Optimize Shaker Screen For Drilling-Fluid Solids Control

Using shaker screen as the primary or only device for removing drill solids from the drilling fluid. It is necessary to optimize both filtration efficiency as well as the screen life to hinder drilled solids entering the drilling fluid. Optimum shaker screen can be obtained by using the understanding on how damage of the filtration cloth arises, on how to reduce it and finally on how the particles in the circulation system influence the general picture of the drilling process. When this knowledge is accepted in the industry, established and implemented in the drilling organisation; it is possible to maintain proper drilling.

Screen opening and mesh sizes

A shaker screen typically may consist of a single metal cloth or be constructed as a series of superpositioned cloths. In some cases these cloths are tensioned in all directions while being melted onto a frame, while in other cases the cloths are melted onto frames without being tensioned. Some screens are tensioned onto the shakers directly without being attached to frames.

The screen cloths are woven with warp wires running along the cloth and weft wires running across the cloth as it is woven. The warp and weft wires can be equal or different, giving a large variation in possible screen cloth designs.

In Fig. 1, it is shown schematically the definitions of the terms aperture width, w, which is the length of the open area, the pitch, p, which is the length from wire centre to next wire centre and last the obvious wire thickness. An example on varying the pitch differently on the warp and weft wires to produce square and oblong apertures is shown in Fig. 2.

Fig. 1. Screen cloth definitions

Traditionally the petroleum industry has used the Mesh number to designate the screens and screen openings. The Mesh number is the number of apertures per English inch. Therefore, this number does not reflect the aperture width. This is clearly shown in the example shown in Fig. 3 where on the left side it is illustrated how the aperture width changes with varying the wire thickness. Similarly it is shown on the right side of Fig. 3 how the flow area is affected if the aperture width and Mesh number is kept constant and the wire thickness is varied.

Fig.2. Example of screen cloth square aperture (left) and oblong aperture (right)

The example shown in Fig. 3 is for a relatively coarse screen, applicable mostly for top screens on double deck shakers. The similar phenomenon on 200 Mesh cloths is shown in Fig. 4. It is shown that the aperture width changes
from the theoretical maximum of 127 micron to smaller values when realistic wire thicknesses are applied. The common belief is that the aperture of a 200 Mesh cloth is around 75 micron. A cut point of 75 micron for a 200 Mesh screen, however, is seldom found.

Figure 3. Example of different aperture widths on a 10 Mesh screen cloth as function of the wire thickness (left) and variations in the Mesh number by varying the wire thickness for a constant aperture opening (right)

For multilayer screens or screens with oblong apertures application of the Mesh concept is even more difficult. There is no simple connection between cloth apertures and cut points. There are several reasons for the lack of relationship between cut points and mesh size. The different cloths are superpositioned in a manual operation, frequently leading to different appearance of wires if looked upon from above. There are no ways other than true measurements to manage the effect of the tortuous flow paths through the complete screen. Discussions about the effect of screen cloth configuration upon cut point are considered outside the scope of this paper and will not be addressed further.

Fig. 4. Aperture width for 200 Mesh cloth as function of the wire thickness Wire thickness

Flow conductance is strongly dependent on the aperture opening. A first order approximation is that the conductance is directly proportional to the area. The solid line in Fig. 5 show how the relative conductance changes with wire thickness for a 200 Mesh screen cloth. This relative conductance is the flow area of the cloth given the wire
thickness divided by the flow area with an aperture of 75 micron. Conductance is of course not directly related to the
flow area. However, its dependency with geometrical factors is dominated by the dependency on drilling fluid viscosity. Therefore, a more accurate treatment here would be of less use. The flow through screens and screen cloths are controlled by the drilling fluid viscosity and even more, by the extensional viscosity of the drilling fluid. The latter is the reason for the fact that conductance can be significantly different for water and oil based drilling fluids.

Fig. 5. Solid line is approximate relative conductance of 200 Mesh
screen cloth as function of wire diameter. Stippled line is an approximation to relative strength of wire. Unity represents 53
micron wire with aperture width equal to 75 micron.

A first order approximation of the wear strength of the cloth is also shown in Fig. 5. The very crude estimate is based on the strength being proportional to the wire cross sectional area. Shown in the figure is the cross sectional area of the wire divided by the wire thickness giving an aperture of 75 micron for a 200 Mesh screen cloth.

Screen suppliers have for years aimed to produce screens with as good conductivity as possible. From the data shown in Fig. 5, it is shown that doubling the conductivity from that of the 75 micron aperture width leads to using a wire thickness with a strength of only ~15% of the wire strength of the 75 micron aperture width wire. Currently, most shale shakers are good enough to handle large flow rates even though the conductance of the screens is not optimized. Therefore, selection of a screen cloth with too thin a wire should no longer be necessary.

