.-PETROLEUM TRANSACTIONSMicrobit StudiesoftheEf~Hydraulics onJOHNR. ECKELMEMBER AIMEABSTRACTIt has Iongbeen known that mud properties a#ect drill-ing rare and that drilling with water is as much as SIXtoseven times jr.mter than with mud. However, it is not knownwhy drilling rates wfth mud are lower, Recent studiesshow what jiuid properties govern tnicrobit drilling ratemid also provide a quantitative correlation of titese jiuid[~roperties and hydraulics with tnicrobit drilling rate.All tiremicrobit tests reported in this paper wet-e madeIti//l a low petvtreabi!ity (about 10 tnd) Iintestone. Theywere conducted at constant bit we!gilt and rotary speedw i t i t varying jiwid properties, fio wrate Qand nozzie di-(Inletmd. Titc tests showed timt {1) driiling rate ina~~ivensystem with con$tatzt circulating rate and nozzle veIocityis a function of the kinematic viscosity [viscosity/densiry,IL (cp)/ p (density.)1 of tiledriliing fluidmeusured at near hinozzle shear rates; (2) tite combined effect oj jiuid proper-ties and hydraulics on tnicrobit drilling rate is defined bya Reyttolds nuinber function (kQp/ alp); (3) jor the samekinetnatic viscosity, drilling rate is independent of solidsCOllItItt; and (4) for tile same kinematic viscosity, driiiingrate isindependent t of j?uidloss.INTRODUCTIONThe fact that mudding up reduces rate of penetrationiswell known, but thereduction varies fromwell towell.The rate withmudmaybe 80percent of thewater rateorit may beaslowas 15 percent. Since this reduction of upto seven-fold takes place under conditions of constant bitweight, rotary speedandmud circulating rate inauniformformation, it can only be a result of the change in fluidproperties.Previous work- (both in field tests and in laboratorymicrobit tests) showed that drilling rate withmud islowerthan with water (Figs. 1and 2). Mlcrobit test6also con-firm, at least qualitatively, the fieldobservation that drill-ing rate slows with increased pressure differential fromborehole to formation (Fig. 3). The qualitative effect ofdrilling rate of bit weight, rotary speed and rate of mudcirculation are also the same for the field and microbkrig. For these reasons, the industry generally accepts mic-robit results as being qualitatively meaningful in termsof fieldapplication.Orbrinai xnanuecrlpt received in Society of Petroleum Englneem OfffceJuly 2%196&Revised manuscript received Feb. 2, 1967. Paper (SPE1620) W%meaented at SPE 41st find ~~1 M=tinS held in D~l*Ilkx., Oct. 2-6, 1966. @kwrfght 1967 Amerkan Institute of bfin!.m?,Bfetaiiuwkal, and P.rtdeum Ehr@neers, Inc.%eferenoee given at end of paper.Discussion of thfs and all following technical papers is invited. Dk-cussion in writing (time copies) may bec ent t n the officeof theJourd of PetroleumTeehnehw. Any dlscussiOn offered aftir ~. W1967, should Imin the formof a newPaper, Nodiecudon should ex-ceed 10 p-srcent of the manuscript being diseumed.jetof FluidProperties andDrilling RateESSO PRODUCrlON RESEARCHHOUSTON, TEX.co.100 1Y90 -.-1-. -~>-.. ..- ,- ....-WATER =100%80--:..--- i____ :. ___[7* .. . ... ... .... ..-...:. .. .. . .I60 -- -:--- --- c0; ; ;P10 11 1MUDWEIGHT - LB/GAL~ 1000[o CONVENTIONAL FLUID COURSESx~ JET NOZZLESf RC)MREF1,SHALEII WELL A WELL B WELL CFIG. 1FIELDTESTDATASIiow] NC EFFECTOF MUD ONDRILLINGRATLg _______ . .. .... ...-=..-.. ._.. .110x Ob, :.0,.,0.:.0.. . ..* -0 *S g 4(J . !. _. _...STORMER VISCOSITY - CPSFIG. 