Restored-State Core Analysisfor the Hutton ReservoirD.J. Wendel, SPE, Conoco inc.W.G. Anderson, l SPE, Conoco Inc.J.D. Meyers, SPE, Conoco Inc.

SUrtmtSW. Wetmbility is a major control pammter in mukipbase flow analyses such as relative permeability. When such analyses~ ~~, tbe ~tive we~b~~ of tie .=$e~Ok mu:t & ~~~ ~ tie M plug. U~OIIUM~Y fOr ~mmw ~ysii COI=am mcreasmgly bang t8@n with wettabihtyalteriug, od-based, surfwtant-mnmining muds. When a cores nettability has beenaltered by such drilling fluids, sfwial procedures must be used to obtain proper analyses. First, the core plug must be cleaned ofwetmbility-altering ccmmtninams, then te.stored to reservoir wetmbilirj using urmonmminmed native crude before accmme relativepermeabtities cm be obmined.

These special cleating 8nd restoration procedures are described for cores from the Hutton field, located in the U.K. secmr of theNorth Sea. Plugs taken from Hutton cores were contaminated with invefi-oil-emukion drilling mud, which made them strongly oil-wet. By use of special cleaning metbcds, the surt%mnts from the drilfittg mud we= removed, and the Hutton reservoir wassubsequently shown to be only slightly oil-wet by use cd relative pmm%bilities on plugs that had been restored to mtive wettabilky.It was also found that some sections of the Hutton cores were neutral-wet m slightly oil-wet even when clean, possibly because ofthe pmence of coal.

Introduction

Nativ4tate Core. Nettability is one of the major control pamtn-eters in multipbase-type flow analyses such m relative perme8bil-W. 1 fbe be~ re8Uk3are obtained with mtive.state OXW,,wherealterations to the wettzbilily of the undisturbed reservoir rock areminimized. The term native-state is used in this paper for anymm that is obtained and stored by methods that preserve the wet-m.biiityof the reservoir, regardless of whether an oil-or water-baseddrilling fluid is used. However, be aware tit some papers do dM-dnguisb on the basis of drilling fluid (e.g., see Tteiber et al.z).Jn tkse papers, native state refa only to a core taken wbb a suitableoil-based dritting mud, while fresh state refers to a core withunaltered wetmbility @ken wilh a suitable wat8r-based mud.

Native-state cmes are obtained hy (1) coring with a mud that ccm-tains no sutfactatm or other chemicals that can alter tie nettabilityand (2) packaging the core m prevent wettabifity alteration. We nor-mafly recommend that a water-based mud with a mi@num of ad-&fives be used to obtain native-state cores. The use of tiesh or saltwater wcidd be based on whether hydrating clays wi!J be encoun-tered during drilling. A typical freshwater mud would contain onlypolymer, bentonite, and weighting material. A saltwater systemwould contain potassium chloride (KCl), polymer, benmnite (m helpform a tilter cake, not for viscosity), and weighting material. Causticmaterials should not be added for pH control because they can reactwith compounds in the etude to 81ternettability. Synthetic forma-tion brine or uncontaminated resemoir cmde can also & med mobtain mtive-state cores. 3 However, reservoir crude is norntallynot used because of the fire baza.rd. Oil-based emulsion muds andother muds containing suttwrants, caustics, 0rg8nic corrosion in-hibitors, or mud thinners shotdd be avoided because they can alterthe wettabiMy of the mres.

Once the core is brought to the surface, it must be protected fromwe.tmbility alteration resulting from loss of light ends or deposi-tion and oxidation of heavy ends. Upon exposure to air, sub8t8ncesin cmde cao oxidize to form polar prcducts that are surfacmntt,81t8ringthe nettability. 3 To avoid thes8 problems, there are sever-rd alternative packaging pmcedum M can k-sused for natkestatecores.4 Cores taken with a rubber sleeve, or fiberglass or poly-vinyl chloride (PVC) liner, can be. presened by cutting the coreinto sections and capping and sealing the ends. For other cores,the preferred method is to immerse them at the welfsite in deox-

.Nw withPel@eumTesting%vice.. .NmvwithParlklaleWd RW-3CUCh[c,C4PyTight19~ 3@e4y01PetroleumEn@ws

SPE Formation Evat.dim, Dwemtw 1987

ygenated formation or synthetic brine in a PvC tube, which is thensealed against Ie&age and the entrance of oxygen. If this prme-dure is not possible, the cores should be wrapped at the wellsite \in fwlYethYleneor polyvinylidene film (Saran WrapTM)and then ...in dumiltum foil. The wrappsd cores are thea seafed with a thicklayer of pan.ftin or a plastic sealer. Even when the nettability isknown to be altered because of Mlimz fluid constituents. coresused for spckd core analysis should & wrapped to prevent fur-ther alteration.

