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he ffects of Mud Rheologyon Annular Hole Cleaning

in Directional WellsSiavomir S Okrajni Tulsa Azar SPE, of Tulsa

Summary The effects of field-measured mud rheological properties on cuttings transport in directional welldri ll ing were studied experimentally. Water and bentonite/polymer muds were used, and angles of annulusinclination ranging from 0 to 90 from vertical. Experimental data were processed to express the cuttingstransport quantitatively through annular cuttings concentration vol ) at steady state.

Three separate regions of hole inclination can be identified regarding cuttings transport: 0 to 45 45 to 55,and 55 to 90. The effect of laminar flow dominates cuttings t ransport in low-angle wells 0 to 45). In highangle wells 55 to 90 ), the effect of turbulent flow predominates. In the range of intermediate inclination 45to 55), turbulent and laminar flow generally have similar effects.

In laminar flow, higher mud yield values and yield-point/plastic-viscosity YP/PV) ratio provide bettercuttings transport. The effect of mud yie ld value is significant in the range of 0 to 45 hole inclination andbecomes small or even negligible in the range of 55 to 9 0. The effects of mud yield value and YP/PV ratioare more significant for lower annular fluid velocities. In turbulent flow, the cuttings transport was generallynot affected by the mud rheological properties.

Introduction and General DiscussionThe problem of cuttings transport was studied by manyinvestigators. An extensive literature review is given byTomren. 1 Recently, increasing attention regarding cuttings transport has been given to directional drilling. Tomren, 1 Iyoho,2 and Becker,3 among others, have

conducted studies in this area.On the basis of detailed analyses of previous and cur

rent work, several factors affect the cuttings transport inan inclined annulus.

Axial an d Radial Components of Particle Slip Velocity . According to gravity laws, only the axial componentof the slip velocity exists in the case of a vertical annulus:

s 1

This situation changes while the annulus is inclined gradually. The component of the slip velocity appears as

= s cos 2

and

s = s sin . . 3)

This situation is shown in Fig . 1Obviously, when the angle of inclination is increased,

the axial component of the slip velocity decreases, reaching zero value at the horizontal posit ion of the annulus.At the same time, the radial component reaches a maximum in the position mentioned. By taking these condi-

Copyright 1986 Society of Petroleum Engineers

SPE Drilling Engineering, August 1986

tions into account , one can say that all factors that maylead to improved cuttings transport by a reduction of theparticle slip velocity will have a diminishing effect whilethe angle of inclination is increased.

Annular Mu d Velocity. The annular mud velocity in vertical drilling has to be sufficient to avoid cuttings settlingand to transport these cuttings to the surface in reasonable time. As discussed earlier, in the case of an inclinedannulus, the axial component of particle slip velocity playsa less important role, and one could conclude that to havea satisfactory transport, the annular mud velocity in thiscase may be lower than in the vertical annulus. This, however, would be a misleading conclusion. The increasingradial component of particle slip velocity pushes the particle toward the lower wall of the annulus, causing a cutt ings particle) bed to form. Consequently, the annularmud velocity has to be suff icient to avoid or at leas t tolimit) the bed formation. Studies show that to limit cuttings bed formation, the annular mud velocity in directional drill ing has to be generally higher than in verticaldrilling. 1 2

Flow Regime an d Regime of Particle Slippage. Whenthe cuttings-transport phenomenon is considered, the regime of flowing mud and vertical slippage should be considered simultaneously. A mud in turbulent flow alwaysinduces turbulent regime of particle slippage, independentof the cuttings shape and dimensions. Therefore, in thiscase, the only factor that determines the particle slip velocity is the momentum forces of the mud; there is noinfluence of mud viscosity. the mud flows in the lami

nar regime, then depending on the cut tings shape and297

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OW

+ I

Vsa=VsVsr=O

vsa=vscos9

Vsr=Vs sin 9

OW

Vsr=VsVsa =0

Fig. 1-Particle settling velocity in an inclined annulus from Tomren 1).

dimensions-either turbulent or laminar regime of slippage may be expected. The laminar regime of slippagewill always provide a lower value of particle slip velocity. One should conclude that laminar flow usually willprovide a better transport than turbulent flow. It shouldbe recalled again, however, that in the case of an inclinedannulus, the significance of the axial component of particle slip velocity decreases, and one may expect that anadvantage of laminar flow will be nullified while the angle of inclination is increased. This has been confirmedpartially by Ref. 2.

