Effect of Drillpipe Rotation on HoleCleaning During Directional-Well Drilling

R. Alfredo Sanchez,* SPE and J.J. Azar, SPE, U. of Tulsa; A.A. Bassal, SPE, Gearhart United Pty. Ltd.;and A.L. Martins, SPE, Petrobras

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SummaryThe effect of drillpipe rotation on hole cleaning durindirectional-well drilling is investigated. An 8 in. diameter wel

bore simulator, 100 ft long, with a 412 in. drillpipe was used forthe study. The variables considered in this experimental workrotary speed, hole inclination, mud rheology, cuttings size,mud flow rate. Over 600 tests were conducted.

The rotary speed was varied from 0 to 175 rpm. High viscosand low viscosity bentonite muds and polymer muds were uwith 1

4 in. crushed limestone and110 in. river gravel cuttings. Fourhole inclinations were considered: 40°, 65°, 80°, and 90° frvertical.

The results show that drillpipe rotation has a significant effon hole cleaning during directional-well drilling, contrary to whhas been published by previous researchers who forced thepipe to rotate about its own axis. The level of enhancement dupipe rotation is a function of the simultaneous combinationmud rheology, cuttings size, and mud flow rate. Also it was oserved that the dynamic behavior of the drillpipe~steady statevibration, unsteady sate vibration, whirling rotation, true axialtation parallel to hole axis, etc.! plays a major role on the significance in the improvement of hole cleaning.

Generally, smaller cuttings are more difficult to transpoHowever, at high rotary speed and with high viscosity muds,smaller cuttings seem to become easier to transport. Generalinclined wells, low viscosity muds clean better than high viscosmuds, depending on cuttings size, viscosity, and rotary splevel.

IntroductionNumerous studies on cuttings transport have been conductethe past two decades. Although several investigators have mobservations on the effect of drillpipe rotation, most have focutheir studies on mud rheology and annular velocities. This isfirst time an extensive experimental study is conducted withsole purpose of investigating the effect of drillpipe rotationhole cleaning.

In the past, the effect of drillpipe rotation was thought tominimal. This belief was based on the results of experimewhich were conducted in flow loops that used centralizers to cstrain the pipe to rotate on its own axis, avoiding any orbmotion. Although the motion of the pipe will change at differepositions along the well, it is now believed that in most casesdrillstring will have both rotary and orbital motion, even whentension. In this case, it is the orbital motion and not the rotatthat improves hole cleaning. When the pipe is rotating only aloits axis, it will cause a shift and a slight increase in the velocprofile in the annular area, causing the velocities on one sid

*Now with Reda Pump.

Copyright © 1999 Society of Petroleum Engineers

This paper (SPE 56406) was revised for publication from paper SPE 37626, first presentedat the 1997 SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 4–6March. Original manuscript received for review 2 April 1997. Revised manuscript received15 January 1999. Revised manuscript approved 11 February 1999.

SPE Journal4 ~2!, June 1999

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the hole to be higher than on the other. Generally, a no slip cdition at the boundaries in the annulus is assumed. These incthe boundary between the hole or casing and the fluid, the bouary between the fluid and the drillpipe, and the boundary betwthe fluid and the cuttings bed. If the pipe is not rotating, tvelocity of the fluid particles at these boundaries is zero. Whenpipe rotates, this boundary condition means that the velocitythe fluid particles adjacent to drillpipe is equal to the rotationspeed of the pipe, perpendicular to the hole axis, resulting ipseudo-helical flow. The minor effects observed in tests cducted under this configuration~using centralizers! indicate thatthe shift and the increase of the annular velocities are minordo not affect cuttings transport significantly. On the other hathe orbital motion of the pipe improves the transport of cuttinsignificantly in two ways: First, the mechanical agitation of tcuttings in an inclined hole sweeps the cuttings resting onlower side of the hole into the upper side, where the annuvelocity is higher. Second, the orbital motion exposes the cuttiunder the drillstring cyclically to the moving fluid particles. Evethough investigators have been aware of this phenomenon, itusually been ignored for several reasons: First, the orbital moof the string will generally reduce the cuttings concentration inannulus, and therefore ignoring it was thought to be a consetive approach. Second, the dynamics of the drillstring in the wbore is still not well understood. Also, the borehole simulatocurrently available that allow simulation of pipe rotation assuthat the pipe does not orbitate. However, the reduction in cutticoncentration and improvement on bed erosion are too high toignored. Furthermore, recent advances in drillstring dynamcould eventually evolve into a complex cuttings transport simutor that accounts for orbital motion.

