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Shaker Applications
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Shaker Applications Most drilling rigs are equipped with at least one shale shaker. The purpose of a shale shaker, as with all drilled solids removal equipment, is to reduce the drilling cost. Most drilling conditions require limiting the quantity and size of drilled solids in the drilling fluid. Shale shakers remove the largest drilled solids that reach the surface, which are the ones that create many wellbore problems if they remain in the drilling fluid.
probably be processed through a 200-mesh screen on most linear motion shale shakers.
SELECTION OF SHAKER SCREENS Proprietary computer programs are available that reportedly allow estimating screen sizes used on specific shale shakers. Most of these computer p r o g r a m s are verified with data obtained from laboratory-prepared drilling fluid with limited property variation. Different drilling fluid ingredients can reduce the capacity of a shaker system and, therefore, predicting screen-mesh sizes that will handle certain flow rates is very difficult. For example, a drilling fluid containing starch is difficult to screen because starch, acting as a good filtration control additive, tends to plug fine mesh screens. Drilling fluids with high gel strengths are also difficult to screen through fine meshes. For these reasons, screen selection for various shale shakers is primarily a trial and error evaluation. The best advice is to contact the manufacturer for recommendations for various geographic areas.
SELECTION OF SHALE SHAKERS The first consideration in selecting a shale shaker is to decide w h e t h e r or not a gumbo slide, or g u m b o - r e m o v a l device, will be needed. This is often necessary when drilling recent sediments. A scalping s h a k e r must be considered next. Scalping shakers are usually needed when large quantities of drilled solids or gumbo reach the surface. Usually, long intervals of 17 ½-in.-diameter holes, with flow rates above 1000 gpm, require scalping shakers in front of fine mesh screens (see Chapter 7). The final consideration is to decide on the type and quantity of m a i n s h a k e r s necessary for processing all the drilling fluid. The goal should be to sieve an unweighted drilling fluid through the finest mesh screen possible. For weighted drilling fluids, the goal should be to screen all drilling fluid through 200-mesh screens (finer screens may remove too much weighting material). Many factors affect the liquid capacity of specific shale shaker and screen combinations. While no publication accounts for all of these variables, some manufacturers publish curves relating the fluid flow capacity to screen sizes as a function of one or two parameters. These curves are usually generated without a comprehensive testing program. Many manufacturers use generalizations to gauge the number of shakers needed based on the maximum flow rate anticipated. For example, flow rates b e t w e e n 300 gpm and 500 gpm can
Cost of Removing Drilled Solids Few wells can be drilled without removing drilled solids (see Chapter 1). Even for 3,000- to 4,000foot wells, problems created by drilled solids, such as lost circulation, stuck pipe, or well control, are more than enough reason to properly process the drilling fluid. In expensive drilling operations, the proper use of solids removal equipment will significantly reduce costs. Although drilled solids can be maintained by simply diluting the drilling fluid to control their acceptable levels or concentrations of drilled solids, the expense and impracticality of this approach are evident using the following example. A 12 ¼in.-diameter hole, 1,000 feet deep, will contain approximately 146 bbls of solids. If these solids are
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to be reduced to a 6% volume target concentration, they must be blended into 2400 bbl ( -~--~ 146 ) of slurry. To create the 2400 bbl of slurry, 2256 bbl of clean drilling fluid must be added to the 146 bbl of solids [146 bbl/2256 + 146 bbl)] = 6% volume. Not only would the cost of the clean drilling fluid be prohibitive, but most drilling rigs do not have the necessary surface volume to build 2256 bbl of clean drilling fluid for every 1,000 feet of hole drilled. (See Chapter 8 for a more complete discussion of dilution calculations.) As demonstrated, it is important to remove as m a n y drilled solids as possible with the shale shaker. Shakers are an important component of this process but they are only one portion of a complete drilled solids removal system. Careful attention to details is the key to developing the most efficient drilled solids removal operation. Complete processing will decrease the cost of accumulating excess drilling fluid, thereby contributing to the ultimate goal of reducing the costs associated with oil well drilling (Chapter 8).
Specific Factors Specific factors that should be considered when designing the shale shaker system include: flow rate, fluid type, rig space, configuration/power, available elevation, and discharge dryness (restrictions). Most programs extrapolate laboratory-generated performance curves to predict field performance. Unfortunately, l a b o r a t o r y - m a n u f a c t u r e d drilling fluid does not duplicate properties of drilling fluid that has been used in a well. High shear rates through drill bit nozzles at elevated temperatures produce colloidal-size particles that are not duplicated in surface-processed drilling fluid.