Restored-State Core. While it is always preferable to n@e.-. . .

multiphase-iype flow measurements on native-stare cores, some-times_sxb c&&areunavailable. This t@ally ccmus because either(1) the core wa3 exposed to oxygen and/Or allowed to dry outz+or (2) the core was cut with a mud containing surfactatms, such asan invert-oikmukion ddling mud. Because of cost factors, manycares will &cut with tivert-oil-emukion drilling mud because thesenmds can reduce drilling time, cost, and problems substantially. !0Because of the sutimnts in such mud8, cores cut with rhese mudsare usually contaminated, which renders them oil-wet and aftersthe mtive resemoir nettability.

When only cores with altered nettability me avaitable, the bestpassible multipbase flow measurements are obtained by restotingthe reservoir wettabfi~ using the three-seep process shown in Fig.1.,7.8,1l-ls The timt step is to clean the core to remove all cOm-pounds from the rock surface. After the core is cleaned, the sec-ond step is to flow reservoir fluids into the core sequentially. Thecom is saturated with the formation brine, followed by a cmtk-oiitlocd to simulate the inflow of oil into the core. Finally, the coreis aged at resemoir tempwmme for a sufficient time to establishadsorption equilibrium. This method has already kn used to re-store iutpmpdy stored cores or previously cleaned c0re3.7,11-13In tbk paper, we will show tbzt tbe methcd can also be used forcorm contaminated with drilling mud surfactants.

Core Cfewting. The frst and most difficult step in wettabili~restoration k to clean the c.oumtnirmtedcore to remove all cOm-Founds from the surface.. All compounds must be removed fromthe core because we have no knowledge of.which compounds wereadsorbed on the original rcxk and which were&posited afterward.The clearing is succe.wfid when thecore is left strongly water-we~because almost all clean reservoir minerals without adsorbed com-pounds are strongly water-wet. (An exception is coal, which maybe weakly water-wet or neutrally wet. 3, U.S. Bureau of Mines

SW

CORING

MEASUREWEITABILITY

z

SPECIALCLEANING

MEASUREWE7TABILITY

SATURATIONWITH FORMATION

FLUIDS

1AGING AT

RESERVOIRCONDITIONS

MEASURENETTABILITY

?REST:FWD-:TATE

PERMEABILITY

Fig. 1Sequence of procedures to obtain relative permelbiIitie5 when weffabllity alteration has occurred.

(USBM) 19,20or Am0tt5 wettabifity measurements are used to veri-fv that the cleaned core is smcmzlywater-wet. Note that the clean-~g requirements for nettability-r&tomtion are more stringent thanthose for routine core analysis where the core is generally consid-ered to be clean when the extract from the core is visibly clean.21

To date, cores for nettability restoration have km successfullycleaned using either distillation-extraction (Dean-Stark and Soxh-let) or flow-through cleaning methcds.= Unfortumtely, determin-ing which solvents will clean the cores successiidly is still ahial-and-srror process, which adds to the time and expense of wet-fability restoration. WetLability is nommlly measured on the con-taminated core., *en remeasured after each cleaning attempt tomonitor progrew. Typically, a serks or mixtures of solvents willbe needed to clean the core. The best choice of solvents dependsheavily on the crude oil, the drilling mud contaminants, and themineral surfaces.. For example, solvents that effectively clean somecmdes from cores often fail in other cases. A survey of the sol-vents and cleaning methods used to clean corss can be found inRef. 22.

Wehlz6fZi!Y Restoration. Once a clean, strongly water-wet coreis obtained, the wetfabilily can be restored (Fig. 1). The core issaturated wifh devxygenated formation or synthetic brine, followedby a crude-oil fled to simulate the intlow of oil into the core. Thecore is then aged at the reservoir temperature to establish adsorp-tion equilibrium. Several expimemem have compared measure-ments made on cores in the mtive, cleaned, and restored states.Jn each expwimmt, me+mt3 in the mtored state were alm03tidenticaf to the previous mtive-state ones, demonstrating that thisprocedure will rsstore wettabilily.8,1&16,18

An example of nettability restoration after com cleaning is shownin Fig. 2, taken fmm Mungan. 14Native-state cores were cut from

510

1.0

0.9 -+..4 NATIVE C(X2 ,

R~lR FLUIDS.4aEr!NE0 WE,

= o. a FURIFIEOFLUIDS

&F.5mRm03E.

