Global warming! It’s in the news almost every day. Growing international scientific consensus highlights the need to understand and curtail global warming, and to mitigate potentially disastrous environmental change. The causes of global warming, whether natural or the result of human activity, are topics of considerable debate among scientists. Of current concern are the “greenhouse” gases in the atmosphere that prevent heat from escaping into space—the “greenhouse effect.” Without it the Earth’s surface would be too cool to support most life forms. The major greenhouse gas is carbon dioxide (CO2). Carbon dioxide is not a hazardous substance but a naturally occurring component of the atmosphere (about 0.04%). Carbon dioxide is part of the carbon cycle—a natural balance between the carbon in the atmosphere, oceans, and the surface rocks and minerals. The carbon cycle has included natural variations in CO2 and climate (for example, CO2 in ice cores has varied with glacial periods over the past 600,000 years). However, unnatural buildup of CO2 increases the greenhouse effect and threatens the equilibrium of the carbon cycle that has operated for millions of years. This ultimately leads to a rise in the average temperature of both the atmosphere and the Earth’s land and sea surfaces—global warming and dramatic climate change.
There is little argument that human activities (burning fossil fuels such as coal, oil, and gas) since the industrial revolution began in the 1700s have contributed at least somewhat to increased levels of CO2 in the atmosphere. A major source of CO2 emissions is coal-fired power plants. Those in Utah emit 33 to 45 million tons of CO2 per year. Engineers are developing economic methods to remove and capture the CO2 from the combustion exhaust at these sites. But what to do with all that CO2 once it is captured? That is a problem the Utah Geological Survey (UGS) has been addressing through various studies over the past six years. These studies investigate how to permanently and safely store (sequester) CO2 geologically. Our studies show CO2 can be sequestered in (1) large folds of rock (anticlines like the San Rafael Swell), (2) coal beds, and (3) deep saline (salty) aquifers, especially near power plants.
Carbon dioxide may also be sequestered in Utah’s many matur ing oil and gas fields. Hydrocarbons occupy the pore spaces (like holes in a sponge) of limestone and sandstone (reservoirs), and accumulate over millions of years in traps (such as anticlines or ancient sandbars and reefs). A key component of a hydrocarbon trap is the seal—a layer of rock (salt or shale, for example) that prevents the oil and gas in the reservoir from escaping to the surface or out of the trap. Once the hydrocarbons have been produced, the depleted or “empty” reservoir rock and trap may be an ideal place to sequester CO2 captured at power plants and shipped via pipeline to the field. One UGS study indicates as much as 1.8 billion tons of CO2 could be sequestered in Utah’s oil and gas fields.
An additional benefit of sequestering CO2 in oil reservoirs is that CO2 can be used to enhance oil recovery from old fields before they are abandoned. When oil is produced, a significant portion (often 60 to 80 percent) remains “stuck” to the rock surrounding the pores. Injecting and “flooding” CO2 into depleted reservoirs allows the CO2 to mix with the remaining oil, loosening it up as it were (becoming what is termed miscible), thereby becoming less viscous and flowing more easily so twice as much oil can be produced. The CO2 is later separated from the oil/CO2 mixture and ultimately re-injected and permanently stored in the reservoirs. This technique has been used for over 30 years but applied, thus far, to only one field in Utah—Aneth, the state’s largest oil field. Located in southeastern San Juan County, Aneth has produced over 425 million barrels of oil. The CO2 used for enhanced oil recovery at Aneth is supplied via a specialized pipeline from a naturally occurring source in Colorado (the lack of pipelines has prevented use of this method in other Utah fields). An additional 15,000 barrels of oil per day may be recovered using the CO2-flood method in this field (about a 140 percent increase in the production rate).
What ultimately will be the fate of the CO2 used at the Aneth or other fields over time? Again, the UGS, in partnerships with industry, university, and state and federal agencies (the Southwest Regional Partnership on Carbon Sequestration), is conducting a demonstration project (funded by the U.S. Department of Energy) to clarify just that. Will the CO2 possibly leak through the seal rocks along unknown faults or natural fracture systems into important ground-water aquifers or to the surface? Will it leak through the cement behind the casing of old oil wells (the field is over 50 years old)? What are the long-term effects of CO2 in contact with the seal rocks? Much of the CO2 will mix and dissolve in the brine (salty water) remaining in the reservoir; the pore spaces of the reservoir rocks are filled will oil, brine, and gas. When this occurs, the brine becomes acidic (carbonic acid). This acid could dissolve both rocks and casing cement. Conversely, some of the CO2 could react with the reservoir rock to create new carbonate minerals such as calcite and dawsonite, a sodium aluminum carbonate, making storage more permanent. Sequestering CO2 in the form of minerals is ideal for long-term storage, but lab studies suggest this is a slow process. These are questions the UGS and its partners hope to answer.
The expansion of a CO2–enhanced oil recovery program by the Aneth operator to parts of the field that have never experienced CO2 injection presents a unique opportunity to monitor the fate of CO2 from the start. The UGS is mapping the surface geology in detail to identify faults and fracture zones. Our partners are analyzing soil gas over the area to note any possible changes in background CO2 levels. We are also mapping the subsurface geology, especially the ground-water aquifers that supply water for the needs of the local communities (Montezuma Creek and Aneth), livestock, and agriculture. Another element of our study is determining the nature of possible CO2 effects on the reservoir seals by analyzing cores from wells in the field and stored at the Utah Core Research Center (see sidebar). Evaluating the reservoir rock observed in these cores and our subsurface interpretations of the field geology will be done by our project partners to model the movement and storage of the injected CO2 over time.
Hydrocarbons have been stored in naturally occurring traps like those at Aneth for millions of years. With this demonstration project, we hope to show that CO2 can also be permanently stored, safely, in a mature field like Aneth, while increasing oil production and therefore revenues to the citizens of Utah. The CO2 produced when the oil is burned will of course go into the atmosphere, but the amount of CO2 ultimately stored in the field will be significantly higher. The project results can then be applied to other fields in Utah and elsewhere to increase domestic oil production and recovery, and simultaneously take a step toward reducing global warming.