2LABORATOiiYDATASHOWINCEFF.SCTor MUD ONDiWLLINCRATE.APRIL, 1967Although not recognized at the time of these previousfieldtests, the degree to which mud affected drilling ratewas alsorelated tobit hydraulics, sincethe reduction duetomud was lesswithjet than withconventional bits (Fig.1). While early microbit work at atmospheric pressureshowed the effect of viscosity on cleaning and drillingrate (Fig. 2), it could shed no light on the long-held be-lief inthe fieldthat fluidloss was aproperty significantlyaffecting penetration rate. Tests todetermine the effect offluid loss on drilling rate required laboratory equipmentwhich would reproduce the press~re environment of arock beingdrilled at depth, including thedifferential fromtheborehole intotheformation.About 12years ago Humble Oil &Refining Co. builtthe first microbit rig that would suitably simulate thedown-hole environment and used it tostudy the effect ofpressure onrock drillability.zThese tests showed that dif-ferential pressure across theborehole wastheonlypressurestrongly affecting rock drillability. The effect of this pres-sure differential on drilling rate at constant overburdenand hydrostatic pressures isshown inFig. 3. Animprovedmicrobit rig (Figs. 4A, Band C) was built during 1963.This rigwas used todetermine the effect on drilling rateof tluidproperties (filtration, density, viscosity) under con-stant mechanical and environmental conditions, and thento determine the interaction of fluid properties and hy-draulics.LABORATORY TEST PROCEDUREBecause a large quantity of uniform rock was requiredfor the microbit tests, a quarry stone, Indiana (Bedford)lime, was selected. Permeability of this rock is about 10md. It was felt that if laboratory results showed PO effectondrilling rate due tofluidlossinarock of this permea-bilityy, theresults wouldbeapplicable tomost fielddrillingsituations sincemost of industry drilling costs areincurredin drilling formations with even lower permeability. Anyfluid loss effect should be greater in 10-md rock thanrocks ofIower permeability.Samples were prepared by the procedure described inRef. 2, except that inour tests the saturating fluidwas 3percent salt water. Arelatively high differential pressure(500 psi) between the borehole and the rock was selectedto maximize filtration effec~ the over-balance carried inroutine fieldoperations is 500 psi or less. For these tests,the bh weight and rotary speed were arbhrily selected,but held constant; weight Wwas 1,000 lb and speed Nwas75rpmfor alltestsreported here.Most of the tests were made at acirculating rate Qof3.5 gallmin using nozzles with a diameter d of % in.Later tests were also run at 8, 10,5 and 16 gal/rein with!:- P, =OVERBURDEN PRESSURE=3000 PSI ; :lQ5KLEdi 12 PH=MUD COLUMNPRESSURE=300C PSI . . . . . .500 1000 1500 2000 2500 3000DIFFERENTIAL PRESSURE - PSI(WELL BOREOVERFORMATION PORE)FXG. 3-EFFECTOF DIFFERENTIALPasssuaa ON DEULLINCRATE.(A)MUDCOLUMN PRESSUREOVERBURDEN PRESSURE [-[(B)n1!i-. FORMATION PRESSURErLccyLND(c)Fxc.4-(A) LABORATOSSY DRIWNGRIG; (B) LABOFtATORYDRILLINCRIG; (C) MICROBITDRILLINGCHAMBER.e42 JOURNAL OF PESTSIOLEUM TECHNOLOGY..both ~8- and Y4 -in. nozzles. Conditions for each point areshowninthefigurelegends, Each point plotted isanaver-ageof tests infromfour tosixrock samples to minimizetheeffect ofvariation in rockdrillability, Data aresummar-izedinTable 1,EFFECT OF VISCOSITYFluids usedwerewater, water solutions of salts andgly-cerin, oils andbothwater-base andoil-emulsion muds. Flu-id density varied from0.803 to 1,42gin/cc. Vkcosity at10,000seconds- ranged from0.77 to 74.0 cp. The theo-logical meaning of viscosity as used in this report is dis-cussed in Appendix A. Viscosities were measured in acapillary viscometer.Tests confirmed thestronginfluenceof viscosityondrill-ingrate found inearlier tests. 