Measurement and analysis of the pressure drop in a vertical annulus in a simulated air drilling operation

It is important to immediately clear cuttings from the face of the bit and to efficiently carry them to the surface in any drilling operation. Cuttings removal is especially critical in air drilling operations because of the relatively low viscosity of the gas compared to drilling mud. Air can effectively clear and remove cuttings, however, if the proper volumetric flow rate and injection pressure are used. One important feature of pneumatic conveying of solid particles in vertical pipes is that pressure drop initially decreases as the air velocity decreases, reaches a minimum, and then increases with further drops in air velocity. Eventually, the air flow can attain such a low velocity that the result is a sharp increase in pressure drop often accompanied by flow instabilities and vibration. The purpose of this study was to experimentally verify the presence of this minimum pressure drop in a simulated air drilling operation and to accurately determine it. The eventual goal is to develop a predictive capability for the velocity that yields this minimum pressure drop since this is an important parameter in optimizing the power requirements. A plexiglass wellbore model was constructed with air and sand feed lines at the bottom. The air-sand mixture flowed through a cyclone at the top to separate the two components and then recycled back into the feed line at the bottom. The pressure drop across the wellbore model was measured at various sand feed rates and air velocities. One of the findings of this study is that a minimum pressure drop does indeed occur at all sand feed rates as the air velocity decreases. We also found that there is a velocity at which the air moves too' slow to lift any of the particles. No more pressure readings are possible when air is moving at the choking velocity. This study is the first in a series of studies designed to foster a better understanding of the air drilling process. Our goal is to better understand the mechanics of air drilling and, ultimately, to be able to select the proper surface equipment for an optimized operation.