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Section 1000 SOLIDS CONTROL HANDBOOK Schlumberger Dowell

Addition/Mixing Systems

January 1998 Page 1 of 15

Addition/Mixing Systems 1 Introduction .........................................................................................................................2

2 Mixing Hoppers ...................................................................................................................2

3 Bulk Systems.......................................................................................................................4

4 Polymer Mixing ...................................................................................................................5

5 Active System Addition.......................................................................................................7

6 Premix System ....................................................................................................................8

7 Water Addition ....................................................................................................................9 7.1 Waste Pit Water ............................................................................................................9

8 Agitation ............................................................................................................................ 10 8.1 Agitator Design ............................................................................................................11 8.2 Agitator Sizing Example............................................................................................... 13

9 Summary............................................................................................................................ 15 FIGURES Fig. 1. Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6. Fig. 7.

Jet/Venturi mixer. .........................................................................................................3 Sidewinder mixer..........................................................................................................4 Jet shear mixer. ...........................................................................................................6 SECO (Echols) homogenizer ring. ...............................................................................7 Horsepower requirements for canted-blade impellers................................................. 12 Horsepower requirements for flat-blade impellers. ..................................................... 12 Floor baffles. .............................................................................................................. 13 TABLES

Table 1 Recommended Turnover Rates................................................................................ 11 Table 2 Impeller Displacement Rates ................................................................................... 14 Table 3 Physical Specifications for Mechanical Mixers.......................................................... 14

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Section 1000 January 1998

SOLIDS CONTROL HANDBOOK


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Schlumberger Dowell

1 Introduction All mud systems require a mixing system for the addition of viscosifiers, weighting agents and chemicals to maintain desired mud properties. The method and location chosen for addition can greatly impact material consumption and the resultant properties of the active system. For example, if bentonite is not completely hydrated before being pumped downhole, the viscosity of the mud at the flowline will be much higher than at the suction pit. Since viscosity negatively impacts solids control equipment performance, inadequate control of viscosity can lead to higher dilution volumes. Polymers present special mixing concerns to prevent the formation of “fish-eyes”; balls of dry polymer encapsulated by a thick, partially-hydrated layer. Unless properly wetted and sheared, a significant portion of the polymer will be lost at the shakers and increase polymer consumption. It is therefore important to ensure that additions are made correctly and in the right location.

2 Mixing Hoppers The most common device used to add dry material to the mud is the venturi mud mixing hopper (Fig. 1). Fluid is supplied to the mixer by a centrifugal pump. The hopper device works by converting pressure head into velocity through a jet nozzle. The downstream venturi increases the shearing action and changes velocity head back into pressure head. Dry material is added where the jet stream crosses the gap between the nozzle and the venturi. Here, a low pressure area creates a slight vacuum. This vacuum, along with gravity, helps draw the material into the fluid stream. The high velocity and high shear rate of the fluid wets and disperses the dry material. To operate at maximum efficiency, both the nozzle and venturi must be correctly sized for the flowrate and head. This type of hopper is available from many manufacturers. “Homemade” versions, usually without a properly-designed venturi, are common.

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Fig. 1. Jet/Venturi mixer. Note: This design reduces dust but entrains more air into the fluid. Another common mixing device is the Sidewinder hopper (Fig. 2), manufactured by Swaco. The operation is much like a hydrocyclone. Fluid is pumped to a tangential inlet which allows pressure energy to be converted to centrifugal force. The spiraling fluid picks up the dry mud where it undergoes shear as it travels twice around the mixing chamber. As the fluid exits the hopper through a tangential outlet, the velocity is converted back into pressure head.

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Section 1000 SOLIDS CONTROL HANDBOOK

January 1998


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Schlumberger Dowell

Fig. 2. Sidewinder mixer. Note: This design does not entrain as much air as the venturi mixer, but creates more dust. Laboratory tests conducted at APR with bentonite showed little difference between the two devices in both capacity and mixing capability as measured by the resultant mud rheology. Since the Sidewinder does not draw air into the hopper, dust can be a problem when adding some materials. Conversely, the Mission Venturi hopper eliminated dust but entrained more air into the mud. Sizing of the jet nozzle and venturi are critical in obtaining maximum performance from venturi mixers; “homemade” versions should be avoided.

