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Section 400 SOLIDS CONTROL HANDBOOK Schlumberger Dowell

Shaker Screens

January 1998 Page 1 of 11

Shaker Screens 1 Introduction .........................................................................................................................2

2 Separation Performance.....................................................................................................2 2.1 Grade Efficiency ............................................................................................................2 2.2 Separation Potential ......................................................................................................3

3 Liquid Throughput Performance........................................................................................4

4 Screen Life ..........................................................................................................................4 4.1 Effect of Screen Composition ........................................................................................4 4.2 Effect of Vibration Pattern..............................................................................................5 4.2.1 Linear Motion .......................................................................................................5 4.2.2 Circular, Elliptical Motion ......................................................................................5

5 Shaker Screen Designations ..............................................................................................5 5.1 Mesh Count ...................................................................................................................5 5.2 API RP13E Screen Designation ....................................................................................6 5.2.1 Screen Name .......................................................................................................6 5.2.2 Equivalent U.S. Sieve Number .............................................................................7 5.2.3 Separation Potential (d50, d16, d84) .......................................................................7 5.2.4 Flow Capacity (Conductance, Non-blanked Area) ................................................8 5.2.5 Transmittance ......................................................................................................8 5.2.6 Aspect Ratio ........................................................................................................9 5.3 Field Procedure to Estimate Cut Point (D50) ................................................................ 10 5.3.1 Equipment.......................................................................................................... 10 5.3.2 Procedure .......................................................................................................... 10

6 Summary............................................................................................................................ 11

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Section 400 SOLIDS CONTROL HANDBOOK

January 1998

Shaker Screens

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FIGURES Fig. 1. Percent separated curve. .............................................................................................3 Fig. 2. Effect of plate opening size on screen blinding.............................................................9 TABLES Table 1 U.S. Sieve Series .......................................................................................................7 Table 2 Blinding Resistance of Common Screens ................................................................10

1 Introduction Shaker screen selection has the largest impact on the overall performance of the shale shaker. It is therefore important to understand the factors which may impact screen performance and how to properly select screens. Shaker screen performance is measured by: 1.

Separation Performance - the size of the solids removed

2.

Liquid Throughput Performance - the capability of the screen to transmit fluid

3.

Service life

2 Separation Performance 2.1 Grade Efficiency The separation performance of a shale shaker screen (or any other solids control device) is commonly represented by its percentseparated, or grade efficiency, curve. This curve is generated from fullscale experimental measurements and depicts the percent solids removed as a function of particle size. It reports the screen's probability of separating any specific particle size with a given shaker under conditions specific to the test. Grade efficiency is the preferred measure of separation performance because it is independent of feed particle size distribution. An example of a percent-separated curve is shown in Fig. 1. In this example, the median size separated by the screen was 145 microns. This means that 50% of the solids with a diameter of 145 microns were removed. A rough estimate of the median cut point (d50) can be made in the field by the wet sieve procedure (see Field Procedure to Estimate Cut Point).

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Fig. 1. Percent separated curve. Note: This curve indicates the percentage of solids removed as a function of particle size.

2.2 Separation Potential APR has developed a method to characterize the relative separation efficiency potential of shaker screens without the expense and time required for full-scale testing. The technique links the relative separation performance of screens to a volume-equivalent distribution of their opening sizes. The screen's openings are measured using PC-based image analysis technology. Each opening in the screen is then represented by a spherical diameter corresponding to an ellipsoidal volume calculated from the image analysis data. The cumulative volume of these ellipsoids, when plotted as a function of spherical diameter, yields a curve which correlates well with the standard grade efficiency curve. This curve represents the “separation potential” of the screen. The word “potential” is used because the screen's separation performance is not measured directly, but implied by the size of the screen's apertures. Note: Grade separation efficiencies as measured on the shaker are subject to specific shaker and flowline conditions. They may not always agree with separation potential values. For example, the separation potential value for a screen with rectangular openings may be pessimistic when drilling clean sand sections producing predominantly spherical sand grains. The image analysis method assumes solids of all shapes and sizes are available to the screen. However, on average, the

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SOLIDS CONTROL HANDBOOK

Shaker Screens

Schlumberger Dowell

separation potential values have been shown to adequately represent the screen's separation performance.

