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. Introduction Conventional drilling of the underpressured, dolomitic limestone in the area created non-prodroductive time (NPT) associated with lost circulation and differentially stuck pipe. More importantly, the driver for this project was the induced reservoir damage caused by fluids and fines migration into the formation. To minimize formation damage, the well would need to be underbalanced drilled (UBD), thereby eliminating fluid loss, differential sticking, and the resulting fluid invasion that caused irrecoverable reservoir damage. The challenge of drilling of this field using underbalanced operations was that the reservoir not only contained methane, but also condensate and 9.0% H2S. The condensate and methane normally does not generate much concern for UBD other than an additional point of separation for the condensate prior to it reaching any atmospheric systems. The decision was made to employ recycling and recovery technology for both the drilling fluid and the lift gas to ensure the UBD operations could be completed without exceeding the EUB imposed flaring limited. Methane was used for the lift gas and was supplied from and recovered to a neighboring gas sales line at an average drillpipe injection rate of 52 sm3/min (1,830 scf/m) and 7,000 kPa (1,020 psi). Operational challenges, such as flush events causing over pressure shut downs on the suction side of the compressors, resulted in less than 1.3% of the cumulative net gas being flared on location. With sour underbalanced operations, there are many things to consider, from the drilling program to the emergency planning zone to the underbalanced surface equipment and results. The objective of this paper is to examine the pre-engineering, implementation, and the success of drilling a non-critical, Level 1 sour reservoir underblanced using recycling and recovery technologies and techniques.