Foam is one of the most frequently used drilling fluids at underbalanced drilling operations.This paper introduces more accurate model than existing models for estimating pressure drop of foam flowing through the bit.The major difference between the proposed and the existing models is that the proposed model includes the effect of foam expansion and velocity change as a function of pressure.It has been observed that pressure drop increases significantly as the upstream pressure and foam average velocity increases when compared with the existing models.For the same flow conditions,pressure drop decreases as the foam quality increases,and as the upstream pressure increases,pressure drop also increases,which the existing models cannot detect this event at all.In some cases,the pressure drop at the bit can be 10 times greater than the pressure drop predicted from existing models.
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