Geologic carbon dioxide (CO2) storage involves the injection of supercritical CO2 into deep geologic formations (injection zones) overlain by competent sealing formations and geologic traps that will prevent the CO2 from escaping. Current research and field studies are focused on developing better understanding 11 major types of geologic storage reservoir classes, each having their own unique opportunities and challenges. Understanding these different storage classes provides insight into how the systems influence fluids flow within these systems today, and how CO2 in geologic storage would be anticipated to flow in the future. The different storage formation classes include: deltaic, coal/shale, fluvial, alluvial, strandplain, turbidite, eolian, lacustrine, clastic shelf, carbonate shallow shelf, and reef. Basaltic interflow zones are also being considered as potential reservoirs. These storage reservoirs contain fluids that may include natural gas, oil, or saline water; any of which may impact CO2 storage differently. The following summarizes the potential for storage and the challenges related to CO2 storage capability for fluids that may be present in more conventional clastic and carbonate reservoirs (saline water, and oil and gas), as well as unconventional reservoirs (unmineable coal seams, organic-rich shales, and basalts):
Potential storage reservoirs may contain layers of porous rock saturated with brine. Brine, or saline water, is present reservoirs in most sedimentary basins worldwide. As a result, formations containing saline water have the highest storage potential resource capacity of all geologic media. However, less is typically known about formations baring only saline due to the lack of sufficient subsurface data. Saline water can contain minerals that could potentially react with the injected CO2 to form solid carbonates. The reactions can increase permanence but could decrease the porosity in the immediate vicinity of an injection well. Researchers seek injection techniques that promote advantageous mineralization reactions.
Natural Gas and Oil
Oil and natural gas can be present in reservoirs containing layer of permeable rock with a layer of low-permeability rock above, such that the low-permeability layer forms a stratigraphic trap that holds the oil and gas in place. The geologic conditions that trap oil and gas are also the conditions that are conducive to CO2sequestration. As a value-added benefit, when CO2 is injected into an oil-bearing formation, it can produce additional oil (10-15 percent). This process, enhanced oil recovery (EOR), begins with the injection of CO2 into an oil reservoir. A small amount of injected CO2 dissolves in the oil, increasing the bulk volume and decreasing the viscosity, thereby facilitating flow to the wellbore. However, commercial practitioners operate their injections with the goal of minimizing the amount of CO2 left in the ground so that the CO2 can be used for another well. NETL's work in this area is focused on EOR and enhanced gas recovery (EGR) injection practices that maximize the amount of CO2 sequestered.
Unmineable Coal Seams
Unmineable coal seams are too deep or too thin to be economically mined. All coals have varying amounts of methane (CH4) adsorbed onto pore surfaces, and wells can be drilled into unmineable coal seams to recover coal bed methane (CBM). Initial CMB recovery methods leave a considerable amount of CH4 in the formation, but this can be increased by injecting CO2. CO2 is adsorbed onto the surface of the coal and the methane is desorbed (enhanced coal bed methane [ECBM] recovery). Coal swelling is a potential barrier to CO2 ECBM due to a sharp drop in permeability, which not only restricts the flow of CO2 into the formation but impedes the recovery of CBM. Angled drilling techniques and fracturing are possible means of overcoming the negative effects of swelling.
Organic-Rich Shale Basins
Shale, the most common sedimentary rock, is characterized by thin horizontal layers of rock with low permeability in the vertical direction. Many shales contain 1-2 percent organic material in the form of hydrocarbons, which provide an adsorption substrate for CO2 storage similar to CO2 storage in coal seams. Research is focused on achieving economically viable CO2 injection rates, given the low permeability of shale.
Basalt formations are geologic formations of solidified lava. Basalt formations have the unique chemical makeup that could potentially convert all of the injected CO2 to a solid mineral form, thereby isolating it from the atmosphere permanently. Research is focused on enhancing and utilizing the mineralization reactions and increasing CO2 flow within a basalt formation.
Geologic storage of oil, gas, and CO2 in the deep subsurface has been naturally occurring for millions of years. For more than 40 years the oil industry has injected CO2 in depleted oil reservoirs for the recovery of additional product through enhanced oil recovery (EOR). Natural analogs to CO2 storage exist throughout the United States, where CO2 has been naturally trapped in confined geologic layers and structures deep below the surface of the Earth. Lessons learned from natural systems, EOR operations, gas storage, and sponsored CO2 storage projects are all important for developing storage technologies for a future CCS industry.
Geologic Storage Research Goals
NETL's Core R&D research in the Geologic Storage Focus Area is based on developing the ability to characterize a geologic formation before CO2-injection to be able to predict the CO2 storage resource and developing CO2injection techniques that achieve broad dispersion of CO2 throughout the formation, overcome low diffusion rates, and avoid fracturing the cap rock. These areas of site characterization and injection techniques are interrelated because improved formation characterization will help determine the best injection procedure. As part of NETL's effort to adequately understand and characterize these potential storage formations, they are supporting the development of tools and protocols to:
Improve the ability to predict storage capacity in closed and open geologic systems to within ± 30 percent.
Assess and minimize the impacts of CO2 and co-contaminants on geophysical processes.
Develop remediation technologies that will prevent or reduce possible releases through existing wellbores and natural pathways.
The following figure illustrates the geologic storage concept and the different areas of research being pursued within the Geologic Storage R&D effort.
Geologic Science and Technologies Research
A future geologic CO2 storage industry will need to augment existing technologies with novel technologies to ensure permanent storage of CO2. The Carbon Sequestration Program looks to support research that will better our scientific understanding of the following areas:
The program is also looking to support research to develop technologies that can improve containment, improve injection operations, increase reservoir storage efficiency, and mitigate releases. The following table summarizes NETL's active projects that are currently addressing the critical geologic storage barriers.