Copyright 1998, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the 1998 SPE/ICoTA Coiled Tubing Roundtableheld in Houston, Texas, 15–16 April 1998.

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AbstractThis paper analyzes representative samples of downhole

and surface measurements collected during a sidetrackoperation with a coiled tubing drilling unit. The downhole dataare classified into operational windows such as drill ahead,running in hole and pulling out of hole. They are correlatedwith drilling reports and surface measurements of hook load,pumping pressures, flow rates, and slackoff. Correlationsbetween the surface and downhole data are used to understandforce transmission during set-down, pick-up and drillingoperations.

IntroductionThe work described in this paper was motivated by a need

to calibrate models of force transmission in Coiled Tubing(CT) with field data. The paper describes the collection,analysis and interpretation of data from a downhole weight onbit and torque measurement device (hereafter referred to as theWOB sub). The data were collected during a sidetrackoperation on the North Slope of Alaska. The operational planconsisted of setting a cement plug in an existing well, drillinga pilot hole through it and exiting from a milled window to thetarget at 10,400 ft. The sidetrack was directionally built tohorizontal at nearly 22° / 100 ft and drilled to 13,100 ft MD.Drilling was suspended 300 ft. short of total depth (TD)because of difficulty in getting WOB.

The data collection project was motivated by the followinggoals-

1. To measure the downhole WOB transmitted to the bitwhile drilling and identify parameters affecting WOB transfer,

2. To determine the accuracy of the tubing force models(TFM) used plan CTD jobs. CT force transmission models

have only been validated with surface hook load data1, 2.Experience with conventional drill string torque and dragmodels has shown that both surface and downhole data arecritical to validate models for successful planning and drillingof highly deviated and extended reach wells3.)

3. Obtain improved estimates of input parameters to theTFM (e.g. friction factor, effects of tortuosity, etc.)

4. To improve understanding of the more complex issuesconcerning the mechanical behavior of CT (e.g., the effects ofresidual curvature, post buckling wall contact forces, etc.).

Not all of these goals were realized in this project becauseof the time and manpower required to analyze the largeamount of downhole and surface data. Also, the behavior ofsome of the data was complex enough to prevent directcomparison with the results of simulations.

The WOB sub recorded tension, torque and temperaturenear the bit while separate transducers recorded internal andannular pressures. Data from these devices were recordeddownhole and transferred to a personal computer during theBHA trips at the surface. A total of 22 megabytes of downholedata were collected during this field test. The sub stoppedrecording data about 500 ft. before drilling was terminated dueto poor transfer of weight to bit. In addition, 3.5 megabytes ofsurface data on hook load, depth indicator readings, pumpingpressures, mud weights, etc. were recorded.

This paper presents representative samples of downholeweight and downhole torque, and surface measurements. Itdiscusses selected portions of this data by classifying theminto operational intervals such as drill ahead, running in hole(RIH) and pulling out of hole (POOH) and correlating themwith events recorded in the drilling reports and surfacemeasurements. To the authors’ knowledge, this paperrepresents a first attempt to understand the mechanics of forcetransmission in CT using both surface and downhole data inthe literature. The present analysis focuses on trends ratherthan actual magnitudes of data. A detailed comparison of thedownhole data with simulator predictions and estimation offriction coefficients is ongoing and may be published in thefuture.

The WOB SubFigure 1 shows a schematic of the WOB sub used to gather

the data. The sub consists of a load carrying member (LCM)

SPE 46008

Weight on Bit in Coiled Tubing Drilling: Collection and Analysis of Field DataC. G. Blount, ARCO Alaska Inc., D. D. Hearn, ARCO Alaska Inc., M. L. Payne, ARCO Exploration and ProductionTechnology, U. B. Sathuvalli, ARCO Exploration and Production Technology, R. Livesay, Hecate Software

2 BLOUNT, HEARN, PAYNE, SATHUVALLI AND LIVESAY SPE 46008

which is subjected to internal and external fluid pressures andWOB. Suitably mounted strain gages measure the load andtorque on the LCM. The electronic data acquisition andmemory units are protected from the internal and externalfluids by protective inner and outer sleeves. These sleevesmake sliding contact with the LCM and do not transmitpressure (or other) loads except for the small friction due tothe sliding O-ring seals. Force balance on the LCM indicatesthat the reading on the WOB sub is the resultant of forces dueto internal and external pressure and the WOB. Denotingupward forces as negative and downward forces as positive,force balance gives