3 Bulk Systems Bulk systems are economical for storing and distributing weighting material in large quantities. There is less waste and trash than when using sacked material. Bulk barite is stored in large vertical tanks, equipped with an air delivery system. Barite is drawn from the tank by a venturi to a bulk hopper which meters the material into the mud hopper. Bulk systems for other dry materials are becoming increasingly popular in offshore applications to eliminate handling and waste associated with sacked material. When consumption is not high enough to justify bulk tanks, hopper systems using 2200 lb “big bags” may be an alternative.

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4 Polymer Mixing Conventional mixing hoppers are not generally adequate for mixing and wetting dry polymers into viscous muds. Problems frequently arise when attempting to mix dry PHPA powder through conventional mixing hoppers regardless of whether the polymer is added directly to the active system or to a concentrated premix. Polymer fish-eyes, excessive viscosities, extensive mixing times and shaker screen blinding are commonly reported. These problems can be reduced by using a liquid form of the product, but liquid formulations contain less active polymer and use an oil as the carrier fluid. Work conducted at APR on the characterization of polymers such as PHPA has led to the following conclusions regarding the mixing and shearing of polymers: 1.

PHPA polymers marketed for use in drilling fluids may contain varying amounts of high molecular weight fractions. Viscosity is a function of molecular weight. Those products with a higher fraction of high molecular weight polymer will be harder to dissolve and generate higher viscosities.

2.

Shear-degradation reduces the molecular weight of many polymers, especially PHPA. Higher shear rates produce lower molecular weights. Below a certain molecular weight, the inhibitive characteristics of PHPA are effectively lost.

A mixing and shearing system consisting of a perforated-wafer type of jet shear mixer coupled with a SECO Homogenizer was found to provide improved mixing and allow preparation of PHPA concentrations to 6 lb/bbl in a 50,000 mg/L chloride brine. The Flo Trend Jet Shear Mixer, pictured in Fig. 3, directs fluid into a mixing chamber through opposing nozzle disks to impart turbulence and increase contact area. Polymer is added into the chamber through a conventional hopper. The mixture is then pulled into a venturi eductor, where further shearing and mixing occurs.

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Schlumberger Dowell

Fig. 3. Jet shear mixer. Note: Designs such as this can improve polymer mixing. The SECO (Echols) homogenizer consists of a perforated ring that fits around the perimeter of the impeller blades in a centrifugal pump (Fig. 4). Test conducted at APR indicate that the SECO homogenizer produces sufficient shear to degrade the higher molecular weight fractions that make the product hard to dissolve, but will not shear-degrade below the molecular weight required for inhibition. The SECO is recommended for premixing polymer to reduce viscosity and elimination fish-eyes. Do not use this device for shearing weighted muds; the high solids content will quickly erode the perforations in the homogenizer ring. Also, barite may be degraded by the homogenizer.

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Fig. 4. SECO (Echols) homogenizer ring. Note: Recommended for shearing polymers. The following guidelines should be followed for building concentrated premix volumes: 1.

When possible, use brine instead of fresh water. Polymers will impart less viscosity in brine than in fresh water.

2.

Mix supplemental material into the brine prior to adding the polymer.

3.

After mixing the supplemental material, the total amount of polymer should be mixed through the jet shear mixer into the brine in a single pass.

4.

After mixing the PHPA, the premix should be sheared through the homogenizer until stable rheological values are achieved.

5 Active System Addition Dry products such as bentonite and barite should not be added, even through a hopper, at the suction compartment nor in the solids removal section. Addition should be made at least one compartment upstream of the suction compartment to allow time for the material to wet and disperse into the active system. Both these compartments must be well-agitated either by mechanical stirrers or mud guns. Mud materials added at the suction can cause air entrainment at the rig pumps and increase the incidence of drill pipe corrosion.

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6 Premix System A premix system is a separate set of tanks with agitators and a hopper for batch mixing mud to desired specifications before addition to the active system. Premix systems are highly recommended for the advantages they provide: 1.

