As more utility contractors are moving up to larger rigs, they are also moving toward the use of solids removal ( solid control ) systems. These solid control systems are not stand-alone units. They are part of the drilling fluids system along with the drilling fluid and the fluid pump.
Solids control equipment and solids control job doesn't like high viscosity fluids. High viscosity fluids don't pass through shaker screens as easily as lower viscosity fluids. They tend to create surface tension that can blind the screen openings and cause the fluids to pass off the end of the shale shakers. The viscosity problem also exists in the hydro-cyclones ( desanders and desilters ). Hydro-cyclone of each size operates most efficiently within a certain pressure range. If the available pressure of the pump feeding hydro-cyclone is being used up in moving the fluid, the efficiency of the hydro-cyclone is greatly reduced.
The viscosity problem may be coming from building too much viscosity with drilling fluid additives ( bentonite and polymers ). However, the problem can easily come from high solids content in the slurry that is being pumped from the sump up to the solids control unit.
Pump more fluid when backreaming. This is one reason that these larger rigs we're using have larger pumps. They are extremely necessary when back-reaming because of the larger reamers we're pulling and, therefore, more soil will be cut. This means more solids being created in the slurry. If you don't have a larger pump, then slow down in order to maintain a lower solids content and therefore, a lower slurry viscosity.
When adding water to the system, add it at the sump pit or at the shale shakers so that the thinning effect from any water additions will help keep the separation equipment more efficient. Add anything that thins the fluid as early as possible, at the sump for example. Anything that thickens the fluid should be added to the system as late as possible, at the pump suction pit for example.
When mixing fluid additives through a venturi or any other type of mixer, mix the material slowly over an entire circulation ( tanks plus hole volume ) or multiple circulations. This will give some assurance that the fluid and slurry is consistent throughout the mud system.