Underbalanced drilling (UBD) has the potential to add value by maximizing productivity and ultimate recovery by reducing formation damage. The benefits of UBD are of course dependent on the ability to maintain underbalanced conditions throughout the entire life of a well, especially during the drilling phase. The ability to maintain underbalanced conditions is complicated because real UBD wells are rarely, if ever, in a "steady-state" condition and are subject to constantly changing or transient flow behavior. Improved understanding of the transient flow behavior will increase the ability of the rigsite engineer to maintain the desired bottomhole pressure and thus minimize potential formation damaging, overbalanced periods. This paper illustrates how detailed transient analysis provides a rigorous engineering basis for selection of the appropriate methods, to maintain optimum downhole conditions by minimizing bottomhole pressure instability.

Methods to mitigate destabilizing transient effects with drill pipe injection are relatively well understood, due to extensive case histories, but less experience is available with concentric casing (CC) injection and thus less is known on options to control pressure instability.

The UbitTS transient flow simulator is used to optimize design and operational parameters to minimize well slugging tendency and pressure instability when the concentric casing injection technique is employed. Reference is made to a generic test well, but the methodology has application to all UBD operations considering CC injection.

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