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SOLIDS CONTROL HANDBOOKDecanting Centrifuges

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Decanting Centrifuges1 Introduction.........................................................................................................................2

2 Principle of Operation.........................................................................................................3

3 Performance Parameters....................................................................................................4

3.1 G-Force .........................................................................................................................4

3.2 Viscosity ........................................................................................................................6

3.3 Cake Dryness ................................................................................................................6

3.4 Pond Depth and Processing Capacity............................................................................7

3.5 Bowl - Conveyor Differential RPM And Torque ..............................................................9

4 Centrifuging Unweighted Mud .........................................................................................104.1 Centrifuging Hydrocyclone Underflow ..........................................................................10

4.2 Operating Guidelines, Centrifuging Unweighted Mud...................................................13

5 Centrifuging Weighted Muds ...........................................................................................135.1 Operating Guidelines, Barite Recovery Mode ..............................................................15

6 Two-Stage Centrifuging....................................................................................................166.1 Field Evaluation of Two-Stage Centrifuging Economics...............................................18

6.1.1 Calculations .......................................................................................................18

7 Centrifuge Selection .........................................................................................................197.1 Equipment Descriptions...............................................................................................23

7.1.1 Hutcheson-Hayes HH5500 (16 X 55) .................................................................237.1.2 Alpha-Laval 418/Swaco HS 518 (14 X 56)..........................................................237.1.3 Derrick DE1000/Sharples P3400/Brandt HS3400 (14 X 50) ...............................237.1.4 Oiltools S3.0 (21 X 62), S2.1 (18 X 56) ..............................................................237.1.5 Bird Design Centrifuges - Sweco SC-4, Broadbent, Brandt CF-2, Derrick DB1 ..24

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SOLIDS CONTROL HANDBOOKDecanting Centrifuges Schlumberger

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7.1.6 Alpha-Laval 414, Swaco 414 (14 X 38), Sharples P3000, Hutcheson HayesHH1430 (14 X 30)...............................................................................................24

8 Summary............................................................................................................................25

FIGURESFig. 1. Centrifuge components. ...............................................................................................3Fig. 2. Centrifuge operation.....................................................................................................4Fig. 3. Effect of G-force on separation.....................................................................................5Fig. 4. Effect of viscosity on separation performance. .............................................................6Fig. 5. Effect of G-force on cuttings dryness. ..........................................................................7Fig. 6. Effect of pond depth on fine solids removal..................................................................8Fig. 7. Effect of pond depth on coarse solids removal. ............................................................9Fig. 8. Economics of centrifuging hydrocyclone underflow. ...................................................11Fig. 9. Fluid routing to centrifuge hydrocyclone underflows. ..................................................12Fig. 10. Internal centrifuge feed compartment design............................................................12Fig. 11. Choice of drilled solids removal from weighted mud.................................................14Fig. 12. Benefits of increased G-force on barite recovery......................................................15Fig. 13. Two stage centrifuging. ............................................................................................17Fig. 14. Centrifuge performance comparison on fine solids distribution.................................20Fig. 15. Centrifuge performance comparison on coarse solids distribution............................21

TABLESTable 1 Recommended Centrifuges for Unweighted Mud .....................................................22Table 2 Recommended Centrifuges for Weighted Mud.........................................................22

1 Introduction

Since their introduction to the oilfield in the early 1950s, decantingcentrifuges have become an increasingly common addition to the solidscontrol system. Centrifuges are capable of removing very fine solids thatcannot be removed by any other mechanical separation device. Inunweighted muds, the centrifuge can greatly improve the separationefficiency of the solids removal system and reduce liquid discharge volumeswhen used in conjunction with hydrocyclones. Increasingly stringentenvironmental restrictions on drilling waste discharge and the incentive ofreduced dilution and disposal volumes have made the use of centrifugeseconomically attractive in many instances. In weighted muds, the centrifugeis used to reclaim barite while removing colloidal solids which can cause highmud viscosity, poor filtercake properties, and decreased penetration rates.The centrifuge is the primary separation device used in a chemically-enhanced dewatering system to reduce liquid discharge volumes.

Unlike other solids removal devices, decanting centrifuges are usually leasedfrom service companies. Very few rigs come equipped with centrifugesbecause they are relatively expensive to purchase and require specialized

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maintenance. A typical oilfield-ready centrifuge may cost $80-$150thousand, depending upon size, performance and design features. Leaserates range from $150 to $300 per day. It is therefore important tounderstand the factors affecting centrifuge performance to economicallyjustify the specific application and to achieve maximum performance.

