Copyright 2000, IADC/SPE Drilling Conference

This paper was prepared for presentation at the 2000 IADC/SPE Drilling Conference held in NewOrleans, Louisiana, 23–25 February 2000.

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AbstractLarge difference exists between slimhole annular hydraulics andconventional well’s.It is one of the key factors which hinders awider application of slimhole drilling.The paper has studiedexperimentally the slimhole annular pressure loss(APL) at theslimhole annulus flow loop test apparatus.Various parameterssuch as drillpipe rotation speed,the eccentricity between pipeand annulus,and mud properties are investigated.Rules of APLchanging with pipe rotation speed and pipe eccentricity areobtained.Test results show that when pipe rotation speed is lower,the APLdecreases slightly with it.While as rotation speed greater than 70rpm,APL increases fast.the APL decreases quickly withincreasing eccentricity.With the annular gap decreases,APLchanges dramatically,and becomes more sensitive to piperotation speed.Through the analysis of data collected in thetest,the regressive models for pipe rotation and pipe eccentricityimposed on APL have been worked out.Contrary to conventional drilling,in slimhole drilling,the annularmud flow regime is not only relied on Reynolds number,but alsoon Taylor number.The empirical equation is presented toidentify the flow regime.For the slimhole size of 6 inch,with pipe rotation speed being140 rpm,and mud flow rate of 16.8 l/s,the ratio of pressuredropped in annulus to that in drillpipe lies between 25% to30%,which is much higher than that in conventional wells.On-site data of Miao 1-40 well in Jilin Oil Field are gathered andused to evaluate the valueness of the APL models developed inthis paper.Moreever,these models have been used to Miao 5-40well of Jilin Oil Field.The difference between the calculatedpressure drop and measured pressure drop is less than 6%.

IntroductionSince the downturn in oil and gas industry in 1985-1986,theneed for reduced exploration and production costs has beenemphasized.Indentifing the advantages of drilling holes withreduced diameters in terms of costs and environmentalimpact.Many oil and gas companies,drilling contractors andservice companies in the world have investigated slimholetechnology for exploration drilling and developmentdrilling.While slimhole drilling can be dated back to 1920s.Inthe late 1950s and 1960s,over 3000 slimholes were drilled in oiland gas field worldwide.Although significant cost savings wereachieved,large holes became common in the 1970s and 1980sdue to:(1)short life of small diameter roller bits;(2)reduced rateof penetration;(3)high oil prices;(4)the misconception of largeholes heading everywhere.However with the advance of newtechniques and equipment,the positive results of slimholedrilling in terms of cost reduction,minimisation of theenvironmental impact of drilling activity,drilling efficiency andsafety and quality of geological evaluation have been widelyaccepted.Slimhole drilling has gained increasing interest in oiland gas industry since 1986.Slimhole drilling is different from conventional oil and gas welldrilling because of (1)high rotation speed of drillpipe;(2)thevery slim annulus,the small annular gap.These main differentialcharacteristics make it absolutely essential the drillinghydraulics study,and make the slimhole hydraulics totallydifferent from convential wells.In slimhole drilling,the “Couetteeffect”resulting from pipe higher rotation speed and “Crescenteffect” due to pipe axis highly deviated from the axis ofwellbore must be taken into account,and these two charactericswhich are of different points between slimhole and conventionalwells governes the slimhole hydraulics calculation accurcy.Early field pratice and labortary studies showed that inconvential well drilling APL rarely exceeds 15% of the totalpump pressure.However in a slimhole up to 30-50%pumppressure is due to pressure loss in the annulus,some peopleconsider it is up to 90%.Therefore slimhole drilling hydraulicsbecomes one of the major concerns.This theoretical study andfield application will allow a better understanding ofphenomenor occuring in the annulus and thus an optimization ofhydraulic parameters for a successful slimhole drilling.

