Underbalanced drilling is quickly becoming an important technology in the Canadian oil and gas sector. While drilling impairment has always been a concern, a significant increase in horizontal completions has brought this issue to the forefront. A number of stimulation techniques are available for overcoming impairment in vertical wells or shallow damage in horizontal wells, however, deeper matrix damage is often difficult to remove in long horizontal sections. With this in mind, Canadian producers have looked to underbalanced drilling to prevent damage caused by fluid leak-off and fines migration.

Many of the oil and gas fields in Canada are subhydrostatic, therefore underbalanced drilling operations often require the entrainment of a gas phase in the drilling fluid to generate the appropriate bottomhole pressures. In most cases, because of safety concerns, nitrogen gas is used.

Two basic styles of nitrified underbalanced drilling are currently used with conventional rig drilling operations. One involves injecting nitrogen at the standpipe to co-mingle with the drilling fluid. The other, referred to as parasite injection, utilizes an external gas injection conduit that allows gasification of the drilling fluid in the vertical annulus. Either approach has specific advantages which must be considered for each application.

An essential component of underbalanced drilling (UBD) success is effective two-phase flow modeling. Initial simulation is required to investigate and optimize various design options in the planning stage.

This is often followed up by providing onsite support to assess actual job parameters and make any required design modifications.

This paper focuses on these two UBD techniques, addressing the advantages and limitations of each. Two-phase flow analysis and job design will be discussed. A summary of the jobs completed, operational details and production results will be given.

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