2 Principle of OperationThe major components of a decanting centrifuge are shown in Fig. 1.Decanting centrifuges separate solids from liquid by imparting highcentrifugal forces on the solid-liquid slurry fed into a bowl rotating at highspeed. The feed stream is pumped into the center of the bowl via a feedtube. The slurry exits the feed tube and enters an acceleration chamberhoused inside the conveyor. It exits the chamber through feed ports andenters the bowl area. Here, the slurry is exposed to a high G-force createdby the bowls rotation. The high G-force causes sedimentation of the feedstream solids. The rotating conveyor has flights similar to threads on a screwwhich auger the settled solids up the conical section of the bowl and out ofthe liquid pool. The gear box causes the conveyor to rotate at a slightlyslower speed than the bowl. The torque needed to turn the conveyor iscarried through the gear box and emerges at a shaft. This shaft is held by ashear pin or other safety device so that excess torque will not be applied tothe gearbox or conveyor. The relatively dry solids continue out of the bowl.The cleaned liquid is decanted off through ports at the opposite end(Fig. 2).

Fig. 1. Centrifuge components.Note: These components are common to most decanting centrifuges used in

oilfield applications.

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Fig. 2. Centrifuge operation.Note: The conveyor augers solids up the conical section of the bowl and out of the

liquid pool.

3 Performance Parameters

The effect of various design and feed parameters on centrifuge performancehave been evaluated by APR. The results of this study are summarized toassist in the selection and operation of centrifuges. Since many centrifugeparameters are related, one aspect of performance cannot be discussedsingularly without implicating others. However, in general, centrifugeperformance is affected by the following parameters in decreasing order ofimportance:

3.1 G-ForceAccording to Stokes Law, particle settling velocity is proportional to G-force:

( )VT

s L =

aDp2 10-6

-

116

where:

VT = Particle terminal velocity, in./sec

a = Bowl acceleration, in./sec2 = .0054812 x bowl Diameter xRPM2

(1 g = 386 in./sec2)Dp = Particle diameter, microns

S = Solids Density, gm/cm3

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L= Feed Slurry Density, gm/cm3

m = Feed Slurry Viscosity, (centipoise = gm/100 cm sec)Since G-force increases with the square of bowl RPM, it is an importantparameter. G-force also increases linearly with bowl diameter. Fig. 3 showshow solids removal efficiency improves with increasing G-force. For a givenparticle size and fluid properties, there is a minimum G-force necessary toinvoke settling. Although high G-force is desirable, the cost is proportional tothe cube of the bowl rpm and there are similar economic limitations on bowldiameter as well. Thus, the required G-force must be obtained from apractical combination of speed and diameter. Most oilfield centrifuges havebowl dimensions from 14 to 28 in. in diameter and lengths from 30 to 55 in.Rotational speeds range from 1000 rpm to 4000 rpm, depending on theapplication. The more expensive, high-G machines can provide up to 3,000Gs. The specifications for each centrifuge are listed in Appendix F,Equipment Specifications.

Fig. 3. Effect of G-force on separation.Note: Higher Gs improve separation performance.

Note, however, that increasing G-force eventually reduces solidsconveyance capacity due to torque limitations. As G-forces increase, moresolids are settled in the bowl and they adhere more tightly. More conveyortorque is required to move the solids out. Once the torque limitations of themachine are reached, conveyance ceases.

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3.2 ViscosityFrom Stokes Law, particle settling velocity is inversely proportional to fluidviscosity. Fig. 4 illustrates the beneficial effects of a feed mud with a lowyield value. This shows the merit of diluting the centrifuge feed to improveperformance. It also helps explain the relatively poor performance ofcentrifuges when processing polymer fluids with characteristically highviscosities at low shear rates.

Fig. 4. Effect of viscosity on separation performance.Note: Higher yield values degrade centrifuge separation performance.

3.3 Cake DrynessDischarge dryness is commonly considered a direct indication of centrifugeperformance. However, test results have shown that cake dryness is morecorrectly a function of particle size and, therefore, is inversely related toseparation efficiency. Test points which yielded the driest solidscorresponded to the lowest efficiency and coarsest D50 separation. Asshown in Fig. 5, solids dryness occurs at a threshold G-force level.Subsequent increases in G-force do not remove additional liquid. Length ofthe dry beach within the centrifuge bowl (a function of pond depth) also haslittle effect on dryness. Dry beach length refers to the distance from thesolids discharge ports to the surface of the fluid pond within the centrifugebowl. But, the small difference in dryness made a significant difference in theappearance of the solids. At 71% by weight, the solids were quite runny andat 76% by weight, the solids seemed much more stackable.