Velocity Distribution Profile in aminar Flow as a Result o f M u d Rheological Properties. The power-law Ostwald s model for a flowing mud is

r= yn 4)

Parameter n characterizes the velocity distribution profile shown in Fig. 2. The profile is more pointed for highervalues of n and more flat for lower values. The shape ofthe profile has a very important practical significance inthe cuttings-transport phenomenon. A flatter profile

Fig. 2-Effect of Parameter n on annular velocity profile.

v.

Fig. 3-Definit ion of annular eccentricity from lyoh0 2

a = 0.5, b =0 , and c -0.5.

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1I FLOW WITH

CONCENTRICSTATIONARY

I BED

.I i; j,

cI uI InC>I

II

InI I 33 ;.

0 I ECCENTRIC

II 1lIJI N PSEUDO HOMOGENEOUSI iii FLOWI i / \ lIJ0 I JUI i=

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TABLE VALUES OF EXPERIMENTAL VARIABLES USED IN. CUTTINGSTRANSPORT EXPERIMENTS

VariableAnnulus length, ftAnnulus x 1 in.Annulus inclination, degreesFlow rate, gal/minInner-pipe rotation, rev/minInner-pipe eccentricityParticle size, ASTM design, in.Particle density, Ibmlft 3

Particle injection rate, Ibm/min average valueMuds used parameters in Table 1)

Range

405 x 1.9

0,20,40,45,50,70,90100,150,175,200

500.0 0.5

0.250163.420.2

1,2,3 15

TABLE 3 VALUES OF EXPERIMENTAL VARIABLES USED INANNULUSCLEANING EXPERIMENTS

VariableAnnulus length, ftAnnulus x 1 in.Annulus inclination, degreesFlow rate used for particle injection, gal/min

Flow rate used for annulus cleaning, gal/minInner-pipe rotation, rev/minInner-pipe eccentricityParticle size, ASTM design, in.Particle injection rate, Ibm/min average valueMuds used parameters in Table 1)

Range

405 x 1.9

30,45,70,9040

175,2000,50,150 0. 5

0.25020.05

1,4,12,13,15

course, a resul t of t ransport; however, its presence affects continuous transport until steady-state conditions arereached. Because of the presence of a cut tings bed, theeffects not observed in a vertical annulus are experiencedin the inclined one.:.-.saltation flow, heterogeneous andpseudohomogeneous flows, etc. Fig. 5 illustrates thesesituations. These effects are caused by a kind of interaction between flowing mud and the cuttings bed that isbeing f or me d. We believe that the better transport observed by Iyoh0 2 in turbulent flow was.provided largely by these interactions. Also, because of the presenceof the cuttings bed, the velocity of particles being transported in an inclined annulus was independent of annularmud velocity in steady-state conditions.

Sliding Down of Cut t ings Bed Along Lower Wal l ofInclined Annulus. Under certain conditions, the cuttingsbed slides along the lower wall of the inclined annulus. 1 2This was observed for 40 and 45 0 angles of inclinationat relatively low annular mud velocities. This effect was

dominant, nullifying the influence of other parameters andresulting in the worst transport highest final annular particle concentration). This observation has an importantpractical significance.

Inner-Pipe Rotation. Beyond the mud axial flow, a tangential flow is experienced while the inner pipe is rotated. A minor effec t of turbulence is observed as a resultof this tangential flow. Furthermore, because of the presence of the cuttings bed, a mechanical action of the rotating pipe on the bed can be expected. These factors shouldinfluence the cuttings transport in the inclined annulus.However, previous work 2 found this to be negligible.

Drilling Rate. The drill ing rate has an important effecton the quantitative aspect of cuttings transport. This was

3

shown to be true in studies concerning a vertical annulusand is also reflected in the predictive models for thecuttings-transport ratios in a directional annulus. 2

Rheological Properties of Flowing Mud. This very important aspect of the problem is the main objective of thepaper and therefore will be discussed in detail later.

Transport and leaning xperimentsTransport Experiments. In the investigations performedby Tomren1 and Iyoho,2 the main at tent ion was givento such basic variables as angle of inclination, annular mudvelocity, flow regime, eccentricity, and inner-pipe rotation. Iyoho also used the mud effective viscosity, whichobviously reflects the actual behavior of flowing mud. I tis still very important in field practice, howevet, to evaluate mud transport ability quickly on the basis of such parameters as apparent viscosity, PV, yield value, and gelstrength.

