The relationship between the size and shape of the particles being separated and the size and shape of the screen openings will influence how fine a separation is made. This is reflected in the percentageseparated curve. If all of the solids being drilled are spherical, then the distribution of the narrowest dimension of the screen openings will establish the percentage-separated curve. For wells with poor drilling practices, cuttings are tumbled in the annulus and arrive well rounded at the surface. For wells that have good cuttings transport in the annulus, the cuttings may be long, thin slivers of rock.

Solids have mobility in a pool of fluid to seek a screen opening large enough to go through. As a result, the conveyance velocity, contact time with the screen, and presence of other solids all affect the ability of the
solids to go through the holes in the screen. These variables therefore affect the percentage-separated curve.

Surface tension of the fluid causes solids to agglomerate together as they exit a pool of fluid. If solids finer than the screen openings make it out of the pool of fluid, then they are held by the surface tension and have very little chance to go through the screen. Adding a spray wash to the last screen panel disperses these patties, which will allow finer solids to be washed through the screen.

Blinding or plugging of screen cloth, as shown in Figure 7.24, dramatically affects not only the amount of fluid that will pass through the screen, but also the separation the screen makes. Many of the screen openings effectively become smaller, and fewer solids will pass through. The screen then makes a much finer separation than originally intended, and the screen capacity decreases significantly.

Reported values for percentage-separated curves are also affected by the way the measurement is made in the laboratory. The greatest error is often the measurement of particle size distribution. Particle sizing by sieve analysis is the best way to characterize solids being screened, since the sieving process is similar to screening. Unfortunately, sieving is a tedious and slow process. Forward laser-light-scattering particle size analyzers such as the Malvern and Cilas granulometers tend to report size distributions somewhat larger than sieve analysis. These instruments report particle sizes in terms of equivalent spherical diameters. Some drilled solids may be more rectangular in shape, so the equivalent spherical diameter may not exactly agree with the sieve analysis. Clay
particles in the 1-micron size are broad, flat surfaces, similar to a tabletop.These are difficult to describe in terms of a diameter.

Particles plugging wire mesh
Figure 7.24.Particles plugging wire mesh

In summary, the percentage-separated curve represents the fraction of solids rejected by the screen as a function of size. From the preceding discussion, it may be noted that the percentage-separated curve is dependent on the conditions that existed when the data were taken. As a result, in actual drilling conditions, the percentage-separated curve probably varies as drilling-fluid properties and the shapes of the solids change and as the screen blinds.

Revised API Designation System

Shale shaker screens made of two or three layers of screen cloth of different mesh sizes present openings that cannot be easily characterized. A technique to describe these openings has been adopted by the API as the ‘‘Recommended Practice for Designations of Shale Shaker Screens,’’ API RP 13C, to be issued soon. This recommended practice supersedes the second edition (1985) of API RP 13E, which was valid for only single-layer screens.
The new designation system was chosen to convey information on screen opening size distribution and the ability of nonvibrating screens to pass fluid. Information for each of the following is legibly stamped on a tag attached to the screen panel in such a way as to be visible after the screen is installed on the shale shaker:
. Manufacturer’s designation
. API number
. Flow capacity
. Screen conductance
. Conductance
. Total nonblanked area

Manufacturer’s Designation
The screen manufacturer may name a particular screen in any manner it desires. This designation is used when ordering a shaker screen with particular characteristics.

API Number
Shaker screen designation has been complicated by the advent of multilayered screens. When two or three screens are layered together, the opening sizes are not uniform. Experience has shown that the flow rate through these layered screens is much higher than anticipated, and the solids-removal rate is maintained. Since the screens have different irregular shapes, the standard mesh equivalent cannot be used to describe the screens. API RP 13C was recently rewritten by a task group composed of most of the authors of this book. The task group selected a mechanical method of designating shale shaker screens and comparing them to equivalent square mesh opening sizes. This section describes this method for determining the API U.S. sieve number equivalent of a shaker screen using a laboratory sieve shaker,
U.S. Standard Test Sieves, and sized grit samples. Screens are rated on the U.S. sieve number scale by the separations they achieve in dry sieving standard grit samples and comparing these separations to the separations of the same standard grit samples with standard U.S. sieves.
For example, a shaker screen that separates the grit sample similar to a U.S. 100 mesh test sieve is designated an API 100. Standard U.S. Test Sieves applicable to this procedure are as follows:

