Drilling fluid for Dome F project in Antarctica

National Institute NationalInstitute of Polar Research Research ofPolar Mem, Natl Inst.PolarRes,,Spec.Issue,49,347-357, 1994 DRILLINGFLUID FOR DOME F PROJECT IN ANTARCTICA ShLljiFunTAi, Tomomi YAMADA2, Rerlji NARusE2, Shiaji MAE], Nobuhiko AzuMA3 and Yoshiyuki Fum' Plysics,thcutc), ofApt)lied ofEngineering, Hbkkoido U}tiversity, Kita-13,Nishi-8,Kita-ku,Sxpporo 060 :institute Sbience,Hbkkaido Lbeiver:sity, ofLowTlemperature Kita-19,?Vishi-8, Kita-ku, kpporo060 ]Department Ehgineering,?VLrguohaUitivensity ofA`techanical of1lechnotqgy, Kbmitomioko-cho,IVtrgaeko940-21 `thtionat institute 9-1O, Ktzga i-chome, ftabashi-ku, ofPolarResearch, Tblya 173 LDepartment Absttact:At Dome F, one ofthe summits ofthe East AntarcticIce Sheet,a yery deep operation isto be carried out by the Japanese Antarctic Research Expedition from1995. Since itwill taketwo yearsto complete the coring up to about 3ooO rn depth, the boreholeshould be fi11ed with properdrilling fluidto preventborehole closure during the operation. This paper isa report on our investigations for searching for the proper dn1]jngfluidwhich can be used in the very cold environment at Dome F, Although the investigations are still in progress,three kinds of fluidwere chosen as drMing fluid candidates. They are: 1)n-butyl acetate,2) "IP-solvent" with densifier and 3) SiliconeOil. Theirproperties were investigated and compared interms of densityand viscosity, which are essential Tequirements for a drillingfluid.As a result, itwas shown that n-butyl acetate and IP-solvent with densifier can be used as the dri11ing fluid.However. the use of ice-conng n-butyl acetate isimpossible without suthcient ventilation to dispose of itsvapor. choice at the coring siteor other When the use of IP-solventwith densifierisconsidered, ofproper densifier isalso a problern. action the 1. Introduction The JapaneseAntarctic ResearchExpedition(JARE)has a at plan fbr ice-coring Dome F, one of the summits of theEast AntarcticIce Sheet(77022'S, 39e37'E,3807 m a.s,1, (AGETA et al,, 1989)).This planiscalled the Dome F ProjecL It is plannedto take two years frem 1995 forthe coring operation to reach the bottomof the icesheet, which is at depth between 2800 m and 3000 m. In order to preventboreholeclosure duringthe coring operation, the borehole should be fiIled with a proper drilling fiuid.So far,various kindsof drilling fluidhave been used fbrice-coring of glaciers and ice sheets. Some reports have been published(e, g. GosiNK et ai., 1991). However, since the annual mean temperature at Dome F is about -580C fluidthat can be (AGETAet al,, 1989),a drilling used insuch a cold environment isrequired forthe project. Consideringtheviscosities at temperatures up to -580C, only a few kindsof fiuid can be used at Dome F. The fundarnental requirements fbrthe drilling fluidare as fo11ows: 1) Physicalprepenies a. The densitysheuld be comparatively the same as icein the ice sheet between about 940 (kglm3) and abeut 960 (kgtm]). b. The viscosity should be smaller than 10 (cp), 2) Effecton the icecore qualities 347 NII-Electronic Library Service National Institute NationalInstitute of Polar Research Research ofPolar 348 S. FuJffA The fluidshould 3) 4) not spoil et aL icecore quality. Effecton the coring is compatible witih the coring The fluid must be one which Effectonhumanhealth forthe peoplewho The fluid must not cause healthhazards equipment 5) Lowcost and volatility 6) No dangeron flammability There isno fluidwhich satisfies all of the stated as candidates liquidsremained 1) N-buty1 acetate after carefu1 equipment. work at thecoring