In this study, particular attention is given to mud yieldvalue, which is believed to be a major factor affectingthe cuttings transport. This effect has been demonstratedby Hussaini and Azar 5 for the case of a vertical annulus. Note, however, that mud behavior cannot be evaluated on the basis of a single parameter i.e., yield valuein this case). Such evaluation is insufficient and sometimes is even misleading. Fo r instance, it is possible todesign an unlimited number of muds with yield values of10 Ibf/l00 ft2 [4.8 Pa], and each one will indicate adifferent behavior. Therefore, it was decided to use theYP/PV ratio as the additional characteristic of the mud.The yield value was assumed to be in the range of 0 to20 Ibf /l 00 ft2 [0 to 9.6 Pa], which usually representsfield use of nondispersed, unweighted muds. Fo r everygiven yield value, three values of YP/PV ratio 0.5, 1

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A- FRAME

ROTARYDRIVE

RETURN EN QINNER PIPE - - - - ~ - - - - - - - - - - - - . I J o - - - ~ C E N T R A L I Z E R

SUCTION LINE I OF 4

DUMPV LVE

SWINGARM

FLOWDIVERTERPARTICLE

COLLECTION.=:::U;= \BASKETS

TANK

Fig. 6-Schematic side view of transport apparatus from Tomren 1 .

SLIDESUPPORT

HOIST

and 2 were established, resulting in a total of 15 mudsystems, including water. See Table 1.

The flow rates considered were 100, 150, 175, and 200gal/min [6.3, 9.5, 11.0, and 12.6 dm 3 Is]. In the case ofa 5 x 1.9-in. [12.7 x4.8-cm] annulus, these flow rates gaveannular mud velocities: 1.91,2.86,3.34, and 3.82 ft/sec[0.58,0.87, 1.02, and 1.16 m/s], respectively. These velocities, along with the established ranges of rheologicalproperties, provided both laminar and turbulent flow inthe annulus.

Although many controversies have arisen regarding thetrue distinction between laminar and turbulent flow of nonNewtonian fluids, the following general equation for critical velocity is used in this study.

V ac

[

6,464/ 1 +3n 2 12 n l a ]1/ 2-n J

1 2+nJ/ l+n J do-di n 79PL

+n

6

Two different positions of inner pipe were run: concentric position E =0 ) and positive eccentric position E +0.5). The hole angles of inclination chosen-fromvertical to horizontal position-were 0, 30, 40, 45, 60,70, and 90.

Because the main purpose of this study was to investigate the effect of mud rheological properties on cuttingstransport, other parameters, such as pipe rotation, particle size, injection rate, and mud type, were kept constant.Consequently, inner-pipe rotation was established to be50 rev/min. The cuttings used in all experiments were obtained by drilling Carthage marble. An American Soc.of Testing Materials ASTM size of 0.25 in. [0.64 cm]

SPE Drilling Engineering, August 1986

was chosen. The cuttings injection rate was establishedto be 200.5Ibm/min [1.140.0038 kg/s]. Every mudwas formulated with bentonite, Ben-Ex, andWL lOO The necessary concentrations were adjustedto obtain desired values of YP and YP/PV ratio. Valuesof all experimental variables used in the transport experiments are shown in Table 2.

Because a cuttings bed is formed, the particle transportv l o i ~in steady-state conditions was independent of flowrate. I, Any reduction in flow rate was compensated forby an increase in bed thickness, resulting in the constantcuttings transport velocity. An erroneous conclusion thatcuttings transport in an inclined annulus is independentof the annulus mud velocity could be made. This is misleading because the amount of cuttings inside the annulus in steady-state conditions annular cuttings concentration varies. Field practice shows that the annular cuttings concentration vol ) is the factor that causes pipesticking, high torque, and drag. Very often, this factordetermines whether drilling will be trouble-free or troublesome. Therefore, in our opinion, the annular cuttings concentration vol ) is the parameter that should be

considered regarding the cuttings transport in directionalwell drilling.

Annulus-Cleaning Experiments. It is common practicein drilling operations to circulate a certain period of timebefore pulling a bit out of the hole to clean an annulusand consequently to avoid pipe sticking as well as to ensure that the bottom will be reached while the bit is runagain into the hole. Often, particularly before casing, acertain volume of high-viscosity mud is pumped into thehole for the above-mentioned reasons. Field experienceshows that this procedure is effective in vertical wells,but would it also be effective in directional wells? To answer this question, some experiments were run where par-

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Fig. 7 Schematic top view of transport apparatus fromTomren 1).

ticle injection was stopped, but fluid circulation continued

to clean the annulus completely. These experiments willbe referred to as annulus-cleaning experiments. Thevariables used are given in Table 3, and the deta iled experimental procedure will be given later.

