In 2005 a sour underbalanced horizontal gas well was designed and drilled in a pristine, environmentally sensitive region of southern Alberta, Canada. There were two primary drivers for the client. One was to remain under reservoir pressure throughout drilling and completion operations and the second driver was to safely recover 9.0% hydrogen sulfide (H2S) gas production and recycle the H2S contaminated drilling fluid. The well was not classified as critical sour because of its proximity to populated areas and a low well productivity of 0.015 E3m3/kPa (0.37 Mscf/psi); however, a cumulative flaring limit of 240 E3m3 (8.5 MMscf) was imposed by Alberta’s Energy and Utility Board (EUB).

Casing and hole sizes were optimized after confirming the well could be successfully drilled underbalanced to total depth (TD). Both well engineering and process engineering were integrated to provide the process and equipment to accommodate the sour well effluent while controlling the downhole multiphase flow and pressure environment. The injection gas for the underbalanced section was supplied via on-site compression from a sales quality gas supply line at a rate of up to 55 sm3/min (1,950 scf/m) at 10 000 kPa (1,450 psi). National Association of Corrosion Engineers (NACE) certified equipment was employed to create an enclosed gas and liquids separation system. The sour return gas was recompressed down an existing pipeline to a gathering station approximately 10 km (6.2 miles). To ensure the safety of the rig floor personnel, the drillstring was purged with an inert gas to the top float, and then bled off back to a low pressure separation system for drillstring connections. The well was successfully drilled underbalanced for 21 days, with 1.3% (8.5 E3M3)(0.3 MMscf) of the cummulative gas being flared. The use of recycled and recovered natural gas has economic benefits when compared to conventional disposable inert gas supplies, especially on a project basis.

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