Flow rate. The flow rate that a particular s h a k e r / screen combination can handle greatly depends on the flow properties of the drilling fluid. The lower the values of plastic viscosity, yield point, gel strength, and mud weight, the finer the mesh size that can be used on a shale shaker. The conductance of the shaker screen provides a guide for the fluid capability but does not reveal how the screen will actually perform. Screens with the same conductance may not be able to handle the same flow rate if used on different shale shakers. Shaker screen selection programs have been developed to predict the quantity of solids that can be removed from a drilling fluid by various shaker screens on specific commercial shakers. Many programs start by assuming that the flow rate of drilled solids reaching the surface is identical with the generation rate of the drilled solids. Unfortunately,
many drilled solids are stored in the well-bore and do not reach the surface in the order in which they are drilled. Frequently, in long stretches of open hole, as many drilled solids enter the drilling fluid from the sides of the wellbore as are generated by a drill bit. One proposed relationship shows that the maximum flow rate that can be handled by a shaker (Q), is inversely proportional to the product of the plastic viscosity (PV), mud weight (MW), and proportional to the screen conductance (K). This relationship answers the question: If a linear motion shale shaker is handling 1250 gpm of a 10.3 ppg drilling fluid, with a PV of 10 cp on a 120-square MG mesh screen, what flow rate could be handled on a 200-square MG mesh screen if the mud weight is increased to 14.0 ppg and the PV becomes 26 cp? (Kx)(PVl)(MW1)Q1 QE = (K1)(PV)(MW2) (1.24 kd / mm)(10 cp)(10.3 ppg) x 1250 gpm Q2 = (0.68 kd / mm)(26 cp)(14.0 ppg) Q2 - 645 gpm The problem with this equation is that it fails to account for other rheological variables. For example, if the gel strength of the 10.3 ppg drilling fluid significantly increased, the shaker could no longer handle the fluid. To further demonstrate, take a shaker that handles 750 gpm of an 11.0 ppg drilling fluid with a certain plastic viscosity (PV). If the yield point is significantly increased, or additives such as PHPA or a high concentration of starch are added to this fluid, the shaker capacity might be only 350 gpm. In both these cases, the PV would change very little but there would be a significant effect on the screening capability. Therefore, the above equation should only be used to predict the flow rate if no other properties in the drilling fluid change other than the mud weight and plastic viscosity. The equation should be used with caution.
Rig configuration.
On some drilling rigs, the derrick rig floor is not high enough to allow some shale shakers to be used because the flow line is not high enough. Whichever shaker is used, consideration must be given to providing sufficient safe p o w e r to the shaker motors. It is best to check with the manufacturer concerning the electrical requirements for individual shakers.
Discharge dryness. In some areas, drilled solids and drilling fluid cannot be discarded at the rig location. This applies to both land and offshore
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rigs. The cost of handling discarded material may require drying the discard. The fine mesh screens discharge much wetter solids than those discarded from very coarse screens (see Chapter 13).
CASCADE SYSTEMS The first cascade system was introduced to the drilling industry in the mid-1970s. A scalper shaker received fluid from the flow line and removed gumbo, or large drilled solids, before the fluid passed through the main shaker using a fine mesh screen (at this time, 80- to 120-mesh screens were the practical limits). The first unit combined a single-deck, elliptical motion shaker mounted directly over a double-deck, circular motion shaker (Figure 4-1). This combination was especially successful offshore where space is limited. One advantage of multiple-deck shale shakers is their ability to reduce solids loading on the lower, fine mesh screen deck. This increases both shaker capacity and screen life. However, capacity may still be exceeded under many drilling conditions. The screen mesh and, thus, the size solids returned to the active system, is often increased to prevent loss of whole mud over the end of the shaker screens. Processing drilling fluid through shale shaker screens, centrifugal pumps, hydrocyclones, and drill bit nozzles can cause degradation of solids and aggravated problems associated with fine solids in the drilling fluid. To remove drilled solids as soon as possible, additional shakers are installed at the flow linemsometimes, as many as six to ten parallel shakersmso that the finest mesh screen may be used. With the finer mesh screens and additional shakers in place, downstream equipment is often erroneously eliminated. It is important to remember that the improved shale shaker
FIGURE 4-1
still remains only one component of a complete drilled solids removal system (see Chapter 7). A system of cascading shale shakers is designed to use one set of screens (or shakers) to scalp large solids and gumbo from the drilling fluid, and another set of lower screens (or shakers) to receive the upper shaker underflow fluid for removal of fine solids. This combination increases the solids removal efficiency of high performance shakers, especially during fast, top-hole drilling or in gumbo-producing formations. The cascade system is used where the solids' loading exceeds the capacity of the fine mesh screen. The advantages of this cascade arrangement include: 1. 2. 3. 4. 5.