Screen wear

The normal way to look at screen wear is related to the function of friction between the drill solids and the screen
cloth in the separation process. Traditionally it has been anticipated that the wear develops from the topside of the
threads in the cloth and in some cases this is true. However, most often the main wear is a result of the friction between the wires of different cloths of multilayer screens. The wear is generated by the weight of the solid material in the drilling fluid or by drilling fluid itself pressing the upper cloth down onto the coarser backing cloths. Friction between the different cloths is then generated because of the relative motion between the cloths and due to the strain of the upper cloth giving a relative motion between the cloths. Most of the wear is thus taking place on the finest threads in the upper cloth or middle cloth where the wear is acting from underneath.

Single layer cloths are often used as scalping screens, meaning screens for the upper deck on multi-deck shakers. In this case the wear is truly a function of the friction between the drill solids and the threads in the cloth. An example of wear experienced from a field operation is shown in Fig. 6.

Fig. 6 Example of wear on a single layer scalping screen.

The wear on the single layer screens are results of the impacts from cuttings particles hitting the screen as well as from the continuous bending action of the screen cloth wires due to the shaker vibration. Furthermore, there is wear arising from the scratching of the cloth by the movement of the particles along the screen as shown in Fig. 6. In the large magnification in the bottom right of this figure it is seen that the threads are flat on the top where the solids have travelled past.

Double and triple layer screens are often used as primary screens; i.e. the screens used for the final filtration of the drilling fluid at the bottom deck of the shakers. Part of the wear is a function of the friction between the cuttings particles and the cloth as being the case for single layer screens. However, the major contribution for the wear may no longer be due to this friction. Dissimilar thickness of the threads in the filtration cloths and the coarser backing cloths cause different stiffness of the cloths as indicated in Fig. 7 and Fig. 8. Therefore, the load of the cuttings onto the fine upper cloth presses the upper cloth down onto the lower and coarser cloths in the screen. These cloths do not deflect equally with the upper cloth, leading to friction forces between the different layers. Due to this, the wear arises first from underneath onto the middle and upper layers.

Fig. 7. Cut of a triple layer shaker screen illustrate the different
wear on the top and middle cloth. The large arrow symbolizes the
direction of gravity forces. The three arrows to the left illustrates
the difference in strain during a load of the top (T), middle (M) and
backing (C) cloths in this screen.

Fig. 8. A section of a triple layer shaker screen: the fine top
(bluish green), middle (brown) and coarser backing cloth (grey),
where the wear from each point of intersection to the coarser
backing cloth puncture the finer over cloth.

On double layer screens the wear will first be observed on the top cloth. On triple layer screens the wear may appear in the middle cloth first. The junction points of the warp and weft wires on the coarse backing cloth often appear like spikes where the first wear first will arise due to the direct contact with the finer middle cloth. These spikes are illustrated as the top points of the bottom wire in Fig. 7. As illustrated in Fig. 9, the wear appears first in the middle of the different cells. Furthermore, it also illustrates that the wear may be more severe in the middle cloth than in the top cloth.

Fig. 9. Illustration of wear details from the middle cloth (left), and
from the top cloth (right). The wear appear in the centre of the
cells.

An example of a cell where the wear has removed most of the top and middle cloth is shown in Fig. 10. The backing cloth is the cloth that is non-damaged throughout the complete cell. Investigating the lower right part of the picture, it is possible to see the top cloth as the light grey cloth on the left side and the middle cloth as the somewhat darker gray cloth on the middle and right side. This part of Fig 10 is magnified in Fig. 11 and the top and middle cloth is therefore more distinct in this picture. It can be seen that the spikes from the bottom cloth wire junctions have penetrated the middle cloth. The top cloth is sufficiently transparent to indicate that the middle cloth is penetrated also in the area where the top cloth to some degree is intact.

Fig. 10. A field example of nearly complete wear on the top and
middle cloths of a three layer screen.

Especially when drilling through sand sections there is a possibility that sand may be entrapped between the top cloth and the under laying cloths during a short or longer period. This sand particle will contribute to wear in two ways. First, the sand particle may hammer onto the two cloths and thereby create wear. Second, the sand with sharp edges may be sandwiched between the two upper cloths increasing the friction between the cloths and thereby increase the wear from inside of the screen.

Fig. 11. A field example of wear on the top and middle cloths of a
three layer screen. Magnification of lower right part of Fig. 10.

Field experience using triple layer shaker screen prefabricated onto frames divided up in smaller cells indicate that the wear very often starts in the centre of the cells. As discussed in previous paragraphs, the wear on over loaded screens evolves from below the fine filtration cloth. For this reason it can be difficult to determine how far the wear has come until suddenly the whole filtration cloths wires tears off.

If a fine mesh screen with damages from wear, like holes, is allowed to be used for a long period it will on longer term
act like being a significantly coarser screen. Therefore, if there is a possibility that the fine mesh screen may be used a “long” time without being changed, there is generally better to use a slightly coarser, but stronger screen.

Field experience

Based on experience from the Norwegian part of the North-Sea, it has for a long time been practiced running coarse
scalping screen over very fine primary screen. Often 17 ½” section have been drilled with a screen configuration of 10 Mesh scalping screen over lower deck primary screens ranging from 200 Mesh to 300 Mesh. Both rectangular and square cloths have been used. In such cases the shaker screen consumption has been extremely high. Only 1.2 m3 drilled formations has been removed per used screen.