t e e +0.5 50WATEA

v 3. 2 ItTUll8ULENT FLOW

6 + - . - - , . -tJ.--l .. . 1 . - . - b - - . L _ _ 0

< - - - - - - - - - o - - - - - - - - ~ - - - - - - - - - - - ~. -

z0

4II:t-Z

Z t e e +0.5u

rpm : 50 AVERAGE..J v. 3. B2 It TRENDui= 2 yp 0 - - 0 9 . 3 0II: PV I and 0 --- 9 . 7 0fII: TURBULENT FLOWS

0 - . - 0 9 9 0

..J I:::>zz

I I I0

0 I 2 3 4 6 7 8 9 10YIELD VALUE. Ibfl100 ft2

Fig. 9 Effect of mud yield value on annular cuttings con-centration turbulent flow .

Fig. a Effect of inclination on annular cuttings concentration turbulent flow .

O k - - - - - j l r - - - - l J ~ - - ~ O O : _ _ - - _ : I O : - - -

ANGLE INCLINATION

Test ProcedureCuttings Transport. Before any test, a mud system wasprepared. The mud pit was filled with 1,600 gal [22.9m 3 ] tap water. Then the water was pretreated with sodaash to remove the calcium ions present. Finally, the estimated from preliminary tests) amounts of Ben-Ex, bentonite, and if necessary) WL-l00 were added through themud hopper to obtain projected values of YP and YP/PVratio. After the mud was mixed accurately, its propertieswere checked with a six-speed rotational Fann V -0 viscometer. I f obtained values of YP and YP/PV ratio

differed from those projected, a correction in composition was made with either water, bentonite, or WL-IOO.I f obtainedvalues agreed with the projected whole range);all six speeds of rheological properties were recorded. Theproperties of all 15 mud systems used are included in Table 1

While the mud system was prepared, the particles werewashed, dried, and sorted according to the ASTM procedure. This procedure was repeated after every four runs,when the percentage of broken particles became considerable.

After the mud and particles were prepared, the pumpwas started and the mud was pumped until the hopper washal f full. Then between 300 and 400 Ibm [136 and 181kg] of particles were lowered carefully into the hopper.

FLUID TANK

MOUNTINGPLATE

ROTARY ~ = - . ,DRIVE

Experimental Setup and ProcedureTest Apparatus. The test apparatus used in this study isreported by Tomren et l 6 and is shown schematicallyin Figs. 6 and 7. I t was designed to meet the followingrequirements: 1) steady-state annular mud flow must prevai l in every test case, and 2) the appara tus must al lowdrilling variables flow rate, drillpipe rotation, drillingrate, hole inclination, annular geometry configuration,

etc.) to be varied and/or controlled within acceptable average field conditions.

The test apparatus consisted of the following majorcomponents: 1) annulus section with enough length toensure steady-state conditions, 2) rotary drive for innerpipe rotation, 3) fluid circulating system, 4) cuttings injectitm system, 5) liquid/cuttings separation system, 6)monitoring and recording system mud flow rate, cuttingsinjection rate, drillpipe rotary speed, average cuttings velocity, annular cuttings concentration), 7) a means ofvarying angle of inclination, 8) a means of varying eccentricity of annulus, 9) a je t -type hopper complete witha spray nozzle for efficient mud mixing, and 10) an automatic relief valve for effective control of pressure surgescreated by cuttings buildup and severe slugging.

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I __ _ _ _ _ _ _ _ _ _ _ _ _ _

i D--._._.-.o-._._._._._.---a

I. ICC 0 5.. . eo yp l y . 3.12 fl i

I TUflIULENT 'LOWS

A'iI :IIAGEI _ N O , 30 I 70 O-.-

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6 . ..........

- - 6 . .

O - . _ . _ . - o . - ~ - = = - - - - - ~

- - - - - - - 0

9 3 0t e e 0. 5r p m 50YP 20LAMINAR FLOWS

i l s0---- I. 91e ~ 2 86

o ~ 3.82

8

~ 6~a:I -~ 5U~oIII J 4ofiita: 3z~ 2

i 7~I IQ.