Improved hydration and less air entrainment with dry solids addition. After mixing dry material in the premix tank, the mud can be agitated until the dry material is fully wetted. This also provides time for entrained air to break out of the mud.

2.

Better control over active system mud properties. The properties of the premixed mud can be tailored to meet desired properties before transferral to the active system. Once properties in the premix have stabilized, the mud can be transferred over a complete circulation to ensure even mud properties in the active circulating system.

3.

Less material consumption. With longer hydration and shearing time, premix tanks offer the benefit of maximizing the yield from bentonite and polymers before addition to the active system. Premix tanks are especially effective for polymer muds and almost essential for oil-based muds. Specialized shearing and mixing equipment (see Polymer Mixing) may be used on the premix tank to properly wet polymers at high concentrations and eliminate fisheyes, thus reducing polymer consumption.

4.

Easier to monitor dilution rates. The volumes added to the active system are usually much easier to monitor when transferring liquids of known quantity from a premix tank. The overall solids removal efficiency can be determined much more readily when accurate measurements can be made of dilution volumes and water additions.

5.

Less manpower requirements. Since the premix is prepared in a batch process, material may be added much more quickly than when making additions over a complete circulation in the active system. Once the material has been added, the premix may be left to agitate and hydrate the slurry. After the desired properties have been achieved, the premix may be metered slowly into the active system. Both the hydration and transferral operations require minimum attention, thus freeing up manpower for other duties.

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7 Water Addition Dilution water is necessary in all water-based mud systems to maintain circulating volume and desired mud properties. Since the amount of dilution is directly linked to solids removal efficiency, the use of a water meter to monitor dilution volumes is strongly recommended. Water will be necessary on the rig floor, in the motor area, and may be required to help move discarded solids to the waste pit. Water must be supplied to the addition section and solids removal sections of the mud pits for both volume maintenance and cleaning. Since water must be supplied to almost every area on the rig, manifolding is obviously required. Ensure that the water meter is located to account for all water streams that will end up in the active system. Water should be recycled wherever possible. Remember that regardless of its purpose, any water used on and around the rig will contribute to the total liquid waste volume. This is especially important on locations where water supply or disposal costs are high. It is imperative that every water line be equipped with a valve and that no leaks are tolerated. Use low volume nozzles on the wash water lines. When possible, wash water should be collected and segregated from the cuttings disposal pit for recycling or for makeup water in the active system. Water should be added at the flowline when necessary to reduce the viscosity of the mud and allow finer screens to be used on the shakers. Any potential degradation in the cuttings size due to viscosity reduction is offset by the increased removal rate. Lower viscosity mud will also improve downstream degasser and hydrocyclone performance. Because centrifuge performance is sensitive to the viscosity of the feed mud, water addition at the centrifuge is usually necessary to achieve optimum performance. Since the centrifuge feed rate is usually much lower than other devices, the beneficial effect of water addition is proportionately greater at the centrifuge. Note, however, that dilution water added to the feed of the barite-recovery centrifuge is discharged with the centrate and the does not contribute to dilution of the active system unless two-stage centrifuging is employed.

7.1 Waste Pit Water In many instances, recycling water from the waste pit makes both economic and environmental sense. The following guidelines can significantly reduce overall rig water consumption: 1.

Recycle waste pit water to the shaker or centrifuge slides to help flow discarded solids back to the waste pit.

2.

Use clean waste pit water as makeup water for the mud. Design the waste pit to access this fluid. This water must be checked for chemical compatibility before addition to the active system.

3.

Use the waste pit water as dilution water for the barite-recovery centrifuge, provided the volume of colloidal solids in the water is low.

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4.

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Segregate wash water and runoff water from the solid waste pit. This is especially important when using salt muds or oil-based muds to control waste pit remediation costs.

