PanCanadian Petroleum's Weyburn carbon dioxide (CO2) injection project will add 25 years to field life and provide rich fodder for a greenhouse gas tuck-away program.

The Weyburn CO2 flood project, in the southeastern corner of Saskatchewan, is up and running, bringing new life to a mature oil field.
But the US $720 million (C $1.1 billion) project also is providing hands-on experience for scientists studying techniques that would enable them to tuck away large quantities of industrially generated greenhouse gases, rather than discharge them into the atmosphere.
In mid-1997, PanCanadian Petroleum Ltd., Calgary, Alberta, as operator for 36 partners in the Weyburn unit, about 78 miles (125 km) southeast of Regina, Saskatchewan, signed a long-term agreement with Dakota Gasification Co. (DSC) of Bismarck, N.D., to supply CO2 for a tertiary enhanced oil recovery (EOR) project at the unit. The CO2 is a byproduct of coal gasification at the DSC-operated Great Plains Synfuels Plant near Beulah, N.D., and is transported across the border to the Canadian field through a purpose-built pipeline.
PanCanadian said the CO2 flood will add 25 years to field life and increase recovery by an additional 120 million bbl of oil. From 1997 production of 18,000 b/d, the company plans to phase up to 30,000 b/d by 2008, and maintain that rate to 2011.
To complete the project, PanCanadian in early 1997 increased its ownership in the Weyburn unit to 69% through an assets swap with Shell Canada, to whom it traded its ownership in the Jumping Pound unit and gas plant near Calgary. The trade allowed PanCanadian to step in as Weyburn operator.
On the northeastern flank of the Williston Basin, the unit covers more than 70 sq miles (181 sq km), and is one of the largest medium-sour (2% H2S) crude oil reservoirs in Canada. Originally, it contained some 1.4 billion bbl of 24° to 34° API gravity oil in place. Discovered in 1955, the field was unitized in 1962 and has been on waterflood since 1964. As of the end of 1999, the Weyburn field had produced about 351 million bbl of oil, or nearly 25% of the original oil in place.
Horizontal wells become injectors
About 10 miles (16 km) southeast of the small town of Weyburn, the unit contains about 1,000 wells, including 137 horizontal wells and 284 lateral legs. PanCanadian has converted 19 patterns of horizontal wells to CO2 injection. Since early October 2000, each well pattern has been injecting about 3 MMcf/d to 7 MMcf/d of CO2 into the producing reservoir. At the program's peak, about 95 MMcf/d of CO2 will be injected, making it the largest project of its kind in Canada and sixth largest in the world.
The field produces mainly from the Mississippian Midale beds of the Charles formation at an average depth of about 4,595 ft (1,400 m). The reservoir generally averages 100 ft (30 m) in thickness, has a temperature of 145°F (63°C) and a pressure of about 3,000 psi. The zone is made up of two parts: the uppermost Marly dolomite and the lower Vuggy limestone. An overlying anhydrite unit forms the top and updip lateral seal to the reservoir. The most porous unit is the Marly, averaging 26% porosity and 30 ft (9 m) in thickness. Permeability is low, averaging 10 md. Horizontal wells drilled in the field since 1991 have targeted the Marly as a zone of bypassed pay. These wells substantiated that the Marly had not been swept by the waterflood as effectively as its underlying counterpart, the Vuggy. The Vuggy averages 70 ft (21 m) in thickness with an average porosity of 11% and permeability of 15 md.
The Marly has a relatively low-flow capacity relative to the Vuggy and correspondingly low sweep efficiency, PanCanadian officials said. The potential for bypassed oil in the Marly is greater with CO2 flooding than it is with waterflooding, they said, because of the comparatively high mobility of CO2.