13gs, 5and 6showdrillingrate vs kinematic viscosity at 3Mand 101/2 gal/rein, re-spectively. Kinematic viscosity is the viscosity ~ cp ofthe fluid under flowconditions divided by density p. Agood experimental correlation between kinematic viscosityand drilling rate was obtained. Fig. 6also shows drillingrate vs Farm apparent viscosity. The correlation here isfair topoor, indicating that normai Farmmeasurements offlowproperties made at lowshear rates (1,020seconds-)do not directly predict drilling rate performance at shearrates as high as 10,000 to 50,000 seconds-. The correla-tions obtained inthese experiments showthat drilling rateis aspecificfunction of kinematic viscosity, when this vis-cosityis measured at theproper shear rates. The best shearrate was determined experimentally and approaches thenozzleshear rate.COMBINED EFFECT OF VISCOSITYAND HYDRAULICSThe correlation of drilling rate and kinematic viscositySpe%ificGravity:;951:061.040.994:.::41:161,201.111,191.421.210,82!50.803::!3;1:0131.021.0251.025;,:::1:0171.0131.0130,995QAPI FluidLoss(cc/30 rein)25\117= 29 ..... L-3.UI.__._.! _ W= 1000 LB N =75 RPM.~I AP=500 PSIt3S (3PMB IS..- .__ ., .-.Iid=J~8 5 I ..1i ._._: .J .. ...- . . .... . ~.... - g 10 :--- ~-- ~ ;; .__ . .yl,*g5 .O?..Q : ! :. :....:._, . . . . . .Oo& ;0 qOi ~ 8 ~. ;\l01-.-&__Jo 10 20 30 40 50 60 70 80KINEMATIC VISCOSITY - CSFJG, 5DRILLINGRATEvsKINEMATIC VIscosrrv Ar 3!4CAL/ MIN.having been established for constant circulating rate andnozzle velocity, tests were run at a three-foId increase incirculation rate. The resulting drillkig rates were thesameas would have been obtained with a three-fold reductionin viscosity. This indicated that a relationship similar tothe Reynolds number (kQp/dK) might control the conl-bined effect of fluidproperties and hydraulics on rate ofpenetration. Further tests using both the standard %-in.nozzles at 3% and 101/2 gal/rein and M-in. nozz[es atthesesamecirculating ratm, plus tests at 8 and 16 gal/reinwith both sets of nozzles, confirmed the empirical corre-lation with the Reynolds number function. This is showninFig, 7which is aplot of drilling time vsthe Reynoldsnumber function inrectangular coordinates, and in Figs.8and 9which are plots of drilling rate vs the Reynoldsnumber function in rectangular and logarithmic coordl-nates. Under the weight-speed conditions of these tests(W=1,000 lb, N= 75 rpm), maximum cleaning andrate appear tooccur above values of about 90to 100forTABLE lLABORATORY DRILLING RESULTS= 3.5gal/reinViscosityat 10,000sec.-(Cp)0.777:.:2:32.3;::8.15;~:17:83;:::0:772.053.45:$10.511.720.037.241.574.00.77NozzleDiw#r?81!8%6%81/8%81?8%8%8%a1/8%8g/81/81/8/8%81/8/8=/81/8/8%av4D;l$g(ft/hr)1;:;;::;6:916.607.155.513.394.24;~3:224.7910,35lg.;:5:824.564.554.214,423.984.083.563.705.88Q =8gal/rein0.995m0.770.995 co0.77&Q= 10.5 gal/reinNozzle Viscosity DrillingDi amet er at 30,000 Rat e(i n.) sec .