Underbalanced drilling (UBD) technology is widely used in oil and gas industry for multiple purposes including minimizing formation damage and controlling lost circulation. One of the important tasks in UBD design is to predict bottom hole pressure under given drilling conditions. This task is very difficult when aerated liquids are utilized. Sophisticated computer programs are often required for this task. However, reliable computer programs are not always available for engineers who are in charge of UBD designs and executions. It is highly desirable to have spreadsheet programs that can accurately perform such predictions.

This paper presents three analytical models that are coded in a spreadsheet program for UBD hydraulics calculations. These three models cover multiphase flow of commonly used drilling fluids in three categories: 1) air, gas, mist, and unstable foam; 2) stable foam; and 3) aerated liquid. Comparisons of these models with field measurements indicated that these analytical models are accurate enough for UBD design.

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