3 Liquid Throughput Performance The liquid throughput capacity of a screen panel is primarily a function of screen conductance and usable area. Conductance describes the ease with which fluid can flow through a unit area of screen cloth. In simplistic terms, it is analogous to permeability with the length in the direction of flow (screen thickness) taken into account. Higher conductances will result in higher flow rates through the screen. Conductance is calculated from the mesh count and wire diameters of the screen cloth by the equations given in Appendix B, Conductance Calculation. Multilayer screens can also be handled by the conductance equation. The inverse of conductance for each screen layer is summed to equal the inverse of the net overall conductance: 1 1 1 1 = + +... Ct C1 C2 Cn This is valid provided that the screen layers used in the composition are designed to remain in contact. Oilfield screens are typically bonded to a perforated metal panel or plastic grid to provide extra strength and improve service life. This practice eliminates some of the usable area through which fluid may pass. Some metal backing plate designs may reduce effective screening area by as much as 40 percent. Because conductance describes screen flow capacity per unit area, the usable unblocked area available for screening must also be considered when comparing the mud processing capacity of shaker screen panels.

4 Screen Life The definition of “acceptable” screen life must be judged within the context of the total solids removal system economics. Besides screen replacement cost, consideration must be given to the costs of drilling mud dilution and waste disposal costs when determining whether longer screen life is warranted at the expense of solids removal efficiency. In weighted mud applications, the economic benefits of improved solids removal efficiency usually outweigh the additional screen costs.

4.1 Effect of Screen Composition Only very general correlations may be made between composition and service life. Unfortunately, features that improved life are usually detrimental to flow capacity. Using wires with greater tensile strength or adding supporting layers

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screen lead to heavier of cloth


Shaker Screens


can both reduce conductance. Increasing support through additional bonding area (smaller plate openings) eliminates usable screening area. Also, support techniques and screen tension can have a major effect on screen life. As a result, screen panels are typically designed to balance flow capacity performance with screen life. Screen life is heavily dependent upon flow line conditions. Solids loading rate, drilled cuttings abrasiveness, and shaker dynamics can easily outweigh composition effects.

4.2 Effect of Vibration Pattern 4.2.1 Linear Motion The abrupt changes in acceleration during the vibration cycle tends to cause screens to wear more quickly unless close attention is paid to tensioning and screen support techniques. Perforated metal backing plates and pretensioned screen panels have been specifically developed to address this problem. Linear motion shakers usually operate at less than 4.0 G's (normal to the screen) to balance screen life with processing capacity. Regardless, the finer screens normally run on linear motion shakers cannot be expected to outlast the coarser screens used in the past. For screens finer than 100 mesh, expect an average service life in excess of 100 hours.

4.2.2 Circular, Elliptical Motion The smooth change in acceleration with respect to direction translates into longer screen life compared to other vibration patterns. However, many circular motion shakers were designed before the advent of fine mesh screens and may provide less support for the screens. This will tend to negate much of the screen life benefit associated with circular motion.

5 Shaker Screen Designations 5.1 Mesh Count Shaker screens have traditionally been assigned mesh count designations by the manufacturer. Unfortunately, they do not adequately describe screen performance in terms of separation efficiency or flow capacity. Mesh count is defined as the number of openings per linear inch of screen cloth. Mesh count does not establish the size of screen openings unless wire diameter is known. The opening size, D, is related to the wire diameter, d, and the mesh count, n, by the following equation:

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D =

Schlumberger Dowell

1 -n d

With the wide variety of wire diameters used to construct the same mesh count, the actual separation efficiencies of screens with the same mesh count designation are rarely consistent: 1.

Manufacturers commonly designate layered screens by a single mesh count number. Experimental separation efficiency tests have revealed that these designations are predominantly optimistic.

2.

Oblong mesh screens may be identified by a single number which may be the sum of mesh counts in both the horizontal and vertical direction. For example, a 60 x 40 mesh screen may be labeled “100 mesh”. This practice is misleading: The opening sizes of a 60 x 40 mesh screen will pass much larger particles than a 100 x 100 square mesh screen.

