An experimental study of the annulus pressure drop in a simulated air drilling operation

A comprehensive experimental wellbore model was built to simulate the multiphase flow of compressed air and sand that occurs in an air drilling process. The model was designed to control the air volumetric flowrate and sand mass flowrate through a plexiglass annulus. The model also permitted observation of the sand and air in the annulus and allowed the measurement of the pressure drop across the annulus.; The effects of sand particle size distribution on the annulus pressure drop were experimentally studied. A minimum pressure drop was observed to occur in the annulus at some lift velocity. This minimum pressure depended on the annulus air velocity and several other operating parameters. The minimum annulus pressure drop and the corresponding optimum annulus air velocity were determined for sand mixtures having different particle size distributions.; The minimum annulus pressure drop is an important design criteria for air drilling operations because it represents the smallest surface power requirements.; The experimental results were compared to annulus pressures predicted by a semiempirical model of the air drilling process developed by Angel. This model failed to predict the observed minimum annulus pressure drops. In addition, the experimental results indicated that an annulus air velocity lower than the value assumed by Angel's model may be required.; Results indicate the parameters most closely related to the minimum annulus pressure drop are the optimum annulus air velocity, the weight fraction of coarse sand in the sand mixture entering the annulus, and the weight fraction of coarse sand in the sand mixture in the annulus. Results also show the minimum pressure is not a strong function of the sand mass flowrate and solids loading. This is perhaps attributable to the dilute nature of the systems studied.; An empirical equation was developed for the minimum annulus pressure drop as a function of these variables. In addition, a simplified form of the equation is presented for field use.