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Fig. 5. Effect of G-force on cuttings dryness.Note: Above a certain threshold G-force, cuttings dryness does not improve.

3.4 Pond Depth and Processing CapacityPond depth controls both fluid residence time and dry beach length. Testdata confirms that increased pond depth residence time increasesseparation. However, increased pond depth reduces centrifuge flow capacity.Maximum flow capacity is controlled by the height of the cake discharge port.When the fluid depth in the centrifuge bowl reaches this height, drilling fluidflows out along with the discarded cake. The flowrate at which liquid spillsout the cake discharge port is called the flood-out point. Since oneobjective of centrifuging is to limit liquid waste, it is obviously notadvantageous to run the centrifuge at a flow rate beyond the flood-out point.

Flooding is controlled by a combination of pond depth and flowrate. Thepond depth is set mechanically by an adjustable weir. The flowrate increasespond height according to the viscous drag forces which increase the fluidhead required to drive the liquid through the centrifuge. The head height isadded to the fixed pond depth to give a total depth of fluid in the bowl. Forexample, consider a centrifuge with a maximum fluid depth of 3 in. beforeflood-out (closed fluid exit ports). If 300 gpm is the maximum flow rate atfloodout with a 1-in. pond depth setting, this means 2 in. of fluid head wasdeveloped. If the pond depth setting is adjusted to 2 in., then only 1 in. offluid head is available before the 3-in. flood-out point is reached. Obviously,

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the maximum flowrate for this pond depth setting will have to be much lessthan 300 gpm.

Maximum flow capacity is achieved when the shallowest pond depth is usedat the expense of separation efficiency. Conversely, deep ponds maximizeseparation efficiency at the cost of flowrate capacity. The best combination isdetermined by the coarseness of the solids to be separated. Fig. 6 illustrateshow, for a fine solids size distribution, a deep pond depth at lower flow ratescan produce almost the same cake rate as a shallow pond depth at higherflow rates. This is due to the improved separation efficiency of the deep pondcase. Fig. 7 shows how, for coarse solids, the higher flow capacity of theshallow pond produces more solids removal than the deep pond case. Theresults suggest that, for coarse particle size distributions as encountered intop hole drilling, shallow pond depths are advantageous, whereas deepponds should be used for all other applications.

Fig. 6. Effect of pond depth on fine solids removal.Note: Deeper ponds are more efficient than shallow ponds when the solids are

very fine.

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Fig. 7. Effect of pond depth on coarse solids removal.Note: Shallow pond depths are preferred for coarse solids distributions.

3.5 Bowl - Conveyor Differential RPM And TorqueDifferential RPM is the difference between the bowl RPM and the conveyorRPM. The differential is provided by the gearbox which transmits power fromthe bowl to the conveyor. Differential RPM is simply calculated by dividingthe bowl RPM by the gearbox ratio. Many centrifuge manufacturers provide abackdrive which can independently alter the DRPM. Backdrive units are, ineffect, hydraulic gear reduction systems used to vary the speed of theconveyor relative to the bowl. On backdrive units, DRPM depends upon therotation of the gearbox pinion and the orientation of the flights on theconveyor. For these units, DRPM may be calculated by:

DRPM = (Bowl RPM - Pinion Speed)/Gearbox ratio.DRPM is important because it determines the velocity at which solids areconveyed through the centrifuge. For example, a DRPM of 50 and a flightpitch of 3-in. yields a conveyance velocity of 150 in./min. Another expressiontakes the flight pitch and number of leads on the conveyor into account todescribe the surface area of the bowl swept by the conveyor flights per unittime. The faster the rate at which the area is swept, the greater the solidscapacity.

As = 2p rcyl x DRPM x SN

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where:

As = swept area/unit time

rcyl = cylindrical bowl radius

S = flight pitch

N = number of leads on the conveyor

This equation suggests that solids capacity can be increased by increasingthe DRPM (lowering the gearbox ratio). Low swept area values could indicatepotential torque problems. For example, centrifuges with 130:1 or highergearbox ratios and centrifuges with 80:1 gearbox ratios with single-leadconveyors may be limited in flowrate by torque.