Standard U.S. Test Sieves

. Hold the test screen securely between the top and bottom parts, which are designed to bolt together and to nest with regular 8-inch U.S. test sieves.
. Arrange the sieves in consecutive order with the coarsest on top and the finest on the bottom. Nest the sieve stack with the sieve pan on the bottom.
. Place the grit test sample on the top sieve, cover it, and shake it for about 5 minutes with the RoTap test sieve shaker. Determine the weight of the grit remaining on the test screen. The fraction of the weight sample retained on the test screen determines the API screen number. Calculate the cumulative weight percentage retained for each individual sieve (beginning with the coarsest) by summing up the results.
. Prepare a plot of cumulative weight percentage retained versus the U.S. sieve opening (in microns) using a linear plot from point to point.
. Sieve and size the test grit through square mesh ASTM (American Society for Testing and Materials) screens. Place equal quantities of five different sizes of the test grit on a test screen on a RoTap for 5 minutes. The quantity and sizes of solids presented to the test screen would be:
(1) no solids from an ASTM 80 mesh (180 microns);
(2) 10 g from an ASTM 100 mesh (150 microns);
(3) 10 g from an ASTM 120 mesh (125 microns);
(4) 10 g from an ASTM 140 mesh (106 microns);
(5) 10 g from an ASTM 170 mesh (90 microns);
(6) 10 g froman ASTM 200 mesh (75 microns).
Present the total sample of 50 g of solids to the test screen, shaken for 5 minutes on a RoTap, and weight the residue on the screen.
. Graphically determine the D100 separation, in microns, from the plot. The value of the D100 separation usually falls between two U.S. sieve openings.
. When the D100 separation falls at a point that is 0.5 or less of the difference between the openings of a finer and the next coarser consecutive U.S. sieve, rate the test screen as the finer U.S. test sieve.
When the D100 separation falls at a point that is more than 0.5 of the difference between the openings of a finer and the next coarser consecutive U.S. sieve, rate the test screen as the coarser U.S. test sieve. For example, if the D100 separation is between a U.S. 170 (90 microns) and a U.S. 200 (75 microns), the test screen is rated as an API 170 if the D100 separation is greater than 82.5 microns, and as an API 200 if the D100 separation is 82.5 microns or less. API numbers are assigned with the following D100 separations, in microns.

API numbers
In the graph that follows, 33 weight percentage of the grit sample was captured on the test screen. This screen would have an API number of 140 and an opening size of 102 microns.
API Designation

API Designation
The designation would be an API number of 140, with the actual separation point of 102 microns in parentheses:
API 140 (102 microns)
The ‘‘mesh’’ designation is now called an API number so that the new designation will be more rig-user friendly. Rig crews recognize ‘‘mesh size’’ even though they may not actually know the definition. The change is necessary, since the API Recommended Practices are being converted to International Standards Organization (ISO) documents. ISO uses the metric system, consequently the number of openings per inch would need to be converted to the number of openings per centimeter or meter. This number would be meaningless to most rig crews.
The API designation number is specified to appear at least three times larger in physical appearance than any other letters or numbers on the screen tag. The ASTM 140 screen has openings of 106 microns, and an ASTM 170 screen has openings of 90 microns. The number in parentheses will indicate that the screen designation was actually measured and provide an indication of how close the openings are to standard screens.

Conductance Measuring Device

Flow Capacity/Screen Conductance

A screen that makes an extra-fine separation is not useful in the drilling industry if it will not pass a high-volume flow rate. The amount of fluid that a screen will process is dependent on the screen construction as well as solids conveyance, solids loading, pool depth, deck motion and acceleration, drilling-fluid properties, and screen blinding. Although it is difficult to calculate the expected fluid processing capacity of a shaker, screens can be ranked according to their ability to transmit fluid.

