

2- Anisotropy ference producedby the oil increases,that causes

3- Degrae of electric isolationby cemantat$on. ‘n’ to ksep increasinguntil the water volume has

4- Occurrenceof an open fracture. been reduced to irreduciblefiha.

‘m’ valuee will range from 1.3 for fracturedrocks Looking ●t the factors ●ffecting ●,m ad m, w

to 3. nom the factors ●bove we can see that it find that we could asstwa that ‘m’ would stay the

would be ● reasonableassumptionfor thie paper to same, ●nd ‘a’ would clmWe ali@tlY~

conai&r ths value of ‘m’ to be constant. Xn casee*-

,-,.—.’:

.

SPE 2009s MOHAMED N. HAW-IEM 3

tioweverto account for the disturbancein the clay is the porosity corrected for wet clay.Mstribution or content of the porous media that If the new point falls on the water line, thiscould influence these parameters,this study will is the EffectivePorositydaal wfth the effectiveporosity ($e), and dealwith (sRw) as a term, and leave the value of (n) 8* If the new point crosses the water &_ne, someto the different users to apply their imperical Hydrocarboncorrection fs then needed . Forvalues. that determine tho Hydrocarboneffect direct-

ion and that is ganerally in the NE direction,TECHNICALAPPROACH so its corracti.onis in the opposite direction

to bring our new point dotinto the water line.This paper’s approach will differ from the origi- That is the EffectivePorosity corrected fornal Pickett work in the followtngways:- Hydrocarbonand shale effect.

I. Model assumed is the Dual Water Model, 9* If the new point stops before the water line ,

~ (3-3a-4)shows the distributionof the then ‘heavymineral’ correction is needed tofree and bound water, and it couldbe summed bring it up to the water line and that willup in the following form:- give you the effectiveporosity, corrected for

shale and heavy mineral effect.$lt= ~e +$ne

10* To find the total porosity, one must first11.Porosityused is the Effectiveporosity (Oe) determine the clay density,thatwould inters-

which is the ‘HydrodynamicallyEffective Poros- ect the extensionof the lime connectingthaity”. water point(l,l),withthe wet clay point (WCP)

at the Dry Clav Point. Any line drawn parallelNote in Fig-3A, that the irreduciblewater satura- to the line connectingthe matrix point to thetion is part of the free water, while the bound dry clay point, f~om a given originalpoint,water is part of the bound water saturation. will intersectthe water line at the

Total Porosity of that point.Fig-4 shows the increaseof shaliness,ina model.

Figure (6) shows actual log values plotted beforecorrection, and figure (7) the same points aftercorrection,on the water line, giving the “EFFEC-

To determina Pe there are numerous methods, one TIVE POROSITY”values used in this study.of tha ways could be obtained through computerprograms in the logging unit, provided a dual III. Rt: True FormationResistivitywater modal is used for clay correction.A graphi-cal method will be introducednext. This term is obtained from the DUS1A graphicalway to get $e ie :- Water Model, where :-

1* Plot (~ ) or Bulk Density Vs (p ), Rt - f(~e,fie), but @ -$e +flneD N then

and construct the graph shown In Fig.(5),. Rt - f(~t) , andinclude in the plot points .:romclean wet Total Water Saturation; Swt- f(Rt,@).zonesa as well as some shaly zo..ws. As we are seeking tha effective Sw, for better

Hydrocarbonsaturationdetermination,2* From the origin ((),0) draw a ta~gent line to then

to the cluster of points to touch them at thair Swe-f(Rt,~e) . . . . . . . . ..(9)

lowest point.Point(O,O)is the [matrixpoint].That line is the [Shalinessline]. The material Balance for hydrocarbon volume in

water wet rock:-3* Froa the point(l,l) [Waterpoint] draw two

lines; one to point (0,0) -that will be the ( l-Swt)jZt- (1-Swe)j%e. . . . . . . . (10)effectiveporosity or clean matrix lina. thenThe second line is a tangent to the cluster of Swe - l-[@(l-Swt)/~e] . . . . . . (11)pofnts to touch them at the most southwest wherepoint. That is the [Wetness Line]. Swe ~ Swt

A* The intersectionbetween both tangents, thewetnass and the shaliness,will give theJWET CLAY POINTI.

