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.


Underbalanced drilling (UBD) is defined as the drilling operations where the drilling fluid pressure in the borehole is less than the pore pressure in the formation rock in the open-hole section. The borehole pressure is intentionally maintained not to balance formation pore fluid pressure by using light drilling fluids. The light fluids used in underbalanced drilling are usually air, gas, foam, and aerated water. However, un-aerated oil, water, even weighted mud can be used for UBD in areas where formation pore pressure gradients are higher than hydrostatic pressure gradient of water.

The increasing coverage of wells being drilled with the UBD is due to many advantages of the technology. These advantages include increased penetration rate, minimized lost circulation, prolonged bit life, minimized differential sticking, improved formation evaluation, reduced formation damage (reduced stimulation requirements), earlier oil production, larger wellbore available to production in offshore, and environmental benefits.

There are three distinct applications for UBD technology: air or gas drilling, stable foam drilling, and aerated liquid drilling operations. Air or gas drilling becomes mist or unstable foam drilling when liquid is added to the system.

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