Experimental study of pneumatic transport of solids in a vertical wellbore annulus

The Drilling-Hydraulics Research Center at Penn State has been conducting studies of the hydrodynamics associated with air drilling since 1990. In this study, an experimental wellbore apparatus was constructed to simulate pneumatic transport processes that occur during 'dry' air drilling operations. The laboratory model constructed provided for the observation of multiphase flow phenomena associated with pneumatic transport in a specific wellbore geometry. The results indicated that 'choking' occurred at low annulus velocities where gravitational effects on the particles predominated and large pressure drops were observed. As the annulus air velocities were increased, a minimum pressure drop was observed. This minimum pressure drop occurred at the optimum air velocity, the flowrate where air drilling is optimized. As the air velocities were increased beyond this point, the pressure drops increased as frictional effects predominated at higher air flowrates. It was observed that optimum air velocity depended primarily on particle size; solids loading increased with increasing solids flowrates and decreased with increasing annulus velocities; solids loading does not appear to be a function of particle size or annulus pressure drop; and when 'choking' phenomena were observed in the experimental apparatus, drillpipe vibrations and annulus pressure surges were considerable.