Solids control is the process of controlling the buildup of undesirable solids in a mud system. The buildup of solids has undesirable effects on drilling fluid performance and the drilling process. Rheological and filtration properties become difficult to control when the concentration of drilled solids (low-gravity solids) becomes excessive. Penetration rates and bit life decrease and hole problems increase with a high concentration of drilled solids. It is estimated that over 80% of mud treatment costs are directly related to drilled solids 20 µm and smaller.
Solids-control equipment on a drilling operation is intended to be operated like a processing plant. In a perfect world, all drilled solids could be removed from a drilling fluid. In the real world, there are limitations as to the size and volume of the solids that can be efficiently removed.
Under typical drilling conditions, low-gravity solids should be maintained below 6 percent by volume.
Sources and Sizes of Solids
The two primary sources of solids (particles) are commercial solids such as barite, and non-commercial solids such as formation cuttings. Formation cuttings are considered contaminants that degrade the performance of the drilling fluid. Solids that remain in the drilling fluid will be reduced in size until they become difficult to remove from the drilling fluid with normal solids control equipment (SCE).
A large proportion of formation solids can usually be removed by mechanical means at the surface. The smaller the particle, the greater the surface area, the greater the effect on drilling fluid properties and the more difficult they are to remove from the drilling fluid. The particle size of drilled solids incorporated into drilling fluid can range from 0.1 to 250 microns (1 micron equals 1/25,400 of an inch or 1/1,000 of a millimeter).
The table below lists the approximate sizes of contaminating solids.
Table 1 Solids Sizes
1.1. Mechanical Solids Removal Equipment
One method of solids control is the use of mechanical solids-removal equipment. Another method, dilution, is discussed later in this chapter. Equipment that removes solids mechanically can be grouped into two major classifications:
• Screening devices (a form of filtration)
• Centrifugal separation devices
Table 2 Solids Control Equipment and Effective Removal Ranges in Microns
Solids Control Device Effective Range
Scalping Shaker (coarse screening) API 18 ≈ 1000 microns Primary Shale Shakers API 325 ≈ 42 microns (in water) Desanders (10 & 12” hydrocyclones) ≈ 40-45 microns (in water) Desilters (4” Hydrocyclone) ≈ 20 – 24 microns (in water)
Decanting Centrifuge ≈ 4 microns
Decanting Centrifuge preceded by Chemical Flocculation ≈ Clear water (in Water Based Fluids)
Screening Devices
The most important solids removal device on a drilling rig is the shale shaker. It is the first and best opportunity to remove drilled solids before they become further degraded over time. Shale shakers contain one or more screens, integrated into a vibrating screen frame. Drilling fluid passes through the screen as it is circulated out of the hole. Shale shakers are available in various configurations with one to three decks, varying amounts of G-force at the screen surface and four basic vibratory motions, as shown below.
Circular Motion
Uniform round motion for the length of the screen frame
Elliptical Motion
Non-uniform oblong motion for the length of the screen frame.
Balanced Elliptical Motion
Uniform oblong motion for the length of the screen frame.
Linear Motion
Straight line motion (balanced elliptical motion with an aspect ratio of infinity to one)
Generally speaking, at this time, any shaker that is able to provide 7 G’s or more at the screen surface is considered to be a Hi-G shaker.
Hi-G force linear motion is superior in transport of solids and screening with all but very wet, sticky cuttings, in which case balanced elliptical motion would have a slight advantage if Hi-G force Balanced Elliptical Motion Shakers existed.
Screen Effectiveness
Two factors that determine the effectiveness of a screen are mesh size and screen design.
Mesh - Historically, screen mesh meant the number of openings per linear inch as measured from the center of the wire. With the introduction of sandwich screens in the 80’s, mesh count became unable to predict how a screen cloth would perform and therefore irrelevant. Variations in how the layers of screen cloth aligned created multiple sizes of screen openings. As a result, the API screen standard was created.
Figure 1 Screen Mesh: variations in hole size due to sandwich screen construction.
API Screens – API rp13c required all screens to be compared against an ASTM test screen and ranked in
accordance with how they were able to classify sized Aluminum Oxide grit. This provided a level playing field for all screen manufacturers. It meant for the first time that any screen labeled API 200 for instance, would have a near identical ability to separate solids. The standard also provided a method of testing conductance, the ability of a given screen to allow fluid to pass through it.
Actual separation on the drilling rig will also be influenced by factors such as particle shape, fluid viscosity, g-force, vibratory motion and condition of the screen frame upon which the screens are mounted. The table below lists the range of solids that a given API Screen number is able to remove.