completely. requirements investigatiens. They are as site. Some fo11ows. 1620" is as 1620" with a densifier. identified a company which manufactures manufactured by Idemitsu, products. The petrochemical can also be used as a fueloil. It has similar suggested that the IP-solvent manufacturer are D60". The densifier candidates as the fueloil components and properties CFC11 or CFC113. Here CFC isthe abbreviation ofChloro Fluoro Carbon. which by Shin-Etsu,a company cs) manufactured 3) Siliconeoil (KF96-1,5 manufactures silicone products. report about n-butyl acetate as the drilling Among these three candidates, a detailed et al. The fluidfordeep drillingwas givenby GosiNK (1991). advantages and disadvanas fbllows. One ofthe advantages ofn-butyl acetate is tages of these threeare summarized 4 cp at ternperatures above --600C). Thus the total thatthe yiscQsity isyery small (below is In addition, no densifier periodof tirne fbrthe coring operation can be minimized, air pollution. The Itcannot be used without sufficient ventilation to minimize necessary, is still is larger than that of n-butyl acetate but viscosity of the IP-solventwith densifier can be used as the acceptable. However, itis not certain yet whether CFC113 andCFCI1 A problem remains becausethe freezing densifier. point of CFCI13 is -35eC and the ef safety for vapor pressure of each CFC isvery high. SiliconeOil has the advantage human health.However, ithas a higherviscosity than theother two candidates and itis 5000 yenlkg). It can be used only when relatively high viscosity is expensive (about acceptable forthe coring operation. conditions in the icesheet at In this paperwe first show the temperatureand pressure in terms of density Dome F. Second,the propertiesof these three candidates are compared fluid.Finally, and viscosity becausethey are essential conditions required forthe drilling 2) A "IP-solvent, "IP-solvent, solvent "Exxon some problemsthat still rernain unsolved are summarized. 2. Environment inthe lce Sheet at Deme F described The drilling fiuidshould be one which can be used in the environment OC. -58 This was mean temperature is expected to be below. At Dome F, the annual NIsHIo deducedfrom the 10 m snow temperature there observed by AGETA et al. (1989). communication) calculated the possibledepth-temperature profile.In the (personal calculation, steady state and accumulation rates equivalent) between2 and 5 cmla (ice were rate was assumed to be 5 the accumulation It is shown as in the ice sheet was estimated. cm!a, the temperature-pressure (T-P) profile hydrostatic Fig.1. Itis seen that boththetemperatureand the internal pressureincrease in assumed. Based on the calculated result when NII-Electronic Library Service National Institute NationalInstitute of Polar Research Research ofPolar Dri1]ing FluidforDome F Prejectin Antarctica : 349 lii"" i l2ooom"11/･ il t.tttttttt"tttttttttttttttt;ttttttttttttlttttttttttt[tttttttttttltt "H" ' ttttttttt cuAasgZEoovdi .........."""".llL."."...--11i""""H.-. ../.rfr]",.,H..""l"".. ' i/ L= ttttttt M 15mo ttttttttttttttttttltttttttttttttttttttttltttttttttttttttttt/tttttttttttttttttttttl i 1500m ' /, LaO 1' i il ttttttttttttilttttttttttttttttttltttttttttttttti tttt't"""t"'ttttttttttt"""'tttttttttttttitttttttttttt 10･--esu9,Vhs ttttt-tt+ttttt ' moocimI i l j l i i i ...."""""i･....-."..ii.