Given the ongoing debate about the causes of global warming, there’s at least one generally accepted fact: “Greenhouse” gases present in the atmosphere prevent heat from escaping into space.
Without this greenhouse effect, of course, the earth would be a mighty cold place, no doubt unsuitable for most life forms.
See Figure 1 and 2. Carbon dioxide (CO2), which is a naturally occurring non-hazardous component of the atmosphere, is also a greenhouse gas. But unnatural buildup of CO2-- such as that from coal-fired power plants -- is thought to have the potential to increase the greenhouse effect and is generally viewed as a threat to the equilibrium of the carbon cycle that has operated for millions of years.
The Utah Geological Survey (UGS) is a partner in a federal government-funded effort to evaluate available technologies to capture and to reduce greenhouse gas emissions in the southwest United States via carbon sequestration, i.e., CO2 storage, principally in underground geological formations.
“We're in the Southwest Regional Partnership (SWP) for carbon sequestration,” said Tom Chidsey, petroleum section chief at the UGS, “and there are partnerships all over the country.”
Chidsey was chairman of the 2003 AAPG Annual Convention in Salt Lake City.
“The Southwest Partnership is one part of a huge thing, with major funding from the federal government,” Chidsey continued. “There are groups such as state surveys, federal laboratories (U.S. Department of Energy) and universities all over the place working on this problem.”
See Figure 3. The UGS has been investigating permanent, safe geological storage, or sequestration, of CO2 for the past six years. The studies determined the gas can be sequestered in:
- Large fold of rock (anticlines like the San Rafael Swell).
- Deep saline aquifers, particularly near power plants.
Carbon dioxide also can be stored in the state’s many aging oil and gas fields, where it can be injected into the partially depleted reservoirs after production ceases.
In fact, a UGS study concluded as much as 1.8 billion tons of CO2 could be sequestered in oil and gas fields in Utah.
When it comes to oil reservoirs, there’s a whole other side to this story.
“The point of what we're doing there,” Chidsey said, “is to try to increase oil production and sequester CO2 at the same time.”
Once a reservoir is produced to the max, as much as 60 percent or more oil may be retained within the pores of the reservoir rock by a combination of capillary, gravity and viscous forces.
Carbon dioxide is soluble in oil, and when dissolved in oil it decreases the viscosity, allowing it to flow more readily and thereby allowing the previously trapped oil to be produced.
The produced CO2 can then be separated from the oil and re-injected into the reservoir for permanent storage. The hydrocarbon reservoir has an indigenous barrier to escape -- the seal.
See Figure 4. This miscible displacement form of enhanced recovery has been used for many years in numerous fields, including the Aneth field in southwestern Utah’s San Juan County.
The CO2is transported via pipeline from a natural source in Colorado.
Aneth has produced more than 425 million barrels of oil, and an additional 15,000 barrels per day may be recovered via CO2 flooding. This would result in a 140 percent increase in the production rate, the UGS reported.
The unknown in this attractive scenario is the ultimate fate of the CO2 at Aneth or other fields over time. To that end the SWP is conducting a demonstration project to attempt to answer a number of questions:
- Will the CO2 leak through seal rocks along unknown faults or natural fracture systems into important groundwater aquifers or to the surface?
- Will it leak through the cement behind the casing of old oil wells?
- What are the long-term effects of CO2in contact with the seal rocks?
According to the UGS, the operator at Aneth has expanded the CO2 enhanced oil recovery program into portions of the field that have not been previously subjected to CO2 injection. This will provide the opportunity to monitor the movement of the CO2 from the beginning.
Hydrocarbons have been stored in naturally occurring reservoir traps for millions of years. Chidsey said the UGS is optimistic the demonstration will show that CO2 also can be stored safely in mature fields like Aneth, thereby reducing its presence in the atmosphere while simultaneously increasing oil production.