PROCEDIJRBOF SW DETBRHINATION

5* Scale the Shaliness line linearly from ZeroIf old logs, of a near by well that were recordedbefore the flooding took place, are available,

at the (0,0) point, to 100* at the wet clay - then construct the [Resistivity- Porosity] plotpoint. disc”’zsviearlier”. If both density and neutron

6* calculate the shale volume Q by any method,logs wede recorded, then follow the proceduredescribed above to coma up with the effective

preferablyby the neutron-densitymethod, andapply it to the shale-linescale, and

porosity,●nd evaluate the value of (m) and (sRw)

measure the equivalentlength on that scale.old. Then follow the following steps from step #l.Wa will discuss both situations,but first;

7* maw fr~ eve~ Qoint a parallel ltne to theshale line , and mark the measured length ofstep 6, starting from the point Itself. that

H

.

4 SATURATION 13VALUA’I’fONOF SIKCXQ13ARY R~VRl?l?D RI?!W?RVOIRS SPE 2#9s—.. —--- .—.—.. .— -- .—— -—— -.-——

If the old 10KS are not available,using new re- The S@xration determinedby thi# method ●greedcorded logs. the most with the cores with a differenceof ~ h,

and by -10% to GST Saturation,and =6* to the UPTSaturations. The differencewith the SPT and the

1* Pick a wet zone, on the new log, that would GST Saturations is influencedby the fact thatstill show the old formationwater, in the example they investigatein the inva&d zone, and as theshown in Fig.(l) that was the upper water zone, as oil in this case is quit heavy the dffferance isthe most llkely explanationshows that the upper not much. Results were also comparedwith comput-portion of the otl zone wae sweptby the flood. arized packages of ~gging Service c~aniee and

it agreed with around the seas tolerance.

CONCLUSIONSCautisn should be noted , in excluding the zonethat the higher resietivewet zone ie not due to * The saturationvalues can be &termined withless porosity. Read several points in both old reasonable accuracy from the Resistivi.ty Veand new wet zones, as wall as the oil zone to be EffectivePorosity Croesplot.That could raault inevaluated,plot them on the Log-Log plot of Rt vs huge savinge due to less cortng needed.~e obtainedby the method explainedbafore.

* Old records of the field are very deeirable to2* Pace a line through the lowest NW old water determine the original formationparameter morepoints. Find the slope (ml); and the value of(al*Rwl)at the intersectionwith ~e-1,

accurately , however, if not ●vailable tha resultscould be still cbtained , as euggasted by thispaper, by searching for a zone that was not flocd-

Slope m - [Lcg(sRw)- Lcg(Ro)] / Log 06 ed and obtaining the originalwater parameters (m).,. (12) and (sRw).

The value of Rol is the value read on the Ro-lineof the vertical projectionof the point (Rtl,$el). * The values of (m) could be accurately deter-

mined, if the effectiveporosity ie used. That3* The new lowest NEwet points should fit a will correct for the shale effect, also will leadparallel line to the old Ro-line. This parallel to Swe.line is displacad away due tls difference of6alinity between old and new waters. If tha mate- * Applicationof such a method is scheduledto berlal balance of the formation was not tested in different lithologies,●ndwith differ-maintained;(i.6. injactisnrate is not equal to ent types of flood recoveries,co test for thethe production rate) and subsidenceoccurred, the saturations.value of (m) could changa for the new water zone.In that case find the mean ovar the new water zone NO14SNCIATURE”‘“points andpaes a parallel line through it , thatwill ba the new Ro-line, for that case, Later. Bound Water : “Layer of water ●dhering to thecompare this line and the value of (m) to another shales, describedby Rwb,Swbnwell were the same subsidence problem did not Free Water : ‘All water not bound, describedbyhappen. Rwf - Resistivityof free watar

includtngthe irreduciblewater5* That new Ro-line becomes the basis for thenew ttaturationevaluation.Its intersection with Total Porosity :*Fractionof unit volume occupied~e-1, gives (a2Rw2) by fluMs* - ~t

