An Experimental Study of Particle Transport In a Deviated Wellbore
Authors: Hagar J.M. , Tian S. , Adewumi Ma , Watson R.W. 
Published: March 2010

Air-drilling is defined as the process of making bore-holes by utilizing air or gas as the circulating fluid. Penn State has been systematically studying the fluid dynamics associated with air-drilling in vertical wells since 1985. Recently this work was expanded to include an investigation of the effects of wellbore angle on key air-drilling design parameters such as annular air velocity and minimum pressure drop. This paper is a preliminary report of the findings of this investigation. An improved understanding of the effects of annular deviations on the pneumatic transport of cuttings will help to develop new design criteria for drilling deviated wells using air as the circulating medium.

An experimental wellbore apparatus was constructed to simulate air-drilling in a deviated wellbore. The apparatus was designed to permit variations ill the volumetric air flow rate delivered via the drill pipe. Changes in penetration rate were simulated by controlling the mass flow rate of the sand particles into the annulus. The model also provided for the visual observation of multiphase flow and the measurement of pressure losses in the annulus space.

The data obtained from experiments conducted vertically were compared to air velocities determined using Angel's model. Significant differences are noted because Angel's model ignores the variation in particle sizes and particle size distribution, and only predicts the minimum air flow rate necessary to clean the wellbore.

The effects of wellbore deviation were examined and analysed. For a given particle size, the effects of even small deviations from vertical were apparent. The analysis further indicated that both the minimum pressure drop and the optimal annular air velocity are strongly dependent on the angle of the annulus. Using this information, empirical equations were developed to predict minimum pressure drop and optimum annular air velocity. These equations are based upon data which can be collected in the field and consequently they may prove helpful in improving air drilling operations in the field.

Introduction

Air-drilling is defined as the process of making bore-holes by utilizing air or gas as the circulating fluid. The advantages of air-drilling over conventional mud-drilling have been well documented (11 –5). Drilling muds are expensive to make and maintain. Moreover, disposal of drilling muds in an environmentally prudent fashion can also be expensive. Air by comparison, is readily available. cost-free, and environmentally benign. Moreover, the use of air as the circulating fluid can result in much higher rates of penetration and higher footage runs per bit. The other noteworthy benefits realized in its correct utilization include better formation parameter estimation, minimized formation damage, minimized well bore damage, and mitigation of lost circulation problems in certain drilling environments. Where feasible, it makes economic sense to use air.

Air-drilling technology with its many advantages is underused, and has not realized its full potential. Recent estimates(6) indicate that about 30% of all wells drilled in the U.S. could use air drilling successfully.