9.. . . . .----.-----r--- --- ----

YPPV

o ~ - - - ~ - - - . . . L - - - - . . . . . . . . . . . - - - - : : - L :

I I I I

9 0 . _ -- - - - 0 - - ~_ 6 ____ . 0

8 -CI I~I I H ..... -. te e 0 .5Z rpm 500~ va o 3.82 i lsa: 6 1 - YP = 20 -I -z LAMINARY FLOWSII I0Z

5 f - 0 - - 0 9 3 00 -/ ; - - -06 9 7 0III 0 0 9 9 0J0i= 4 1 - a:ita:zz

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uc +Q 5rpm, SOv. .3 .12 ft

e > - - < TURBULENT FLOW WATER)

~ - ~ LAMINARFLOW IYP S 20.J . l t

i

~ c C .0

r l 0 1 \ 0

i YP . 2 0~ . I LAM'NAR FLOWS ..L>i .. I t l

0 - - - 0 1.91

L a.a2u

i ~ .-- __ _ o _ _ ~ P'c

- r ' - ' - o - ' - ' ~~0 zo 40 10 10

ANGLE INCLINATION. de . . .

Fig. 18 Combined effects of inclination and annular mudvelocity on annular cuttings concentration (laminar flow).

.

il 1

i: . I: /,/ /~ ....IT':I I _ - - -I o l - - - - - - : . - - - . . - - - - - - , k - - - - t -;: - 40 SO 10

ANGLE 7 WCLINATION,de . ,

Fig. 19 Effect of flow regime on annular cuttings concentra tion in the whole range of inclinations.

again. This kind of interaction may be one of the factorsthat provides a better transport in turbulent flow underhigher angles of inclination. Another factor that certainly contributes to a better transport is the nonexistence ofa pointed profile in turbulent flow.

As discussed earlier, an increase in mud yield value inlaminar flow improves the transport only at lower anglesof inclination. A different trend is observed while theYP/PV ratio is increased. As shown in Fig. 14, the slopesof all three cUrves are almost the same, which means thatincreasing the YP/PV ratio improves the transport at lower, as well as higher, angles of inclination. Table 1 showsthat the higher the YP/PV ratio is, the lower the valueof the parameter. As indicated earlier, lower values of yield flatter velocity profiles in laminar flow. In therange of lower inclinations, the flatter velocity profileslimit the percentage of annulus cross section that is available for faster settling of particles and consequently pro

vide better transport. This effect is not as significant inthe range of higher angles of inclination where the axialcomponent of particle slip velocity is diminishing. In thisrange, however, a flatter velocity profile provides higherpoint velocities at the lower wall of the annulus wherethe cuttings bed is being formed. Consequently, bettertransport in this range is observed as well.

Figs. 15 and 16 show combined effects of annular mudvelocity and yield value (Fig. 16) or YP/PV ratio (Fig.15) on the annular particle concentration in laminar flow.

A comparison of the slopes of the curves shows that theeffects of yield value and YP/PV ratio are morepronounced for lower annular mud velocities.

Special attention must be given to the range of inclina

tions within 40 to 45 . Fig. 17 shows that the annularparticle concentration for angles less than 45 0 is completely independent of yield value if the annular mud velocityis relatively low. This is caused by the cuttings bed sliding down along the lower wall of the annulus. This effectis dominant and nullifies the effects of other parameters.Note that this is observed only when relatively low mudannular v e l o c i t i ~ sare used (see Fig. 18). Remember thatall experiments in this study Were performed with Luciteoutside pipe. I t is hard to say whether the sliding-downeffect is likely to happen in a directiortal well where anannulus may be composed of rocks, rocks covered withmud cake, or metallic walls of casing. Perhaps this problem needs to be investigated, particularly for the caseswhen a lubricating additive is one ofthe mud ingredients.

Fig. 19 shows the summary effect of flow regimes onannular cuttings concentrations in the whole range of annulus inclinations. Three regions are clearly visible. Inthe 0 to 45 region, the effect of laminar flow ispredominant. In the 45 to 55 region, there is no appreciable difference between the effects of laminar or turbulentflow. In the 55 to 90 region, the effect of turbulent flowpredominates.

q\

\\\

\\

\

'a....-------_-O- a

t : cc +0 .5r p m . 50YP 2 0

.2LAMINAR FLOWS

o . ~ va 3 . 8 2 ft /s0 - - - 0 va . 2 .86ff l

--.o

if

_ ....v v

v .12 f t l,.-10 LAIIINM FLOWSIYP020. ~ o l l- ~ F L O W S l W A T E R

o . +Q 5 0 0

i r - - - - r - - - - r - - - - - - , - - - - , - - - - - - ,i~

.~0: ~~ . . ji . .../;/It: ~ < i J t 1

- - - - - - ~ - --:::::::----~ ooF::....----; . - - - - - : 4 0 : ; - - - ~ I O ; ; - - - - t I O : - - - - . . . . . J

ANGLE Of INCLWATION.det,

Fig. 20 Combined effects of flow regime and inner-pipeeccentricity on annular cuttings concentration.

