Sour Gas Well Completion Practices in the Foothills, Western Canada
Authors: Millian M.R. 
Published: December 2007

Shell Canada Resources Ltd. Currently completes and stimulates sour natural gas wells up to 20,000 ft (6000 m) deep in the foothills region of Alberta, western Canada. In addition, sour gas wells recently were recompleted as dual tubing string producers in the foothills, Limestone field. This paper reviews completions and stimulation practices used in these wells.

Introduction

Shell Canada's active exploration and development well program in the foothills region of Alberta has been directed primarily at the sour natural gas reservoirs found in the porous carbonate formations of the region. Fig. 1 illustrates the geographical location and extent of the foothills play, which extends approximately 400 miles (600 km) northward from the U.S. border and runs along the eastern edge of the Rocky Mountains, Shell currently produces from Waterton, Jumping Pound, Burnt Timber, and Limestone sour gas fields.

The primary zones of interest in the foothills produce sour natural gas and condensate with a hydrogen sulfide (H2S) content between 5 and 30% (50 and 300 mol/kmol), and a carbon dioxide (CO2) content between 5 and 15% (50 and 150 mol/kmol). The sour reservoir fluids are found mostly in formations of Mississippian and Devonian age, such as the Mississippian Turner Valley (Mt), Mississippian Pekisko (Mp), Devonian Nisku (Dn), Devonian Reef (Dr), and Devonian Wabamun (Dwa). In general, the productive formation rocks are dolomites [CaMg(CaCO3)2] or partially dolomitic limestones (CaCO3). Porosity normally is less than 10%, and permeability ranges from about 0.1 to 10 md, with an average of 2 md. Reservoir pressures are slightly less than hydrostatic, and bottomhole temperatures (BHT's) are typically 160 to 250°F (70 to 120°C), depending on depth.

Exploration targets have increased in depth over the past 2 years, and wells up to 20,000 ft (6000 m) deep currently are completed. Completion practices and stimulation techniques used in these deep sour gas wells are reviewed in this paper.

Another area of recent activity has been the development of sour gas production from the Limestone field. This field came on stream in Nov. 1980, shortly after six wells had been recompleted as dual tubing string producers on segregated Mississippian and Devonian intervals. (There are a total of 10 production wells tied into the Limestone gathering system; four are as originally completed with single production tubing strings.) The average depth of completion intervals in Limestone range from 10,000 to 13,800 (3050 to 4050 m). The recompletion of the Limestone wells as dual tubing string, sour gas producers also is addressed, including description of the completion techniques and stimulation methods.

Completion Techniques

Deep Sour Gas Wells.

In general, the deep sour gas wells are cased with 7-in. (178-mm) API long thread and coupling (LT&C) production casing from surface to total depth (TD). However, in completions deeper than about 18,000 ft (5500 m), 7-in. (178-mm) production casing is set at about 16,500 ft (5000 m), with a 5-in. (127-mm) flushjoing production liner from above the 7-in. (178-mm) casing shoe to TD. Casing grades used for the sour gas conditions include K55, L80, and S0090. Grades K55 and L80 are modified in accordance with Shell Canada's material specifications for sour gas service. These specifications are additional to API Specs. 5A1 and 5AC2 for Grades K55/J55 and L80, respectively, and entail modification to the heat treatment, process of manufacture, and physical properties as outlined in Table 1.

Deep Sour Gas Wells.

In general, the deep sour gas wells are cased with 7-in. (178-mm) API long thread and coupling (LT&C) production casing from surface to total depth (TD). However, in completions deeper than about 18,000 ft (5500 m), 7-in. (178-mm) production casing is set at about 16,500 ft (5000 m), with a 5-in. (127-mm) flushjoing production liner from above the 7-in. (178-mm) casing shoe to TD. Casing grades used for the sour gas conditions include K55, L80, and S0090. Grades K55 and L80 are modified in accordance with Shell Canada's material specifications for sour gas service. These specifications are additional to API Specs. 5A1 and 5AC2 for Grades K55/J55 and L80, respectively, and entail modification to the heat treatment, process of manufacture, and physical properties as outlined in Table 1.