Test data indicates that increasing DRPM reduces torque. Also, torqueincreases as feed median particle size increases. Despite the common beliefthat high DRPM values agitate the pond and inhibit sedimentation, testresults indicate that the effect of DRPM on solids removal efficiency is slight,provided sufficient differential exists to remove the solids.

4 Centrifuging Unweighted MudCentrifuging unweighted muds provides two major benefits: 1) The removalof drilled solids that are too fine to be removed by any other solids removaldevice, and 2) a relatively dry discharge. Although the centrifuge cannotremove ultrafine, colloidal solids, it is important to remove the fine solidsbefore they degrade into these submicron particles. As a rule, at least 25%of the circulating rate should be centrifuged. It is usually uneconomic (andlogistically unfeasible) to process the entire circulating rate. Regardless, thebenefits of centrifuging to remove fine solids cannot be understated.

High-G, high capacity centrifuges are recommended to maximize separationperformance. Refer to the discussion on centrifuge selection, appearing laterin this chapter. Since separation efficiency varies inversely with feed rate andresidence time, the optimum feed rate is not necessarily the highest possiblerate. Rather, it is the combination of pond depth and feed rate that producesthe highest solids discharge rate. The maximum efficient processing rate fora large oilfield centrifuge will seldom exceed 250 gpm, even for relativelycoarse drilled solids and low fluid viscosities. If the particle size distribution isvery fine, more solids may be removed with a lower feed rate and deeperpond depths.

4.1 Centrifuging Hydrocyclone UnderflowWhen liquid discharge must be strictly controlled due to high mud cost, highliquid disposal cost or limited reserve pit capacity, the centrifuge shouldprocess the underflow of the desilter cones. In this configuration, thehydrocyclones are used to concentrate solids to the centrifuge which thenseparates the drill cuttings from the free liquid and colloidal solids. System

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performance can be improved by opening the cone apexes to dischargemore liquid. This improves the separation efficiency of the cones andproduces a less viscous slurry at the underflow. Fig. 8 gives an example ofhow centrifuging desilter underflow becomes economic with increasing mudcost and desilter underflow rates. Enough centrifuge capacity must beavailable to process slightly more than the cone underflow rate. Additionalmakeup volume should be provided from the active system downstream ofthe hydrocyclone feed.

Because the hydrocyclone underflow must be segregated from the activesystem, a separate centrifuge feed compartment is required. Fig. 9 andFig. 10 illustrate two designs for the centrifuge feed compartment. Thecompartment should be small (

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Fig. 9. Fluid routing to centrifuge hydrocyclone underflows.Note: The disilter underflow is segregated from the active system for

processing by the centrifuge.

Fig. 10. Internal centrifuge feed compartment design.Note: The dense desilter underflow will displace the lighter active system

mud from the centrifuge feed compartment.

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4.2 Operating Guidelines, Centrifuging Unweighted Mud1. When processing the active system, the centrifuge feed should be

taken from the desilter discharge compartment or downstream. Thecentrate should be returned to next downstream compartment.

2. Provide enough centrifuges to process at least 25% of the circulationrate. Large, high-G units are usually required.

3. Run at maximum bowl RPM to achieve highest G-force and bestseparation.

4. Operate the centrifuge just below the flood-out point.5. The best feed rate and pond depth will depend on the size distribution

of the drilled solids. Use shallow ponds and high feed rates whencoarse solids predominate. Conversely, deeper ponds and lower feedrates are more efficient when fine drilled solids are to be removed. Fieldexperimentation is necessary to optimize centrifuge setup.

6. Always wash out the centrifuge on shutdown.

7. If the centrifuge is to used on both unweighted and weighted muds, rigup to allow either option. Both the centrate and solids streams shouldbe rigged up to allow each to be discarded or returned to the activesystem.

8. The solids discharge chute should be angled at greater than 45 toprevent solids buildup. If this is not possible, a wash line may benecessary to assist in moving the solids. On land-based operations, usethe reserve pit as a source for wash fluid. Do not create unnecessaryreserve pit volume by using rig water.

5 Centrifuging Weighted MudsThe centrifuge is used in weighted mud applications to recover valuableweighting material from mud which must be discharged due to unacceptablecolloidal solids content. The centrifuge settles out barite and coarse drilledsolids which are returned to the active mud system to maintain density. Therelatively clean centrate containing liquid and colloidal solids is discarded.These colloidal solids cause many drilling fluid problems, such as highsurge/swab pressures and ECDs, differential sticking, and high chemicalcosts. Usually, the value of the weighting agent in these mud systems makesit economic to recover the weighting agent from the whole mud before it isdiscarded. Fig. 11 gives an example of the economics of centrifugingweighted muds.

