It is generally recognized that oil influx from pay zones creates high-temperature fluid streams in Underbalanced drilling (UBD) and increases drilling cost due to bore hole complications. However there is lack of an accurate and simple method to predict the fluid temperature that is necessary for taking actions to prevent down hole complications. This study fills the gap. A simple analytical method was developed to replace sophisticated mathematics involving numerical solution procedure. The analytical model includes closed form equations for predicting the fluid temperature profiles inside the drill string and in the annulus. The analytical model considers formation oil influx, entrained cuttings and Joule-Thomason cooling effect at the drill bit. A field case study and a sensitivity analysis with the analytical model were conducted. The field case study with the analytical model indicates that the model can match temperature measurement perfectly by tuning three parameters namely temperature drop at bit, formation oil influx rate, and thermal conductivity of cement sheath. Result of sensitivity analysis shows that the temperature profiles in UBD is very sensitive to the temperature drop at bit and sensitive to the formation oil influx rate and thermal conductivity of cement sheath, especially in the low range of influx rate and thermal conductivity. More clean data sets are required to further validate the newly developed analytical model. Once fully validated, this analytical model can be used in the petroleum industry to improve Underbalanced drilling performance by reducing drilling complications and thus cost of operations.

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