Cut Points

In general, screens on shale shakers reject solids larger than their opening sizes and retain the drilling fluid and smaller solids. Drilling fluid properties, as well as screen conditions, may affect screen performance. For example, high gel strengths and high surface tensions tend to bridge small screen openings and prevent screens from passing small solids and liquid; filtration-control additives, such as starch, tend to plug screen openings and prevent small solids and liquid from passing; and in an oil-based drilling fluid, water-wet, fine-mesh screens may reject a large portion of the drilling fluid flowing onto the screen.

When 50% of the mass of a particular solid size is found in the underflow of a screen and 50% of the mass of that size is found in the overflow, that size is said to be the D50 or 50% cut point. Cut point curves, or a percent separated curve, is a graphical representation of the actual measured separation of solids made by the screen. For example, a D20 cut point would be the size where 20% of the mass of solids of that size are returned to the drilling fluid (pass through the shaker screen) and 80% of the mass of that size solid is rejected from the system (discarded). The measurements required to determine cut point curves are flow rate, percent solids, and particle-size distribution on the feed and discard flow streams. Alternatively, the underflow and discard flow streams can be used. The mass, or weight of solids, in these two streams can be combined to mathematically represent the input stream. In order to ensure that the measurements are accurate, it is best to measure all three flow streams and check continuity. Chapter 9 provides the calculations that must be made to obtain percent separated, or cut point, curves.

Drilled solids enter the wellbore from the sides of the hole (represented by the largest solids) where the drill bit is drilling the rock (represented by the smaller solids), and all drilled solids are subjected to deterioration as they move up the an-nulus (represented by the various ranges of sizes).

For illustration purposes, assume that six different size spheres reach the surface and present to a very coarse mesh screen. The dimensions discussed are in inches instead of microns to assist visualization. The number of spheres in each size category is shown in Table 6-6.

The total volume of spheres in Table 6-6 represents approximately 252 gal of solids. This is equivalent to less than 100 feet of a 10-inch diameter hole. In a wellbore, the mass of the smaller sizes can and usually do exceed the mass of the larger sizes. Table 6-6 shows the proposed distribution of spheres to be presented to a very coarse

TABLE 6-4. Typical Market Grade and Tensile Bolting Cloth Shaker Screen Characteristics

Separation Potential Screen Designation in Microns Conductance

TABLE 6-4. Typical Market Grade and Tensile Bolting Cloth Shaker Screen Characteristics

Mesh

d16

d50

^84

Kd/mm

Market Grade

10x10

1678

1727

1777

49.68

20x20

839

864

880

15.93

30x30

501

516

531

8.32

40x40

370

381

392

4.80

50x50

271

279

287

2.88

60x60

227

234

241

2.40

80x80

172

177

182

1.91

100x100

136

140

144

1.44

120x120

114

117

120

1.24

150x150

102

105

108

200x200

72

74

76

0.68

250x250

59

62

63

0.58

325x325

43

44

45

0.44

Tensile Bolting Cloth

20x20

1011

1041

1071

40.93

30x30

662

681

700

24.33

40x40

457

470

483

11.63

50x50

357

368

379

7.94

60x60

301

310

319

5.60

70x70

261

269

277

5.25

80x80

218

224

230

3.88

94x94

175

180

185

2.84

105x105

160

165

170

2.77

120x120

143

147

151

2.51

145x145

116

119

122

2.03

165x165

104

107

110

1.86

200x200

84

86

88

1.49

230x230

72

74

76

1.30

TABLE 6-5. Comparison of Open Area with Conductance

Conductance

Open Area

Market Grade Screen

Mesh

(Kd/mm)

(%)

50 x 50

2.88

30.3

100 x 100

1.44

30.3

150 x 150

0.68

33.6

200 x 200

0.58

36

TABLE 6-6

Sphere diameter (inches)

2.0

1.5

1.0

0.5

0.25

0.125

Number of spheres

28

130

880

14,100

226,000

3,610,000

Volume of spheres

4

8

16

32

64

128

Mass of solids (lb)

88

177

354

707

1414

2828

screen. The mass distribution of these solids will be used to create a cut point curve for this screen. Four gallons of 2.0-inch diameter spheres would have a mass of 88 lb. The 128 gal of ^-inch diameter spheres would represent 2828 lb of solids. The number of spheres increases from 28 to over 3 million. This also is similar to a distribution of solids in a wellbore. During transport to the surface, solids grind smaller increasing plastic viscosity and sometimes yield point. Plastic viscosity indicates liquid phase viscosity and size, shape, and number of solids. Plastic viscosity can increase without an increase in solids content because of drilled solids degradation. (Usually, this indicates the need for a centrifuge.)

The solids concentration traveling up an annu-lus is assumed to be 10% volume, these 252 gal of spheres would be contained in 2520 gal of drilling fluid. For illustration purposes, the flow rate onto the screen will be assumed to be 400 gal/ min and the flow rate off the screen will be assumed to be 20 gal/min. The screen would be presented with all of the solids in 6.3 min. For this illustration, the screen will be assumed to have f-inch square openings. The screen discard would consist of spheres larger than the opening size as well as liquid that wet these spheres. Drilling fluid clings to the screen discard and small solids also adhere to the large solids discarded from a rig shaker. Actually, some of the large solids find their way through or around shaker screens and appear in the retained drilling fluid. (The hydrocyclone underflow opening is frequently plugged even downstream from a fine-mesh shaker screen. This indicates that large solids are entering the drilling fluid system.)

To obtain a cut point curve for the spheres arriving at the surface, the mass of each size of spheres in the discard would be compared to the mass of that size presented to the screen. Table 6-7 presents the mass captured in the discard.

The cut point curve, plotted in Figure 6-4, indicates that the D 50 cut point would be around finch. This means that 50% of that size particle would be found in the discard and 50% would be found in the retained fluid. Some spheres smaller than the screen mesh are discarded even though they could pass through the screen. In reality, screen discards contain many solids that could pass through the shaker screen. Liquid drilling fluid (whole mud) is carried off the shaker screen with the discard. Finer mesh screens (150 or 200 mesh screens) discard more drilling fluid (or whole mud) than coarser mesh screens (such as 40 to 80 mesh).

A detailed procedure for determining cut points is presented in Chapter 9.

Cut Point Curve

0 0.5 1 1.5 2 Ball Diameter: Inches

FIGURE 6-4

TABLE 6-7. Data for Cut Point Curve

Size (in.)

Mass Presented (lb)

Mass Discarded (lb)

Fraction Discarded (% wt)

1/8

2828

226

8

1/4

1414

212

15

1/2

707

600

85

1

354

354

100

1.5

177

177

100

2.0

88

88

100

+1 0

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  • medhanie
    How to cut the weight of drilling fluids with a cetrifuge?
    6 years ago

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