IADC/SPE 59265

Experimental Study of Slimhole Annular Pressure Loss and Its Field ApplicationsHaige Wang,Yinao Su,Yangmin Bai,Zhenguo Gao,RIPED, CNPC,Fengmin Zhang,Jilin Oil Field of CNPC

2 HAIGE WANG IADC/SPE 59265

Experimental Study and Results AnalysisExperiment was conducted in slimhole annulus test apparatus inPetroleum University,China.The annular flow loop consists of a5 inch outer pipe,while inner pipe has two size 3 inch and 3-1/2inch.The test apparatus is 6 meters long with outer pipe beingtransparentIn this test,following factors are considered:(1)High rotation speed of inner pipe.The highest rotation speedis up to 230 rpm.In mining industry the highest rotation speedcan be 900 rpm.In China,there is no special slimhole rigs forslimhole drilling up to now,thus the rotation speed rarelyexceeds 200 rpm.In Jilin oil field ,Northeast of China ,the rotaryspeed of rig usually is 140-150 rpm ,occasionally reaches to170-180 rpm.(2)Effect of annular gap.In this test ,both of 19 mm and 25 mmannular gap are considered.(3)Effect of pipe eccentricity.Three position of drillpipe ofwhich eccentricity is 0.4,0.5 and 0.6 respectively are used tosimulate drillpipe eccentricity on APL.The objective of the experiment is to simulate different factorson APL of silmhole drilling.Various parameters are investigatedduring the study,which includes annular mud velocities between0.336 m/s and 2.36 m/s,pipe rotation speed was selected as0,30,40,60,70,90,110,150,190 and 230 rpm.water and threeMMH drilling fluids were used as simulated mud.Thousands oftest data were gathered to further analyses the relationship ofdifferent factors with APL.Effects of Different Factors on APL in Slimhole Drilling

Effect of Annular Mud Flow Velocity on APL.Figure 1shows that APL in slimhole changes with annular mudvelocities.The lines indicates the APL increases with increasingvelocities for each eccentricity.

Effect of Drillpipe Rotation Speed on APL.Figure 2 plotsAPL changing with pipe rotation speed N.Test results show thatAPL increases with N increasing.While at lower rotationspeed(N<30 rpm),APL decreases with N increaing,but this effectcan be ignored.At higher rotation speed(N>70 rpm),rotationspeed has a significant influence on APL.Obviously,large errorswill produce if this effect is not considered in slimholehydraulics design.

Effect of Drillpipe Eccentricity on APL.In slimhole,rotatingdrillpipe doesn’t remain concentric.In Conventionaldrilling,drillpipes are small in front of hole diameter anddrillpipe position doesn’t affect APL.But in slimholeconfigurations,ratios between pipes and hole diameter oftengreater than 0.6,sometimes come close to 1.For such ratios asused in slimhole,the eccentric position of drillpipe inside thehole can significantly influence annular flow and APL.More thepipe is eccentric,more the APL decreases. Figure 3 shows the effect of eccentricity on APL.The resultssupport the above hypothesis.Moreever,there is a two-factorinteraction between rotation speed and eccentricity,with Nincreasing,effect of E increases.

Effect of Annular Slim Clearance on APL.ApL is sensitiveto rotation speed with slim clearance decreasing.Figure 4illustrates this relationship of annular gap with APL.

Estimation of Coefficients in APL Model.The major difference between APL in slimhole drilling andconventional wells lies in that the influence of drillpipe rotationspeed and eccentricity are greater,so this two factors should betaken into account in APL calculation in slimhole well drilling.The APL in slimhole can be written as:

( )σρ

−××=

1

2

21lim RLVf

ffPS (1)

While f1 is the coefficient taking into account the drillpiperotation speed. f2 is the coefficient taking into account thedrillpipe eccentricity.

Determination of the Coefficient f1.In slimhole,both the twodimensionless groups,named Reynolds number Re and Taylornumber Ta influence the axial flow and tangential flowregimes.Therefore the complicated relationship of rotation speedN on APL can be transferred to seeking for the links betweenReynolds number ,Taylor number and f1. Figure 5 illustrates the dimensionless velocity VW (ratio ofaxial velocity to tangential velocity)on f1.Results suggest that f1increases with dimensionless velocity VW.But the trend impliesthat with the further increasing of VW, f1 will decrease.f1 can be written as:

( )ea RTff ,1 = (2)

Through the regressive analysis for large amount data,thecoefficient f1 can be represented as:

( ) ( )ca

be TRaf =1 (3)

while the constants of a,band c can be determined throughanalysis of test results.For different flow regimes,a,b and c takesdifferent values.