-.....i･ ms()c)rTi i wnl･ o -50 -eo l, ttttttttttttttttt 1'ttHtt'ttttitttttttttttHttttttt"tttt't l i i i [i -40 .eo temperature(oc) .oo Flg.1. Estimatedtemperature-pressure (7:･Il}profle in theieesheetatDome projilewas calculated communication). byDr. F, IV)siilo {Pensonal forthe -le F, 77ietemperature Bounciat:y conditions eatculation are asfollowsr the suij2iceY]ow vetocity (u.] iszero; sucflxce ,slope @ iszero; su(lkce temperature is -58eC;accumutation rate is5 cnvti; geothermatheatflzax is 1 (llFCV. the assumed accumulation rate is one which makes the calculated temperature colder. 7Viisvalue was selected to give the most eonservative estimate. deeperlayers. 3. Density Densityisone of the most essential requirements fbr the drilling fluid.When the hydrostatic in the borehole that is filled with pressure thedrilling fluidissmaller than that in the surrounding ice sheet, the diameterofthe boreholebecomessmaller, Conversely, when the hydrostatic pressure in the boreholeis largerthan that in the ice sheet, the diameterof the boreholeexpands, Inthissection, we first compare the density of the three drilling fluidcandidates. Second,the difference in the hydrostaticpressurebetween the boreholeand the surrounding icesheet isestimated basedon the densitydata.And finally, the annual strain rate ofthe boreholediameteriscalculated. Based on these estimates, we will show thata density betweenabout 940 (kglmi) and about 960 (kgfm') is desirab]e and thatthreecandidates can satisfy thisrequirement, 3,1,Density qfthefluid Firstof all, the density of the drilling fluidcandidates are shown in Fig. 2. The densityof ice is also shown fbreomparison. For the IP-solvent with densifier, CFCIl is assumed as the densifier. Three cases of the volume fraction between the volurne of IPsolyent and CFCI 1 were investigated. In the figure, pressuredependenceisnot considered except although the aetual drilling fluid is to [e2] be subject to a hydrostatic pressureup to 27 (mx!m2) in the ice sheet, GoslNK et al. (1991) describedthatthe densityof n-butyl NII-Electronic Library Service National Institute NationalInstitute of Polar Research ofPolarResearch 35e S. FuJITAet at. of iceis increasesnearly 3% at 3000 m depth.In contrast, since the bulkmodulus 1970),itresults in a densityincreaseof about at -100C (FLETcHER, about 10× 109OIfmi) O.27% at 3000 m depth.However, these effects were not considered in the datainFig,2. acetate 950=ENc ) oooMvh･--co= esoov BOO 60-se oo eo -2o o temperature (oC)-10 vehen versus temperature. (IVisthedensity drilling,fluid candidates ofice Fig.2.Density ofthe acetate (Gosnw et aL, isthe densityofn-butyl hydrostatic (Zny) pressure is net considered. the itseU'without the densijier. TViedatawasfrom l991). (e)is the densityofIP-solvent IP-solvent with denspter, are the density mant{t2icturer ofthe qf'IP-soivent. ld0-ldsp 7he densitiesare simple averages of were considered. Three cases ofvolumefraction is not considered. te1)is bothfluids. Increasein the density"'hen the.fluids are mixed when is ignored, is that Oil when hydrostatic te2) the density pressure ofSilicone The data on SiticonOii }v'ere given b.vthe hydrostatic pressure is considered. mantij2icturer. betweenthe boreholeand the icesheet 3.2.DUferencein the hyd}'ostaticpressure in the hydrostaticpressurebetween the Based on the datain Fig. 2, the difference boreheleand the icesheet (AP,(MNIm]))was calculated assuming that the boreholeis fluidat depthsbelow 100 m. At depthabove 100 m, the borehole fi11ed with driiling becausethe icesheet isexpected to be firnthere. Figure3 cannot be fi11ed with the fiuid there are also inaddition to the case ofa real fluid, AP versus depth.In the figure, shows with imaginary fluids which have various includedcases in which the boreholeisfilled At shallow depths, the hydrostatic densitiesbetween 920 (kgfm3) and 1000 (kglm]). than that inthe borehole becausethe upper surface pressurein the icesheet isalways larger fluidwith densitylargerthan about isat 100 m depth. Only when of the drilling fluid increaseswith depth. In addition, only when fluidwith density 920 (kglm3) isused, tt!P NII-Electronic Library Service National Institute NationalInstitute of Polar Research ofPolarResearch DrillingFluidforDome F PrQjectinAntarctica 351 largerthan about 950 (kgfm]) is AP becomes positivein deeper i.e. the hydrostatic in the boreholeused, islargerthan thatinthe surrounding icelayers; pressure sheet there. 