Shale shaker should remove as many drilled solids and as little drilling mud as possible. These dual objectives require that cutting (or drilled solids) convey off the screen while simultaneously separating and removing most of drilling mud from 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 mud as possible defeats the ultimate objective of reducing drilling costs.
Cutting size greatly influence the quantity of drilling mud that trends to adhere to the solids .As an extreme example, consider a golfball-size drilled solid coated with drilling mud. Even with a viscous fluid, the volume of fluid would be very small compared with the volume of the solids.
If the solids are sand-sized, the fluid volume increases as the solids surface area increase. For silt-size or ultra-fine solids, the volume of fluid coating the solids may even be larger than the solids volume. When we use coarse screen, more drilling fluid will return to dirty tank; When we use ultra-fine screen like 200 mesh, less drilling mud would returen to mud tank.
Drilling fluid is rheologically complex system. At bottom of the hole, faster drilling is possible if the fluid has a low viscosity. In the annulus, the higher viscosity of drilling fluid, the easier to transport drilling solids.
When the flow of mud 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. Typiclly, low shear rate viscosities of drilling fluids range from 300 to 400 centipoise up to 1000 to centipoise. As the shear rate (or usually the velocity) increases, drilling mud viscosity of 1500 centipoise, the plastic viscosity (or high shear rate viscosity) could be as low as 10 centipoise.
The shale shaker is the first stage of solids control equipment in drilling mud management. In theory, it can remove cuttings above 75 μm brought from the drilling well to leave the liquid with smaller particles into the tanks below, which will flow into next grade solids control equipment for further treatment.
Drilling fluid flows downward, on and through shaker screen. If the shaker screen is stationary, a signficant head would need to be applied to the drilling mud to force it through the screen. For example, imagine pouring honey onto a 200 mesh screen, honey at room temperature has a viscosity around 100 to 200 centiposie. the flow through the screen would be very slow. If the screen is moved rapidly upward through the honey, more fluid would flow in a given period of time.
The introduction of vibration to this process apllies upward and downward force to the honey. The upward stroke moves the screen rapidly through honey. These same force 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 upwward motion of the shaker screen force fluid downward through the shaker openings and moves solids upward. When the screen moves on downward stroke, solids do not follow the screen. They are, instead, propelled forward along the screen. This is the theory behind elliptical and linear motion shale shakers.
Screens are moved upward through the fluid with the elliptical and linear motion shale shakers. The linear motion shaker has an advantage because solids can be tranported out of a pool of fluid and discharged from the solids removal system. The pool of fluid 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 shale shaker.
The movement of the shaker screen through the drilling mud causes the screen to shear the fluid. This decreases the effective viscosity and is an effective component to allow shale shakers to process drilling mud.
The upward movement of the shaker screen through the fluid is similar to pumping the drilling mud through the screen opening. If the fluid gells on the screen wires, the effctive opening is decreased. This is the same as pumping drilling mud through a smaller diameter pipe. With the same head applied, less fluid flows through a smaller pipe in a given period od time than a larger pipe. if a shaker screen becomes water-wet while processing an oil-base drilling mud, the water ring around the screen opening effectively decreases the opening size available to pass the fluid. This would reduce the flow capacity of the shale shaker too.
The scalping screen has to be sized to remove the smallest solids possible and allow 100 percent of the recirculating volume through the screen. Linear shakers allow the use of smaller micron scalping screens, as opposed to elliptical ones.