8 Agitation Agitation is necessary to keep weight material suspended and ensure a homogeneous drilling fluid. Agitation also prevents solids buildup in the mud tanks. All removal compartments except the sand trap should be well-stirred. Mechanical (paddle type) stirrers are efficient mixers and are recommended, especially in the solids removal section. Mud guns impart shear which may degrade the drilled solids. Mechanical agitation ensures that the solids control equipment cannot be bypassed. Mud guns are acceptable in the Addition-Suction compartments downstream of the solids control equipment. In the addition section, mud guns may help shear and blend newly-added mud materials. Mechanical stirrers must be correctly sized. They must be large enough to adequately mix the fluid and not so large to cause aeration of the mud. The following method for sizing agitators was developed by the Brandt Company. This agitator sizing method is based on the desired turnover rate (TOR). The TOR is the time required, in seconds, for all of the fluid in the tank to move past the agitator blades: TOR = Vt / D x 60 where: Vt

=

Tank volume in gallons (L x W x H, in feet x 7.481)

D

=

Impeller displacement in gal/min (from Table 2)

The mud area to agitate should be divided into squares. For example, a 10 ft x 30 ft tank should be divided into 3 equal parts, each 10 ft x 10 ft. The TOR would be based on the volume of each 10 ft x 10 ft area. For proper agitation, the TOR should be between 35 and 90. A TOR less than 35 may result in vortices; a TOR over 90 will lead to solids settling. Table 1 gives the recommended TORs for various mud compartment uses.

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Table 1 Recommended Turnover Rates Tank Type

Recommended TOR

Solids Removal

Suction

Reserve

Pill

50-75

65-85

50-80

40-65

8.1 Agitator Design 1.

Calculate volume, Vt, (gal). 3

Vt = L x W x H, in feet x 7.481 gal/ft . 2.

From Table 1, determine the required TOR, (sec).

3.

Calculate the required impeller displacement, D, (gpm). D = Vt x 60/TOR

4.

Choose an impeller from Table 2 with impeller displacement closest to the value calculated in Step 3. For tank depths > 5 ft, use a cantedblade (angled blade) impeller. Flat-blade (vertical blade) impellers may be used in shallower tanks.

5.

Locate the impeller diameter corresponding to the chosen impeller displacement on Table 2. Using Fig. 5 (Canted-Blade) or Fig. 6 (FlatBlade), enter the chart at the impeller diameter and follow the horizontal line until it intersects the maximum anticipated mud weight curve. Read the recommended horsepower.

6.

Determine the recommended agitator shaft length from Table 3.

7.

Canted-blade impellers should be located so that the distance between the tank bottom and the lower edge of the impeller blades is equal to 0.75 times the impeller blade diameter. Flat-blade impellers should be placed 6 in. above the bottom of the tank, or 2 in. above the bottom shaft stabilizer.

8.

Baffles, as shown in Fig. 7 are highly recommended for flat-bottomed tanks to help direct the flow towards the corners and eliminate “dead areas” in the tank. A baffle is a steel plate 12 in. long, mounted on the tank floor and extending 6 in. above the top of the agitator blades. The baffles should be installed 6 in. from the agitator blade tips along a line from the agitator shaft to each corner of the compartment.

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Schlumberger Dowell

Fig. 5. Horsepower requirements for canted-blade impellers. (Courtesy of Brandt)

Fig. 6. Horsepower requirements for flat-blade impellers. (Courtesy of Brandt)

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Fig. 7. Floor baffles. Note: These are recommended to eliminate “dead areas” in flat-bottomed tanks.

8.2 Agitator Sizing Example Given:

Suction Tank, 9 ft L x 7 ft W x 9 ft H 18 ppg mud

1.

Vt = (9 x 7 x 9) x 7.481 = 4242 gal

2.

Recommended TOR, from Table 1: 75 sec

3.

Impeller Displacement Rate: D = (60) (4242)/75 = 3394 gpm

4.

Since tank depth > 5 ft, a canted-blade impeller is selected. From Table 2, nearest D = 3764 gpm, Impeller Diameter = 32 in.

5.

From Fig. 6, for 32 in. diameter and 18 ppg mud, required agitator horsepower = 5 HP (MA5).

6.

From Table 3, for model MA5 agitator and 9 ft tank depth, shaft length reduction = 10 in. Total Shaft Length = 9 ft x 12 in./ft - 10 in. = 98 in.