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Fig. 11. Choice of drilled solids removal from weighted mud.Note: This example shows the economic advantage of recovering barite.

Ideally, the barite recovery process should remove only colloidal solidswithout losing the larger particle sizes used as weighting material. Discardingpotentially reusable barite increases barite use and drilling fluid cost. Baritelosses can be reduced when the centrifuge makes the maximum liquid/solidsseparation. As discussed in the previous section, this means operating thecentrifuge at high G-force. Fig. 12 shows the effect of G-force on the amountof barite discarded in the centrate. At 20 gpm, the difference in barite lossesis 4.58 lb/min. Based on 10 hours per day centrifuging and barite cost of$6.50 per 100 lb, high G-force centrifuging should save $175 per day.Centrifuges are usually torque-limited in weighted muds due to the highsolids content. Typically, torque is reduced by slowing bowl RPM. Thisreduces G-force and DRPM, resulting in less effective liquid/solidsseparation and the likelihood of increased torque from reduced solidsconveyance.

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Fig. 12. Benefits of increased G-force on barite recovery.Note: Less barite is lost in the centrifuge centrate with increased G-force.

5.1 Operating Guidelines, Barite Recovery Mode1. The following procedures are recommended to reduce torque when

operating centrifuges in barite recovery mode to maximize liquid/solidsseparation:

A. For a given flowrate, increase the pond depth until the recoveredsolids become runny. Buoyant force reduces the torque neededto convey solids out of the centrifuge. A shallow pond creates along beach section. Once the solids exit the pool, the extra energyrequired to convey these solids results in higher torque.

B. Process weighted mud continuously at a reduced feed rate ratherthan intermittently at higher feed rates. This reduces solids loadsand results in less torque. It also increases residence time whichwill result in finer separation.

C. At higher mud weights, use hydrocyclones to reduce the solidsloading in the feed mud to the centrifuge. The cone underflow isreturned to the active system. The overflow, containing fewersolids, is fed to the centrifuge. Since solids concentration isreduced, torque from conveying settled solids is reduced andpermits higher G-force centrifuging.

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2. Provide sufficient centrifuge capacity to process 5-15% of the rigcirculation rate. Centrifuge capacity is reduced in weighted mud; the25% target recommended for unweighted mud is usually difficult toattain in weighted mud.

3. Add as much dilution water as possible to the centrifuge feed to reducethe mud viscosity and improve centrifuge separation performance.

4. Return the solids to a well-agitated compartment upstream of thesuction and mixing tanks.

5. Use a high weir between the barite return compartment and the nextdownstream compartment to keep the fluid level high. This will promotebetter mixing.

6. Always wash out the centrifuge on shutdown.

7. Routinely check the centrifuge performance by measuring the flow rateand solids composition of the cake and centrate.

6 Two-Stage CentrifugingTwo-stage centrifuging is used in weighted muds when the liquid phasecannot be discarded for economic or environmental reasons. The mostfrequent application is in weighted, oil-based muds where the expensiveliquid phase cannot be discarded. The first centrifuge recovers weightingmaterial from the weighted mud as discussed in the previous section onsingle-stage centrifuging for barite recovery. The centrate, instead of beingdiscarded, is fed to a second centrifuge operating at higher G-force. Thiscentrifuge is used to discard the solids and return the cleaned liquid phaseinto the active mud system (Fig. 13).

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Fig. 13. Two stage centrifuging.Note: The first centrifuging recovers barite; the second centrifuge dries its centrate

and recovers valuable fluid.

For two-stage centrifuging to be efficient, the first centrifuge must make agood separation since most of the solids in its centrate will be discarded. Thepoorer the separation, the more barite which will be carried over in thecentrate and discarded by the second centrifuge. Similarly, the secondcentrifuge must operate at the highest possible G-force to remove the mostsolids. Pond depths should also be deepened to just under the flood-outpoint for the best separation efficiency.

Economics of two-stage centrifuging are site-dependent. Variables such astime, drilling fluid, buy-back agreements, and well plans contribute to theoverall economics. Field experience has been mixed on the cost-effectiveness. As a rough rule of thumb, oil-based muds with bariteconcentrations greater than 4 lb/gal (i.e., 12 ppg mud) are usually candidatesfor two-stage centrifuging. Below this concentration, centrifuging to strip allsolids including barite may be more economical, especially at lower mudweights. At intermediate mud weights, dump and dilute may be a viableoption depending upon the conditions of the buy-back agreement. Dumpand dilute in this case means transferring mud laden with low gravity solidsfrom the active system to storage tanks for return to the mud company.Clean whole mud is used to replace the dumped mud in the active system.

