The difficulty of maintaining the wellbore pressure should be overcome by precisely predicting the wellbore pressure. The empirical models used widely by UBD simulators give a large error and may not be extendable to other UBD cases. For this reason, Mechanistic modeling has been applied to two phase flow with a number of assumed values for the existent coefficients varying from one UBD case to another. Therefore, in this paper, a new comprehensive mechanistic model is developed mathematically for Iranian Oil fields to match the characteristics of the corresponding UBD jobs in Iran by modifying the constant values of Lage et al, Perez Telez at al., Mousavi et al. and Ansari et al. mechanist models. It is compared with Ansari et al, the specifically developed mechanistic model for use in South of Iran.

Since the mechanistic models use mathematical relations with assumptions for the values of the constants rather than pure empirical study of two phase flow, the mechanistic models developed and validated with data outside Iran are not good enough to be used. In this paper different constant values have been tested and the ones which have led to the least average absolute error have been picked up to present a specific mechanistic model for South of Iran.

The empirical models which have been developed outside Iran cause erroneous results. The results illustrate that the new mechanistic model predicts BHCP with an error much less than Ansari et al. Mechanistic Model.

In this paper, it is shown that the development of mechanistic models to predict the flow behavior of two phase flow and transition patterns is a necessity to escape the incompatibility of correlations with the regional flow restrictions. Also, the Ansari et al model's lower performance has been diagnosed and tried to be removed. Very few mechanistic models have been developed to respond to the fluid flow parameter prediction requirements for use in UB drilled wells of Iran.

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