1.0 O.5rvAE2E o.ova<e.s .1,Oo sw1000 2000 15oo depth(m) rtuld p=iooo :: lthil :lasinary 25003000 (ngltne} i/ i$/-. g IS iff/ wtth ' ' eo '･' 9`O er n Fig. ac･e,i"skEEEiig'%' 3.7he dtOlerence inthe 1ij,db'estatic between the boreholeand the ice sheetfor variotts pressure cbJilling,fluids, Positivevalue means that the lydrostaticpressure in the boreholeislarger than that in the ice sheet. (bD""(bsy are imaginar),Yiuids between 1000 (kgthtV oj'clensity and920 isforn-butylacetate, (kgfmV.(lt) icIJ-ic" isforIP-solventwithdensgfier. Three volume.fractions were considened. 7hese estimates are based on the datain Fig,2, borehole ofthe Based on Fig, 3,the annual strain rate of the borehole diameterwas estimated. The results are shown inFig. 4. For the estimation, itwas assumed that a boreholewas made at 3,3. Strainrate a time, and the strain after one year was calculated. betweenstress and strain rate observed at Mizuho The activation expressed as energy of the creep of fo11ows. To calculate the strain rate, the relation Station(NARusEet al., 1988) was icegivenby PATERsoN (1977) was also used. "A exp( e R7L )Xn. used. It is (1) a effective shear strain rate = effective A-4529 shear stress (MNfmi) NII-Electronic Library Service National Institute NationalInstitute of Polar Research ofPolarResearch 352 S. FuJITAet al, n=2.87 energy obtained by PATERsoN (1977) R: gascenstant n: temperatureindegreesKelvin changes inthe diameter Using the planneddiameterof the boreholeat Dome F, 13.5 (cm), Figure 4 indicates that when the densityis between 940 of the boreholewere calculated. Nthe annual changes inthe diameterare smaller than O.5mm, and 960 (kglm'), (kg/m3) the case of with densifier can satisfy this condition. Although buty1acetate and IP-solvent itwill be between(a4)and (a5). Silicone Oilwas not includedinthisfigure, activation {l};54 (kJlmol): o = pt = o O.OIO A ¢ Tasvo- =-1.0 a N' Bo･s as O.oo5 L 9 9O.O =di "- co L O-h o.ooo -= es o= oO.5 U` co O ".oo5 >･.-- o" tu o ot O o- =1.0 OOOIOo A 5001OOO 1500 am depth (m> 25oo30oo - - (al) imagjnarynuldwithp=tooo {xpim3) B B iss･ v :: Ige3 :2gg :: [:gmsge, -- (a6)pt900 n-buY aeetate - (b) IP-solvent and CFCII, 82:18 {ct) - = l::ge,lve:l :g8ge,l lig8i･fo, l%l and changes in the borehotediameter,Strain annuat strain rates due to the dtfilerence in the bydrostatic pressurebetweenthe boreholeand the are the cases ofthe imaginary.fluid that have icesheet shown in Fig. 3. laIV-'(laq) is the case ",hen n-butyl acetate densitybetween IOOO (kghnsp and 900 (kghnV.(lij are cases when IP-solvent with densijier isused, isused, ft'JJ'"ic3,V Flg.4.77ie calculated occurs 4. Viscosity Low viscosity forthe drill fluid.Sincethe isalso one ofthe essentiai requirements determinesthe drill's loweringvelocity and pull-upvelocity,ingtotal period oftime viscosity NII-Electronic Library Service National Institute NationalInstitute of Polar Research Research ofPolar DrillingFluidforDome F ProjectinAntarctica 353 the coring operation will be determined by it, Although itisdifficult to the exact viscosity of the drilling fluidunder high pressurein the borehole,we