( )( )

Sub Rdg. = WOB -

WOB +

p D p D D p D

p p D

o o o o i i i

i o i

π π π

π4 4 4

4

2 2 2 2

2

+ − +

= −

(1)where and are the annular and internal pressuresrespectively. This implies that

[ ]WOB = Sub Rdg. - ( p p Di o i− )π4

2 (2)

It can be verified that the sub reading given by Eq. (1) isindependent of configuration of BHA. The WOB as calculatedby Eq. (2) is hereafter referred to as the “ Corrected WOB.”The sub records a tensile force on the LCM as a positivenumber and a compressive value as a negative number.Therefore, a negative value of “corrected WOB” indicatescompression in the sub. Positive values of “corrected WOB”indicate a force opposite to the direction shown in Fig. 1. Forexample, while RIH or POOH, the “corrected WOB” is apositive number.

The WOB sub was installed in twelve of the fourteenbottom hole assemblies used to drill the well. The typicalBHA arrangement consisted of a mill or bit followed by theequalizing sub, MWD unit, the non magnetic drill collars,flow tube and the flex sub. The WOB sub was installed abovethe flex sub in most cases. The WOB sub recorded the axialload, bit torque, time and the temperature. Data was recordedat intervals that varied from one second to ten minutes, and itwas downloaded each time the BHA was retrieved at surface.Pressure transducers also recorded the internal and annularpressures near the bit.

Data from measurements at the surface were recordedseveral times a minute throughout the operation. The clockson the WOB sub, the pressure gages and the surfacemeasurement system were not synchronized. Since the fourdata sets (WOB and torque, internal pressure, externalpressure, and surface measurements) could not besynchronized, the raw data was interpolated to accommodatethe data sampled at the fastest rate.

Data AnalysisFigures 2 and 3 show the wellbore schematic and

trajectory respectively. As illustrated in Fig. 3, the wellpathconsists of six distinct sections. The well is highly deviatedand consists of vertical, inclined, curved (build and drop) and

horizontal sections. The rapid change in the direction is due tothe build section of the sidetrack drilled by the CT.

When the CT is run into hole, the sub is expected to readthe resultant of the weight of BHA section below the sub, thepressure forces and the drag on the BHA. Figure 4 shows thecorrected WOB and sub reading plotted (on the left and righthand vertical axes respectively) as functions of the measureddepth in the vertical hole section for two similarly constructedBHAs. The measured depths for all the figures in this paperwere obtained from the surface depth indicator. The figureillustrates several key points-1. The sub reads a consistent steady tensile load (~220 – 350lbf) for both BHA configurations.2. There are large spikes in the sub readings in the first 1000feet. These spikes are caused by shallow hole testing of theMWD tool. The shallow hole test involves increasing thepump pressure and checking the response of the MWD tool.The drilling reports for this operation confirm the shallow holetests. Note the relatively smooth (spike-less) curve forcorrected WOB which compensates for these hydrauliceffects.3. The sub records repeatable data, i.e. similar readings forsimilar BHAs and serves as a check for the data.

Figure 5 shows the sub reading and inclination in thecurved (dropping section) of the hole. The sub and thecorrected WOB increase as the inclination decreases due toincrease in the axial component of weight and a correspondingdecrease in drag. Figure 6 shows similar data for pulling out ofhole with BHA # 1. Note that the sub reading increases withdecreasing inclination, as expected.

Figure 7 shows the corrected WOB during pilot holedrilling and compares it with the hookload at surface. Thepilot hole is a “dropping” hole. The daily drilling report forthis period indicates drilling ahead at an ROP of 15 to 20 fphwith approximately 1- 2 klbf WOB. The data indicates anaverage downhole (corrected) WOB of less than 200 lbf. Thedata in Fig. 7 is re-plotted in Fig. 8 to illustrate thetransmission of weight to bit. The data shows the absolutemagnitude of downhole corrected WOB as a function of thesurface slackoff. Note that all data points do not lie below themaximum weight transfer line.