5.2 API RP13E Screen Designation Recently, a new performance-based screen designation system has been developed. This designation system has been adopted by the API RP13E as a Recommended Practice for Shale Shaker Screen Cloth Designations. The API has recommended that all screens be labeled with the following information: Screen Name Separation Potential (d50, d16, d84) Flow Capacity (Conductance, Total Non-Blanked Area) A comprehensive list of screen designations for most shakers is included in Appendix D, Screen Designations. The screen designations include additional information not specified by the API to further define screen performance. Each of the designation components are described in detail below:

5.2.1 Screen Name This is the “mesh count” designation or part number used by the manufacturer to identify the screen. Typically, it consists of a mesh count number preceded by a letter code which may describe the screen's cloth type or layering technique. For example, MG100 signifies a 100 x 100 mesh “market grade” bolting cloth, a PWP HP100 signifies a perforated plate, triple-layer screen composed of oblong mesh screen cloth.

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5.2.2 Equivalent U.S. Sieve Number This is the U.S. Sieve Number which has the same median opening size, or d50, as the screen. Table 1 lists the opening sizes of the standard U.S. Sieve series. In cases where no actual U.S. Sieve exists for a given opening size, the equivalent U.S. Sieve Number is a linearly-interpolated value. This value provides a simple scale by which to quickly rank the separation potential of screens. Caution should be exercised when using this value to compare screens of different type since it represents only the median separation potential of the screen.

Table 1 U.S. Sieve Series U.S. Sieve Number

Opening Size Microns

U.S. Sieve Number

Opening Size Microns

3.5

5660

40

420

4

4760

45

350

5

4000

50

297

6

3360

60

250

7

2830

70

210

8

2380

80

177

10

2000

100

149

12

1680

120

125

14

1410

140

105

16

1190

170

88

18

1000

200

74

20

840

230

62

25

710

270

53

30

590

325

44

35

500

400

37

5.2.3 Separation Potential (d50, d16, d84) The separation potential of the screen is represented by 3 points on the separation potential curve, labeled d16, d50 and d84 (Fig. 1). These points are the spherical diameters, in microns, corresponding to 16, 50 and 84 percent of the cumulative ellipsoidal volume distribution of hole sizes present in the screen. It must be stressed that these values provide a relative measure

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SOLIDS CONTROL HANDBOOK

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Schlumberger Dowell

of a screen's potential ability to remove solids. They may not necessarily agree with measured grade efficiency cut points for a given application. d50 The d50 is the median aperture size of the screen on a volume-equivalent basis. In experimental grade efficiency terms, it is analogous to the size of solid that has a 50% probability of separation. The d50 is typically used as a single value indicator of separation efficiency performance. Because of it's importance, the d50 is listed first. d16, d84 The d16 and d84 values indicate the range of hole sizes present in the screen. The d16 and d84 values can be important when the removal of fines from an unweighted mud is desired, or when the removal of barite is a concern. The deviation from the d50 describes the screen's implied separation characteristics. As the difference between the d16 and d50 increases, it is more likely that some solids finer than the d50 will likely be removed. Conversely, a smaller percentage of solids coarser than the d50 may be removed as the difference between the d84 and d50 increases. A multilayered screen will generally have a larger spread between the d16 and d84 values than a single mesh screen with the same d50.

5.2.4 Flow Capacity (Conductance, Non-blanked Area) The calculated conductance is reported in units of kilodarcies/millimeter for the total screen composition. Non-blanked area is the total effective screening area per panel, in units of square feet. Note: Support rails on the shaker deck can reduce the usable area of screens not mounted on metal backing plates. This area reduction is not included in the calculation of usable area because it is not a function of screen panel construction and will vary with the shaker type.

5.2.5 Transmittance Transmittance represents the net flow capacity of individual screens. It is the product of conductance and unblocked screening area. Transmittance permits the comparison of individual screens which differ in usable screening area.