to cemplete estimate that the IP-solvent and n-butyl acetate exist as fluid under (rm!m2)and at temperatures typicalofthe icesheet of Dome F, lnthis section, we firstdescribethis experiment. Second, the viscosity of the drilling fluid candidates under atmospheric pressure are compared. Finally. based on these data,the tota1 periodoftime to comptete the 3000-m coring isestimated, experimentally confirmed to 50 pressureup 4.1,77iefluidunderhighpressure Firstof all, we brieflydescribethe experiment by which we confirmed that the drilling fluidwill not freezeunder pressures up to 27 (MN!m!) and at temperatures up to -58 (OC) at Dome F. The fluid was compressed ina cylindrical high-pressure cell together with a small ironball.The innersize of the cell was 15 mm in diameter and 7 mm in width. The diameterofthe ironballwas about 2 mm, The pressure was varied betweenO and 100 (MN!mi) at temperatures between-200C and -60eC. We confirmed thatthe fluid exists as fluid by shaking the cell and by observing the ironballto roll. In the experiment, with a load-cell and the temperature was controlled pressure was detected with nitrogen gas. The temperatures and the pressuresat which the rolling of the iron ballwas observed are shown in Fig.5, In the figure,the T-P profileat Dome F (in Fig.1) isalso shown for reference. As forthe n-butyl acetate and IP-solvent,we observed thatunder a pressureas low as below 50 (MNhn2) the fluiddidnot freezeand the steel ball rolled when the cell was shaken, 1ooouAENz m di eoEvo s oo$9Q D D es D eo OLv>=o 20 , l , co.-o-N co m o . m o --e- - -60 -50 -40 P.-T. protile atDome buY acetate IP-solwmt -30 temperature(ec) Fig.5, Temperatures F -20 pressures at which roUing ofthe iron batl was highzpressurecell, (2pen squares are the results fornbuo,l acetate. Solid circtes are rhe results .forIP-solvent rwithout dens4fieV. P-Tprofteat Dome F anFig.1) is atso sheu,n,for rEIlerence. observed in and the NII-Electronic Library Service National Institute NationalInstitute of Polar Research ofPolarResearch 354 S. FuJITA et al. 4.2,urscosity The viscosity fluidcandidate drilling of each togetherinFig, 6 as isshown a function except inthe case of pressureis not considered with densifier, the viscosity of the Oil ((d2) inthe figure),As for the IP-solvent Silicone itself are also shown. Sincethe viscosity of the CFCI 1 itself and thatofCFC11 IP-solvent ismuch smaller than that ofIP-solvent, the calculated viscosity ofthe mixture ofthe two is Among the three candidates, the viscosity of n-butyl smaller than that of IP-solvent. Oilisthe largest. acetate isthe smallest. ThatofSilicone The oftemperature. effect ofhydrostatic 14 12 A "tt::t.l S-b.i .. 10ae>8･.co--86.9>4 [gll sc) (,) 2 '1(e)'e"-`-----"-s -s - g ---=x= o -60 -se -4o -3o -2o -10 temperature(eC) [gl Tf.i::,., rpP&!Kls,fletate /-- (c)IP-solvent and CFCI1, 80i20 ----･ S"iconOilKF96L-1.5c$ ( P isignored.) (dl) -･--･ SMcen OilKF96L-1.5cs (P is considered.) -A- (d2) (e)CFCI1 et aL 1991), viscosity of'the drilling.ITuid candidates. ki)isthat ctfn-butyt acetate (Gosm'K h.vthe authors, and measured fe)isthat ofthe mixture oj'M-solvent (Zijisthat of'IP-sotvent and (27), CFCII. T7teseare catculated values based on (Zij (dJland (d2)are those ofSiiicone isthat qlf'CFCi1. OiL without and with the ofQiet (la) qf'lydrostatic'pressure,respectively. Fig. 6.lhe the coring operation as a.fiinction ofviscosity 4.3.Tbtalperiod oftimefor Based on the viscosity in Fig, 6, the totalperiodof time to complete the 3000 m loweringand pull-up coring was estimated. The total period isdeterminedby the drill's in tirne of the drill at the top and at