Throughout the planning and hazard analysis phases of unconventional drilling techniques, one question continues to arise: When is it appropriate to switch from conventional drilling to underbalanced operations?

The one condition that must exist before choosing an underbalanced operation is first encountering a need for the application of the technique. That the problem cannot be solved using conventional, less expensive means must also be determined. In many fields with no historic use of underbalanced techniques, one or more attempts to solve the problem utilizing conventional techniques are carried out until those methods are shown to be ineffective.

When the appropriate conditions have been satisfied, several other criteria exist that affect the decision to make the switch from conventional circulation to underbalanced methods, including Safety, Economics and Logistics.

Using data and examples from MudCap Drilling (MCD) scenarios from around the world, this paper demonstrates the process and techniques that have been utilized to successfully determine when the critical point has arrived to make the switch.

The development of a simple flow chart that can be utilized at the field location in making the determination to switch based on satisfaction of pre-determined criteria is also demonstrated.

Work and presentations to date have concentrated on answering the questions of ‘Why’ and ‘How’ to perform various underbalanced operations, with very little emphasis on ‘When’ to perform the underbalanced operation. While this answer is often self-evident, in many cases there is substantial leeway in determining what conditions should be present. This is particularly true when the "Why" has no bearing on preventing formation damage.

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