Higher overall solids loading on the system Reduced solids loading on fine mesh screens Finer screen separations Longer screen life Lower fluid well costs
There are three basic designs of cascade shaker systems: 1. The separate unit concept 2. The integral unit with multiple vibratory motions 3. The integral unit with a single vibratory motion The choice of which design to use depends on many factors, including space limitations, performance objectives, and overall cost.
Separate Unit The separate unit system mounts usable rig shakers (elliptical or circular motion) on stands above newly installed linear motion shakers (Figure 4-2). Fluid from the rig shakers (or scalping shakers) is routed to the possum belly (back tank)
FIGURE 4-2
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of a linear motion shaker. Line size and potential head losses must be considered with this arrangement to avoid overflow and loss of drilling fluid. This design may reduce overall cost by using existing equipment. This concept, where space is available, has the a d v a n t a g e s of highly visible screening surfaces and ease of access for repairs. The disadvantage of separate individual shakers is the space needed for mounting the shakers and the extra piping needed to tie the scalping shakers to the flow-line shakers.
The Integral Unit with Multiple Vibratory Motions This design type combines the two units of the separate unit system into a single, integral unit mounted on a single skid. Usually, a circular motion s h a k e r is m o u n t e d above a linear motion shaker on a common skid (Figure 4-3). The underflow (liquid mud) from the top scalping shaker is directed through a flowback, sheetmetal pan onto the back of the bottom flow-line shaker. The primary advantages of this design are reduced installation costs and space requirements. The internal backflow pan eliminates the manifold and piping needed for the two separate units. By mounting the units above one another, normally the flow line entry height and the weir height of the mud flow on the scalping shaker, is lower than that of separate units.
The disadvantage of this design is the limited visibility of the upper and lower screens, due to the height of the scalper shaker and the minimum clearance b e t w e e n the upper and lower shakers. On some models, it is also difficult to perform routine maintenance and replace vibratory drive components.
The Integral Unit with a Single Vibratory Motion This design consists of an integral unit with a single vibratory motion, as shown in Figure 4-4. Typically, the units use linear motion shakers and incorporate a scalping screen in the upper part of the basket. The lower bed consists of a fine screen mesh, flow-line shaker unit and the upper scalper section is designed with a smaller-width bed, using coarse mesh screens. Most of the available single vibratory motion shakers use hook striptype screens for the upper scalping deck and the lower section deck designs vary from hook strip to pretension framed screens. Compared to the cascade shaker units, this design significantly lowers the weir height of the mud inlet to the upper screening area. Conversely, as with the cascade shaker units, the visibility and access to the fine screen deck of the single vibratory unit is limited by the slope of the upper scalping deck.
Summary Cascading systems use one set of shakers to scalp large solids a n d / o r gumbo from the drilling fluid and another set of shakers to remove fine
FIGURE 4-3
FIGURE 4-4
SHAKER APPLICATIONS 1 1 3
solids. Their application is primarily during fast, tophole drilling, or in gumbo formations. This system was designed to handle high solids loading, which occurs when rapidly drilling large-diameter holes or when gumbo arrives at the surface. The introduction of linear motion shale shakers has allowed development of fine screen cascade systems capable of 200-mesh separations at the flow line. Scalping screens, as coarse as 10 mesh, may be used to avoid dispersing solids before arriving at the linear motion shaker. This is particularly important in areas where high circulation rates and large amounts of drilled solids are encountered. After either the flow rate or solids-loading is reduced in deeper parts of the bore hole, the scalping shaker should be used only as an insurance device. When the linear motion shaker, with the finest mesh screen available, can handle all of the flow and solids arriving at the surface, the need for the cascade system disappears. When this occurs, the inclination may be to discontinue the use of the scalping screen unit. However, even when the fine mesh screen can process all of the fluid, screens should be maintained on the scalper shaker for use as needed. These scalping screens can be changed to a relatively medium-coarse mesh (12 or 40 mesh) and still be used to protect the finer mesh on the main shaker.
DRYER SHAKERS The dryer shaker, or dryer, is a linear motion shaker used to minimize the volume of liquid associated with drilled cuttings discharged from the main rig shakers and hydrocyclones. The liquid removed by the dryers is returned to the active system. Because of environmental efforts to reduce liquid waste disposal, dryers were introduced with c l o s e d loop mud s y s t e m s . Two methods are available to minimize liquid discharge: chemical and mechanical. (The chemical system, called a dewatering unit, is described in Chapter 13.) Linear motion shakers are used to mechanically reduce the liquid discard. These systems may be used independently or in combination. The dryer is used to deliquify drilled cuttings initially separated by another piece of solids separation equipment. These drilled solids can be the discharge from a main shaker or a bank of hydrocyclones. Dryers recover liquid discharged with solids in normal liquid-solids separation that would have been previously discarded from the mud system. This liquid contains colloidal solids and the effect on drilling fluid properties must be considered since dewatering systems are frequently needed to flocculate, coagulate, and remove these solids.
