Abstract

Foam is one kind of widely used drilling fluids for underbalanced drilling due to its high cuttings-carrying capacity and low lost circulation rate. However, temperature and pressure change continuously during drilling operations, making the accurate prediction of the bottomhole pressure more difficult. Besides, for fluids near the bottom, CO2 or N2 could change phase state among gas, liquid, and supercritical condition. Thus, density, rheology, and thermal properties of foam fluids could change significantly.

In this study, a transient thermal model is developed to predict pressure and temperature profiles in horizontal wells. The velocity and pressure are calculated based on continuity and momentum equations. Temperature profiles of foam and formation are obtained by simultaneously solving energy equations for different thermal regions in the wellbore-formation system. Foam properties are calculated using the state-of-the-art equation of state with the effect of phase transition.

The effects of four important parameters on the wellbore temperature and pressure distributions are analyzed. As the annulus outlet foam quality and geothermal gradient increase, wellbore pressure decreases accordingly for both CO2 foam flow and N2 foam flow. For both CO2 and N2 foam flow in horizontal wells, the phase state of CO2 and N2 phases are the gas state and supercritical state. And the critical well depths for CO2 and N2 foam flow are different due to their different critical temperature values.

This work provides a transient model to simulate flow and thermal behaviors of CO2 foam and N2 foam during both drilling and making a connection in the horizontal wellbore. Operators may use this model to select the most suitable foam fluids to optimize injection parameters.

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