The overall shale shaker screen consumption, based on information from the drilling fluid and shaker screen suppliers and supported by internal data, show that an average of 2.7m³ drilled formations has been removed per used screen. This is with a “trash limit” of 20% damaged cloth. Damaged meaning here damaged or repaired with plugs or sealing compound.

To reduce the high wear and tear on the primary screen, it is necessary to reduce the weight load from the drilled solids before it reaches the fine filtration cloth. The scalping deck should be used to reduce the weight load by removing most of the formation on this deck. Therefore, this deck should generally be used with much finer screens than 10 Mesh. When performed correctly it is possible to reduce the wear on the primary screens with roughly 90%.

As described earlier the mesh definition is the number of wires per 1 inch of a cloth. Therefore the cut-point will be influenced of the wire thickness and do not follow the same scale as the different mesh sizes do. As shown in Fig. 12, the open areas for each mesh size do not follow the same scale as the Mesh description does. Using Mesh as an exact definition of a filtration quality for a cloth is not sufficient and can be confusing for the daily handling on the rigs. Cut-point and the particle sizes where 16% and 84 % of the particles go through the screen are in reality the parameters necessary to optimize solids control.

Fig. 12. Shaker scalping screen: Visual presentation for the
proportion between mesh sizes vs. open areas, measured in
micron, for square cloth.

Tables 1 and 2 show examples of cut-points and mask area, respectively. A reduction in cut-point and the affiliated
reduction in the area of one mask for the change from 10 Mesh to 60 Mesh screen give a reduction in particle size of 1/7.8 and a reduction in area of 1/60.2. Note that this number can change differently if another type of screen had been evaluated in the examples.

Table 1. Example of cut-point related to Mesh number. Each
column show size of cut-point for each Mesh number compared
to the cut-point of the upper Mesh size in each column.

Table 2. Example of area of one mask as function of Mesh
number for the same cloths as described in Table £01. Area is
given in 1000 square micron. Each column show size of area for
each Mesh number compared to the area of the upper Mesh size
in each column.

As described earlier, the weight of drilled solid particles hammering on the primary filtration cloth determine the rate of wear. The weight of single particles passing through examples of scalping screens and the ratio between these are shown in Table 3. A comparison of data for the 10 and 60 Mesh screens, show that the mass of one particle is reduced to less than 1/450 by changing from 10 Mesh to 60 Mesh on the scalping screens. This change is illustrated in Fig. 13 where the size of two spheres, one having the maximum size for a 10 Mesh mask and the other have the maximum size for a 60 Mesh size are illustrated.

Table 3. Example of relative weight of particle flowing through the
screen as function of Mesh number for the same cloths as
described in Table £01. Each column show particle weight for
each Mesh number compared to the particle weight of the upper
Mesh size in each column.

With this knowledge in place, field experiences from several drilled sections where the shaker screen consumption have been reduced and 680 m³ drilled formation has been removed for each used shaker screen. This is 250 times more material being removed per screen before they are worn out.

Fig. 13. Visual presentation of volume and weight of one particles
passing thru one mask in the filtration cloth and the proportion
between 10 and 60 mesh.

During the drilling operation, the rig personnel measure manually, the size of punctured area in the cloth with an
“Open-Area” template. The average open or punctured area in the cloth for each drilled m3 formation was close to 1.25cm². With a “trash limit” for the shaker screen of 20% damaged area, this will in this case be approximately 850 cm2 filtration cloth of open/punctured area. This method does not measure the repaired screen area.

As long as the measuring of the “Open-Area” is another way of measuring screen wear, these numbers cannot be
directly compared to the historical numbers (2.7m³/screen). However, it clearly illustrates the potential that lies within optimizing running of solids control equipment.

Understanding the mechanisms of wear have changed the operating procedures of the solids control system and thereby reduced the screen consumption simultaneously with improving the quality of the drilling fluid. In this case the rig personnel repaired the shaker screen continuously as tendency of wear arose. In the overall picture this had also another positive effect for the rig personnel. The work load for operating the solids control equipment was reduced because most of the drilled formation was separated out on the first circulation. This also lead to positive effects on the whole drilling operation by reducing the equivalent circulating density (ECD), pump pressures and lowering the fluid consumption compared to similar sections drilled.

A final comment which has to be added to the discussion in this paper is that finer cloths on the scalping deck is not the sole solution for obtaining optimum drilling fluid solids control in the different drilling operations. In some cases it is desirable to have a more continuous particle size distribution in the drilling fluid potentially avoiding down hole losses. It is essential, however, to have a full control of the particles in the system, which can only be achieved by optimal operation of the solids control equipment.

Conclusion

The mechanisms for wear on shaker screens are outlined. Furthermore, it is explained how double deck shakers should be operated to minimize wear on the primary screens at the same time as the solids removal from the drilling fluids are optimised. It is shown how use of this knowledge has improved field operations by increasing significantly the amount of formation material being removed from the drilling fluid in the first circulation.