The performance of CaCI2/CaBr2 clear brine fluid was compared with that of lignosulfonatewater-based and oil-invert-emulsion-based fluids in a series of full-scale laboratory drilling tests. Sandstone and shale rock samples were drilled with a clean CaCI2/CaBr2 brine, a solids-contaminated CaCI2/CaBr2 brine, a "Yater mud, and an oil mud, all at 13-lbm/gal [1558-kg/m3] density. Penetration rates were measured under various conditions of weight on bit, fluid flow rate, and differential wellbore/pore pressure. Drilling fluid filtration into the sandstone was measured under both drilling and circulating (nondrilling) conditions. After drilling, 2-in. [5.08-cm] radial cores taken from the sandstone specimens were studied for permeability damage caused by the drilling fluids. In a separate test, shale specimens were exposed to circulating drilling fluids to examine wellbore stability effects.