0 0 - - - - - - ; 2 0 ~ - - - . . . , 4 1 , , - 0---- :60:1, ;------ ; ' : : -0 .....JANGLE OF INCLINATION, d8 Qrees

Fig. 21 Combined effects of inclination and annular mud

velocity on cleaning rate (laminar flow).

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I -

j 1

- .....0- . . ; . - -=

0. 5 . ....,;;;;:..

I,, . eov. 5 . 2f t l l

d TuRBULENT FLOWSI _ MUOYP I,

~ O

~ WATER DTMNO

00I

40 eo eo_ L E 111I1CLlNATll1N,. . . . _

Fig. 22 Comblned effects of inclination and mud yieldvalue on cleaning rate (turbulent flow).

Fig. 20 shows the effect of eccentricity on the annularparticle concentration in the whole range of inclinations.The effect is negative-i.e., higher annular particle concentrations are observed when the inside pipe is positionedeccentrically. However, this effect is small under lowerangles of inclination for either laminar or turbulent flow.It remains rather small in the range of higher angles ofinclination while the turbulent flow is used.

Quite a different situation is Observed in regard to eccentricity effect while the laminar flow prevails. Muchhigher annular cuttings concenttations are observed foreccentric inside pipe. This is caused by two factors thathave been discussed earlier: (1) lower point velocities inthe vicinity of the lower wall of the annulus as a resultof eccentricity and (2) pointed velocity profile in laminarflow. Effects are more pronounced in the range of higherinclinations where a considerable cuttings bed is formed.

Annulus Cleaning ResultsGenerally, the trends found in the annulus-cleaning experiments are similar to those found in the above-discussedtransport experiments. Fig. 21 shows that the annuluscleaning rate is higher for higher annular mud velocityfor the whole range of inclinations. The annulus cleaning rate is generally not affected by mud rheological properties (yield value) if the mud flows in turbulent regime(Fig. 22). I f the mud flows in laminar regime, a higherannulus cleaning rate is observed for the muds having

I I~ ,,,

,,I

,,

te e - 0 5

1 r p M 50 oz v. . 3 . 8 2 t t l . ......... 0 - -0 TURBULENT FLOW WATER ......... 0....- _ _ _ _ -- 0

oJ0 0 - 0 0 LAMINAR FLOW(YP 20 2

I

0 0I I

10 oo eoANGLE OF INCLINATION.d.. .

Fig. 24 Combined effects of inclination and flow regimeon cleaning rate.

306

t e e . 0. 5r p m 50

v 3.82 ftLAMINAR FLOWS

0 - - 0 YP.20 ,15-2

0 - . - 0 yp 16 ~ 2

ASSUMED TREND

1 r ; 2 ~ ; i 4 \ ; _- - - - . 1 0 - - - - ; . 0 . - - - - -ANGLE OF INCLINATION,de/ilr .. .

Fig. 23 Combined effects of inclination and mud yieldvalue on cleaning rate (laminar flow).

higher yield values (Fig. 23). The effect of mud yieldvalue becomes slight while the angle of inclination is increased.

Fig. 24 shows the summary effects of flow regimes onthe annulus cleaning rate in the whole range of inclinations considered. The results are similar to those of thetransport experiment results. A conclusion may be drawnfrom Fig. 24: application of high-viscosity muds to cleanan annulus completely in directional drilling is reasonable only if the angle of inclination does not exceed 45 0

Fig. 25 shows the effect of rotary speed on annularcleaning rate. An increase in revolutions per minute provides a higher cleaning rate. A comparison of the slopesof all three curves shows that this effect is morepronounced (and becomes significant) under higher angles of inclination. As discussed earlier, inner-pipe rotation induces some additional turbulence in flowing mud.Beyond that, the rotating inner pipe has a mechanical, destructive influence on the cuttings bed. This influence isthe main factor that provides a higher cleaning rate forhigher revolutions per minute, particularly under higherangles of inclination where a considerable cuttings bedis formed. Thus it is not advisable to raise the revolutions per minute after drilling is stopped to gain betterannulus cleaning. However, maintaining the revolutionsper minute used during drilling may help in faster annulus cleaning.

......

- - - _ ....... ,g

,,; --,--i --: 2 --ii l ..