The dryer family incorporates equipment long used as independent units: the main linear motion shaker, the desander, and the desilter. They are combined in several configurations to discharge their discard across fine mesh screens of a linear motion shaker to capture the associated liquid. These units, formerly identified as mud cleaners, are mounted on the mud tanks, usually in line with the primary linear motion shaker. They can also be connected to the flow line to assist with fine screening when not being used as dryers. Their pumps obtain suction from the same c o m p a r t m e n t s as d e s a n d e r s and desilters and discharge their overflow into the proper downstream compartments. A linear motion dryer may be used to remove the excess liquid from the main shaker discharge. The flow rate across a linear motion dryer is substantially smaller than the flow rate across the main shaker. This lower flow rate permits the use of finer mesh screens and removal of the excess fluid by the linear motion dryer. This dryer is usually mounted at a lower level than other solids separation equipment to allow gravity to transport the solids. Whether using a slide or conveyor, the cuttings are deposited into a large hopper located above the screen that replaces the back tank or possum belly. As the cuttings are transported along the screen, they are again deliquified. This excess fluid, together with the fine solids that passed through the screens, is collected in a shallow tank that takes the places of a normal sump. This liquid is pumped to a catch tank, that acts as the feed for a centrifuge, or back to the active system. A dryer unit can be used to remove the excess fluid from the underflow of a bank of hydrocyclones (desanders or desilters). This arrangement resembles a mud cleaner system. When used in this configuration, the dryer unit may be used on either a weighted or unweighted mud system. The liquid recovered by the linear motion shaker under the hydrocyclones can be processed by a centrifuge as previously described. The perfection of the linear motion shaker for drilling fluid use, coupled with advanced fine screen manufacturing technology, has made these dryers very efficient. In most configurations, the dryers use the same style of screens, motors, and/ or motor/vibration combinations as other linear motion shakers. Depending on the fluid, saving previously lost liquid from the discard may be financially advantageous. The dryer discard is relatively dry and, therefore, may be removed with a backhoe and dump truck rather than a vacuum truck. It is important to remember that drilling fluid properties must be properly monitored when the
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recovered liquid is returned to the active system. Large quantities of colloidal solids may be recovered with the liquid and this could affect plastic viscosity, yield point, and gel strengths of a drilling fluid.
NON-OILFIELD DRILLING USES OF SHALE SHAKERS T r e n c h l e s s drilling is one of the fastest growing areas for shale shaker use other than drilling oil and gas wells. Trenchless drilling encompasses several areas of use: Micro-tunneling--Micro-tunneling has become
very popular in Europe and is being used more frequently in the United States. Micro tunneling is the horizontal boring of a largediameter hole (from 27 inches up to 10 feet) while simultaneously laying pipe. Typically, this technique is used in cities when laying or replacing water and sewer pipe beneath buildings and heavily traveled roads. To p r e p a r e for these operations, largediameter, vertical holes, or "caissons," are excavated allowing the drilling equipment and hydraulic rams to be set up at the desired depth. The caisson is excavated slightly below the equipment level creating a sump for the returned drilling fluid and associated drilled solids. The returns are pumped to the surface by a submersible pump to a compact solids
removal system, which typically consists of a shale shaker and mud cleaner mounted over a small tank. • River crossing--This technique facilitates run-
ning a pipeline under a river. First, a smalldiameter hole is directionally drilled under the riverbed. The pipe for the pipeline is then attached to the end of the drill string and pulled back under the river while a larger hole is back-reamed to accomodate the pipe. When laying l a r g e - d i a m e t e r pipelines, a substantial solids control system must be established with multiple shakers, desanders, desilters, and centrifuges. Additionally, the use of mud cleaners will reduce drilling fluid disposal volumes. • Road crossingmPipelines or cables often need
to be placed beneath roads. Drilling beneath a road does not require disrupting traffic or destroying the road surface. Frequently, the hole volume is small enough that solids removal equipment is not necessary. Should drilling fluid accumulation cause a problem, or in the case of large-diameter holes and wide roadbeds, a shaker or mud cleaner is used. • Fiber-optic cablesmFiber-optic cables are of-
ten required in residential or business areas where drilling fluid and drilled solids must be contained. Since these cables do not require large-diameter holes, solids control systems usually consist of only a small tank, pump package, and a small shaker.