As will be shown, the benefit of pipe rotation to hole cleaniis a function mainly of rotary speed, hole inclination, and florate, mud rheology, and cuttings size. The latter two have the leffect.

ExperimentsOver 600 tests were conducted at The University of Tu~TUDRP! cuttings transport facility. A full scale 8 in.3100 ft

wellbore simulator~Fig. 1! with 4 12 in. drillpipe was used. A data

acquisition system records information every second. The inmation recorded allows monitoring of cuttings concentrati~mass in the test section! throughout the duration of each test.detailed description of the facility is given in Refs. 2 and 3.

Test Procedure.The test procedure was designed to studyeffect of pipe rotation on:

1. cuttings concentration while drilling~at steady state conditions!,

2. bed erosion after drilling has stopped,3. cuttings transport patterns.

The following sequence of steps was used. First, a flow ratestablished and the data acquisition started while the drillpipstatic. Injection of cuttings is then started and continued usteady state is reached, i.e., a constant concentration in the alus has been established. Ten minutes into the steady state c

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tion, pipe rotation is started. An increase in the cuttings collectrate occurs until a new steady state is reached and held for an10 min. Then the injection of cuttings is stopped and erosionthe bed begins. The test is terminated once no more cuttingsleaving the annulus~collection rate5injection rate50!. The cut-tings concentration at this point may or may not be zero, depeing on the operating conditions.Figs. 2 and 3show two tests inprogress. Stage 1 represents the accumulation process, whecuttings concentration in the annulus increases from zero unreaches a constant value. In stage 2, the cuttings injection raequal to the cuttings collection rate and the cuttings mass inannulus remains constant. Pipe rotation starts in stage 3 andtinues until the end of the test. Erosion of the bed beginscontinues until a new steady state is reached. This is the beginof stage 4 in which the cuttings mass in the test section remconstant. At the end of stage 4, the cuttings injection ratestopped, resulting in further bed erosion. The erosion is showstage 5 where the cuttings concentration decreases to its lovalue. Fig. 2 shows that under the given conditions, rotary spof 50 rpm is not enough to remove all the cuttings from the

Fig. 1–TUDRP cuttings-transport loop.

102 Sanchezet al.: Effect of Drillpipe Rotation

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nulus. However, Fig. 3 shows how under the same conditiorotary speed of 90 rpm does allow all the cuttings to be remov

Mud Systems.A total of five mud systems were used: two betonite muds and three polymer muds. The two bentonite systhad PV/YP equal to 7/7 and 20/20~PV5h600-h300,YP5h300-PV!. The three polymer muds had low, medium, ahigh viscosities~unfortunately the actual rheological propertieare proprietary information we are unable to publish at this tim!.

Cuttings Type and Size.Two types of cuttings were selected14 in. crushed limestone cuttings with a density of 2.56 g/cm3 and1

10 in. river gravel with a density of 2.64 g/cm3. The settling ve-locities of these cuttings in the mud systems at static conditivaried between 1.5 and 10.5 in./sec.

Pipe Motion. The drillpipe was constrained at its ends only, alowing it to assume several types of motion depending on sevconditions. Pure rotation~motion consisting of rotation about itown axis only! existed at very low speed and the pipe did staycontact with the hole. Furthermore, at low speeds, this motionindependent of the rest of the conditions. At higher speeds,drillpipe would begin an orbital motion. The speed at which thoccurs and the actual path of the drillpipe depends on bed heThe higher the bed height, the higher the speed required toorbital motion, and the lower the deviation from pure rotatioThe reason for this phenomenon is that the bed acts as a damp~aconstraint! whose coefficient~strength! depends on its height. Inan extreme case, a drillpipe that is buried in a cuttings bed woremain in pure rotation even at the higher speeds.

