Greenhouse gases (GHGs) trap solar energy within the Earth’s atmosphere and prevent it from escaping back into space, warming the planet via a process known as the greenhouse effect. Without these gases, the Earth would not be warm enough for human life. However, if the greenhouse effect intensifies, the planet could become warmer than usual, causing problems for humans, plants, and animals. GHGs, most of which occur naturally, include carbon dioxide, water vapor, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. Carbon dioxide (CO2), the gas that provides the fizz in carbonated beverages, is the second most abundant GHG.
Carbon capture is the process of separating CO2 from emission sources such as coal-fired power plants, the largest stationary sources of CO2.
Carbon sequestration/geologic storage is the storage of CO2 in a way that permanently separates it from the atmosphere. Geologic storage involves injecting CO2 into underground reservoirs (depleted oil and gas reservoirs, deep saline formations, and unmineable coal seams) that securely contain the CO2 beneath impermeable, overlying rock formations (caprocks or seals) and separate it from shallower fresh water aquifers.
According to the U.S. Department of Energy (DOE), power plants and other large point sources account for nearly one third of U.S. carbon emissions. CCUS technologies will stabilize and ultimately reduce these emissions, thereby stabilizing and reducing atmospheric CO2 concentrations, while in some cases also contributing to enhanced oil recovery efforts.
Site characterization is the process scientists use to assess the suitability of a potential storage reservoir. This process includes conducting geophysical surveys, drilling test wells, and using sophisticated computer models to predict where the injected CO2 will migrate, how efficiently the storage volume will be filled, and how well the storage site will perform over time. A geologic site ideal for CO2 sequestration would offer large storage capacity per unit of reservoir volume, caprock capable of preventing CO2 leakage and escape, minimal seismic activity, and the absence of other valuable natural resources (oil, gas, geothermal, etc.).
The Wyoming Carbon Underground Storage Project (WY-CUSP) is a pioneering research effort designed to investigate and characterize two potential carbon storage reservoirs (the Weber and Madison formations, both deep saline aquifers) on the Rock Springs Uplift (RSU) in southwestern Wyoming. WY-CUSP is managed by the Carbon Management Institute (CMI), a part of the University of Wyoming School of Energy Resources (SER). Scientists from UW, the Wyoming State Geological Survey, and Los Alamos National Laboratory, along with industry partners Baker Hughes, Geokinetics, Emtek, and ExxonMobil, are collaborating on the project. WY-CUSP will result in a detailed characterization of the potential storage reservoirs and will facilitate planning efforts for commercial-scale CO2 storage.
Today’s site characterization studies of promising geologic storage formations will enable us to evaluate the potential of these formations to safely and permanently store CO2. It is important to complete these studies before siting, constructing, and operating commercial-scale sequestration facilities. Site characterization studies will accelerate the design and implementation of an optimal, safe CO2 storage demonstration on the RSU.
A saline reservoir is a deep underground geologic formation that contains highly mineralized brines (salt water) unsuitable for drinking, agricultural use, or industrial use.
At the potential storage site on the east flank of the Rock Springs Uplift, the Weber and Madison formations lie more than 11,000 feet below the ground surface. The two formations have a combined thickness of 1,100+ feet at the site.
Preliminary data indicated that the RSU could store 26 billion tons of CO2; however, more recent work suggests the capacity is closer to 13 billion tons. Wyoming’s largest source of anthropogenic CO2, the coal-fired Jim Bridger power plant, emits approximately 18 million tons of CO2 each year, and is located very near the site. WY-CUSP researchers have modeled the injection of 15 million tons of CO2 per year for 50 years at the test site. Completion of the detailed site characterization work will provide more accurate data and models to support or modify these estimates.
One of the most significant obstacles to implementing commercial-scale geological CO2 sequestration in deep saline reservoirs is displaced fluid management. Preliminary research indicates that in order to prevent CO2 leakage or escape, a CO2 injection operation on the RSU will have to bring displaced brine to the surface and treat it for industrial, agricultural, and/or residential use. Accordingly, WY-CUSP will use the new characterization information to design an efficient, customized displaced water treatment facility.
The U.S. Department of Energy (DOE) and many other organizations across the globe have funded research and development related to CCUS science and the economic impacts of widespread CCUS implementation.
The National Energy Technology Laboratory (NETL), part of DOE’s national laboratory system, has a carbon sequestration program designed to develop technologies to capture, purify, and store CO2 in order to reduce greenhouse gas emissions without adversely affecting energy use or hindering economic growth. NETL funds seven regional carbon sequestration partnerships, along with other site characterization projects such as WY-CUSP, in order to validate various CCUS technology options and confirm their usefulness. For more information on DOE/NETL-funded CCUS projects, visit www.netl.doe.gov.
