Summary State-of-the-art foam and mist drilling suggests a need for predictive models for volumetric requirements that properly account for frictional losses caused by the solid phase in solids/foam slurry flow, settling velocities of such solids, and pressure drop across bit nozzles during foam flow. The objective of this paper is to fulfill this need. A model that predicts pressure drop across bit nozzles for foam and mist is presented. It accounts for the compressibility of foam but assumes negligible pressure losses resulting from friction and change in elevation. A model has been developed for predicting minimum volumetric requirements for foam and mist drilling operations. It accounts for the frictional losses caused by the solid phase, pressure drop across bit nozzles, and particle-settling velocity. The technique offers a high degree of flexibility in the selection of wellhead injection pressures and volumetric injection rates. Field application of this work can be accomplished by two primarily graphical methods that depend on compressor specifications: variable-backpressure and constant-backpressure schedules. Charts are presented for 7.875- and 9.00-in. [20.0- and 22.9-cm] hole sizes, and for 0.500-, 0.75-, and 1.00-in. [1.27-, 1.91-, and 2.54-cm] cutting sizes. Penetration rates range from 30 to 90 ft/hr [9 to 27 m/h]. Results indicate that volumetric requirements increase with increasing hole size, depth, and particle size. Increases in penetration rate cause only minor increases in volumetric requirements. All foam-drilling and well-cleanout operations can be accomplished within the laminar flow region with adherence to 0.55 minimum bottomhole and 0.96 maximum annular foam quality. Annular backpressures greater than atmospheric pressure are needed to maintain a bottomhole foam quality of 0.55 or more while reaching reasonable depths. To maintain constant depth as backpressure increases, however, both wellhead injection pressure and gas injection rate must be increased, and liquid flow rate decreased.