An Experimental Study of Pneumatic Transport of Solids in a Vertical Wellbore Annulus
Authors: Temple R.C. , Watson R.W. , Adewumi M.A. 
Published: December 2007

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.

Introduction

In many areas of the United States, air drilling has been used extensively for drilling oil and gas wells resulting in substantial savings in both well costs and rig time. The advantages of air over drilling mud have been indicated to include significantly higher penetration rates, 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. Moreover, when compared to drilling mud, air is cost free and environmentally benign. Where feasible, it makes sense from both an economic and environmental perspective to use air.

With the many advantages cited for the use of air it remains and under used technology. Recent estimates indicate that about 30% of all wells drilled in the U.S. could use air drilling successfully. Presently, the actual figure is about 10%. The primary reason that air drilling has not gained widespread acceptance and usage is the lack of a scientifically-based engineering method for drilling design". 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. The presence of rock cuttings and the effects of different particle sizes, shapes, and densities are ignored. Field observations and hydrodynamic models indicate that Angel's method tends to underestimate air flowrates for optimal hole cleaning. Hence, larger drill cuttings never obtain the velocity necessary to reach the surface. Instead they fall back down the annulus for further attrition and regrinding at the drill bit face. Dust-like particle sizes are usually observed at the blooey line of a typical air drilling operation. At the field-level then, the effects of regrinding the cuttings are significant. The drill bit penetration rate is reduced, bit wear is increased, and borehole cleaning is poor. The result is an unnecessary waste of time, energy, and money.

To avoid the possibility of regrinding the drill cuttings, some drilling contractors increase their air compressor capacity at the well site. These additional air volumes adequately lift the larger cuttings to the surface and hole cleaning is satisfactory. However, since the optimum air flowrate is not known, excessive air is often delivered to the wellbore. When this occurs, another costly set of problems develop: borehole wash-out, excessive drillstring wear, and additional wasted energy at the air compressors(s).

The single most important factor to consider in planning an air drilling operation is determining the optimal volume of air needed to drill the well. Currently, drilling engineers and contractors lack the design tools necessary to optimize air drilling procedures. Consequently, the technical advantages and economic benefits of air drilling are not fully realized by the oil and gas industry.

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