Determination of the Coefficient f2.To solve the f2 inequation (1),analyzing the former test results, f2 for BinghamFluid at laminar and turbulent flow regimes can be written as:

( )σ,,,2 EYPff PV= (4)

For Power-Law fluid,we have

( )σ,,2 Enff = (5)Flow Regimes Determination in slimhole DrillingIn slimhole drilling ,annular mud flow regimes depend not onlyon Reynolds number which governs axial flow behavior,but alsoon Taylor number which determines tangential flow behavior.Inthe absence of axial flow,the critical Reynolds number isconstant,while both the axial flow and tangential flowexist,critical Reynolds number decreases with Taylor numberincreasing. Critical Taylor number decreases with Reynoldsnumber increasing too. From above analysis,combined with experimental data,theempirical relationship between Re and Ta to determine flowregimes in slimhole annuli is:

)( ea RfT ≥ (6)

Figure 6 shows this flow regime curve.While above thecurve,turbulence occures,below the curve,laminar flow takesplace. By integrating equation (1),(3),(4),(5) and (6) ,APL inslimhole can be easily solved.

IADC/SPE 59265 EXPERIMENTAL STUDY OF SLIMHOLE ANNULAR PRESSURE LOSS AND ITS FIELD APPLICATIONS 3

A Computer Package for Slimhole Hydraulics Optimizationand Mud Rheological Parameters CalculationA computer package has been developed in the drillingdepartment of RIPED,CNPC that provides a useful tool for thefield engineer to optimize drilling hydraulics and rheologicalparameters in order to ensure safe drilling. The software packagehas been designed and built in modules to allow the user to: (1)Define from viscometer data the appropriate rheologicalmodel(Power-Law,Bingham Plastic,Casson,Herschel-Bulkley orRobertson-Stiff model)for a drilling fluid. (2)Predict the rheological parameter at given depth,combinedwith mud circulating temperature model and HPHT rheologicalproperties model. (3)Determination of flow regime both in annuli and drillpipe. (4)Sensitivity analysis of different factors on APL. (5)Optimizing flow rate and drill bit nozzle size when pumppressure is given;Computing APL and ECD as flow rate isgiven.

Application of this software to Jilin oil field in China hasproven the accuracy of the models. Moreover ,this software canalso be applied to conventional wells for hydraulicsoptimization.

Application of Model to Slimhole Drilling in Jilin OilFieldRatio of APL to That of Pressure Loss in DrillpipeTaking Miao 1-40 slimhole in Miao 1-40 platform of Jilin oilfield as an example to illustrate the relationship between APLand pressure loss within drillpipe. While Miao 1-40 platform isused to drill cluster wells, four slim directional wells are drilledfrom this flatform. Table A-1 shows the calculation results. Wecan see under routine operation, as rotation speed is 140 rpm,flow rate is 16.8 l/s, borehole is six inches in diameter, and 3-1/2inches drillpipe is used, the ratio of APL to that of pressure lossin pipe is up to 25-30%, which is far exceeding the ratio of 10%in conventional wells. Therefore,it is absolutely necessary tocalculate accurately of APL for controlling the well,optimizinghydraulics,and avoiding excessive pressure against theformation.

Table A-1 Ratio of APL to That of Pressure Loss WithinDrillpipe

WellDepth

m

FlowRatel/s

RotationSpeed

N

PPipe

atmAPLatm

RatioAPL/PPipe

%140 62.3 15.8 15.36993 16.8270 62.3 21.33 34.24140 76.93 19.71 25.621350 16.8270 76.93 27.59 35.87140 92.26 25.15 27.261724 16.8270 92.26 36.47 39.53

Model Predictions against Miao 1-40 Field DataSlimhole cluster development drilling began in July 1997 atXinmiao structure located in east of Xinmiao countryside,Qianguo country, Jilin Province.Miao 1-40 slimhole is the first

well drilled from Miao 1-40 platform. The well began drilling atJuly 7,1997,reached the total depth at July 19,1997,andcompleted well at July 21,1997.After gathering detail data of thewell, the model was verified against on-site data. Table A-2shows the calculation results. From this table, we can see lessthan 6% error occuring between predicted values and measuredones.Table A-2 Verification of the Slimhole Pressure Loss Modelon Miao 1-40 WellWell Depthm

