Whitney Canyon Sour Gas Well Completion Techniques
Authors: Kilstrom Kevin J. 
Published: December 2007

Kilstrom, Kevin J., Amoco Production Co.

Sour gas wells require special considerations for successful production. Corrosion, safety considerations, prolific gas rates, the casing program, and hydrate problems affect the final completion design for wells in the Whitney Canyon sour gas field. A completion technique that addresses all these constraints yet maintains a degree of simplicity and therefore presents less risk is being used in Whitney Canyon well completions. Additionally, problems that do not exist in Whitney Canyon, such as salt or sulfur deposition, can be handled with this same basic system.

Introduction

The economics associated with exploration of hydrocarbons in the U.S. have improved during the past 8 years, resulting in more deep drilling and frequent discovery of sour gas. One such discovery is the Whitney Canyon field located in the Overthrust Belt of southwest Wyoming, about 15 miles (24 km) north-northeast of Evanston (Fig. 1). The field was discovered in 1977 with the drilling of the Amoco/Chevron/Gulf Working Interest Unit Well No. 1, which reached a total depth of 10,691 ft (3258.6 m) in the Permian Phosphoria formation. Sour gas was recovered in the drilling fluid, and subsequent failure of tubular goods resulted in plugging back the well and completing the Triassic Thaynes formation in the 9,178- to 9,266-ft (2797.5- to 2824.3-m) interval for 4.3 MMcf/D (121.8 × 10(3) m3/d) of sweet gas and 96 B/D (15.3 m3/d) of condensate. The field is on the Absaroka thrust plate, which has resulted in placement of Ordovician Big Horn above Cretaceous rock in this vicinity (Fig. 2). The Ryckman Creek field, which produces oil and gas from the Jurassic Nugget and sweet gas from the Thaynes formation, lies 5 to 6 miles (8.0 to 9.7 km) to the east. The Carter Creek field has been shown to be a continuation of the Whitney Canyon field to the north (Fig. 3). A second well was spudded to test the deeper formations in the Whitney Canyon play and reached a total depth of 16,393 ft (4996.6 m) in the subthrust Cretaceous. Testing in this well verified substantial quantities of sour gas in the Ordovician Big Horn and Mississippian Mission Canyon carbonates and indicated sour gas in the Pennsylvanian Weber sandstone. Subsequent field development has indicated commercial quantities of gas in four of the Paleozoic Era formations. These include the Devonian Darby, Mission Canyon, Mississippian Lodgepole, and Big Horn formations. Limited quantities of sweet gas have been tested in the subthrust Cretaceous, and noncommercial sour gas production was indicated from Triassic Dinwoody and Permian Phosphoria. The approximately 4,500-ft (1372-m) thick Paleozoic interval lies at a depth range of 10,000 to 16,000 ft (4877 to 3048 m) in the Whitney Canyon area and exhibits normal pressure gradients of 0.45 to 0.50 psi/ft (3.1 to 3.4 kPa/m) and a maximum temperature of 250 degrees F (121 degrees C). Drilling has taken place for 13 miles (21 km) along the Whitney Canyon/Carter Creek trend, indicating a sizeable productive area (Fig. 3). The reserves of sour gas are trapped in the folding associated with the thrust faulting in the area and are held in primary and secondary porosity (matrix porosity in the 5 to 10% range). Permeability is generally low [matrix permeability less than 0.1 md (9.869 × 10(-5) mum2)] and production is highly dependent on extensive natural fracturing. The gas composition in the Whitney Canyon field varies from formation to formation, with all of the Paleozoics indicating H2S content from 1 to 15 mol%. The Mississippian Age Mission Canyon formation has indicated the largest reserves in the field and one 30-ft (9.1-m) interval in the Champlin 457 A Amoco No. 1 well tested at 33 MMcf/D (934.5 × 10(3) m3/d) (Table 1). The Mission Canyon formation is a 1,000-ft (304.8-m) thick carbonate section with a gas composition of approximately 15% H2S, 5% CO2, and 70% methane, and the balance is ethane and heavier hydrocarbons. To develop a successful completion design for the Whitney Canyon field, we first defined all factors that would be detrimental to production.

JPT

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