Conductance is a measure of the ease with which fluid flows through a screen cloth. It is analogous to permeability per unit thickness of the screen, C¼k(darcy)/l(mm). To calculate the flow through a porous medium, Darcy’s law is used as follows:

V = K*Δρ/(μ*l)

Now conductance, C, can be calculated where Q=V*A as follows:


. C=conductance (darcy/cm)
. K=permeability (darcy)
. l=screen thickness (cm)
. V=velocity (cm/sec)
. μ=fluid viscosity (cP)
. Δρ=pressure drop across screen (atm)
. Q=volume flow rate (cm3/sec)
. A=screen area (cm2)

Higher conductances mean that for a given pressure drop across the screen, more fluid is able to pass through the screen.
To measure the conductance, a 50-gal container of motor oil is mounted above the test screen. A flow valve is adjusted so that some of the oil overflows the screen into catch pans outside the apparatus.
The oil that flows through the screen is captured in a container on a balance. The weight of the container and oil is observed and recorded. When the flow becomes steady and uniform, the weight of the oil flowing through the screen is measured as a function of time. The temperature of the oil is measured and is kept constant. The density and viscosity of the oil as a function of temperature is determined prior to the test.

From the height of overflow fluid above the test screen, the head can be measured. From the density/temperature charts, the pressure applied to the screen can be calculated. From the density/temperature charts and the weight measurements, the volume of motor oil flowing per unit of time can be calculated. Care is taken to ensure a low flow rate to prevent turbulence in the oil flowing through the screen. From these measurements and the equations described above, the permeability per unit thickness of the screen can be calculated. This is the conductance.

Total Nonblanked Area

Continuous cloth screens present all available screen area to the drilling fluid to remove solids. Panels are popular because screen tears are minimized and limited to only one small area of the screening surface. The screen panels, however, remove some of the screening area that would be available with continuous cloth screens. The nonblanked area allows an evaluation of the surface area available for liquid transmission through the screen.