E2sERWIR FLUIDS

$0.7 - T= l5&F

~

.0.6 -g

~ 0s -~,

#K 04 -

5i=~- -

02 -

0.1 -

0k3203040= 6070vATER 3ATLWTION, PERCENT WE vOWME

~g. 2Comparison of relative permeabilities measured onx reservoir core in the native, cleaned, and restored~tate=. 74 The advancing contact angle, OA, m-ured on aIiat quartz surface was S7 for the rsservoir fluids and 33for brine and a refined 011.

a Pennsylvanian 3and3t0nereservoir witi lea3e crude.oil, then storedin lease crude to preserve wettabili~. Unsteady-state relative per-meability was measured on native-state plugs with brine and livecrude oil at reservoir temperature (138F [dOC]) and a pressurehigh enough to keep!all gases in solution. The cores were thencleaned with benzene, followed by toluene, and then dried. Rela-tive ~rmeabilities were remeasured on fbe cleaned cores witi syn-thetic formation brine and a refined mineral oil. Finally, the coreswere saturated with brine, driven to irreducible water saturation(IV/S) with etude, and aged at the reservoir temperm.m? for 6 daysto restore the nettability. The relative permeability for these coresin the restored state was tlen measured. Fig. 2 shows Omtit is verysimilar to the mtive-mm relative permeability, implying that thewettabilky was successfully restored. Mungan then repeated thecleaning, restoration, and relative pcnneabtity measurements onthe.same cores a second time, obtaining identical results.

Based on Craigs rule-of-thumb method for wettabili~, thecleaned relative prmeabifity is significantly more water-wet than

=24 Craigs method is listedthe native- or restored-state ones. ,here.

1. Interstidid water samratiom are u3u3flygreater than 20 to 25 %PV in a water-ivet rock, but less than 10% PV in an ol-wet wk.

2. Water $anuation at which oil and wafir relative pwmeabili-ties are equal i3 generally greater than 50% for water-wet coresand less than 50% for oil-wet ones.

3. lle relative permeability to water at flmdout is gememllylessthan 30% in water-wet rocks, but from 50 w lCO%in oil-wet ones.

Mungan ASOmade contact angle measurements that demonsmatedthat the crude affected wettabili~. The advancing contact angle,@A,measured on a flat quartz surface was 87 [1.5 rad] for thereservoir fluids and 33 [0.57 i-ad] for brine and the retimed oil.

SPE Formation Evaluation.December 1937

. . .

When crude 01 used for wettabii~ @oration is obtained, precau-tions should be taken to minimize altemtiom to the crude. TIE sam-ple must be taken before any &ctants or other chemicals are addedto treat the cmde. It should be taken as long as possible after anywell treatments, to allow time for these chemicals to be flushedfrom tie well. Finally, the crude should be sealed in airtight con-mfners as scon as possible to minimize Oxi&tion and loss of lightends.

The aging time required to m-establish reservoir wettabili~ var-ies depending on the crude, brine, and reservoir rock. Generally,we fml that cores should be aged for 1,OXthours (40 &ys) at thereservoir temperature. TM aging pe.ricd was chosen for two rea-sons (1) several experiments have shown that d.tnes to 1,W

fhowz are re@red to reach wettabiliiy e@ibrium,3.7.8. 1.2.5-27and(2) 1,WHI hems is roughly the length of time required for the cOn-tact an le measured on a flat surface to approach its equifibtiumvaIue.~24.28.29 fn some cases, the resonation time can be signiti-cantIy less than 1,000 hours. As mentioned, Mungan 14 was ableto restore the nettability after aging for 6 days, while wettabliiyof the rmk/oilibrine system used in Refs. 15, 16, and 18 was re-stored after only 3 days. Salathiel 17was able to restore a ndxed-wettability state to samples after 3 days. Cuiec et al. 12 describetwo reservoirs in which the wettzbility was restored after only afew hours, with no tier cbznge in the wenabfily for aging timesas long as l,tW2 hours.

There are two basic options to determine the agingtime to re-store wettabJky. We fml that it is most convenient to age all coresfor 1,OXt hours, which is roughly the maximum time that tbe ex-periments discussed above required to achieve wetting equilibri-um. While cores may be aged for a Fried longer than tie minimumnecessaIY, this is not a serious drawback because the aging coresrequire midmal atfenfion. Another possibility is to determine theminimum aging time by measuring the wettabiliV of the cores withthe USBM or Amott methcds at frequent intervals during the ag-~g P:ri~. The ag~g k stOp@ when the wettabilify reaches itsequthbnum value. Although this minimizes aging time, it is muchless convenient becsuse it is labor-intensive and requires frequentdisturbances to the pIug.

