Pneumatic Transport of Solids Using Dry Air
Authors: Temple R.C. , Watson R.W. , Adewumi M.A. 
Published: March 2010

Abstract
Air drilling is defined as the process of making boreholes by utilizing air or gas as the circulating fluid. Studies of the hydrodynamics associated with the air drilling process have been ongoing at Penn State since 1985. In this study, dry air was utilized as the circulating medium. Field and laboratory tests have indicated that coincidental to expansion of air across bit-nozzles is the formation of water droplets which accumulate in the wellbore and along the walls of the drill-pipe and hole. The presence of this free-water can become a source of error in studies of the pneumatic transport of solids. In this study, humidity and air density were measured and incorporated into the results.
The use of Ottawa sand for laboratory investigations of pneumatic transport can yield erroneous results because of the size attrition realized in circulating the sand. To eliminate the problem with particle attrition, the experiments were conducted with sintered bauxite spheres. The experiments indicated that "choking" occurred at low annular velocities where gravitational effects on the particles predominated and large pressure drops were observed. As the annular air velocities were increased, a minimum pressure drop was observed. This minimum pressure drop occurred at the optimum air velocity where air drilling is optimized. As the air velocities were further increased, the pressure drops increased as the frictional effects predominated. Further, it was observed that in these experiments, optimum air velocity depended primarily on particle size. Minimum pressure drops also depends on particle size and on solids mass flow rate. Optimum air velocities and minimum annulus pressure drops increased when larger particle sizes were utilized.

Introduction
In the United States and Canada, air drilling has been used extensively for the drilling of oil and gas wells. The use of this technique has resulted in significant savings in both well costs and rig time. These cost savings have resulted in the development of reservoirs that would otherwise not be developed using conventional mud drilling systems. Perhaps the greatest advantage of air drilling, when compared to drilling with mud, is the increased rate of penetration(1–3). Other advantages to air drilling include longer bit life, minimization of damage to the wellbore, and elimination of some of the problems commonly associated with mud drilling such as lost circulation(4–8).
With the many advantages cited for the use of air or gas it remains an under used technology. It has been estimated that air drilling could be used in more than 30% of all land wells drilled in the United States(9). Presently, the actual figure is estimated to be about 10%. The primary reason that air drilling has not gained widespread acceptance and usage in the oil and gas industry is the lack of a scientifically-based engineering method for drill design(10). Current air drilling design methods are based on empiricism and consist primarily of rules-of-thumb. Procedures such as Angel's method and its derivatives are too simplistic because they assume a single phase friction factor for a two-phase system.