Geologic formations with the potential to store CO2 are layers of rock deep underground that are “sealed” by one or more layers of non-porous rock above them. After project operators inject CO2 (in a liquid – or supercritical – phase) into the target storage formations, the CO2 may migrate upward through the reservoir rock layers until it reaches the non-porous, impermeable seal and becomes trapped. CO2 may also become trapped in other ways: it can dissolve in brine, become trapped in the pores of the reservoir rock, or react with minerals to form solid compounds (mineralization). WY-CUSP researchers are investigating all potential CO2 trapping mechanisms at the RSU site.
CO2 also makes water more acidic. Consequently, injected CO2 can dissolve minerals, including some that contain heavy metals. Tests have shown that these dissolved minerals usually do not leave the storage site. On the RSU, the CO2 storage reservoirs lie more than two miles underground, and the chances of any mobilized compounds reaching drinking water aquifers or the surface are remote.
Environmental and safety concerns are minimal with careful site selection, a rigorous monitoring and verification program, an effective regulatory system, and appropriate mitigation plans to stop or control CO2 releases should they occur.
CO2 is not toxic, flammable, or explosive. CO2 is not a poison like carbon monoxide (CO) and poses no health risk at ambient levels (350–500 ppm) or moderately elevated concentrations (5,000 ppm). In fact, low levels of CO2 are necessary for all life. Concentrations above 100,000 ppm are considered life-threatening.
Careful project siting will minimize the chance that an unsafe geological formation with an uncharacterized seal will be used for CO2 sequestration, but nothing in nature is absolutely risk-free. Therefore, leak detection systems will be deployed prior to injection. Monitoring wells will be drilled to detect CO2 leakage, and tracers injected with the CO2 will also be monitored. These techniques have been used successfully in the oil and gas industry for more than 50 years. If a leak is detected, project operators will apply technologies used in the oil and gas industry to manage fluid movement in reservoirs.
No known groundwater contamination has occurred from CO2 injection. For 30 years, CO2 has been used in EOR projects without contaminating groundwater. With sound site characterization research, project siting, permitting, and regulation, the risk of contamination is remote. However, unintended CO2 leakage could impact groundwater quality. Specifically, dissolved CO2 could alter groundwater pH, possibly mobilizing metals, sulfate, or chloride, and potentially giving groundwater an odd color, odor, or taste.
It is also unlikely, but possible, that brines displaced by geologic CO2 storage could migrate into and degrade groundwater aquifers. This risk is site-specific and therefore most effectively addressed on a site-specific basis through the site selection process and regulatory controls on injection pressures.
One of the key requirements for a geologic storage site is that it be stable enough to prevent CO2 leakage or escape, with minimal seismic activity, faulting, and fracturing. Site characterization studies, including WY-CUSP, aim to avoid storing CO2 in areas of potential earthquake activity.
Once stored in deep geologic formations, the CO2 remains in a liquid (supercritical) state due to high pressure. The CO2 is trapped in very small (less than 1 millimeter across) pore spaces between the grains of rock that make up the geologic storage reservoirs. Like original oil and gas reserves that have remained in place for millions of years through numerous earthquake cycles, stored CO2 is unlikely to present an unmanageable risk, even in the event of an earthquake.
Human activity such as dam building, mining, nuclear weapons testing, oil and gas extraction, and fluid injection can induce minor, or on rare occasions, moderate earthquakes. Most instances involving fluid injection can be avoided through proper site selection, injection design, pressure management, and operation.
It is difficult to accurately estimate the capital cost and long-term cost of electricity associated with CCUS. Nevertheless, CCUS could prove one of the most cost-effective ways to reduce GHG emissions while allowing the continued use of inexpensive, abundant fossil fuels. The continued ability to use Wyoming’s vast coal resources is important because Wyoming’s coal industry alone contributes $1.2 billion annually to state revenues. Additionally, captured CO2 could serve to help maximize the recovery of stranded oil in depleted oil fields.
If federal or state governments regulate CO2 emissions or establish market incentives to reduce CO2 emissions (e.g. carbon tax, emissions offsets, and emissions credits), industry and other economic sectors could create demand for available, cost-effective geologic storage sites and technology. In order to meet this demand, CCUS research must stay ahead of such policy and regulatory changes.
WY-CUSP is particularly important to the long-term health of Wyoming’s economy. Understanding our ability to store commercial volumes of CO2 on the RSU will be essential to the continued operation of existing coal-fired power plants, the world’s largest collection of soda-ash (trona) facilities, and the Wyoming coal industry, as well as any future development of new enhanced oil recovery projects and coal-to-chemical plants in Wyoming.