Table 3 API Screens vs Solids Sizes
D100 separation µm API screen number D100 separation µm API screen number
> 3 075,0 to 3 675,0 API 6 > 231,0 to 275,0 API 60
> 2 580,0 to 3 075,0 API 7 > 196,0 to 231,0 API 70
> 2 180,0 to 2 580,0 API 8 > 165,0 to 196,0 API 80
> 1 850,0 to 2 180,0 API 10 > 137,5 to 165,0 API 100
> 1 550,0 to 1 850,0 API 12 > 116,5 to 137,5 API 120
> 1 290,0 to 1 550,0 API 14 > 98,0 to 116,5 API 140
> 1 090,0 to 1 290,0 API 16 > 82,5 to 98,0 API 170
> 925,0 to 1 090,0 API 18 > 69,0 to 82,5 API 200
> 780,0 to 925,0 API 20 > 58,0 to 69,0 API 230
D100 separation µm API screen number D100 separation µm API screen number
> 550,0 to 655,0 API 30 > 41,5 to 49,0 API 325
> 462,5 to 550,0 API 35 > 35,0 to 41,5 API 400
> 390,0 to 462,5 API 40 > 28,5 to 35,0 API 450
> 327,5 to 390,0 API 45 > 22,5 to 28,5 API 500
> 275,0 to 327,5 API 50 > 18,5 to 22,5 API 635
Screen Labeling: Every API compliant screen has two tags prominently displayed on the screen and the box in which they came. The minimum information required by API is as per the example below.
API #
(## microns) Mfg. Part No.
Country of Mfg.
Manufacturer’s Name Non-Blanked Area: ##.## ft2
Conductance: #.# kD/mm Batch No. 123-03/15/05 Conforms to API RP13C
Figure 2 Correct Screen Labeling
Shale Shaker Design
Shale shakers must collect drilling fluid with cuttings from the flow line, and then meet the following design objectives:
• Distribute the flow of drilling fluid evenly across the available shakers
• Distribute the flow of drilling fluid evenly across the screen
• Hold the screens tightly to a screen frame without screen movement or flexing
• Distribute the vibration of the screen frame to the screen surface
• Provide efficient solids transport off the screen
• Collect drilling fluid that has passed through the screen and direct it toward a pit or trough Screen Construction
Screens are available in two- and three-dimensional designs (i.e., flat and corrugated).
Two-dimensional screens include:
• Panel screens: with two or three layers of screen cloth on a backing screen, bound on each side by a one-piece, double-folded hook strip
• Perforated plate screens: two or three layers of screen cloth bonded to a perforated, metal plate that provides support and is repairable.
• Pre-tensioned screen panels: Stiff screen panel with 2 or more layers of screen cloth bonded to a metal or plastic panel.
Three-dimensional Screens include Pyramid™ and Pyramid Plus™:
Three-dimensional screens use a perforated plate to provide structural strength with a corrugated surface running parallel with the flow of fluid. Three dimensional screens provide more usable (non-blanked) screen area than two-dimensional screens. This can be confirmed by comparing the conductance figures on the API screen labels.
Figure 3 Two-dimensional vs Three-dimensional Screens
Vibration forces solids into the trough on three-dimensional screens allowing improved conductance while flat screens allow a uniform bed of solids which impedes fluid throughput.
High Temperature Screens
Water-based fluids with relatively high glycol concentrations and/or high temperatures can be prone to delamination. Although more pronounced in three-dimensional screens due to their relatively higher mass, delamination also occurs in flat panel screen construction. This screen type is designed for fluid temperatures up to 110C and/or glycol fluid systems.
1.2. Centrifugal Separation Devices
All centrifugal separations are governed by Stokes Law.
The two types of centrifugal separation devices most commonly used for removing drilled solids are:
• Hydrocyclones
• Decanting centrifuges Hydrocyclones
Hydrocyclones, like all centrifugal devices separate solids by their relative mass. These conical solids separation devices in which hydraulic energy is converted to centrifugal force are fed by a centrifugal pump through the feed inlet tangentially into the feed chamber. The centrifugal forces thus developed multiply the settling velocity of the higher mass solids, forcing them toward the wall of the cone. The ‘beach’ is the area of the cone in which particles come into contact with the side wall of the cone. The lighter particles move inward and upward in a spiraling vortex to the overflow opening at the top. The discharge at the top is the overflow or effluent; the discharge at the bottom is the underflow. The underflow should be in a fine spray with a slight suction at its center. A rope
discharge with no air suction is undesirable. Figure 11-4 illustrates the hydrocyclone process. The ability to adjust the vortex and thereby change the pressure equilibrium inside the cone provides a level of adjustment to assist in optimizing the performance of a cone.
The diameter and length (think residence time inside the cone) plus the fluid velocity determines the cut obtained.
Lower feed manifold pressures result in coarser separation and reduced capacity. Most hydrocyclones are advertized as being effective with 75 feet of head. (Manifold pressure in psi should be roughly 4 times the mud weight in pounds per gallon).
S t o k e s L a w - D e t e r m i n a t i o n o f s e t t l i n g r a t e s ( f o r s p h e r i c a l p a r t i c l e s ) U t = ( g D s 2 ( s - l ) ) / 1 8 µ
W h e r e :
U t = t e r m i n a l , o r s e t t l i n g , v e l o c i t y p s = d e n s i t y o f t h e s o l i d
g = a c c e l e r a t i o n o f g r a v i t y p l = d e n s i t y o f t h e l i q u i d D s = d i a m e t e r o f t h e s o l i d µ = v i s c o s i t y o f t h e l i q u i d
S e t t l i n g r a t e s a n d t h e r e f o r e c e n t r i f u g a l s e p a r a t i o n e f f i c i e n c i e s a r e d e p e n d e n t u p o n t h e va r i a b l e s a b o ve . We may be unable to change the density of the solid, the density of the liquid or the diameter of the solids but in many devices, G-forces and the retention time can be adjusted to our benefit.