The most suitable fluid for drilling and underreaming operations in a. highly deviated gas well was identified by an extensive laboratory study. Two nondam aging fluid formulas were proposed. The most suitable viscosifier, effective fluid-loss reducer, and appropriate particle-size distribution, of bridging Solids, for each fluid were investigated and identified in the laboratory. All drilling and completion phases proceeded as planned, and no fluid losses were recorded while drilling, underreaming, or gravel packing.