An updated approach to underbalanced well design has been developed with a view to enhancing the ability to recover gas to process (pipeline, gas plant, offset well, temporary storage, etc.). With the growing trend towards reducing greenhouse gas emissions, this technique evaluates the range of anticipated operating conditions (gas inflow and wellhead pressures) as a function of liquid and gas injection rates and the productivity index of the reservoir.

Typically, underbalanced drilling projects have been designed to minimize gas injection requirements down the drill string or annulus (concentric injection or parasite injection) for equipment efficiency and erosional considerations. This means that wellhead pressure is typically kept to a minimum, maintaining sufficient pressure to aid in shipping liquids from the separator. An exception to this is where elevated wellhead pressures may be desired to increase bottom hole pressure and reduce inflow from the reservoir, however, the objective in this instance is to control bottom hole pressure.

The typical approach to designing for underbalanced drilling parameters is normally based upon a graph of bottom hole pressure on the y-axis as a function of gas rate on the x-axis. It is usual to overlay the hole cleaning parameters – based upon minimum annular liquid velocity – and minimum and maximum equivalent flow rates through the motor. In addition, the target bottom hole circulating pressure limitations (usually derived as a function of reservoir pressure) define the operating window from which the required liquid and gas injection rates can be determined.

This updated approach addresses many issues such as equipment sizing and expected production rates while drilling through to effective pressure management at surface to aid in well stability. Furthermore, the approach lays down the fundamental groundwork for future underbalanced control algorithms.

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