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5.2.6 Aspect Ratio Aspect ratio describes the average shape of the screen openings. It is the volume-weighted average length-to-width ratio of the screen openings. Aspect ratio serves as an indicator of screen composition and provides information about the screen's potential resistance to blinding. Rectangular, or oblong, mesh screens have been customarily employed to reduce the “blinding” problems exhibited by square mesh screens when drilling sand sections. The “near-size” sand grains lodge in the square mesh screen apertures and reduce mud processing capability. The longer slots in the oblong screens are more likely to be only partially blocked by these spherical particles and thus tend to resist blinding. Aspect ratios in excess of 1.5 are typical of oblong mesh screens (both single and multilayered designs) used in the oil field. Single layer square mesh screens have aspect ratios near unity. Layered, unbonded, square mesh “sandwich” screens have the capacity to “actively deblind” (remove particles) by the interactive movement between the layers. This feature is lost when the layers are bonded together to improve screen life. Laboratory tests have shown that blinding increases substantially when the apertures in the metal backing plate or plastic grid have dimensions of less than 4 x 4 in. Fig. 2 shows how blinding severely restricts the flow capacity of the shaker when smaller opening dimensions in the screen panel are used.

Fig. 2. Effect of plate opening size on screen blinding. Note: Plate openings with dimensions less than 4 x 4 in. lose their deblinding ability. CONFIDENTIAL


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Shaker Screens

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Some improvement in blinding resistance over single layer square mesh cloth is still apparent in bonded, multilayer square mesh screens: Stacking one screen cloth over a slightly coarser cloth results in a wide range of hole sizes and shapes. Only the portion of the screen with openings near in size to the sand will tend to be blinded. Aspect ratios of layered square mesh screen compositions range from 1.3 to 1.5. The relationship between screen composition and blinding resistance is summarized in Table 2.

Table 2 Blinding Resistance of Common Screens Screen Panel Composition

Aspect Ratio

Blinding Resistance

Single or double layer, square mesh

< 1.2

poor

Triple layer, square mesh, bonded

1.3-1.5

fair

Triple layer, square mesh, unbonded

1.3-1.5

best*

> 1.5

better

Rectangular mesh, all types

* provides “active” deblinding through layer interaction

5.3 Field Procedure to Estimate Cut Point (D50) Note: This procedure provides only a rough approximation of the cutpoint. It assumes that the mass flowrate of the solids discard is negligible compared to the feed and screen unders. Results may be inaccurate under high solids loading.

5.3.1 Equipment ·

U.S. Test Sieves (Enough sizes to bracket expected cut)

·

Sample Containers

·

Sand Content Tube and Funnel

5.3.2 Procedure 1.

Take equal sized samples of both feed and unders. Avoid taking unders samples at the point where the fluid enters the sand trap. Where possible, take them from directly under the screen.

2.

Wet sieve each sample and measure the volume retained on each sieve using sand content tube.

3.

Calculate the percent separated for each test sieve by the following method:

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%Separated =

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Feed Vol. - Unders Vol. x100 Feed Vol.

4.

Plot through the midpoint of each sieve range as a function of volume percent removed.

5.

Read the median cut point (d50).

6 Summary ·

Shaker screens control the separation performance of the shale shaker.

·

Separation performance may be measured by two methods: A.

and

liquid

throughput

Percent-separated or grade efficiency. Generated from full-scale measurements, a grade efficiency curve represents the screen’s probability of separating any specific particle size under the specific conditions of the test. The median separation of the screen, commonly called the “d50” or “cut point,” represents the particle size that has a 50% probability of being removed. A field procedure is provided to estimate the d50 of the shaker screens.

B.

Separation potential. This method uses the range of opening sizes in the screen to indicate the relative separation performance of the screen. Because the screen is visually analyzed, separation potential is independent of operating conditions. This method has been adopted by the API as a Recommended Practice for Shaker Screen Cloth Designations under API RP13E.

·

Liquid throughput performance is represented by the screen’s conductance and usable screening area. Conductance, calculated from the physical dimensions of the screen composition, is analogous to the screen’s permeability. The conductance equations are included in Appendix B, Conductance Calculation. Usable screening area is the area in the screen panel available for fluid flow.

·

Mesh count designations do not adequately describe performance because wire diameters and opening size.

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