1350 1494 1550 1600 1680 1724

RotationSpeed rpm

140 140 140 140 140 140

Flow Rate l/s 16.8 16.8 16.8 16.8 16.8 16.8Nozzle mm 9,10,

119,10,

119,10,

119,10,

119,10,

119,10,

11N3 3 1 1 1.5 2 2N6 4 3 3 4 3.5 3N100 25 17 21 23 19 17N200 36 26.5 30 35 28 27N300 46 32 38 44 36 35

Vis

com

eter

read

ings

N600 70 51 60 67 55 54Mud Densitykg/m3

1260 1240 1260 1260 1250 1260

MeasuredStandpipepressure atm

118 112 120 122 135 140

PredictedStandpipepressure atm

118.2 118.0 125.7 128.3 129.3 132.6

Error % 0.14 5.37 4.78 5.13 4.26 5.28

Model Application to Miao 5-40 SlimholeAccording to the data gathered from the first and the secondslimholes in Miao 1-40 patform,the hydraulic parameters in thethird slim well,Miao 5-40 well,was designed as followings: (1)According to the maximum pump pressure rate of 16Mpa,Miao 5-40 slimhole can only be drilled to measured depthof 1300 meters by use of jet nozzles combination of 9x9x10x10mm,10x10x11x11 mm,11.2x12x12 mm and M150 mm cylinderliner. As well depth exceeding 1300 meters ,cylinder linerneeded to be changed. (2)Total standpipe pressure was computed at the design depthof 1856 meters by use of M130-mm cylinder liner. While exceptthe nozzle combination of 8x 8x 8 mm, all the othercombinations including 9 x 10 x 11 mm, 11x11x11mm, 8 x 9x10 mm, 11.2 x12 x12 mm, 9 x 9 x 10 x 10 mm,10 x 10 x 11 x 11 mm, can meet the requirement of notexceeding pump pressure rating.

Conclusions(1)The flow experiments performed at the slimhole annularflow test apparatus cover annular laminar flow regime andturbulent flow regimes that are relevant to slimhole drilling.Rules of different factors on APL were obtained.

4 HAIGE WANG IADC/SPE 59265

(2)Empirical models of drillpipe rotation speed and eccentricityon APL have been worked out.(3)Slimhole annular mud flow regimes depends not only onReynolds number,but also on Taylor number. The empiricalmodel for flow regime transition has been presented.(4)A computer package for optimizing slimhole hydraulics andrheological models has been developed. Model predictions canhelp the drilling engineers to define flow regimes and to adaptfluid rheology and flow rate for a given well geometry in orderto obtain safe drilling conditions.(5)During slimhole drilling,ratio of APL to that of pressuredropped in drillpipe is up to 25-30%.(6)Model predictions were validated in slimhole Miao 1-40 wellin Jilin oil field with errors less than 6%.Moreever modelpredictions have been applied to Miao 5-40 slimhole hydraulicsdesigning.

NomenclatureE=drillpipe eccentricity ,dimensionlessf=Fanning friction factor,dimensionlessf1=Pressure ratio with and without drillpipe

rotation,dimensionless.f2= Pressure ratio with and without drillpipe

eccentricity,dimensionless.L=wellbore section,mn=Power-Law Flow Index,dimensionlessN=Drillpipe rotation speed,rpmPV=Bingham Plastic viscosity,Pa.sPPipe=Pressure loss within drillpipe,atmPSlim=Slimhole annular pressure loss,atmR=wellbore radius,mRe=Reynolds number,dimensionlessTa= Taylor number,dimensionlessV=annular mud flow velocity,m/sVW=Velocity Ratio of axial to that of tangential,dimensionlessYP= Bingham Yield Point,PaF=ratio of inner diameter to outer diameter,dimensionlessD=mud density,kg/m3

References1.Fultz,J.D. Slimhole Drilling in Harsh Environments SPE Drilling

Engineering 1991.92.G.Thonhauser The Applicability of Current SlimholeHydraulics Algorithm under Field Conditions—A CriticalReview SPE 304893.U.Cartalos Field validated Hydraulics Model PredictionsGiven Guidelines for Optimal Annular Flow in SlimholeDrilling SPE 351314.Haige,Wang Theory,Calculation and Application of SlimholeDrilling Hydraulics Postdoctoral Thesis ofRIPED,Beijing,China oct.19975.Haige Wang,Yinao Su practical Techniques for solvingEccentric Annular Pressure Loss Oil Drilling & ProductionDec,1997 Vol.19(6)

IADC/SPE 59265 EXPERIMENTAL STUDY OF SLIMHOLE ANNULAR PRESSURE LOSS AND ITS FIELD APPLICATIONS 5

6 HAIGE WANG IADC/SPE 59265

IADC/SPE 59265 EXPERIMENTAL STUDY OF SLIMHOLE ANNULAR PRESSURE LOSS AND ITS FIELD APPLICATIONS 7