the bottomof the borehole velocity and the stationary ef the drilling In order to determine the relation between the viscosity each run". which was developed fbrthe coring at Dome F, an fluid and the velocity of thc drill at Rikubetsu in Hokkaido in was canied out at the test site fbr ice-coring cxperiment As a result, a fi;ee-fa11 velocity of O,7 mlsec, was obtained ina borehole February,1993. fi11ed with fluid of viscosity 2.7cp. The clearance between the boreholewall and the drill "drill was 5.S mm, Based on this relation, total hours to complete the arbitrary depthswere calculated for NII-Electronic Library Service National Institute NationalInstitute of Polar Research ofPolarResearch Drilling Fluid forDome F Project inAntarctica 355 7000 7000 oooo 6000 5000 5000 4ooO 4000 3000 30oo 2000 2000 1ooe 1OOO Aco-=o=voE･.-- o o 5ootOOO 2000 1500 depth (m) 250030oo Fle'g. 7.Tbtal period oj'time coring to reach the arbitrary depth. kO is the case Ohoursy of'ice when the n-butyl acetate isused as the driliing.fluid, i,ythe ease when (lij iP-sotventrvith denspteris used. (Ll) isthe case when Silicone Oit isused, For the calcuiatien, the towering velocity ofthe drillin the borehole n'as ealcutated using eqs, (Z)and (ll),T7ie times the lowening pull-upvelocit.v was assumed to be ,l.2 vetocit.v. TVtestationat), time (ij' " veas assumed to be the driU at the top and the bottom oj'the boreholeat eaeh run 40 minutes. Itwas assumed that toweringveloc'itv. targer than i (mlS)is impossibte, "drill eachwerefluid. assumed The results are shown betweenthe viscosity in Fig,7. For the calculation, (n)and the lowering velocity followingrelations v), (the (2)(3) vng1,9, V･n =- 1.3, The constants (2)isa model 1.9and 1.3 were based on the experimental result at Rikubetsu.Equation holds when the frictiondrag due to the drill isdominatedby the shear frictionofthe fluid inthe laminarfiowbetweentheboreholewalt and the drill. Inthis case, eq. (2)holdsbecausethe friction dragisproportionalto the drill velocity and the viscosity, and becausethe only driving forceofthe drill's lowering,the weight of the drill, isalways constant. Freefa11 velocity is always attained when the two are equal, On the other hand, eq. (3) isa model which holdswhen the fuction drag by the turbulentflowin the fiuid between the boreholewall and thedrill isdominantand when the dragdue to the collision betweenthe fluidand the drill at the front of the drill isdominant.Inthis case, the friction drag is proportionalto the squa,re of v, and we assume that the friction drag is still to Since the friction drag is constant when the free velocity proportionaln. fa11 isattained, eq. (3) holds.The actual relatien between v and n must have components expressed by eqs. (2)and (3).Thus, v fbrfluidswhich have various viscosity values was calculated with both of the equatiens as two extreme cases, The other conditions used forthe calculations are described inthe figurecaption. which NII-Electronic Library Service National Institute NationalInstitute of Polar Research Research ofPolar 356 S.FuJiTAet aL ln Fig,7, itcan be seen that ittakes about 2700 hoursto complete 3000 m coring when n-butyl acetate isused. Ittakes about 3200-･3700 hours when IP-solventwith Oil isused. Ifitisassumed is used. Ittakes more than 41OO hourswhen Siiicene densifier that 4000 heursisthe upper limit forcoring in the two years coring operation, only n-butyl are acceptable, acetate and IP-solventwith densifier S. Unsolved Prob]ems Based remain as the on above can IP-soivent with densifier Silicone oil can remain only Dome F, estimates, n-butyl acetate and fluidcandidates the drilling high viscosity foruse at isacceptable. However, some when relatively solved before n-butyl acetate and final deterrnination, the propersolutions Befbrethe used. urgently still remain problerns which must be IP-solvent with densifier can be to the problemsbelow must be glven. 