Common Screen Cloth Weaves

Some of the common cloth weaves available in the petroleum drilling industry are the plain square weave, the plain rectangular weave, and the modified rectangular weave. These are simple over/under weaves in both directions, which can be made from the same or different wire diameters. By making the spacing between the wires the same in both directions, a square weave is created. By making the spacing in one direction longer than the spacing in the other direction, a rectangular weave is made. Plain square and rectangular weaves are often referred to by the
number of wires (the same as openings) in each direction per linear inch. This is the mesh count. Mesh is determined by starting at one wire center and counting the number of openings along the screen grid to another wire center 1 linear inch away. For example, an 8 mesh screen has 8 openings per inch in two directions at right angles to each other. When counting mesh, a magnifying glass scale designed for the purpose is helpful.
Use of a single number for screen description implies square mesh. For example, ‘‘20 mesh’’ is usually understood to describe a screen having 20 openings per inch in either direction along the screen grid. Oblong mesh screens are generally labeled with two numbers. A ‘‘6020 mesh,’’ for example, is usually understood to have 60 openings per inch in one direction and 20 openings per inch in the perpendicular direction.
Referring to a 6020 mesh screen as an ‘‘oblong 80 mesh’’ is confusing and inaccurate. The actual separation that a screen makes is largely determined by the size of the openings in the screen. The opening size of a square mesh screen is the distance between wires measured along the screen grid, expressed in either fractions of an inch or microns. Screen opening size is most often stated in microns. One inch equals 25,400 microns. Specifying the mesh count does not specify the opening size! This is because both the number of wires per inch and the size of the wires determine the opening size. If the mesh count and wire diameter are known,the opening size can be calculated as follows:
D =25,400{(1/n)-d}
. D=opening size, in microns
. n=mesh count, in number of wires per inch (1 per inch)
. d=wire diameter, in inches
The preceding equation indicates that screens of the same mesh count may have different-size openings depending on the diameter of the wire used to weave the screen cloth. Smaller-diameter wire results in larger screen openings, and larger particles will pass through the screen. Such a screen will pass more drilling fluid than an equivalent mesh screen made of larger-diameter wires.
A specialty weave screen is available that consists of large-diameter wires in the long direction and multiple bunches of fine wires in the narrow direction. The long, narrow openings provide low flow resistance and remove spherical and chunky solids.
Layered screens were introduced into the industry in the late seventies. Layered screens are often chosen because they provide a high liquid throughput and a resistance to blinding by drilled solids lodging in the openings. A layered screen is the result of two or more wire cloths overlaying each other. Square and rectangular cloths can be layered.
Reducing the diameter of the wires increases liquid throughput. A large assortment of opening sizes and shapes are produced by the multiple screen layers and the particular screen wire diameter. Layered screens have a wide variety of opening shapes and sizes, and therefore a wide variety of sizes of particles pass through the screen.
In 1993 a three-dimensional surface screen was introduced into the industry. This screen surface is corrugated and supported by a rigid frame for use primarily on linear motion shale shakers. As drilling fluid flows down these screens, the solids are moved along in the valleys, and the vertical surfaces provide additional area for drilling fluid to pass. This increases the fluid capacity of a particular mesh size when compared with a flat surface screen.
In summary, specifying the mesh count of a screen does not indicate screen separation performance, since screen opening size, not mesh count, determines the sizes of particles separated by the screen. Because there are almost an infinite number of mesh counts and wire diameters, screen manufacturers have simplified the selection by offering several standard types of cloth series, such as MG, tensile bolting cloth (TBC), and extra-fine wire cloth (XF), as shown in Table 7.4. Notice in this table that an MG 80 cloth has an opening size of 181 microns, whereas
a TBC 80 has an opening size of 222 microns. The MG 80 cloth has a smaller opening size than the TBC 80 because the MG cloth’s heavier wires take up some of its opening space. As a result, an MG 80 cloth can remove smaller solids than a TBC 80. Furthermore, as a result of the larger wires, the MG 80 cloth will be more resistant to abrasion and will last longer. The major drawback of the MG 80 compared with the TBC 80 is that it will allow less than half the flow rate. That is (see Table 7.5), the screen conductance (ability to transmit fluid) for the
TBC 80 is 7.04 kilodarcy/mm, whereas for the MG 80 it is 2.91 kilodarcy/ mm. Similar comparisons can be made between the separation/fluid conductance of the TBCs relative to the XF cloths. For instance, a single layer of XF 180 screen cloth has almost the same opening size as a single layer of TBC 165. The XF 180 screen could pass 72% more flow!
The screen life of the XF 180 will most likely be shorter than the TBC 165, since the wire diameter of the XF 180 is 30.5 microns and that of the TBC 165 is 48.3 microns. Also, the larger openings would remove fewer drilled solids even though they would pass a larger quantity of fluid.
The National Bureau of Standards has a sieve series that is often used to describe screen opening sizes (Table 7.4). The opening size of this test series plots on uniform increments on semilog paper, making it ideal for use in plotting particle size distributions. Shaker screens used in the industry may be assigned an equivalent National Bureau of Standards sieve mesh count according to their opening sizes as shown in Table 7.4.
From the discussion above, it should be abundantly clear that mesh count alone does not specify the screen opening size. As a result, if mesh count is used, it must be accompanied by a designation of wire diameter, such asMG(mesh count) þ mesh count, TBC þ mesh count, or National Bureau of Standards Test Sieves equivalent mesh count. One other complicating factor enters with shale shaker screens: Layered screens do not have uniform opening sizes in either direction of the screen.
This is the reason that the API has developed a procedure to identify screens. Just as opening size has been used to measure separation performance, the percentage of open area of a single-layered screen has been used to indicate liquid throughput. The percentage of open area, or the portion.

Table 7.4


Tensile-Bolting-Cloth Shaker Screen Characteristics

d=16:84% of particles this size will pass through the screen;
d=50:50% of particles this size will pass through the screen;
d=84:16% of particles this size will pass through the screen.

of screen surface not block by wire, is calulated as follows:
P=percentage of open area
O=length of opening in one direction along the screen grid (inches)
o=length of opening along screen grid perpendicular to the O direction (inches)
D=diameter of wire perpendicular to the O direction (inches)
d=diameter of wire perpendicular to the o direction (inches)

Although open area can be used to indicate the ability of a screen to transmit fluid, a better measure of the ability of a screen to pass fluid is the conductance (or equivalent permeability of the screen cloth). Conductance takes into account both the openings and the drag of the fluid on the wires. (Conductance is discussed later in this text.) For years there was confusion in screen designations. Mesh count and percentage of open area simply did not adequately quantify screen cloth performance. Deceptive marketing practices were common. Furthermore,
with the advent of the layered cloths, which have a range of hole sizes, there simply were no standards against which to compare screens.