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Shaker Applications Most drilling rigs are equipped with at least one shale shaker. The purpose of a shale shaker, as with all drilled solids removal equipment, is to reduce the drilling cost. Most drilling conditions require limiting the quantity and size of drilled solids in the drilling fluid. Shale shakers remove the largest drilled solids that reach the surface, which are the ones that create many wellbore problems if they remain in the drilling fluid.
probably be processed through a 200-mesh screen on most linear motion shale shakers.
SELECTION OF SHAKER SCREENS Proprietary computer programs are available that reportedly allow estimating screen sizes used on specific shale shakers. Most of these computer p r o g r a m s are verified with data obtained from laboratory-prepared drilling fluid with limited property variation. Different drilling fluid ingredients can reduce the capacity of a shaker system and, therefore, predicting screen-mesh sizes that will handle certain flow rates is very difficult. For example, a drilling fluid containing starch is difficult to screen because starch, acting as a good filtration control additive, tends to plug fine mesh screens. Drilling fluids with high gel strengths are also difficult to screen through fine meshes. For these reasons, screen selection for various shale shakers is primarily a trial and error evaluation. The best advice is to contact the manufacturer for recommendations for various geographic areas.
SELECTION OF SHALE SHAKERS The first consideration in selecting a shale shaker is to decide w h e t h e r or not a gumbo slide, or g u m b o - r e m o v a l device, will be needed. This is often necessary when drilling recent sediments. A scalping s h a k e r must be considered next. Scalping shakers are usually needed when large quantities of drilled solids or gumbo reach the surface. Usually, long intervals of 17 ½-in.-diameter holes, with flow rates above 1000 gpm, require scalping shakers in front of fine mesh screens (see Chapter 7). The final consideration is to decide on the type and quantity of m a i n s h a k e r s necessary for processing all the drilling fluid. The goal should be to sieve an unweighted drilling fluid through the finest mesh screen possible. For weighted drilling fluids, the goal should be to screen all drilling fluid through 200-mesh screens (finer screens may remove too much weighting material). Many factors affect the liquid capacity of specific shale shaker and screen combinations. While no publication accounts for all of these variables, some manufacturers publish curves relating the fluid flow capacity to screen sizes as a function of one or two parameters. These curves are usually generated without a comprehensive testing program. Many manufacturers use generalizations to gauge the number of shakers needed based on the maximum flow rate anticipated. For example, flow rates b e t w e e n 300 gpm and 500 gpm can
Cost of Removing Drilled Solids Few wells can be drilled without removing drilled solids (see Chapter 1). Even for 3,000- to 4,000foot wells, problems created by drilled solids, such as lost circulation, stuck pipe, or well control, are more than enough reason to properly process the drilling fluid. In expensive drilling operations, the proper use of solids removal equipment will significantly reduce costs. Although drilled solids can be maintained by simply diluting the drilling fluid to control their acceptable levels or concentrations of drilled solids, the expense and impracticality of this approach are evident using the following example. A 12 ¼in.-diameter hole, 1,000 feet deep, will contain approximately 146 bbls of solids. If these solids are
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to be reduced to a 6% volume target concentration, they must be blended into 2400 bbl ( -~--~ 146 ) of slurry. To create the 2400 bbl of slurry, 2256 bbl of clean drilling fluid must be added to the 146 bbl of solids [146 bbl/2256 + 146 bbl)] = 6% volume. Not only would the cost of the clean drilling fluid be prohibitive, but most drilling rigs do not have the necessary surface volume to build 2256 bbl of clean drilling fluid for every 1,000 feet of hole drilled. (See Chapter 8 for a more complete discussion of dilution calculations.) As demonstrated, it is important to remove as m a n y drilled solids as possible with the shale shaker. Shakers are an important component of this process but they are only one portion of a complete drilled solids removal system. Careful attention to details is the key to developing the most efficient drilled solids removal operation. Complete processing will decrease the cost of accumulating excess drilling fluid, thereby contributing to the ultimate goal of reducing the costs associated with oil well drilling (Chapter 8).
Specific Factors Specific factors that should be considered when designing the shale shaker system include: flow rate, fluid type, rig space, configuration/power, available elevation, and discharge dryness (restrictions). Most programs extrapolate laboratory-generated performance curves to predict field performance. Unfortunately, l a b o r a t o r y - m a n u f a c t u r e d drilling fluid does not duplicate properties of drilling fluid that has been used in a well. High shear rates through drill bit nozzles at elevated temperatures produce colloidal-size particles that are not duplicated in surface-processed drilling fluid.