The core can be agsd either at the reservoir pressure with live~@1416,18,26 or at ambient pressure with dead cmde. 8.27~.nlive crude oils and reservoir pressure are used, the solubilities ofthe wettabili@tering compounds should have their reservoirvalues. It is possible tit the wettabili~ will differ when dead crudesat ambient pressure are used. However, it is not currently knownwhether the difference is in ortsnt.

8Laretu et al. 27 and Cuiec found that it is sometimes possible

to speed up the approach to wetting equilibrium by saturating thecore with oil alone. In principle, the procedure is faster becausethe wettabilily-akm-ing compounds no longer have to diffuse acrossa water layer to adsorb on the rock. However, this procedure shouldnot be used when restoring reservoir nettability, becaw it can giveim.ccurzte results. 3 Instead, the core must be saturated with brinebefore the crude-oil flcod for two reasons. First, the brine cbends-try, including ionic composition and pH, can be ve~ important indetermining the wettabiihy. Second, the brine is nece,rsa~ to re-store cores having mixed wetfzbility, 17 where the large pores areoil-wet and the small ones are water-wet. During the aging proc-ess, the small pores must contain water to prevent the depositionof an oil-wet fdm, which wotdd give the entire mm a uniform wet-tzbility, rather tlmn its mtive mixed wettabili~. 3

Salatbiel 17 generated mixed-nettability samples by aging out-crop Boise sandstone core with brine and a mixture of dead EastTexas crude and heptane for 3 days. A mixture of dead crude andheptane was used because it would deposit stable, oil-wet fdms onsurfaces on which it was in direct contact during this short period.Oil-wet tibns generafed after 3 days with only dead crude were muchless stable and could be displaced by brine atler a relatively briefcontact. Note that if SaMtdel had wanted to restore rhe weftabilityof an East Texas reservoir core, it would have been uecessmy tosaturafe the core with brine snd crude, then age the core at rezer-voir conditions for a longm period of time. However, SaMhiel wasstudying displacement mechanisms in mixe+wettabili~ cores andfid not need to reproduce the reservoir wettabdily exactly. Cores

SPE FormationEvaluation,December 1987

were aged with brine and a cmdelheptane mixture at mom condi-tion because this wa3 the most convenient methcd to generzte ,.

ndxed-wettabtity systems.

.%lathiel found that the oil recove~ from wateztlceds of theseaged samples was dep+mdenton the brine saturation during aging,with maximum recovery when the core omtzined approximately13 to 20% PV of brine during aging. With lower water saturations,the smaller pores became oil-wet and trapped oil during the water-flced., incres?.ing the residual oil saturation. On fhe other hand, forwater saturations larger than 20% PV duxing aging, portions of thekwger pores retnzined water-wet. It appesrs that this dismpted theomdmdty of the oihmt surfaces in the larger pares, mzking it easierfor portions of the 01 to become disconnected and trapped duringthe wztei-flccd. We recommend that the water saturation during ag-ing be obtained by using reservoir crude to oiltload an iltitiy 1LX3%brine-samcated plug to IWS simulating the inflow of oil into an in-ifially water-ftied reservok.

Wetkzbi@ Assay. The two quantitative meihcds suitable for as-saying the wettabiliiy of reservoir Coresz are the USBMmethcdlgn and the Amott methods We chose the USBM IMIIdto determine the wetfabilhy of cores. Tbe test is relatively quick,requiring 1 &y to test four to eight core plugs. A major advantageover the AMott test is its sensitivity near neutral nettability. TheUSBM test compares the work necessary for one fluid to displacethe other. Because of the favorable free energy change, the workrequired for the wetting fluid to displace the nonwetting fluid fromthe core is less than the work required for theopposite displace-ment. It bar been shown thzt the required work is pmpadoml tothe area under the capillary-pressure curve. 30.31In other words,when a core is water-wet, the area under the brine-drive capiUzry-pressure curve (when water displaces oil) is smaller-than the areaunder the capillary-pressure curve for the reverse displacement.fn fact, if the water-wetting is strong enough, most of the waferwill imbibe into the com spartanmurly, and the ares under the brine.drive curve wiU be very small.