- (c p) (ft/hr):.;;2:634,337.578.341::;21.5%8%81/81/81/8?4a#:*28.4 3,8748.7 4.370.77 12.81Q = 16sral/min -.!80.77 13.2%0.77 33.025.713.613.79.867.188.346.895.474.83APRIL, 1967 84athe Reynolds number func[ion. At the lower end of thescafe, there appears to be only mechanical cleaning duetotheoffset andskewinthebit for values lessthan abouttwofor the Reynolds number function,EFFECT OF SOLIDSWhile both solids type and amount affect viscosity andreduce drilling rate, the same viscosity obtained withoutsolids reduces rate by the same amount. Fluids with andwithout solids are not distinguishable in Figs, 5 and 6,for example. Solids content, then, does not independentlyaffect drilling rate. Appendix Bisan additional discussionof the effect of solids andfluidloss ondri[ling rate.EFFECI OF FLUID LOSSAPI fluid loss varied from 2.4 to more than 100,000cc, extrapolated 30-minute volume. As Fig. 10 shows,API fluidloss did not influence rate of penetration. Con-sidering the range of fluids used in these tests, the datastrongIy suggest that fluidloss, no matter howmeasured,can have little if anyeffect ondrilling rate inlowperme-ability formations. Further, the magnitude of the effectof viscosity previously discussed appears to account forall of the variation observed in drilling rate. In high[ypermeable formations, somelaboratory data indicate are-duc(ion indrilling rate withlowfluidloss, This condition,however, probably applies to a small part of our drillingopcrutiuns- and costs.30---------- ------------------------7------":------.------7-----W=1OOOLB N=75 RPMLIP K 500Psl25 -----10,5 GPMEdK 1/6; 20G KINEMATICVISCOSITY- FLOWCOND.o FANN-APPARENT VISCOSITYEG a~ 15 .c1m0 Iz ,~ ,0 G Q,.__!__ _..__.;._.,.. . . .zI ,Q0;O .;::5 Gm.+-;---.I0-o .LO 20 40 60 80 100 120 140 160VISCOSITY - CP, CSl% G--DINLLINGRATEvsKXNEMATICVISCOSITYANDFANNAWARENTVISCOSITY AT 10~2GAL/M I N,60 j,I , I , 1~ IGAL/MIN DIAMETERg so ..; : ;- .~@*. . . 0:3-1/2 ~ G 40 8-...: -- . -- ; _: _..!_w10-1/2 o I1:I *A A 30 %;:@-i- ; - ! -!- 16,,~ 20 q !j I. W= IOOOLB N~75RJMz o00AlAP=500 PSI2 10 j- mJ%:-E\ ..+--4.III I ldm.; ...x_ _ ----- .vo00 10 20 30 40 50 60 TO 80 90 130 140REYNOLDS NUMBER (K~)FUNCTIONFrG. 7DRILLINGTMEvs REYNOLDSNUMBEaFUNCTION.844FUTURE WORKAdditional microbit work is planned to determine thecombined effect of bit weight, rotary speed and cleaningon drilling rate, Basedon past fieldand laboratory exper-ience, the essential shape of the curves inFigs. 5through10should be preserved, but the actual scale may changewith weight, speed, pressure and rock type.Abetter representation of the drilling-rate equation isrequired. The formof this equation canbest bedevelopedinthe laboratory where thevariables are easier tocontrolthan inthefield. Fieldverification of thelaboratory resultsis then a relatively simple matter.The usual equation for drilling rate RisR= KWN Q,....... . ..(1)restricted to constant cleaning in a particular formation.Our worksuggests that usingL_.~25.uW=1OOOLB N=75 RPM ~ ..: u :AP=S00 PSI ;!;20 .; ; :... ; . :.; :.;.,,,i:!,*,,g15 :... ;.. _ .. .. . GA?MIN bIAMETER., .,o8A g 1/8 ~g I (J ,0 .,.-,$~ :::3-1/2 ~ G*10-1/2 ] -S5.8A A16, Vvoi;~-~- o 10 20 30 40 50 60 70 80 90 130 140()REYNOLDSNUMBER K wFUNCTION+FIG. 8-DSNLLXNGRATE vs REYNOLDS NUMBER FUNCTION.1 0 080~AL/MINI1k 60 DIAMETER:::, : ::~~g 1/8 1/4 ,!%49; .. ,,. ,(~ To3-1/20* 1:lo-1/2u*. !:4. :.:xo.E8AAo~ 10 -; 160-7: :-::::.g :--: ~ ~- :..;; F$.~:._-:% -;-;:-~ 4_6,-: . M..-# .- . ..-. --. . L..an0w =ltiOLB NK 75 RPM-2-... .. . .....+- ..... . .AP=500 PSI,,: 0.060.1 0.2 0.40.6 1.0 2 4 6 10 20 4060100()REYNOLDS NUMBER K ~FUNCTIONd~Ftc. 9DKI LLI NG RATsvs RI~YNOLW NUMIU:R III NCHON.)WKlf3.5GPM.d=1/8 IIAPI FLUIDLOSS - CCS/30MIN.Fxc, N-DRILLING RATEvs FLUIDLoss (API).JOURNAL OF PETROLEUM TECHNOLOGYaR()/(@p c,2