Figure 9 shows the correlation between the slack off anddownhole sub measurements for a period of twelve hours.According to the drilling reports, the tubing was run into holewith a new BHA at the beginning of the interval shown in Fig.9. Drilling commenced at 10,646 ft MD with 4.5 klbf slack offat surface and a rate of penetration of 200 ft/hr. Thirty minuteslater, the mud flow rate was reduced to 2.1 bpm to preventhole enlargement, with a corresponding decrease in the WOBand ROP. Five hours later, the drilling report indicates“…10,793 ft. difficult drilling shale…Stacking all availableweight…little progress.” This section of the drilling intervalends with the comments “…10706 ft., littleprogress…projected inc. at bit 94 deg. POH for bit/motorchange.” Examination of Fig. 9 shows two clusters of points.The lower cluster of points shows an approximately linearrelationship (with a slope approximately equal to 1) indicating

SPE 46008 WEIGHT ON BIT IN COILED TUBING DRILLING: COLLECTION AND ANALYSIS OF FIELD DATA 3

that all the slack off at the surface is transmitted downhole. Asthe slackoff increases further, the slope of the line decreasessince a portion of the slackoff is used to overcome drag. Thedrag may be caused by the weight of the string in inclined holeand wall contact forces in hole sections where buckling hasoccurred. The upper cluster of points tends asymptotically to 8klbf and the downhole weight is no longer responsive toslackoff. This is possibly due to helical buckling and theaccompanying increase in drag.

Recall that at any point in the CT string, drag is roughlyproportional to the axial force before buckling and isproportional to the square of the axial force after helicalbuckling. In general, the effect of slackoff on the downholeWOB is characterized by two distinct trends In the absence ofbuckling, downhole WOB is a linear function of the surfaceslack off. Since drag increases rapidly with the onset of helicalbuckling, the efficiency of weight transmission to the bit for agiven slackoff decreases and WOB tends asymptotically to thelockup load. The exact relationship between downhole WOBand the surface slackoff can be examined by using themethods outlined in Reference 4. It is conjectured that theasymptotic region of the WOB versus slackoff may bedescribed by the equation

( )WOB SO= − −α β1 e

where SO is the slackoff, and α and β are constants thatdepend on the wellbore and string geometry, coefficient offriction, and string stiffness. This functional dependence ofWOB on slackoff and the data in Fig. 9 suggest that α is themaximum WOB that can be obtained for a given slackoff.Furthermore, by using dimensional analysis, it can be shownthat the constant β is of the order of (µrcso)/EI where µ is thecoefficient of friction, rc is the radial clearance between theCT and wellbore, and EI is the bending stiffness of the string.Interestingly the factor (µrcso)/EI is dimensionally similar tothe reciprocal of the Euler buckling load for an unconstrainedcolumn. This functional behavior of the downhole WOB andits limiting value is currently being investigated.

Figure 10 shows torque as measured by the sub versusdownhole pressure drop determined from the differencebetween the internal and annular pressures near the bit. Thedrilling reports indicate that the data shown in this figurerepresents drilling ahead at 10,689 ft. MD, with 3 klbf WOBand a mud flow rate of 2.1 bpm. The ROP at this point isrecorded as 90 ft/hr. The performance curves of motors similarto downhole motor are also shown in this figure. Comparisonof the downhole data trend and motor performance indicatesthat the bit is drilling ahead with a torque of 150 – 300 ft-lbf.Finally, Fig. 11 represents the downhole data for drilling aportion of the horizontal section. The drilling report does notindicate the magnitude of weight stacked at the surface.However, the reports confirm drilling in the horizontal sectionat an ROP of 100 – 150 ft/hr. It is clear that the downhole subresponds to the slackoff at surface. The figures appear toindicate a correspondence between the slackoff weight and thedownhole data, as confirmed by the drilling reports.Unfortunately, the large amount of scatter in the data for the

horizontal section beyond this point prevented furtheranalysis.

Finally, a complete understanding of the data presentedhere requires careful comparison with the predicted valuesfrom a TFM. This additional analysis is ongoing and isexpected to be the subject of a future paper.

