Underbalanced drilling is one of the most widely used drilling technologies while drilling poor permeability, low pressure and depleted reservoirs. Advantages inherent in application of this drilling technique which are major factors affecting cost, time and drilling quality can be significantly hindered due to poor hole cleaning performance. Experimental study of hole cleaning performance in underbalanced drilling at downhole temperatures26 0 C, 40 0 C and 60 0 C and water influx of 0.25m 3 /s

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... in a simulated vertical well using gasified mud was carried out. Results showed that increasing down-hole temperature by 34 0 C above 26 0 C (ambient temperature) caused 42% and 21.6% increase in annular concentration at flow rates 0.3m 3 /s and 1.6m 3 /s respectively. Increasing mud flow rate by 1.3m 3 /s above 0.3m 3 /s at downhole temperatures:26 0 C and 60 0 C and water influx at 0.25m 3 /s caused 68% and 47.6% decrease in annular cutting concentration. Increase in downhole temperature at constant flow rate resulted in poor hole cleaning performance due to increased in annular cutting concentration. Also, increasing mud circulation rate at a given downhole temperature led to an improved hole cleaning performance due to decrease in annular cutting concentration. Sensitivity studies of effect of mud viscosity, gas/liquid ratio, cutting density and water influx at 40 0 C revealed that increasing mud viscosity and gas/liquid ratio significantly improved the hole cleaning performance while cutting density and water influx cause poor hole cleaning. However, effect of increasing water influx becomes problematic only when the volume is excessively high and miscible with the circulating mud.