Another option is to do nothing except screen the mud and dilute whenpossible to maintain mud properties. The decision to employ this alternativeshould be made judiciously. It is usually better to err on the side of caution.Over time, low gravity solids will become a large percentage of the weighting

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material. Filtercake thickness, mud viscosity, and material consumption alsomay increase. However, this may be the least expensive alternative whendrilling time is short and hole sizes are small. Oil-based muds are quitesolids-tolerant and can withstand some buildup of low-gravity solids. Thisoption is not generally recommended for water-based fluids.

6.1 Field Evaluation of Two-Stage Centrifuging EconomicsAn evaluation of two-stage centrifuging economics can be made in the field.This method computes the total centrifuging cost (including rental cost andthe value of the mud and barite discarded). This cost is compared to thevalue of the whole mud that must be discarded to remove an equivalentamount of low gravity solids. The following data is required:

Active System Mud Density, rm (ppg)Barite Concentration, HGS (lb/bbl)*Low Gravity Solids Concentration, LGS (lb/bbl)*

Centrifuge Discard Sludge Density, rdis (ppg)Operating Time, t (hrs)Centrifuge Discard Rate, Qcen (lb/hr)*Barite Concentration, HGSdis (lb/bbl)*Low Gravity Solids Concentration, LGSdis(lb/bbl)

Costs Barite Unit Cost, Cb ($/sack)Liquid Mud Cost, Clm ($/bbl)Centrifuge Rental Cost, Ccen ($/day)

*Barite and Low Gravity Solids concentrations in lb/bbl of whole mud aredetermined from retort analysis and must be corrected for salt content.

6.1.1 Calculations

Two-Stage Centrifuging Cost1. Mass Flow Rate of Drilled Solids, (lb/hr): Mds = Qdis x LGSdis2. Mass Flow Rate of Barite, (lb/hr): Mbar = Qdis x HGSdis3. Mud Discard Rate, (bbl/hr): Qliq = Qdis - Mds/928 - Mbar/14714. Value of Discarded Barite, ($/hr): $/hr(bar) = Mbar x Cb/100

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5. Value of Discarded Liquid Mud, ($/hr): $/hr(mud) = Qliq x Clm6. Total Two-Stage Centrifuging Cost, ($/hr), ($/day): $/hr = Ccen/24 + (4) + (5) *$/day = Ccen + t ((4) + (5)) * Disposal/treatment cost of the centrifuge discard should be added to

the Total Cost, if applicable. Use Qcen for total hourly sludge rate.

Equivalent Whole Mud Disposal Cost7. Whole Mud Discard Rate, (bbl/hr): Qmud = Mds/LGS8. Mass Flow Rate of Barite Losses, (lb/hr): Mbar(m) = Qmud * HGS9. Liquid Mud Losses, (lb/hr): Qliq(m) = Qmud - Mds/928 - Mbar(m)/147110. Barite Cost, ($/hr): $/hr(bar) = Mbar(m) x $/sack x 1/10011. Liquid Mud Cost, ($/hr): $/hr(liq) = Qliq(m) * $/bbl12. Hourly Cost of Discarded Mud, ($/hr): $/hr = (10) + (11)13. *Daily Cost of Discarded Mud, ($/day): $/day = t x (13)*If using oil-based mud, the buy-back value of the discarded mud should besubtracted from the daily disposal cost.

7 Centrifuge SelectionGenerally, the following features on a centrifuge are highly recommended:

1. Accelerator for the feed to decrease turbulence.

2. Tungsten carbide feed port entries to prevent erosion.

3. Tungsten carbide tiles on the conveyor to improve wear resistance.

4. Universally adjustable pond dams to fine-tune centrifuge performance.5. Stainless steel bowl and conveyor to reduce corrosion problems.