Before fbe testis run, plugs are prepared by centigation un-der oil at high speed to drive them to IWS. The USBM methcdthen has two steps Otst use a modified version of the mcedpre de-

8scribed by Hassler and Bnmne#2 and SIobcd et al. to calcutatethe cemmifugalcapillary pressures. The USBM mekd uses tie aver-age saturatio~ in the plug.m In conhast, tbe centrifug.zl capillary-

. . .

pressure cm-w is based on the saturation at the face of the core,which is cdctdsted from the average saturation. 32.33

In the fit step, plugs are placed in brine and centrifuged at in-crementally increasing speed until a czpillzmypresure of 10 psi[69 kpa] is reached. This step is known 22 me brine drive, ke-cause brine dkplaces oil from the core. At each incremental CSpil.lzry presmre, the avenge saturation of the plug is caktdstexi timthe volume of expelled oil. In the second step, the plugs are placedin oil and cenhifuged. In this oil-drive step, oil wi13displace water.As in the fist step, the capilkiry presmm and average SatUrS,tionare measured until a cspilkrry pressure of 10 psi [69 kpa] is reached.In ezcb case, the curves are linearly exn-zpolafed or truncated ifthe last pressure is not exactly 10 psi [69 kpa].

Tbe USBM method then uses the ratio of aress under the twocapillary-pressure curves to calcukte a nettability index by use of

w=10g(A1/A2) . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...(l)

where Al and A2 we the aress under the oil- and brine-drivecurves, respectively. Wkt Wi3 greater than zezo, the core is w2ter-wet, and when iPis less than zero, the core is ol-wet. A wetfabili-ty index near zero mans that the core is neutrally wet. The largerthe absolute value of W, the greafer the wetdng preference. Gener-ally, there is some variability in the value of Wfor idmticslly treatedplugs. Our work indicates that the stsnckird devistion of the meas-urement is about 0.1. ~

Hutton and Murchison Mud Program and CoringThe Murchison and Hutton fields are located in the U.K. sectorof the North Ses. Before the Murchison production wells weredrilled, a comprefmmive review of the drilling and mud programsW= ~detien. IQ One of the conclusions was that the Murcbi-

511

mNa&04 0.045

1WELL H3/21, 10,121 FT.

+=28.OZ, K,=2.757. rnD&=1,600 mD at S4=17.+

~ .8 -

i65aw .6 xlxw0-

w 4 >Fhz

E .5

2+-

S

>0m

4

.5

ig. 4-Core cleaning results for Berea plugs contaminated with an invett-oil-emulsfon mu,iltrate.

SFE Formation Ewluation, Decanber 1987 513

I TABLE 5USBM NETTABILITY INDEX,CLEANED HUTTON PLUGS (Well H5/13)Cleaning We&bility

Formation Method Index .

Upper Middle Shaly Three-step -0.05Lower Middle Shaly Thr-step + 0.23Massive Three-step + o.&4Bazal Twstep + 0.52

~

mm-step. thre. SU.mwe Dw.mstark exmtil..s-lol.m. (20hours), followd by glaclti ac3tic add (23 hews), followti by ethanolTW-WP. m!nfugal flushing wth warm chloroform (6 hours), folldwdby DeaG-S.srkMramon (~ hours)witi WMW solvent(49,5% tcluenel49.5% melhanolli% ammonium hydmx[d.).

was the thke-step, while 2-methoxyethyl ether was the next bestfor the Guelph dolomite, indicating the effects of the mineraf surface.

Hutton Cores. After cleaning methods using Berea plugs were iden-tified (Fig. 4), Hutton plugs were cleaned by simifar mehds. Ingeneral, cleaned Hutton plugs were water-wet, indicalitg tit clezm-ing was successful. In one zone, bowever, the cleaned plugs re-mained mxtrally wet. As discussed below, this maybe the resultof organic materiaf in the core. Plugs with 1- and 1!%-in.[2.5- and3.8-cm] diameters were cut adjacent to each other. Synthetic for-mation brine was used to lubricate and to cool the core dtill. Thel-in. [2.5-cm] diameter plugs were used for wettabiiiiy measure-ments, witilelhe adjacent 1Ih-in. [3.8-ctn] -diameter pIugs wereused for relative permeab~iy measurement. The ends of the plugswere trimmed dry and bmshed to remove rock cuttings. The amountof carbonate in the plugs was measured on the end cuts from thesample.