The motion of the drillpipe was very realistic. The differetypes of motion observed are probably the closest representaof downhole conditions in an experimental cuttings transport flloop. One of the most realistic phenomena observed was‘‘creeping’’ of the drillpipe up the casing wall, until gravity wouldprevent it from continuing. The combined result of its momentuand gravity would throw the pipe back to the center~horizontally!of the hole.

Fig. 2–Test in progress, 50 rev/min.

SPE Journal, Vol. 4, No. 2, June 1999

Fig. 3–Test in progress, 90 rev/min.

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Results and DiscussionIn Figs. 4–15, the cuttings weight in the test section at steastate conditions is plotted against rotary speed. The graphs sdifferent flow rates for the three angles and two types of cuttiused. The injection rate was kept constant at 35 ft/hr. For cparison purposes, the scale of the axes are the same in all grThe results show that the level of improvement in hole cleaninga result of drillpipe rotation can vary from moderate to significadepending on several variables: cuttings size, flow rate, hangle, mud rheology, and drillstring rotary speed. The figushow that rotating the pipe can result in as much as 80% reducin the cuttings concentration. The following discusses the effecthese variables on the level of improvement.

Rotary Speed and Flow Rate.As flow rate increases hole cleaning improvement increases, as expected, at all rotary speHowever, the level of improvement was different at different rtary speeds. For example, at the highest inclination~90°, Figs. 4to 7! and low flow rates~upper two curves!, the reduction incuttings concentration due to pipe rotation was higher in the uprange of pipe speeds, above 100 rpm. In other words, at low flrates, increasing pipe rotation from 100 to 150 rpm was m

Fig. 4–Cuttings weight in test section, Set 1.

Sanchezet al.: Effect of Drillpipe Rotation

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significant than from 25 to 75 rpm. Looking at the higher flow ra~lower curves, Figs. 4–7!, a change in the shape of the curves cbe noticed. In the lower flow rates, the slope of the curves inregion between 0 and 75 rpm is less~less negative! than above100 rpm. However, at higher flow rates, the opposite is observthe low speed regions have higher~more negative! slopes. Onereason for this phenomenon is that at low flow rates, the pipemore cuttings to act on, even after being rotated. In higher flrates, the bed height is lower and the initial rotary speeds decrit the most. At a lower inclination~65°, Figs. 8–11!, this patterndisappears, but still the higher the rotary speed, the least thenular cuttings concentration. In the 40° tests~Figs. 12–15!, thebenefit of pipe rotation was independent of the rotary speed,creasing rotation from 0 to 75 rpm produced the same benefitfrom 100 to 150 rpm. Although the reduction in annular concetration due to pipe rotation is the least at this angle, it is ssignificant.

Rotary Speed and Mud Rheology.In general, rotary speed haslightly higher improvement on hole cleaning for the higher vcosity muds tested. This was true for the polymer and bentomud systems. It is believed that the most enhancement in

Fig. 5–Cuttings weight in test section, Set 2.

SPE Journal, Vol. 4, No. 2, June 1999 103

Fig. 6–Cuttings weight in test section, Set 3.

Fig. 7–Cuttings weight in test section, Set 4.

Fig. 8–Cuttings weight in test section, Set 5.

Fig. 9–Cuttings weight in test section, Set 6.

104 Sanchezet al.: Effect of Drillpipe Rotation

Fig. 10–Cuttings weight in test section, Set 7.

Fig. 11–Cuttings weight in test section, Set 8.

Fig. 12–Cuttings weight in test section, Set 9.

Fig. 13–Cuttings weight in test section, Set 10.

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cleaning due to pipe rotation is a result of the mechanical agitaof cuttings on the low side of the wellbore and also whetherpipe lifts off from the low side to the high side as it rotates. Itspeculated by the present authors that for finer drilled solsmaller than what was used in the present work, more significeffect of rotary speed on hole cleaning will be experiencedhigher viscosity muds.

Rotary Speed and Hole Angle.The major improvements in holecleaning from pipe rotation were observed in the higher inclitions. The lowest improvement was achieved at 40°~Figs. 12–15!, while the highest was in the horizontal tests~Figs. 4–7!. Apossible reason is that at high angles the mechanical agitatiomore significant.