6. High G-force to ensure maximum separation performance.

Since centrifuges are normally leased, quality of service in the local areashould be a primary consideration when selecting centrifuges. A centrifuge

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with all the features listed above will not be much use if it cannot be keptrunning because of poor maintenance. Contract length should also beconsidered. For example, hard-facing on the conveyor instead of tungstencarbide tiles, or a carbon steel bowl instead of stainless steel is entirelyacceptable if the centrifuge is to be leased for a short term and maintenancecosts are borne by the contractor. Conversely, when drilling in remote areasor under harsh conditions, the features listed above will help ensurecontinued trouble-free operation. Regardless, a full inspection should beperformed before the centrifuge is accepted for lease.

The coarseness of the solids can also influence centrifuge selection. Asshown in Fig. 14, when the solids distribution is fine, a small high Gmachine such as the Sharples 14 x 30 may remove more solids at a lowerfeed rate than a large bowl, low G machine such as the Bird 24 x 38.Conversely, the larger bowl machine will provide superior performance whenthe solids are coarse (Fig. 15).

Fig. 14. Centrifuge performance comparison on fine solids distribution.Note: The smaller 14 x 30 High G centrifuge is more efficient when solids are

fine.

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Fig. 15. Centrifuge performance comparison on coarse solids distribution.Note: The high flow capacity of larger Low G machines is preferred in the presence

of coarse solids.

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Table 1 and Table 2 contain recommended common oilfield centrifuges.Table 1 lists centrifuges recommended for unweighted mud applications.Table 2 lists centrifuges recommended for weighted mud applications. Thesetables are provided as a guideline only. Centrifuges not listed in these tablesmay provide equivalent performance provided the performance criteriadiscussed previously are met. For example, Sharples builds largercentrifuges than the P3400. These larger centrifuges will provide superiorperformance, but very few are available for drilling application and are notlisted here.

Table 1 Recommended Centrifuges for Unweighted MudCentrifuge Bowl Size Maximum Bowl RPM

Hutcheson Hayes HH5500 16 in. x 55 in. 3250Alpha-Laval 418 14 in. x 56 in. 4000Swaco HS 518 14 in x 56 in. 3313Derrick DE1000 14 in. x 50 in. 4000Drexel-Brandt HS3400 14 in. x 50 in. 3250Sharples P3400 14 in. x 50 in. 3250Oiltools S3.0 21 in. x 62 in. 1800Sweco SC-4 24 in. x 40 in. 1950Broadbent 24 in. x 38 in. 1900Derrick DB-1 24 in. x 40 in. 2000Drexel-Brandt CF2 24 in. x 38 in. 1900

Table 2 Recommended Centrifuges for Weighted Mud

Centrifuge Bowl SizeAlpha-Laval 414 14 in. x 38 in.Swaco 414 14 in. x 38 in.Sweco SC-4 24 in. x 40 in.Broadbent 24 in. x 38 in.Hutcheson Hayes HH1430 14 in. x 30 in.Oiltools S3.0s 18 in. x 56 in.Sharples P3000 14 in. x 30 in.Sweco SC-2 18 in. x 30 in.

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7.1 Equipment Descriptions

7.1.1 Hutcheson-Hayes HH5500 (16 X 55)This machine has all of the recommended features including a high capacitygearbox to minimize gearbox failure. In most applications, it has more flowcapacity and separating power than the other centrifuges listed. The 5500can be run at 3250 RPM in unweighted muds and approximately 2600 RPMin weighted fluids. A variable speed controller on the main drive isrecommended for dual purpose applications. Because it is a relatively newproduct, few HH5500s are currently available as rental units.

7.1.2 Alpha-Laval 418/Swaco HS 518 (14 X 56)The Alpha-Laval 418 and the Swaco HS 518 have the same basic design,although the Alpha-Laval has a higher maximum bowl speed and shouldhave more separating power than the slower Swaco unit. Plate type ponddams require total replacement for adjusting pond depth. Adjustment forthree differential RPMs is provided. The flight spacing and 60:1 gearbox ratiogive reasonable performance. The 418 and 518 are not recommended forbarite recovery; the long bowl length is prone to developing high torque inweighted muds.

7.1.3 Derrick DE1000/Sharples P3400/Brandt HS3400 (14 X 50)These Sharples-designed centrifuges have most of the desired designfeatures and will provide superior performance in unweighted muds.Recently, Sharples was bought out by Alfa-Laval, who reportedly willdiscontinue production of the Sharples P3400 and P3000 in favor of theirown 418 and 414 models. The long bowl is susceptible to developing hightorque in weighted muds and is not normally recommended for bariterecovery. Derrick now manufactures this design with some mechanicalimprovements, fully-tiled conveyor flights, and the option of variable mainand back drives. With the variable drive, Derrick has been successful inusing this machine as a barite-recovery unit.