Flow rate. The flow rate that a particular s h a k e r / screen combination can handle greatly depends on the flow properties of the drilling fluid. The lower the values of plastic viscosity, yield point, gel strength, and mud weight, the finer the mesh size that can be used on a shale shaker. The conductance of the shaker screen provides a guide for the fluid capability but does not reveal how the screen will actually perform. Screens with the same conductance may not be able to handle the same flow rate if used on different shale shakers. Shaker screen selection programs have been developed to predict the quantity of solids that can be removed from a drilling fluid by various shaker screens on specific commercial shakers. Many programs start by assuming that the flow rate of drilled solids reaching the surface is identical with the generation rate of the drilled solids. Unfortunately,
many drilled solids are stored in the well-bore and do not reach the surface in the order in which they are drilled. Frequently, in long stretches of open hole, as many drilled solids enter the drilling fluid from the sides of the wellbore as are generated by a drill bit. One proposed relationship shows that the maximum flow rate that can be handled by a shaker (Q), is inversely proportional to the product of the plastic viscosity (PV), mud weight (MW), and proportional to the screen conductance (K). This relationship answers the question: If a linear motion shale shaker is handling 1250 gpm of a 10.3 ppg drilling fluid, with a PV of 10 cp on a 120-square MG mesh screen, what flow rate could be handled on a 200-square MG mesh screen if the mud weight is increased to 14.0 ppg and the PV becomes 26 cp? (Kx)(PVl)(MW1)Q1 QE = (K1)(PV)(MW2) (1.24 kd / mm)(10 cp)(10.3 ppg) x 1250 gpm Q2 = (0.68 kd / mm)(26 cp)(14.0 ppg) Q2 - 645 gpm The problem with this equation is that it fails to account for other rheological variables. For example, if the gel strength of the 10.3 ppg drilling fluid significantly increased, the shaker could no longer handle the fluid. To further demonstrate, take a shaker that handles 750 gpm of an 11.0 ppg drilling fluid with a certain plastic viscosity (PV). If the yield point is significantly increased, or additives such as PHPA or a high concentration of starch are added to this fluid, the shaker capacity might be only 350 gpm. In both these cases, the PV would change very little but there would be a significant effect on the screening capability. Therefore, the above equation should only be used to predict the flow rate if no other properties in the drilling fluid change other than the mud weight and plastic viscosity. The equation should be used with caution.
Rig configuration.
On some drilling rigs, the derrick rig floor is not high enough to allow some shale shakers to be used because the flow line is not high enough. Whichever shaker is used, consideration must be given to providing sufficient safe p o w e r to the shaker motors. It is best to check with the manufacturer concerning the electrical requirements for individual shakers.
Discharge dryness. In some areas, drilled solids and drilling fluid cannot be discarded at the rig location. This applies to both land and offshore
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rigs. The cost of handling discarded material may require drying the discard. The fine mesh screens discharge much wetter solids than those discarded from very coarse screens (see Chapter 13).
CASCADE SYSTEMS The first cascade system was introduced to the drilling industry in the mid-1970s. A scalper shaker received fluid from the flow line and removed gumbo, or large drilled solids, before the fluid passed through the main shaker using a fine mesh screen (at this time, 80- to 120-mesh screens were the practical limits). The first unit combined a single-deck, elliptical motion shaker mounted directly over a double-deck, circular motion shaker (Figure 4-1). This combination was especially successful offshore where space is limited. One advantage of multiple-deck shale shakers is their ability to reduce solids loading on the lower, fine mesh screen deck. This increases both shaker capacity and screen life. However, capacity may still be exceeded under many drilling conditions. The screen mesh and, thus, the size solids returned to the active system, is often increased to prevent loss of whole mud over the end of the shaker screens. Processing drilling fluid through shale shaker screens, centrifugal pumps, hydrocyclones, and drill bit nozzles can cause degradation of solids and aggravated problems associated with fine solids in the drilling fluid. To remove drilled solids as soon as possible, additional shakers are installed at the flow linemsometimes, as many as six to ten parallel shakersmso that the finest mesh screen may be used. With the finer mesh screens and additional shakers in place, downstream equipment is often erroneously eliminated. It is important to remember that the improved shale shaker
FIGURE 4-1
still remains only one component of a complete drilled solids removal system (see Chapter 7). A system of cascading shale shakers is designed to use one set of screens (or shakers) to scalp large solids and gumbo from the drilling fluid, and another set of lower screens (or shakers) to receive the upper shaker underflow fluid for removal of fine solids. This combination increases the solids removal efficiency of high performance shakers, especially during fast, top-hole drilling or in gumbo-producing formations. The cascade system is used where the solids' loading exceeds the capacity of the fine mesh screen. The advantages of this cascade arrangement include: 1. 2. 3. 4. 5.