Rotary Speed and Cuttings Size.Figs. 4–7 show the results fothe 90° tests. For the thinner mud (PV/YP57/7), the cuttingsconcentration with the larger cuttings~Fig. 4! was lower than withthe smaller cuttings~Fig. 5!. For the thicker mud (PV/YP520/20), the smaller cuttings were still harder to transport,

Fig. 14–Cuttings weight in test section, Set 11.

Sanchezet al.: Effect of Drillpipe Rotation

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the difference in the concentration of the two sizes was less. Thfigures show that at 90°, the size of the cuttings, for the sizes uhere, does not affect the level of improvement caused by the dpipe rotation.

Figs. 8–11 show the same plots for the 65° tests. At this innation, cuttings size only affected the level of improvement whusing the thicker mud. The average reduction in cuttings conctration as a result of pipe rotation was higher with the 20/20 mthan with the 7/7 mud. Still the difference was small and the saconclusion can be reached as for the 90° angle. Similar resultsbe observed from Figs. 12–15 for the 40° angle.

Bed ErosionDrillpipe rotation has a major effect on hole cleaning after drillinhas stopped. The benefits of rotating the pipe are in the resicuttings concentration and in the time it takes to clean the hBy residual cuttings concentration it is meant the amount of ctings left in the annulus that cannot be cleaned even after drilceases. At low flow rates, the annular velocities are not henough to erode 100% of the cuttings bed. A residual concen

Fig. 15–Cuttings weight in test section, Set 12.

Fig. 16–Bed erosion at high inclination.

SPE Journal, Vol. 4, No. 2, June 1999 105

Fig. 17–Bed erosion at high inclination, 125 rev/min.

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tion was generally left when the pipe was not rotating. Howevwhen the pipe was rotating, in most of the tests, it was possiblclean the annulus completely, even at low rotary speeds. Alsotime it takes to clean the hole can be drastically reduced withrotary speeds. The bed erosion process is illustrated inFigs.16–19 for the XCD mud. In Fig. 16, the test was started withpipe rotation until the steady state condition was reached, wclose to 975 lb. of cuttings in the test section. At this point, tinjection rate was stopped and bed erosion began, continuingthe cuttings concentration had dropped to 625 lb.~no more cut-tings leaving the annulus!. The pipe was then rotated at 50 rpmreinitiating erosion, and resulting in an additional 125 lb. drop

106 Sanchezet al.: Effect of Drillpipe Rotation

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cuttings in the annulus. Similarly, increasing the pipe rotation75 and 125 rpm reduced the cuttings weight to 475 and 400respectively. No further erosion~cleaning! was possible at thegiven flow rate. From the graph it can be seen that in this caswas the initial 50 rpm rotation that had the most benefit incleaning process. A similar test was conducted under the sconditions and is shown in Fig. 17. In this case, the pipe wrotated during the entire test. The cuttings weight in the test stion at steady state~before stopping cuttings injection! was only750 lb., approximately, compared to 975 lb. for the previous teStopping cuttings injection caused the bed to be eroded dowabout 375 lb.

Fig. 18–Bed erosion at low inclination, 0 rev/min.

SPE Journal, Vol. 4, No. 2, June 1999

Fig. 19–Bed erosion at low inclination, 125 rev/min.

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Figs. 18 and 19 show bed erosion at 40° inclination. Fig.shows a test with no pipe rotation. After the injection rate wstopped, the bed was eroded from 425 to 75 lb. approximatComparing with Fig. 19, it can be seen that at this inclination protation did not affect cuttings concentration~at steady state obed erosion after cuttings injection had stopped! as much as in thehigher 65° and 90° tests.

Cuttings Transport PatternsIn high inclinations, above 50°, the main effect of pipe rotationthe cuttings transport patterns is the formation of an unsteadyleading to individual dunes as rotary speed increases. Thisobserved even at the lowest rotary speeds and at all flow ratethe higher speeds, above 125 rpm, some cuttings can actualover the pipe from one side of the annulus to the other. At lowinclinations, 40°, the bed is about to start sliding down and roing the pipe seem to help initiate it. In general, at this inclinatrotation caused more disorder than in lower angles.