7.1.4 Oiltools S3.0 (21 X 62), S2.1 (18 X 56)These centrifuges are manufactured by Humbolt-Wedag in Germany under aBird Machine Co. license. Both can provide reasonably good separationperformance provided they are operated at maximum RPM in unweightedmuds. Both units are fully hydraulic which helps prevent solids overloadingand bowl plugging in barite recovery mode. The S2.1 is normally supplied asthe barite recovery centrifuge. The quality of local service is an importantconsideration for these centrifuges as with all hydraulic units.

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7.1.5 Bird Design Centrifuges - Sweco SC-4, Broadbent, Brandt CF-2,Derrick DB1

This 24 x 38 in. oilfield centrifuge was originally a Bird design. Modificationsto the basic design have been made in accordance to the specific companysdesign concepts. Both Sweco and Broadbent build their machines in-house.Brandt purchases the centrifuge rotating assembly and skids it out in-house.

These machines will have suitable separation performance above 1900RPM. All except the Brandt have x-rayed welds to certify operation at thishigher RPM. The Derrick machine has a longer bowl (24 x 45 in.) and willperform best in unweighted muds. Gearbox ratio and conveyor flight designvary significantly. The Sweco and Broadbent machines have 60:1 gearboxratios and double-lead conveyors that provide good performance in bariterecovery operations. The Brandt CF-2 has a widely-spaced, single-leadconveyor with a 140:1 gearbox more suited to unweighted mud. The optionalhydraulics package offered by Sweco is recommended when the SC-4 isused as the high-g centrifuge in two-stage operations. It will provide a slightlyhigher bowl RPM and adjustable differential RPM to maximize separationperformance.

7.1.6 Alpha-Laval 414, Swaco 414 (14 X 38), Sharples P3000,Hutcheson Hayes HH1430 (14 X 30)

These machines have the same features as their longer-bowledcounterparts. The shorter bowl length reduces retention time and makesthese centrifuges less susceptible to high torque in barite recoveryapplications. The larger machines are recommended for unweighted mudapplications.

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8 Summary

With the emphasis on reduced waste volumes and improved solidsremoval efficiency, the centrifuge has become an integral part of thedrilling solids removal system. Centrifuges are capable of removing veryfine solids that cannot be removed by any other mechanical solidremoval device. The solids discharge is relatively dry.

Laboratory tests indicate that centrifuge performance is chiefly afunction of G-force, pond depth, bowl-conveyor differential rpm and mudviscosity. G-force, a function of bowl rpm and diameter has the greatestimpact on separation efficiency. Pond depth controls both fluidresidence time and flow capacity. Differential rpm is a factor in solidsconveyance and torque limitations. Increasing yield values detrimentallyaffect separation efficiency.

Once a minimum threshold G-force is reached, cake dryness isrelatively unaffected by G-force. However, a minor difference in drynessmay change the appearance of the solids from runny to stackable.

Large, high G-force machines are recommended for centrifugingunweighted muds. Use deep pond depths and lower flow rates for finesolids distributions. Coarse solids distributions may be more efficientlyprocessed using shallow pond depths and higher flow rates.

Centrifuging hydrocyclone underflows becomes increasingly economicas mud formulation and waste disposal costs increase. The centrifugeshould process in excess of the hydrocyclone underflow rate. Twodesigns for centrifuge catch tanks are shown. A low-G, high capacitycentrifuge is recommended for these coarse solids.

The centrifuge is used in weighted mud to recover valuable weightingmaterial from mud which must be discharged due to unacceptablecolloidal solids content. The economics of barite-recovery centrifugingis usually positive when the liquid phase is inexpensive and disposalcosts are not prohibitive. G-force should be maximized to improve bariterecovery.

Two-stage centrifuging is necessary in weighted muds when liquiddischarge must be minimized. The first centrifuge recovers barite. Itseffluent is fed to a second centrifuge operating a maximum Gs, whichdiscards solids and returns the liquid phase. Colloidal solids are notremoved. The economics of two-stage centrifuging are site-dependent.A method for monitoring the cost effectiveness of two-stage centrifugingis presented in this section.

Recommended features on a centrifuge include: 1) Accelerator for thefeed, 2) tungsten carbide feed port entries and conveyor tiles, 3)universally adjustable pond dams, and 4) stainless steel bowl andconveyor. However, quality of service is paramount. Recommendedcentrifuges for both unweighted and weighted muds are listed.

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