Underbalanced drilling holds several important advantages compared to conventional drilling technology. These include elimination of formation damage, faster penetration rate, ability for evaluation of reservoir productivity during the drilling process. As underbalanced drilling technology matures, it has also been used more and more. However, many aspects of underbalanced drilling technology remain poorly understood. The model presented in this paper seeks to understand the mechanisms involved in the transport of cuttings in underbalanced drilling.

The model simulates the transport of drill cuttings in an annulus of arbitrary eccentricity, and includes a wide range of transport phenomena including cuttings deposition and re-suspension, formation and movement of cuttings bed. The model consists of conservation equations for the fluid and cuttings components in the suspension, and the cuttings deposit bed. Interaction between the suspension and the cuttings deposit bed, and between the fluid and cuttings components in the suspension, are incorporated. Solution of the model determines the distribution of fluid and cuttings concentration, velocity, fluid pressure, velocity profile of cuttings deposit bed at different times.

The model is used to determine the critical transport velocity for different hydrodynamic conditions. Results from the model agree quite closely, qualitatively and quantitatively, with experimental data obtained from a cuttings transport flow loop at JNOC's Kashiwazaki Test Field in Japan. These results show the importance of slippage in the formation of the cuttings deposit bed. The model is useful in evaluating the minimum flow rate for effective cuttings removal in underbalanced drilling.

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