Conclusions1. Pipe rotation has a significant effect on hole cleaning.2. The reduction in the cuttings weight in the annulus can be

high as 80°.3. The decrease in cuttings concentration is a function of ro

speed, hole inclination, and flow rate.4. In general, at 90° from vertical and low flow rates hig

rotary speeds produce the most benefits. The opposite is truhigh flow rates. In lower inclinations, no critical range of rotaspeeds was identified but higher meant better.

5. Pipe rotation also improves bed erosion once drilling hstopped. Both the residual concentration and the erosion timereduced.

6. The formation of unsteady beds and dunes were observeeven the lowest rotary speeds.

7. The motion of the drillpipe determines the contributionpipe rotation to hole cleaning. Orbital motion is needed for snificant improvement to occur.

Sanchezet al.: Effect of Drillpipe Rotation

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AcknowledgmentsThe authors wish to thank Shell International Exploration aProduction, Petrobras, BDM-DOE, and TUDRP for financial suport and permission to publish the results of this study.

References1. Jalukar, L.S.: ‘‘A Study of Hole Size Effect on Critical and Subcrit

cal Drilling Fluid Velocities in Cuttings Transport for Inclined Wellbores,’’ MS Thesis, U. of Tulsa, Tulsa, OK~1993!.

2. Larsen, T.: ‘‘A Study of the Critical Fluid Velocity in Cuttings Transport for Inclined Wellbores,’’ MS Thesis, U. of Tulsa, Tulsa, O~1990!.

3. Bassal, A.: ‘‘The Effect of Drillpipe Rotation on Cuttings Transpoin Inclined Wellbores,’’ MS Thesis, U. of Tulsa, Tulsa, OK~1996!.

4. Martin, M. et al.: ‘‘Transport of Cuttings in Directional Wells,’’ pa-per SPE 16083, Presented at the 1987 SPE Drilling Conference,Orleans, 15–18 March.

5. Kenny, P., and Hemphill, T.: ‘‘Hole-Cleaning Capabilities of aEster-Based Drilling Fluid System,’’SPEDC~3 March 1996!.

6. Kenny, P.et al.: ‘‘Hole Cleaning Modelling: What’s ‘n’ Got To DoWith It?,’’ paper SPE 35099, Presented at the 1996 SPE DrillConference, New Orleans, 12–15 March.

7. Luo, Y. et al.: ‘‘Flow Rate Predictions for Cleaning DeviatedWells,’’ paper SPE 23884, Presented at the 1992 SPE Drilling Cference, New Orleans, 18–21 February.

Metric Conversion Factorsin. 3 2.54* E100 5 cmft 3 3.048* E201 5 m

lbm 3 4.535 924 E201 5 kggal 3 2.31* E102 5 m3

*Conversion factor is exact. SPEJ

R. Alfredo Sanchez is an application engineer with RedaPump. Electronic-mail: alfredo–sanchez@redapump.com. Hepreviously was a MS degree candidate at the U. of Tulsa,where he conducted theoretical and experimental researchon cuttings transport. Sanchez holds a BS degree in mechani-

SPE Journal, Vol. 4, No. 2, June 1999 107

cal engineering and a MS degree in petroleum engineering,both from the U. of Tulsa. J.J. Azar is Professor of petroleumengineering at the U. of Tulsa and a lecturer and consultant indrilling engineering. He has extensive experience in appliedindustrial drilling research. He holds a PhD degree in mechani-cal engineering from the U. of Oklahoma. Azar received the1997 Distinguished Achievement Award for Petroleum Engi-neering Faculty and the 1998 Drilling Engineering Award. Cur-rently the chairman of the Multilateral Technology Technical

108 Sanchezet al.: Effect of Drillpipe Rotation

Interest Group and a member of the Distinguished Achieve-ment Award for Petroleum Engineering Faculty Committee, healso served on an 1985–86 Annual Meeting Technical Commit-tee, as the 1990–92 U. of Tulsa Student Chapter Faculty Spon-sor, and as a 1990–93 member of the Career Guidance Com-mittee. Biographical information for Adel Ali Bassel isunavailable. Andre L. Martins is a petroleum chemist withPetrobras in Rio de Janeiro.

SPE Journal, Vol. 4, No. 2, June 1999