EffectivePorosity :“Fraction of unit volume6* Simply for any oil zone point of a value containing the free water(Rti,jlei),drop vertically to thenew Ro line and and Hydrocarbonsa- ~,read Roi,then the EffectiveWater Saturationis :: Rt - True formationRe6istivity (oh@.

Rw -Resistivity of FormationWater (ohm).Swen - Roi/Rtl Rwf. Resistivltyof Free FormationWater.

Rwb - Reeistivityof Sound Water.7* The value of tha maturation exponent (n)could be elways aesumed the old value as it’s Swt - Total Water Saturation.

change, if ●ny, will change the maturation values ‘Fractionof total porosity occupiedby

obtainedvery slightly .water - both bound and free’

RESULTS COMPARISONSwe - EffectiveWater Saturation.

aFractionof @I occupiedby water-.

Figures 7A and 8 show ●ct@ &ta points frOa 2S* . go@ Water Saturation.

different wel16 that showed two different poei-Vraction of ~t occupied by bound wat*rm.

tio-l~for the new water line, one was less resls-tiv~ (fig.- k), and the other more re8istive.

n - SaturationExponent in Archie’s equstton.

In both cases the Swe wa8 calculatedto the newm - Ce?aentatfon,Shapa, or TortuousityFactor.

lines os shown on the ftgures. RNoma=llatrix density (gm/ce).

Reeults were compared to differant sources ofmeasurement, ●uch ●e core Saturationvalusa, EPTand GST logs Saturations.

-

*

● ●

“. - ..VV. ” LV8US lruvu lx. Ku-Aim SsaLra

RHOlog- Log density value (bulk density) - gin/cc. 5- Fricke,H. :“llm Electric Conductivityand Cap-acity of Disperse Systems”

Dt -Matrix transit time (u/ft). Physics,Volume #1. August 1931❑a

Dt = Log interval transit time, 6- Norris,R.L.and Biggs,W. “ Using Log Derivedlog Values of Water Saturationand Porosity”,

Trana., SPWLA, 1967REFERENCES

7- Pirson,S. :“FactorsWhich Effect True Formation1- Archie,G.E.:”The ElectricalResistivityLcg Resistivity”.Oil & Gas Journal Nov.-1947

es an Aid in DeterminingSome ReservoirCharacteristics”.AIMME Ott.1941. 8- Porter,C.and Carothers,J.:‘FormationFactor -

Porosity Relation Derived from Well Log Data” ,2- Ransom,R. :“Tha Bulk Volume Water Concept of Trane. SPWLh 1970

ResistivityWell Log Interpretation’.Lag Analyst - Jan.197& 9- Wexman,M. and Smits,L.:”ElectricalConductivi-

tiee in Oil Bearing Shaly Sands”. SPEJ June 683- Serra,O. :“Fundamentals@fWell Log

Interpretation”Part Two, ‘ELSEVIER’ 10- Wyllie,M. and Gregory,A, : ‘FormationFactorsBook Series - 1989 of UnconeolidstedPorous Media: Influenceof

Particle Shape and Effect of Cementation”.4- Atkinson and Smith :“The Sigrdficanceof Parc- Paper #223-G, AIMS, Houston,TexasOctober 19S2

icle Shape in Formation ResistivityFactor -Porosity Relationship”. JPT 1961 11- Serra,O. ‘SedimentaryEnvironmentsFrom Wire-

line Logs” Schlumberger 1989

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