Higher overall solids loading on the system Reduced solids loading on fine mesh screens Finer screen separations Longer screen life Lower fluid well costs
There are three basic designs of cascade shaker systems: 1. The separate unit concept 2. The integral unit with multiple vibratory motions 3. The integral unit with a single vibratory motion The choice of which design to use depends on many factors, including space limitations, performance objectives, and overall cost.
Separate Unit The separate unit system mounts usable rig shakers (elliptical or circular motion) on stands above newly installed linear motion shakers (Figure 4-2). Fluid from the rig shakers (or scalping shakers) is routed to the possum belly (back tank)
FIGURE 4-2
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of a linear motion shaker. Line size and potential head losses must be considered with this arrangement to avoid overflow and loss of drilling fluid. This design may reduce overall cost by using existing equipment. This concept, where space is available, has the a d v a n t a g e s of highly visible screening surfaces and ease of access for repairs. The disadvantage of separate individual shakers is the space needed for mounting the shakers and the extra piping needed to tie the scalping shakers to the flow-line shakers.
The Integral Unit with Multiple Vibratory Motions This design type combines the two units of the separate unit system into a single, integral unit mounted on a single skid. Usually, a circular motion s h a k e r is m o u n t e d above a linear motion shaker on a common skid (Figure 4-3). The underflow (liquid mud) from the top scalping shaker is directed through a flowback, sheetmetal pan onto the back of the bottom flow-line shaker. The primary advantages of this design are reduced installation costs and space requirements. The internal backflow pan eliminates the manifold and piping needed for the two separate units. By mounting the units above one another, normally the flow line entry height and the weir height of the mud flow on the scalping shaker, is lower than that of separate units.
The disadvantage of this design is the limited visibility of the upper and lower screens, due to the height of the scalper shaker and the minimum clearance b e t w e e n the upper and lower shakers. On some models, it is also difficult to perform routine maintenance and replace vibratory drive components.
The Integral Unit with a Single Vibratory Motion This design consists of an integral unit with a single vibratory motion, as shown in Figure 4-4. Typically, the units use linear motion shakers and incorporate a scalping screen in the upper part of the basket. The lower bed consists of a fine screen mesh, flow-line shaker unit and the upper scalper section is designed with a smaller-width bed, using coarse mesh screens. Most of the available single vibratory motion shakers use hook striptype screens for the upper scalping deck and the lower section deck designs vary from hook strip to pretension framed screens. Compared to the cascade shaker units, this design significantly lowers the weir height of the mud inlet to the upper screening area. Conversely, as with the cascade shaker units, the visibility and access to the fine screen deck of the single vibratory unit is limited by the slope of the upper scalping deck.
Summary Cascading systems use one set of shakers to scalp large solids a n d / o r gumbo from the drilling fluid and another set of shakers to remove fine
FIGURE 4-3
FIGURE 4-4
SHAKER APPLICATIONS 1 1 3
solids. Their application is primarily during fast, tophole drilling, or in gumbo formations. This system was designed to handle high solids loading, which occurs when rapidly drilling large-diameter holes or when gumbo arrives at the surface. The introduction of linear motion shale shakers has allowed development of fine screen cascade systems capable of 200-mesh separations at the flow line. Scalping screens, as coarse as 10 mesh, may be used to avoid dispersing solids before arriving at the linear motion shaker. This is particularly important in areas where high circulation rates and large amounts of drilled solids are encountered. After either the flow rate or solids-loading is reduced in deeper parts of the bore hole, the scalping shaker should be used only as an insurance device. When the linear motion shaker, with the finest mesh screen available, can handle all of the flow and solids arriving at the surface, the need for the cascade system disappears. When this occurs, the inclination may be to discontinue the use of the scalping screen unit. However, even when the fine mesh screen can process all of the fluid, screens should be maintained on the scalper shaker for use as needed. These scalping screens can be changed to a relatively medium-coarse mesh (12 or 40 mesh) and still be used to protect the finer mesh on the main shaker.
DRYER SHAKERS The dryer shaker, or dryer, is a linear motion shaker used to minimize the volume of liquid associated with drilled cuttings discharged from the main rig shakers and hydrocyclones. The liquid removed by the dryers is returned to the active system. Because of environmental efforts to reduce liquid waste disposal, dryers were introduced with c l o s e d loop mud s y s t e m s . Two methods are available to minimize liquid discharge: chemical and mechanical. (The chemical system, called a dewatering unit, is described in Chapter 13.) Linear motion shakers are used to mechanically reduce the liquid discard. These systems may be used independently or in combination. The dryer is used to deliquify drilled cuttings initially separated by another piece of solids separation equipment. These drilled solids can be the discharge from a main shaker or a bank of hydrocyclones. Dryers recover liquid discharged with solids in normal liquid-solids separation that would have been previously discarded from the mud system. This liquid contains colloidal solids and the effect on drilling fluid properties must be considered since dewatering systems are frequently needed to flocculate, coagulate, and remove these solids.