5.1.NLbutylacetate site at Dome F is One of the problemsisthe highvapor pressure.When the drilling -200C, kept at the air concentration of n-butyl acetate vapor is assumed to be possible 1300 ppm (GosrNK et al,, 1991). The vapor pressure at -200C isabout approximately from Fig. 6 in GoslNK et aL, 1991), Although the fire-safety O,8mmHg (extrapolated in GosiNK et al. (1991), there still remains the questionsand answers are discussed problemswith respect to the physical and mental health of the peoplewho work at the coring site. Sincethe Occupational Health and Safety Administration (OSHA)limit is l50 ppm, itis impossibleto use n-butyl acetate without sufficient ventilation and!or without some means of removing the n-butyl acetate vapor. 5,2, IP-solventwith denspter candidates, One of the problems in using CFCI1 and CFCI13 are the densifier CFC11 isitshighvapor pressure.The boiling pointof CFC1 1 under 1013 hPa is23.80C, Under 600 hPa; this isthe anticipated approximate pressure at Dome F; itisabout 90C, -20eC at which the vapor pressure of We have not confirmed yetthat itcan be used at -58aC, Vapor isabout 12 mmHg at the 10 m snow CFCI1 is about 120 mmHg. pressure temperatureat Dome F, As forCFCI13, one of the problemsis itsfreezingpoint at -350C, We have not confirmed appears in the mixture of IP-solvent and CFCI13 at yet whether sediment 1013 hPa is47.570C, temperatures below -350C, The boilingpoint of CFCI 13 under -200C, the vapor pressure isabout 39 mmHg. Under 600 hPa,itisabout 33eC. At Itshould also be noted that HCFC (Hydro ChloroFluoroCarbon)was developed fbr which harms the ozone layer of the use in placeof CFC becauseCFC is a substance the manufacturer of HCFC are atmosphere, The allowable exposure limits(AEL)set by 10 ppm and 100 ppm forHCFC123 and HCFCI41b, respectively, whereas thatofCFCI1 and CFCI13 is 1000 ppm. In addition, HCFC has the same problem as CFC has,as also described above. Moreover, recently ithas become clear that at leastHCFC141b harms the ozone layer. Thus, there seems to be no merit in using these HCFCs in placeof CFC as the densifier. NII-Electronic Library Service National Institute NationalInstitute of Polar Research Research ofPolar Dri11ing FluidforDome F PrejectinAntarctica 357 Aeknowledgments The authors thank Ms. Yukari TAKEucHt at Institute of Low TemperatureScience for her helpinmeasuring the fluidviscosity. The dataconcerning CFC and HCFC are based on infbrmation providedby Du Pont Mitsui Fluorochemicals, . References AGETA, Y,, KAMIyAMA, K,, OKuHIRA, F, and Fum, Y. (1989): Geomorphologicaland glaciological aspects around the highestdome inQueenMaud Land, East Antarctica. Proc. NIPR Symp. PolarMeteorol.Glaciol,, 2,8896. FLETcHER, N.H. (1970): The ChemicalPhysicsofIce. Cambridge,CambridgeUniversityPress, GosiNK, T.A.,KELLEy, J.J., Kocl, B,R. and BuRToN,T.W, (l991):Butyl acetate, an alternative dri11ing fluidfor deepice-coring 37, 170-176. prejects,J. Glaciol., NARusE, R., OKuHIRA,K.,OHMAE, H., KAwADA, K. and NAKAwo, M, (1988): Closurerate of a 700 m deep bore holeat Mizuho Station, EastAntarctica.Ann. Glaciol., 11, 100-103, PA'rERsoN, W.S.B. {1977): Secondary and tertiarycreep ofGlacier iceas measured by boreholeclo$ure rate. Rev. Geophys. Space Phys., 15,47-55. 19,1993; Revisedmanuscript (1?eceived Aprii received Alovember 24, 1995) NII-Electronic Library Service