Underbalanced drilling is considered a technique for improving productivity, since the target zone in the well is drilled with bottom-hole drilling pressure lower than pore pressure in the target zone. These differences in pressure may minimize the risk of formation damage if the bottom hole circulating pressure is maintained lower than reservoir pressure at all times. For this reason, it is very important to have good knowledge of pore pressures and bottom hole circulating pressures and their variations while drilling.

The ability to accurately predict bottom hole pressure is critical for both designing an underbalanced drilling operation as well as for predicting the effect of variations during the actual drilling operation. Commercial models are available to calculate wellbore hydraulics for fluids used in underbalanced operations. Modeling is employed for designing and monitoring underbalanced drilling operations and for modifying operational parameters in response to changing conditions. It is also very important to realize that, in case the bottom hole pressure signal from the downhole tool is lost, it is necessary to rely on the model being used. If calibration has already been made and predictions agree with actual measurements there is more confidence in the model and the prediction that the system is underbalanced. Otherwise underbalanced conditions may not be guaranteed, and formation damage could occur.

Field data from oil and gas wells are used in this paper for comparing the prediction of bottom hole pressure using different wellbore hydraulic simulators against the actual pressure while drilling underbalanced. Results show which simulator and correlations give better predictions based upon characteristics of the wells and fluids type. The source of differences in results is examined and recommendations are made.

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