7.

Impeller location above tank bottom = 0.75 x 32 = 24 in.

8.

Total Agitator Weight = 98/12 x 15.1 lbs/ft + 580 lbs + 50 lbs = 753 lbs.

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Table 2 Impeller Displacement Rates Diameter (in.)

Impeller Displacement Rate* GPM at 57.5 (60 Hz)

Impeller Displacement Rate GPM at 48 rpm (50Hz)

Canted-Blade

Flat-Blade

Canted-Blade

Flat-Blade

246 560 1051 1941 2839 4365 6273 8411 11300 14401 18630

177 404 760 1373 2060 3142 4510 6081 8288 10445 13440

205 467 877 1620 2370 3644 5237 7023 9435 12024 15552

Weights (lbs)

12 16 20 24 28 32 36 40 44 48 52

11 15 19 21 38 50 61 74 101 118 126

213 484 909 1645 2468 3764 5402 7284 9928 12512 16100 3

3

* D = AB x V x 7.481 gal/ft , where AB = projected blade area, ft , V = impeller velocity, ft/min Canted-blade area based on 60× angle Brandt data

Table 3 Physical Specifications for Mechanical Mixers

Model

MA1* MA2* MA3* MA5 MA7.5 MA10 MA15 MA20 MA25

HP

1.0 2.0 3.0 5.0 7.5 10.0 15.0 20.0 25.0

Shaft Dia. (in.)

Minimum Impeller Dia. (in.)

1-1/2 1-1/2 1-3/4 2-3/8 2-3/8 3 3 3-1/4 3-1/2

12 20 24 28 32 32 36 40 40

Weight

Shaft Length Reduction (in.)**

Shaft (lb/ft)

Agitator (lbs)

Free

6.0 6.0 8.2 15.1 15.1 24.0 24.0 28.1 32.7

200 310 406 580 1200 1224 1830 1898 3130

9 9 10 11-1/2 22-1/2 22-1/2 26-5/8 27 33

Stablized 9 9 10 10 12 12 13-1/8 13-1/2 13-1/2

* Bottom shaft stabilizer required at 6 ft, all others require bottom stabilizer at 8 ft. ** Shaft Length = Distance from tank bottom to top of agitator support beams - shaft length reduction. Brandt data

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9 Summary ·

Addition/mixing systems must be correctly designed to minimize material consumption and ensure complete and even mixing.

·

The two most common mixing hoppers are the venturi type and the Sidewinder hopper. Laboratory tests conducted with bentonite showed little difference between the two devices in both capacity and mixing capability. The Sidewinder does not entrain air like the venturi hopper, but dust can be a problem when adding some materials.

·

Bulk systems are economical for storing and distributing material required in large quantities. There is less waste and trash compared to sacked material. Bulk systems are also becoming popular for the accurate metering of dry material and chemicals in low dosages.

·

Mixing polymers such as PHPA present additional problems such as polymer fish-eyes, extensive mixing times, and shaker screen blinding. Polymers with a higher fraction of high molecular weight polymer will be harder to dissolve and generate higher viscosities. Higher shear rates produce lower molecular weights, but below a certain molecular weight, the inhibitive characteristics of PHPA are lost.

·

A mixing and shearing system consisting of a perforated-wafer type of jet shear mixer, coupled with a SECO Homogenizer, was found to provide improved polymer mixing. Guidelines for building concentrated premix volumes are provided.

·

Premix systems are highly recommended for the numerous advantages they provide: A.

Improved hydration

B.

Better control over active system mud properties

C.

Less material consumption

D.

Easier to monitor dilution rates

E.

Less manpower requirements

·

All dilution water streams should be metered to monitor solids removal efficiency. Water should be added at the flowline to reduce viscosity and improve shaker performance. Any water used on the rig will contribute to the total liquid waste volume. No leaks should be tolerated. Use low volume nozzles on the wash water lines. Recycle water where possible.

·

Mechanical (paddle type) agitators are recommended in the solids removal section of the active system. Mud guns are acceptable in the addition/suction compartments only. A procedure is provided to correctly size mechanical stirrers.

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