A shale shaker’s capacity has been reached when excessive amounts of drilling fluid (or drilling-fluid liquid phase) first begins discharging over the end of the shaker. The capacity is determined by the combination of two factors:
1. The fluid limit is the maximum fluid flow rate that can be processed through the shaker screen.
2. The solids limit is the maximum amount of solids that can be conveyed off of the end of the shaker. The two limits are interrelated in that the amount of fluid that can be processed will decrease as the amount of solids increases.
Any shale shaker/screen combination has a fluids-only capacity (i.e., no solids are present that can be separated by the screen) that is dependent on the characteristics of the shaker (g factor, vibration frequency, type of motion, and angle of the screen deck), of the screen (area and conductance), and of the fluid properties (viscosity characteristics, density, additives, and fluid type). The mechanical features of the shaker are discussed later in this chapter. The fluid-only capacity is the fluid limit with zero removable solids. For the sake of the current discussion, the drilling fluid is assumed to be a fluid with no solids larger than the openings in the shaker screen, although this is not true in many real instances.
The screen cloth can be considered to be a permeable medium with a permeability and thickness (conductance) and an effective filtration area.
The fluid capacity will decrease as the fluid viscosity increases (plastic viscosity is important but yield and gel strengths can have a significant impact as well). Capacity will also increase as the fluid density increases due to increased pressure on the screen surface acting as a force to drive fluid through the screen.
The solids limit can be encountered at any time but occurs most often during the drilling of large-diameter holes and soft, sticky formations and during periods of high penetration rates. A relationship exists between the fluid limit and the solids limit. As the fluid flow rate increases, the solids limit decreases. As the solids loading increases, the fluid limit decreases.