The dryer family incorporates equipment long used as independent units: the main linear motion shaker, the desander, and the desilter. They are combined in several configurations to discharge their discard across fine mesh screens of a linear motion shaker to capture the associated liquid. These units, formerly identified as mud cleaners, are mounted on the mud tanks, usually in line with the primary linear motion shaker. They can also be connected to the flow line to assist with fine screening when not being used as dryers. Their pumps obtain suction from the same c o m p a r t m e n t s as d e s a n d e r s and desilters and discharge their overflow into the proper downstream compartments. A linear motion dryer may be used to remove the excess liquid from the main shaker discharge. The flow rate across a linear motion dryer is substantially smaller than the flow rate across the main shaker. This lower flow rate permits the use of finer mesh screens and removal of the excess fluid by the linear motion dryer. This dryer is usually mounted at a lower level than other solids separation equipment to allow gravity to transport the solids. Whether using a slide or conveyor, the cuttings are deposited into a large hopper located above the screen that replaces the back tank or possum belly. As the cuttings are transported along the screen, they are again deliquified. This excess fluid, together with the fine solids that passed through the screens, is collected in a shallow tank that takes the places of a normal sump. This liquid is pumped to a catch tank, that acts as the feed for a centrifuge, or back to the active system. A dryer unit can be used to remove the excess fluid from the underflow of a bank of hydrocyclones (desanders or desilters). This arrangement resembles a mud cleaner system. When used in this configuration, the dryer unit may be used on either a weighted or unweighted mud system. The liquid recovered by the linear motion shaker under the hydrocyclones can be processed by a centrifuge as previously described. The perfection of the linear motion shaker for drilling fluid use, coupled with advanced fine screen manufacturing technology, has made these dryers very efficient. In most configurations, the dryers use the same style of screens, motors, and/ or motor/vibration combinations as other linear motion shakers. Depending on the fluid, saving previously lost liquid from the discard may be financially advantageous. The dryer discard is relatively dry and, therefore, may be removed with a backhoe and dump truck rather than a vacuum truck. It is important to remember that drilling fluid properties must be properly monitored when the
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SHALE SHAKERS AND DRILLING FLUID SYSTEMS
recovered liquid is returned to the active system. Large quantities of colloidal solids may be recovered with the liquid and this could affect plastic viscosity, yield point, and gel strengths of a drilling fluid.
NON-OILFIELD DRILLING USES OF SHALE SHAKERS T r e n c h l e s s drilling is one of the fastest growing areas for shale shaker use other than drilling oil and gas wells. Trenchless drilling encompasses several areas of use: Micro-tunneling--Micro-tunneling has become
very popular in Europe and is being used more frequently in the United States. Micro tunneling is the horizontal boring of a largediameter hole (from 27 inches up to 10 feet) while simultaneously laying pipe. Typically, this technique is used in cities when laying or replacing water and sewer pipe beneath buildings and heavily traveled roads. To p r e p a r e for these operations, largediameter, vertical holes, or "caissons," are excavated allowing the drilling equipment and hydraulic rams to be set up at the desired depth. The caisson is excavated slightly below the equipment level creating a sump for the returned drilling fluid and associated drilled solids. The returns are pumped to the surface by a submersible pump to a compact solids
removal system, which typically consists of a shale shaker and mud cleaner mounted over a small tank. • River crossing--This technique facilitates run-
ning a pipeline under a river. First, a smalldiameter hole is directionally drilled under the riverbed. The pipe for the pipeline is then attached to the end of the drill string and pulled back under the river while a larger hole is back-reamed to accomodate the pipe. When laying l a r g e - d i a m e t e r pipelines, a substantial solids control system must be established with multiple shakers, desanders, desilters, and centrifuges. Additionally, the use of mud cleaners will reduce drilling fluid disposal volumes. • Road crossingmPipelines or cables often need
to be placed beneath roads. Drilling beneath a road does not require disrupting traffic or destroying the road surface. Frequently, the hole volume is small enough that solids removal equipment is not necessary. Should drilling fluid accumulation cause a problem, or in the case of large-diameter holes and wide roadbeds, a shaker or mud cleaner is used. • Fiber-optic cablesmFiber-optic cables are of-
ten required in residential or business areas where drilling fluid and drilled solids must be contained. Since these cables do not require large-diameter holes, solids control systems usually consist of only a small tank, pump package, and a small shaker.