Global Climate Change and Energy: Case Study
The Weyburn Oil Field—Enhanced Oil Recovery
The Weyburn oil field, operated by EnCana, Canada’s largest oil company, is 130 km (80 mi) southeast of the city of Regina in Saskatchewan province. The Weyburn oil field was discovered in 1954 with an estimated 1.4 billion barrels of original oil in place. Oil production started in 1955 and rose to about 31,500 barrels of oil per day in 1963. Starting in 1964, water was pumped into injection wells in order to increase oil production. By 1966 production peaked at about 47,200 barrels per day. Over the next 20 years, production declined steadily, dropping to just 9,400 barrels per day by 1986. Additional vertical and horizontal wells were drilled. This increased production to approximately 22,000 barrels per day.
By 1998, roughly 330 million barrels of oil had been produced. This amounted to about 23% of the oil in the reservoir. Production was again declining rapidly. It was predicted that, unless a new solution could be found to enhance oil recovery, total production would be no more than 350 million barrels—just 25% of the original oil in place.
The Great Plains Synfuels Plant
To encourage the development of alternative fuel sources, the United States government supported the building of the Great Plains Synfuels Plant near Beulah, North Dakota. Commercial operations started in 1984. The goal was to produce methane (CH4) from coal. Every day, more than 16,000 tons of crushed lignite coal is fed into “gasifiers” where it is mixed with steam and oxygen and then partially burned at a temperature of 1200°C (2200°F). This breaks down the coal to produce a mixture of gases. The gas is cooled to condense tar, water, and other impurities. Then it is passed through methanol at -70°C (-94°F). This separates the synthetic natural gas (SNG)—mostly CH4—from other compounds—mostly carbon dioxide (CO2).
The daily production is 3,050 tons of SNG, which is fed through gas pipelines to customers, and 13,000 tons of waste gas, 96% of which is CO2. Many Synfuels plants release their waste gas into the atmosphere, contributing to the greenhouse effect and global warming. Waste gas from the Great Plains plant is fed into a 330 km (205 mi) pipeline to Weyburn, where it not only is disposed of safely but also helps to produce more oil.
The Weyburn CO2 Enhanced Oil Recovery Operation
In 1997, the Dakota Gasification Company (DGC) agreed to send all of the waste gas (96% CO2) from its Great Plains Synfuels Plant through a pipeline to the Weyburn oil field.
Delivery of the first CO2 to Weyburn commenced in September 2000. The gas in the pipeline is at very high pressure (about 152 bars), which makes it a supercritical fluid. Supercritical fluids are gases under such high pressures that the vapor (gas) phase becomes as dense as the liquid phase. Supercritical fluids have high density but flow easily like gases, and so are ideal for transporting through pipelines. The Weyburn oil field has a total of 720 wells. The vertical wells were drilled in a “9-spot” grid pattern—eight producing wells in a square around an injection well—and typically have a spacing of around 150 m (500 ft). The high-pressure CO2 is being pumped into 37 injection wells, helping oil to flow toward 145 active producer wells.
The level of purity of the CO2 supplied is ideal for use in enhanced oil recovery (EOR). This is because CO2 dissolves more readily into oil when small impurities are present. Hydrogen sulfide (H2S), which makes up 2.5% of the injection gas, is particularly beneficial at helping CO2 to mix with oil.
When CO2 supercritical fluid is pumped at high pressure into the reservoir, the CO2 mixes with the oil, causing it to swell and become less viscous. The swelling forces oil out of the pores in the rocks, so that it can flow more easily. Water is pumped into the injection wells, alternating with CO2, to push the released oil toward producer wells. Some CO2 comes back out of the ground at producer wells. This is recycled, compressed, and reinjected along with gas from the pipeline.
It is predicted that the CO2 EOR operation will enable an additional 130 million barrels of oil to be produced, extending the field’s commercial life by approximately 25 years. It is also anticipated that about 20 million tons of CO2 will be injected and become permanently stored 1,400 m (4,600 ft) underground over the lifetime of this project. There is worldwide interest in this test of the viability of underground storage for large-scale reduction in CO2 emissions to the atmosphere. The Weyburn CO2 Monitoring and Storage Project is funded by several international energy companies, the U.S. and Canadian governments, and the European Union. The main concern is whether the CO2 will stay in place.
Weyburn is an excellent test site because, since 1955, thorough geological tests have been made and the results stored. There are rock core samples from 1,200 boreholes, plus time-lapse seismic analysis and borehole logging. Researchers are also sampling groundwater to test for CO2 leaks in wells. So far, no leaks have been detected and none of the gas has escaped to the surface. The Canadian government believes that CO2 storage deep underground will help it to meet its targets under the 1997 Kyoto Protocol, which requires a reduction in greenhouse gas emissions by an average of 5% between 2008 and 2012. In the case of Weyburn, the CO2 is from coal that came from under the ground, so it is effectively being put back where it came from.
Another CO2 capture and storage project currently under way is in the Sleipner field in the North Sea.