JD - tCC +0_5

u ---- v. z 3.82 ftlsI y _ - - - - 0 - - 0 30 yp , 20yp

6 - - l l . 45- PV 20 - - . . 0 70 - LAMINAR FLOWS

50 100 150rpm

Fig. 25 Effect of inner-pipe rotation on cleaning rate (lami-nar flOW).

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Suggested ield GuidelinesWe hope this study will be a useful contribution in optimizing annular hole cleaning in directional well drilling. It is not our intention, however, to assert that the fieldguidelines given below are universal . These should beconsidered with the complex technical, geological, andeconomic problems associated with directional well drilling. The suggested field guidelines follow.

a cutt ings t ransport problem exists, flow rateshould be increased to its limiting value for all ranges ofinclinations, particularly in the range of higher angles (55to 90). Assuming that the sliding-down effect of the cuttings bed also occurs during drilling, then the above recommendation becomes critical for 40 to 45 angles ofinclination.

2. For the range of lower inclinations 0 to 45), laminar flow inside an annulus and an increase in mud yieldvalue to its limiting value are recommended. Assumingthat the field range of mud yield values is within 0 to 20I bf /lo o ft2 [0 to 9 .6 Pal. then a mud yield value of 20Ibf/loo ft2 [9.6 Pal and an increase of the YP/PV ratioto its limiting value are recommended. Assuming that the

range of YP/PV ratios within 0 to 2 is the range commonly used in the field, then the YP/PV ratio of 2 is recommended.

3. In the range of intermediate inclinations (45 to 55 0),either turbulent or laminar flow may be used. When a highflow rate is in use, a larger amount of mud materials (bentonite and chemicals) is usually necessary to mix the mudto provide laminar flow. Therefore, just for this economical reason, the turbulent flow would be preferable, Because the cuttings transport in turbulent flow is not affectedby mud rheological properties, an optional (lower) mudparameter may be used. Remember, however, that staticmud parameters such as mud gel strength are usually desirable even if turbulent flow is preferable. Therefore,this parameter should be considered in the mud design. the turbulent flow cannot be used because of other adverse effects (e.g., borehole wall instabili ty), then thesame recommendations described above for low-anglewells (0 to 45 should be considered in laminar flow.

4. For the range of high-angle wells (55 to 90), turbulent flo.\\ is preferable. Generally, the same recommendations as those described in Item 3 are applicable in thisregion for turbulent flow. However, the requirements toensure a mud gel strength are less important. the turbulent flow cannot be used, then, of course, laminar flowmust be considered. Because the cuttings transport in laminar flow in this region is almost unaffected by mud yieldvalue, lower yield value that provides the laminar flowis rel iable economically and may be used. At the sametime, the YP/PV ratio is still important in this region andshould be maintained as high as possible.

5. Application of high-viscosity muds to clean the annulus after drilling is stopped appears to be beneficial onlyin low-angle wells (0 to 45 .

onclusions1. The annular cuttings concentration (vol ) is the pa

rameter that should be considered in the assessment ofdrilling-fluid cuttings transport in directional wells.

2. Considering the wide range of hole inclinations, three

separate regions can be identified regarding cuttings trans-

SPE Drilling Engineering, August 1986

port: Region 1 (0 to 45 , Region 2 (45 to 55 andRegion 3 (55 to 90).

3. Under turbulent regime, the cut tings t ransport isge,nerally not affected by the mud rheological properties(yIeld value and YP/PV ratio) in all three regions .

4. Under laminar regime, higher mud yield valuereduces the annular cuttings concentrations and providesbetter transp?rt . The effect of mud yield value is verypronounced m the range of low-angle wells (Region

and becomes slight or even negligible in the range of highangle wells (Region 3).

5. In laminar flow, the annular cuttings concentrationis lower for higher YP/PV ratios. This is true for the entire range o,f hole inclinations investigated in this study.

6. In lammar flow, the effects of mud yield value andYP/PV ratio are more pronounced for lower annular mudvelocities.

7. The worst cuttings transport (highest annular cuttings~ o n c e n t r t i o nwas experienced at angles of inclination the 40 to 45 range. This is t rue when relatively lowflow rates are used.

8. The effect of annulus eccentricity on cuttings transport rather small for low-angle wells (Regions and2) in either laminar or turbulent flow. The effect becomesmoderate in Region 3 under turbulent flow and significant when the flow becomes laminar.

9. The laminar flow has a predominating effect on cuttings transport in the range oflow-angle wells (0 to 45while in high-angle wells (55 to 90 , the effect o t u r u ~lent flow is predominant. In the range of intermediate inclination (45 to 55), turbulent and laminar flow havegenerally similar effects.

