It has been widely recognized that maintaining adequate air/gas injection rate is vitally important to achieving hole cleaning both against drilling cutting and liquid accumulations. It is highly imperative to accurately predict the annulus pressure drop for determining the critical air/gas injection rate required for cleaning formation both cutting and liquid influx during underbalanced drilling.

A new numerical model for determining annulus pressure drop model has been developed that is required for accurately predicting minimum air/gas injection rate for simultaneous removal of both cutting and liquid during underbalanced drilling. The new model has incorporated the influence of cutting volume which has been overlooked by most researchers. Since the required minimum flow rate needs to be determined at the surface condition, temperature and pressure dependent variables such as gas formation volume factor, (Bg), Oil formation factor, (Bo), and water formation factor, Bw has been considered in the model. The results generated show that the existing models have underestimated the required minimum air/gas flow rate for continuous lifting of both cutting and liquid during underbalanced drilling. Engineering charts have also been generated for predicting cutting and liquid-carrying capacity of air/gas that is injected into the borehole at various rates. This project provides drilling engineers the necessary knowledge and a useful tool for minimizing complications in air/gas drilling operations

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