The effects of solids in unweighted fluids, regardless of what their base fluid may be, do not present a problem unless their concentration is allowed to reach excessive levels. With water-based fluids, many operators limit drilled-solids content to 5% by volume. Assuming the presence of approximately 2% bentonite, this is a total of 7% low-gravity solids. Most experts agree that in water-based fluids, concentrations of lowgravity solids in excess of 10% make hole trouble likely. Therefore, in unweighted water-based fluids, drilled-solids content of 8% or more by volume can be considered to be excessive. Any solids-related problems encountered with these fluids are the result of depending entirely on drilled solids to achieve densities in excess of 9.4–9.6 lb/gal, the density of a fluid containing 9–10% low-gravity solids in freshwater. At lesser concentrations of low-gravity solids, particle size appears to be irrelevant.
NAFs can tolerate higher solids concentrations. Drilled-solids concentrations as high as 12% by volume may be acceptable with these fluids when they are unweighted. This corresponds to an unweighted density of about 9.2 lb/gal.
Solids problems are much more frequent and serious in weighted fluids because of their higher solids content, their reduced tolerance for fines, and the fact that degradation of larger particles leads to a progressive increase in the concentration of fine particles. Excessive concentrations of fine and colloidal solids are known to reduce penetration rates and, by reducing filter cake quality, lead to troublesome hole conditions by increasing torque, drag, and the risk of sticking the drill string. In order to avoid these problems, barite—or an alternative weighting agent—should be used when the desired density is above the 9.6 lb/gal that can be reached with water-based fluids, or 9.2 lb/gal with NAFs.