The amount of carbomte in the rock determined the cleaningmethcd to use. The three-step methcd was used to clean all plugs

> ,8 - RESTORED-SATEf-

f=a~ .2 -cK

ot

o .2 .4 .6 .8

WATER SATURATION, PV

ig. E-Restored-state ws. cleaned relative permeablllth+utton Well H5113, Massive Sand zone, 10,077 ft. PI:Ieaned by the three-step method. The USSM wetfabilltyiex of the adiacent-wetfabilitv Dlua was +0.64 after cleanilw water-wet - -

TABLE 6--CORE CLEANING RESULTS,CONTAMINATED HUTTON PLUGS FROM THE

fdEUTRALLY WET ZONE (well HII12)

USBMCleaning Method Nettability IndexContaminated, not cleaned -0.42Dean-Stark with spetial solvent for

20 hours -0.15Three-step. -0.14DeanSt& with tokmne followed by

steamsoak for 72 hours -0.08Two-$tept -0.07

.Sp+clal soivent. 49.S% methaml, 49.5% Mum., and 1% ammoniumhydmide.

..mmSep. *,ee ,,mWe Man.swk sxtracms-tol...e (m h..~l.followed by glacld amlio add (20 hours], followed by .0!,..1 (20 hours].

Tw.s!e$.=chlomfom cenbifuw (6 hours),fdl!umd by axtrac!lowith w%@ sdvet (2Chmm).

containing less than 0.5 W% carbomte. For all Hutton cleaning,the time for each Dean-Stark extraction was increased to 20 hours.Because the glacial acetic acid used in the thre+step metlmd candiss61ve carbomte, plugs containing 0.5 W% or more carbomtewere cleaned by an alternative me,thod,which was a two-step prcces.sccmsisdne of chlomfotm centrifiwation for 6 hours, followed byDean-S&k extraction with a ~ird solvent for 20 hours. Af&extraction, the z2111pleswere placE@in an unituntiditied oven at250E [121C] for approximately 8 hour$ to drive of ftbesolvenbthen cooled and stored in a desiccator.

As Fig. 4 shows, two S-SEof solvents were more.effective in C1ean-ing contaminated Berea core than the two-step process describedabove extraction with 50% meihmol/50% chlomfmm and exhac-tion with tohtene followed by metbanolichlomform. The reason thatthese solvents were not used to clean Hutton core is that they weresuggested late in the swdy. By the time they were tested in Berea,most of the Hutton core cleaning bad already been completed.

Typical postcleaning nettability measurements of Hutton core.are shown in Table 5 for WeU H5113. Average wetmbilily for@

-. .

1

> ,8i-3E~ .6zuwCL

Ld 4>Fa

~ .2cc

o,

T WELL H5/13, 10,102 F..+., CLEANED-STATE6= 25+. mD d ,%-31 .8!i: + RESTOREDSTATE& 202, ,0 at S.-29.2$+=24.8%, K,= 338. m[I

.2 .4 .6 .8WATER SATURATION, PV

Fig. 6Restored-state vs. cleaned relative pemteabll{iies,Hutton Well H5{13. Massive Sand zone, 10,102 ft. Plugclaaed by the two-step method. The USBM tiettab[lity in-dex of the adjacent-wettabifity plug was + 0.45 after cleaning,or water-wet.

514 SPE Formation Evaluation, December 1987

u>

1 t

T!sWELL H5/13, 10,206 Fi-.+.. CLEANED-STATE

& 857. ,nD at 3+-31,62- RESTORED-STATS

K.- 950. mD at S+-30.024=21 .2%, KA=l ,086. ml

!.

,;

~

o1 I

o .2 .4 .6 .8

WATER SATURATION, PVi9. 7Re6tored-state vs. cleaned relative permeabltities,iutton Well H5/13, Basal Sand zone, 10,206 ft. Plug cleaned)y the two-step method. The USSM nettability index of theIdjacent-nettability plug was +0.52 after cleaning, orvater-wet.

zones before ckaning was 0.25, mifdly oil-wet. The Massive andBa3ai plugs were strongly water-wet *r cleaning, while the LowerMiddle Shaly was mildlv water-wet. In mite of rereated attemnts.we were u~ble to de~ the Upper Mi&le Sba2~ plugs to morethan a madly wet condition. Adjacent plugs were cut from WetlH 1/12 in the same neutmtty wet zone and cleaned by several differ.ent methods. As shown in Table 6, none of the cleaning methcdsrendered the plugs water-wet (USBM in&x greater than zeto).

