Shale shakers should remove as many drilled solids and as little drilling fluid as possible. These dual objectives require that cuttings (or drilled solids) convey off the screen while simultaneously separating and removing most of the drilling fluid from the cuttings. Frequently, the only stated objective of a shale shaker is to remove the maximum quantity of drilled solids. Disregarding the need to conserve as much drilling fluid as possible defeats the ultimate objective of reducing drilling costs.
Cutting sizes greatly influence the quantity of drilling fluid that tends to adhere to the solids. As an extreme example, consider a golfball-size drilled solid coated with drilling fluid, Even with a viscous fluid, the volume of fluid would be very small compared with the volume of the solid. If the solids are sand-sized, the fluid-film volume increases as the solids surface area increases. For silt-size or ultra-Tine solids, the volume of liquid coating the solids may even be larger than the solids volume. More drilling fluid returns to the system when very coarse screens are used than when screens as fine as 200 mesh are used,
Drilling fluid is a Theologically complex system, At the bottom of the hole, faster drilling is possible if the fluid has a low viscosity. In the annu-lus, drilled solids are transported better if the fluid has a high viscosity. When the flow stops, a gel structure slowly builds to prevent cuttings or weighting agents from settling. Drilling fluid is usually constructed to perform these functions. This means that the fluid viscosity depends on the history and shear within the fluid. Typically, low-shear-rale viscosities of drilling fluids range from 300 to 400 centipoise up to 1000 to 1500 centipoise. As the shear rate (or usually the velocity) increases, drilling fluid viscosity decreases. Even with a low-shear-rate viscosity of 1500 centipoise, the plastic viscosity (or high-shear-rale viscosity) could be as low as 10 centipoise.
Drilling fluid flows downward, on and through shaker screens. If the shaker screen is stationary, a significant head would need to be applied to the drilling fluid to force it through the screen. For example, imagine pouring honey onto a 200-mesh screen (Figure 2-1). Honey at room temperature has a viscosity around 100 to 200 centipoise. The flow through the screen would be very slow. If the screen is moved rapidly upward through the honey, more fluid would (low in a given period of time (Figure 2-2). The introduction of vibration to this process applies upward and downward forces to the honey. The upward stroke moves the screen rapidly through the honey. These same forces of
vibration affect drilling fluid in a similar manner. The upward stroke moves drilling fluid through the screen. Solids do not follow the screens on the downward stroke and, therefore, are propelled from the screen surface.
The upward motion of the shaker screen forces fluid downward through the shaker openings and moves solids upward. When the screen moves on
Ihe downward stroke, soiids do not follow the screen. They are, instead, propelled forward along the screen. This is the theory behind elliptical, circular, and linear motion screens.
Screens are moved upward through the fluid with the elliptical, circular, and linear motion shale shakers. The linear motion shaker has an advantage because solids can be transported out of a pool of liquid and discharged from the system. The pool of liquid creates two advantages: it provides an additional head to the fluid and also provides inertia, or resistance, to the fluid as the screen moves upward. This significantly increases the flow capacity of the shaker.
The movement of the shaker screen through the drilling fluid causes the screen to shear the fluid. This decreases the effective viscosity and is an effective component to allow shakers to process drilling fluid.
The upward movement of' the shaker screen through the fluid is similar to pumping the drilling fluid through the screen opening. If the fluid gells on the screen wires, the effective opening is decreased. This is the same as pumping drilling fluid through a smaller diameter pipe. With the same head applied, less fluid flows through a smaller pipe in a given period of time than a larger pipe. If a shaker screen becomes water-wet while processing an oil-base drilling fluid, the water ring around the screen opening effectively decreases the opening size available to pass the fluid. This, too, would reduce the flow capacity of the shaker.
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