ConclusionsAnalysis of selected portions of downhole and surface data

collected in this study indicates the following-1. The downhole data follows expected trends for operationssuch as RIH, POOH and drill ahead operations.2. The dependence of downhole WOB on the surface slackoffillustrates the onset of buckling and the tendency for eventuallockup.3. The scatter in the downhole data is considerable. Themagnitude of the scatter is of the order of resolution of thedownhole weight measurement device.4. The WOB sub is essentially a strain gage based load cellcapable of measuring loads from 25,000 lbf in tension to10,000 lbf in compression. Since the downhole weights are ofthe order of a few klbf, a cell with a smaller range and greaterresolution may have to be used. This may require significantre-design of the tool. Further, well defined calibrationprocedures to benchmark tool measurements and identifyeffects of downhole temperature must be developed prior to afield test.5. Commercial TFMs used to plan CTD operations need to beverified with the downhole and surface data. This dataanalysis reveals the lack of an important feature in CTsimulators, i.e., the calculation of the downhole WOB for agiven slackoff at surface. Currently, this calculation has to beiteratively performed to generate a curve similar to Fig. 9.6. Future analysis should include the effects of hydraulics onbit behavior.

NomenclatureBHA = Bottom Hole AssemblyCT = Coiled TubingDi= Inside diameter of WOB sub, inDo= Outside diameter of WOB sub, inE = Young’s modulus, psiI = Moment of inertia, in4LCM = Load Carrying Memberµ= coefficent of frictionMD = Measured Depth, ftMWD = Measurement while DrillingPOOH = Pulling out of holepi= Pressure in the CT, psipo= Pressure in the CT-hole annulus, psirc = Radial clearance between CT and wellbore, inRIH = Running in HoleROP = Rate of Penetration, ft /hrSub Rdg.= Reading recorded by the WOB sub, lbfTFM = Tubing Force ModelWOB = Weight on Bit, lbf

4 BLOUNT, HEARN, PAYNE, SATHUVALLI AND LIVESAY SPE 46008

AcknowledgmentsWe are grateful to Lamar Gantt, Erin Oba and themanagement of ARCO Alaska Inc. and ARCO E & PTechnology for sponsoring this study and the writing of thispaper. We also thank Mr. Barry Dean and Mr. LorenHauglund of Dowell Schlumberger for their help with the CTforce simulator and preliminary data analysis. We also thankMr. Fred Pomeroy of Petredat, Inc. for his assistance inreducing the large amounts of raw data into files ofmanageable size.References1. Van Adrichem, W and Newman, K. R., “Validation ofCoiled Tubing Predictions in Horizontal Wells,” JPT (Feb.1993) 155.2. Bhalla, K., “Coiled Tubing Extended Reach Technology,”paper SPE 30404 presented at the 1995 SPE offshore EuropeConference, Aberdeen, Sep. 5-8.3. Payne, M. L. and Abbassian, F., “Advanced Torque andDrag Considerations in Extended-Reach Wells”, paper SPE35102 presented at the 1996 IADC/SPE Conference, NewOrleans, Mar. 12-15.4. Suryanarayana, P. V. R., “Maximum Set Down Weight andOverpull in Coiled Tubing Simulations,”, paper SPE 36348presented at the 1996 1st North American Round CoiledTubing Roundtable, Montgomery, TX, February 25-26.

SI Metric Conversion Factorsft x 3.048 E-01 = min2 x 6.4516 E-04 = m2

ft /hr x 1.13864 E-2 = m / hrpsi x 6.89476 E+00 = kPalbf x 4.448222 E+00 = Nft-lbf x 1.355818 E+00 = N-m

Fig. 1 Schematic of the WOB sub

Fig. 2 Wellbore and Sidetrack Schematic

Fig. 3 Wellbore inclination and azimuth

Fig. 4 Running in Hole- Vertical Section

SPE 46008 WEIGHT ON BIT IN COILED TUBING DRILLING: COLLECTION AND ANALYSIS OF FIELD DATA 5

Fig. 5 - Running in Hole- Dropping Hole Section

Fig. 6 - Pulling out of Hole

Fig. 7 – Hook load and WOB- Pilot Hole Drilling

Fig. 8 - Weight Transmission- Pilot Hole Drilling

Fig. 9 – Weight Transmission- Sidetrack Entrance

Fig. 10 – Downhole Torque and Pressure Drop

6 BLOUNT, HEARN, PAYNE, SATHUVALLI AND LIVESAY SPE 46008

Fig. 11 – Drilling the Horizontal Section