CO2 supply key to the project
Miscible and near-miscible CO2 injection is not new to western Canada. Shell Canada maintains a large CO2-based water-alternating-gas program at the Midale unit about 100 miles (161 km) southeast of Regina, not far from Weyburn. Shell also has a smaller CO2 injection program ongoing at the Harmaton East field in Alberta. Finally, a small miscible CO2 project was conducted in the Joffre field about 10 miles (16 km) east of Red Deer, Alberta, and 110 miles (177 km) south of Edmonton, Alberta. Most of the miscible gas used in these projects went to the fields by truck for injection.
However, PanCanadian considered the Weyburn unit "ideal" for CO2 injection, and the sheer size of the project warranted a large supply source, hence the off-gas supplied by the $2.1 billion synfuels plant in North Dakota owned and operated by DSC, which is a wholly owned subsidiary of Basin Electric Power Cooperative.
The Great Plains plant was the first commercial-scale synthetic fuels project in the United States based on conversion from coal. It began production in 1984 and produces about 125 MMcf/d of pipeline-quality synthetic natural gas, plus quantities of anhydrous ammonia and other chemical byproducts. Daily coal consumption is about 18,000 tons of lignite from local mines.
The heart of the plant is the processing unit, where coal is gasified by the action of steam and oxygen at high temperature. That gas is then cooled to condense the byproducts, allowing their removal. About 30% of the synthesis gas is passed into a shift reactor to increase the proportion of hydrogen to carbon monoxide to an appropriate level for conversion to methane. After cooling, this stream is purified by a methanol wash, which removes the CO2, sulfur and naphtha. The plant's main output is a mixture of hydrogen, carbon monoxide and methane, which is dispatched to gas users.
However, the waste gas from the methanol wash is 96% CO2. After treatment, about 14,300 ton/day of CO2 is then available. In the past, much of this CO2 was vented into the atmosphere. Now it is sold to PanCanadian for the Weyburn project, and the pipeline is the connection.
DSC owns and operates the CO2 pipeline to Weyburn, having built it during the summer of 1999. The line is about 205 miles (330 km) long. It is made up of a US section, which runs 168 miles (270-km) of 12-in. pipe from the plant to the border, and a Canadian section, which strings about 37 miles (60 km) of 10-in. pipe from the border to a receiving terminal inside the southern boundary of the Weyburn unit. There, PanCanadian takes ownership of the gas.
During its life, the Weyburn project will help reduce net CO2 emissions from the North Dakota plant by an estimated 15.5 million tons, said PanCanadian officials. Scientists deem this the equivalent to taking about 3.2 million cars off the road for 1 year.
Researchers mull sequestration
Almost immediately after PanCanadian announced its intentions at Weyburn, the scientific community seized upon the idea of studying the project from start to finish. It offered an excellent opportunity to scrutinize the results of storing CO2 in an oil reservoir.
So in the interest of learning more about what happens to it once injected, the Weyburn unit is the site of a major international CO2 sequestration research study conducted under the auspices of the International Energy Agency's (IEA's) Greenhouse Gas R&D Program.
Joining IEA in the project are the Saskatchewan Energy & Mines Ministry and Natural Resources Canada, along with several corporations, including BP, DGC and SaskPower, among others. Finally, coordinating the study is the Petroleum Technology Research Centre, a Regina-based R&D joint venture sponsored by the first two organizations mentioned above, along with the Saskatchewan Research Council and the University of Regina.
The $10 million (C $15 million) CO2 monitoring project is unique, said officials, because scientists and researchers were allowed to collect background information at Weyburn before the CO2 flood began. Detailed geological records and samples and almost 50 years of production history were available for the Weyburn field. Additionally, just prior to making the decision to go ahead with the project, PanCanadian completed a major 3-D geophysical data acquisition program, the data from which were made available to researchers. Geochemical samples also were collected to provide baseline information in advance of the injection program.
This information will enable the research team to compare before-and-after results and help them better understand the interaction and relationships between oil recovery and CO2 storage. The lessons learned from the monitoring work could help set the standards for verifying CO2 sequestration as an economical technology option for reducing emissions of the gas - particularly from industrial processes - into the atmosphere.

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