The plugs that we were unable to clean maybe naturally m-trally wet. X-ray pbotodectron spectroscopy and ion suipping showthat approximately 50% of the rock smtlce in the neutrally wetzpne is covered by a thin layer of organic matter, less than 300A [30 MI] thick, which may be caused by diffusion of organiccompounds released during diagenesis fmm the small, organic,detrital particles of coal scattered throughout the zone. Work intie flotation industry indicates that clean coal is weakly ivater-wetor neutrally wet. 3 Cuiec er aL 12 and Cuiec 13cleaned unpresewedcores (with no drilliig mud mntamimtion) before restoratim ofnettability. fn four cases, where cores contained large amounts ofuncxtracfable carbon, they were likewise able to dean the coresordy to neutral wettabiliiy.

The h@esis of organic compounds from coal coating the sand-grain surfaces is somewhat clouded. Thin-sections from both water-wet and neutmlly wet (after cleaning) Huttcm zones show that bothcontain approximately eqwd amounts and distribution of wcxdy coal,algal co?l, and pyrite. Although an amempt was made to use Augerelectron spectroscopy to determine the origin of the organic coat-ing in the neutdy wet zone, this failed because the electron beamcaused the samples to become electrically charged. Consequently,at this time, we are uncertain what causes the postcleaning neutralwetness of Huttons Upper Middle SbaIy zone.

Nettability Res20rati0nTo rest~re nettability, 1%-in. [3.8-cm] diameter plugs were tirstcleaned by the same medmd as the adjacent-nettability plugs, eitherthe two-s@p or three-step method, depending on the amount of car-bomte. The cleaned samples were then dried and the porosity andair permeability measured. After clea@g, the relative permeabiky

SPE FormationEvaluation,December 1987

1WELL H5/13, 9,916 iT.

--+-, CLE4NED-STATE!+4970. r,D.1 %-24,32

~ ,8 - - RESTORED-STATEK.-4.88 1. mD at %..2O.3Z

i *=23,4% K.=6,412. m[E

~ .6 -

&IAL

>

F-

q .2 -u

o0

WAYER S~TURfTION;8 PVFig.8Restored.state vs. cleaned relative permeabititieHutton Well H5/13, Upper Middle Shaly zone, 9,916 ft. PILcleaned by the thre~step method. The USBM nettability iliex of the adjacenkwettability plug was -0.05 after CIW3nin!m neutrally wet.

.. .

was measumd with brine and mineral oil for comparison with tielater restored-state relative permeability. Tbe dry plugs were placedin a Hassler-type core holder at a conftig pressure of 2,W0 psi[14 MPa] and the cnntlned airpmneabii was measure-d.Tbe samp-les were then evamwed ofairand saturated with synthetic for-mation brine by metering to a pore pressure of 403 psi [2.8 MPa].Under a backpressure of 202 psi [1.4 MPa], brine was pumpedthrough the samples to remove any trapped gas, and a lCO%brinepermeability measured. The plugs were driven to IWS with a 25-cpP5-pPa.s] re6ned mineral oil. Next, watdoil relative permea-bfities wore calculated from a room-temperature, constznt-ratewaterflood by the 3BN metbcd. .35 The relative permeability ap-pzrztus k computer+ontro!.led, nuking mezmrements of presmredrop and oil prcducdon approximately once a second, allowing ca-lculation of more than the usual number of points on the relativepermeability curves. After the relative permeability measurements,the samples were cleaned by Dean-Stark exuaction with toluene,followed by cenmifugation with chloroform. The samples were thendried in an uuhmnidified oven at 250F [121C] for approximately 8 hours, and stored in a desiccator until needed.

The nettability of the cleaned Hutton plugs was then restored.The dry plugs were placed in a Hassler-type core holder at a con-fining pressure of 2,0(KIpsi [14 M%] and saturated with sy@beticHutton bdne by the pmcecbmmfm the cleaned plug described atwve,The plugs were then driven to IWS with 0.45-#m-tiltered dead Hut-ton crude oil. The samples were removed from the core holdersand stored in dead Hutton crude onder a 2GQ-psi[1.4-MF%I]met@eor nitrogemblanket. fbis pEssure was used to prevent oxygemfromcontacting the crude and mres and does not represent an attemptto use live cmde oil for restoration. The samples were heated mreservoir temperature (220F [105C]) and aged for approximate-ly 1,1X!41hours (4I &ys). To facilitate rearrangement bf tie fluidsas the nettability changed, cmde oif was occas.ior@ly tlWhedthrough the plugs.

A&r 40 &ys, the crude was flushed out of the plugs W@ a 25-cp[25-mPa.s] mineral oil and an oil permeability at IWS measured.We tested this mineral oil on native-state plugs tiom a number ofdifferent reservoirs and found that tie wetoabiliiy was not altered

515

by flushing the plug with it. As expkiined before, tie wafer-oif rela-tive pwmeabilities were measured by the umteady-stats JBNmstlrod. At the end of the watsrffcod, the effective permeabilirjto brine at the residual oil samration was measured, then the sam-ples were cleaned by DeAn-Smk extraction with toluene to deter-mine saturations.