10. A measureable torque was observed in the transport experiments because of the presence of the cuttingsbed.

11. The cleaning rate of the settled cuttings in the an

nulus after cuttings injection has ceased seems to increasewith an increase in rotary speed, particularly in Region 3.12. Generally, i t can be concluded that the trends ob

served during annulus-cleaning experiments are identicalto those observed in the cuttings-transport experiments.

13. Mud flow rate has a dominant effect on annular holecleaning.

Nomenclaturee = annular particle concentration, percent

i = inner-pipe diameter, in. [mm] o = outer-pipe or hole diameter, in. [mm] p = particle size, in. [mm]

e = inner-pipe offset relative to hole center, in.[mm]

h = local annular clearance or slot height, in.[mm]

= power-law consistency index, Ibf-sec n /ft2[Pa sn]

= equivalent consistency index for annularflow, Ibf-sec n /ft2 [Pa s n]

= length of annulus, ft [m]n = power-law exponent, dimensionless

t p = pressure drop, psi [Palr = annulus radius, in. [mm] = average (nominal) annular velocity, ft/sec

[m/s]

307

8/9/2019 00014178

12/12

vac = critical annular velocity, ft/sec [m/ s]vm = mature velocity, ft/sec [m/ s]vs = particle slip velocity, ft/sec [m/ s]

v sa = axial component of particle slip velocity,ft/sec [m/s]

v = radial component of particle slip velocity,ft/sec [m/s]

Iy I = absolute value of y coordinate = shear rate, seconds- 1

= pipe/hole eccentricity, percent = inclination angle, degrees

PL = mud density, liquid, Ibm/ft 3 [kg/m 3 ]7 = shear stress, Ibf/ft 2 [Pal

cknowledgmentsWe thank the member companies of the U. of Tulsa Drilling Research Projects (Amoco Production Co., AppliedDrilling Technology Inc., Aramco Services Co., Arco Oiland Gas, Chevron Oil Field Research Co. , Conoco,Dowell, Exxon Production Research Co. , IMP, IN

TEVEP S.A., Mobil R&D Corp., PERTAMINA, Petrobras/Cenpes, Petro Canada, Sandia Nat . Laboratories,Schlumberger Cambridge Research, Shell DevelopmentCo., Sohio, Texaco U.S.A., and Union Oil) who providedfunds for this research.

References1 Tomren, P.H.: T he Transport of Drilled Cuttings in an Inclined

Eccentric Annulus, MS thesis, U. of Tulsa, Tulsa, OK (1979).

2. Iyoho, A.W.: Drilled-Cuttings Transport by Non-NewtonianDrill ing Fluids Through Inclined, Eccentric Annuli, PhDdissertation, U. of Tulsa, Tulsa, OK (1980).

3. Becker, T.E.: T he Effect of Mud Weight and Hole GeometryVariations on Cuttings Transport in Directional Drilling, MSthesis, U. of Tulsa, Tulsa, OK (1982).

4. Iyoho, A.W. and Azar, J.J.: A n Accurate Slot Flow Model forNon-Newtonian Fluid Flow Through Eccentric Annuli, SP J (Oct.1981) 565-72.

5. Hussa ini, S .M. and Azar, J.J.: Experimental Study of DrilledCuttings Transport Using Common Drilling Muds, SP J (Feb.1983) 11-20.

6. Tomren, P.H., Iyoho, A.W., and Azar, U .: Experimental Studyof Cuttings Transport in Directional Wells, SP (Feb. 1986)43-56.

Metric onversion Factorscp X 1.0* E-03 = Pa sft x 3.048* E - O l = m

gal/min X 6.309 020 E-02 = dm 3 /sin. X 2.54* E OO = cm

Ibf/IOO ft2 X 4.788 026 E-Ol = PaIbf-sec/ft 2 X 4.788026 E+Ol = Pa s

Ibm X 4.535 924 E-Ol = kgIbm/ft 3 X 1.601 846 E+Ol = kg/m 3Ibm/gal X 1.198264 E+02 = kg/m 3

Conversion factor is exact. SP

Original manuscript received in the Society of Petroleum Engineers office Sept. 22,1985. Paper accepted for publication Feb. 24, 1986. Revised manuscript received April15.1986. Paper SPE 14178) first presented the 1985 SPE Annual Technical Con-ference and Exhibition held in Las Vegas, Sept. 22-25.