As oil well drilling encountered more and more difficult conditions, hole problems finally became undeniably associated with excessive drilled solids. Many years ago, a controversy raged concerning the effect of drilled solids on the cost of a well. Many thought that drilled solids were beneficial as an inexpensive substitute for weighting agents. Frequently, production horizons near the surface were normally pressured and could be drilled with unweighted drilling fluids. Usually, these drilling conditions were relatively trouble free, and a poor-quality drilling fluid was used for drilling. Of course, drilling performances and well productivity could be enhanced with better-quality drilling fluids, but those effects were difficult to quantify. As these areas graduated from unweighted drilling fluids to weighted drilling fluids, better drillingfluid properties were required to prevent trouble. The primary problem was that large quantities of drilled solids were intolerable. The drilling trouble costs could easily be traced to failure to limit drilled-solids concentration. This provided the impetus for most drilling rigs to upgrade their surface systems handling drilling fluids. The benefits of a clean drilling fluid have been well stated in previous chapters and have been well validated.