Relative Permeability Measurement~ical results for the cleaned- and restored-state felative permea-bility measurement are shown in Figs. 5 dnrough 8 for plugs fromWelf 33s/13. AU relative pan-abilities are based on the oil per-meabii at fWS. By Craigs nd~f-thumb methcds for determiningnettability from relative pemneabtiLY,a>X the cIeaIIed Kkativepermeabilities shown in Figs. 5 through 7 are s@onglywater-wet.This is confirmed by USBM nettability measurements on adjacentplugs cleaned by the same methcd.

The restored-state plugs are significantly less wafer-wet than thecleaned plugs, as shown in Figs. 5 through 7. The water saturationat which water and oil relative permea.bilities afe equal is lower,and the water relative Wrmeability at flcodout is higher, indicatingthat the restored plug is more 0il-wet.24 Based on our experiencewith other rswvoirs, we believe that these relative permeabilityresults indicste that the restored cores are neutrally wet to UtOdfyoil-wet. Unfortumtely, we did not measure USBM wettabifity onthe re.stored-sfiti ratnples..Relative parrmmbiiitymmmrsmenfs weremade on 1%-in. [3.8-cm] -diametir plugs. Because of sample-sizelimitations of the centrifuge, wetfabilily measurements were madeon adjacxmt l-in. [2.5-cm] -diameter plugs. Only the 1%-in.[3.8-cM] relative permeability plugs were restored. We feel thatthe wettabifity of the restopi-state plugs should be measured, sowe now restore both sets of plugs when restoring tbe wettabfityof contaminated reservoirs.

In Fig. 8, the cleaned- and restored-stats relative pmea.bilitiesare similar. This plug was taken thm the Upper Middle Shaly zone,which we believe may tw mhmdfy, neutzdly wet as @cussfd earli-er. The USBM wettabilily index of the adjacent-wettabiIiv plugafter cleaning war -0.05, or neuualfy wet. The plug became slightlymore oil-wet during restoration, because the water relative perme-ability increased and the oif relative permeability decreased at anygiven water saturation.

Conclusions1. Because of cost fictors, many offshore cores wifl continue to

be cut by use of invert-oil-emtdsion drilfing muds. Because of sur-facfants in such muds, these cores cannot b used for any analysisthat is affected by wettabilily, SUCAas capilkwy pressure or relativepermeability, until they are cleaned and restored. The proceduresnecessary for this cleaning and restoration are described here forcores taken tiom the Hutton reservoir.

2. the bsst cleaning metbcd for coIe contaminated by driltingmud COmtiNenLS,such as surfactants, must sdll be determined bytrial and error. Cleaning metbcdf that we found to be effective forthe particular sttrfactads used include the three-step metbcd, ex-traction with methanOl/ctdOrOfOnn,and chloroform centrifugationfollowed by extraction with a special solvent (tvm+tep).

3. when cores are cmtandmted, the best possible mukiphaw-type flow measurements are obtained by restoring the mtive wet-tabtily of the cores with the prxdues discussed. However, thereare sevemf disadvantages to the use of restored-state cores in #aceof native-state cores. First, the procedure adds gfeatly to the costand time for s@al core analyses. Second, a successful cleaningmeotod must be determined by a triaf-and-srmr prccess. F@Y,the cleaning metbcd may damage the clays in the cores. For thesereasons, we strongly recommend that nztive-state cores be used formultipbase flow analyses whenever possible.

NomencfaturtrA, = area under the oil-drive curveAZ = area under tbe brine-drive CUIVekA = air pamieabiiify, md ,k. = oil pmn.eabfity at IWS, md

km = oil fefative pmweabti~, dimensionless

516

km = water relative permdilir, dimensionless

Sti = initial water saturationW = wettabilily index

6A = advancing contact angled+~ = p-mosi~

AcknowledgmentThanksaregivento Samb BeJitz and Ray Luis for performing mostof the experimental work. We also thank the management of thefollowing companies for permission to pubtish this papm Conocofnc., Ccmcno (U. K.) Ltd., Bfitoil plc, Gtdf Oil Corp., Amera&Hess Ltd., Amoco (U.K.) Exploration Co., Enterprise Oil PIC,Mobd North Sea Ltd., and Texas Eastern North Sea Inc.

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