The Ultra-PF System is a specialty drill-in and completion fluid designed for use in potassium and/or cesium formate brines. Standard Ultra-PF Systems formulated with xanthan gum biopolymer and Formatrol® , a proprietary starch derivative, provide enhanced low shear rate viscosities and high temperature-high pressure filtration control up to 330°F. The thermal stability of Ultra-PF Systems may be further increased to 360°F with the addition of Formate Stabilizer, Thermasal®-A, and Thermasal-B. Fluid systems formulated in potassium formate range in density from 9.5 to 13.0 lb/gal at low solids levels, and up to 14.6 lb/gal with the addition of higher density weighting solids. Cesium formate compositions reach densities of 17.5 lb/gal with a minimum concentration of solids. In addition to drilling into the payzone, Ultra-PF can be utilized in underreaming, lost circulation pills, and pre/post gravel pack fluid loss pills.