Wind, solar and biofuels have a long way to go before they’re sufficiently reliable to replace fossil fuels as the world’s primary energy source, but in the meantime, carbon capture and storage will play an integral role in the global transition to sustainable energy.
“The need we have now and the need we will have in the near future is reliability,” said Hannes Leetaru, AAPG Member, principal petroleum geologist and head of Petroleum Geology at the Illinois State Geological Survey. “Right now, renewables are not necessarily reliable. Solar needs the sun. Wind farms need the wind.”
The Carbon Storage Assurance Facility Enterprise, or CarbonSAFE initiative, spearheaded by the U.S. Department of Energy and the National Energy Technology Laboratory, is helping to spur research and new opportunities to capture and store carbon dioxide both onshore and offshore.
While fossil fuels remain the world’s predominant, reliable energy source, they are, of course, known producers of carbon dioxide. CCS projects are designed to separate and capture carbon dioxide from the atmospheric emissions of industrial processes and provide safe, permanent CO2 storage in deep underground storage complexes.
CarbonSAFE initiative projects focus on the development of geologic storage sites for the storage of 50 million-plus metric tons of carbon dioxide from industrial sources, according to the Department of Energy. CCS consists of multiple technologies that have the potential to benefit many industries, including electrical power generation and other industrial sources.
Leetaru noted also that CCS might become an important component in the generation of biofuels. In Decatur, Ill., a project funded by the Department of Energy is currently capturing carbon dioxide during the production of ethanol and safely injecting it into the Cambrian, Mount Simon Sandstone at about 7,000 feet below the surface.
Finding Carbon Sequestration Sites
Across the nation, many geologists, including AAPG members, have embraced CarbonSAFE’s challenge to find and implement successful CCS projects.
Conn Wethington, AAPG member and doctoral candidate at the Boone Pickens School of Geology at Oklahoma State University, has been studying a potential CO2 sequestration site in east-central Mississippi that holds significant promise. The site was chosen by his adviser, Jack Pashin, AAPG Member, professor and Devon Chair of Basin Research.
Referring to notable commercial CCS projects by Equinor (formerly Statoil) in the North Sea, Wethington believes that Mississippi will soon be home to successful CCS sites as well. The storage site he has been researching using hard data, such as well logs and cores, is located at the Kemper County Energy Facility. It has the storage potential of 22 metric tons per square mile, with an estimated storage resource of 1.2 gigatons within the entire storage complex.
The sandstone reservoir averages 28.5 percent porosity and 3.6 darcys permeability, with a maximum of 16 darcys. Its stacked storage potential includes more than 1,300 feet of Saline reservoir, Wethington said.
Its seals are located in the Fredericksburg and Washita Formations and include mudstones in the Tuscaloosa marine shale, the chalk of the Selma Group and the Porters Creek Clay, which overlay the Cretaceous sandstone section.
Such characteristics are highly favorable for a CCS reservoir, Wethington said. “My observation is this is a world-class CO2 sequestration site,” he said.
Having performed the necessary due diligence for injection feasibility, Wethington is currently wrapping up a report on his findings. Based on the data, Southern Company, which hosts the research site, will be able to apply for the requisite permits to begin injections in a Class VI well – one of just a handful in the country dedicated to the geologic sequestration of carbon dioxide into deep rock Formations.
“This is a new frontier in geology. It’s an original project with original data,” Wethington said. “This is a very impactful project.”
While operators have been injecting CO2 into wells since the 1970s, the main purpose was for enhanced oil recovery. Carbon dioxide has been injected into un-minable coal seams as well.
Yet, the CarbonSAFE initiative focuses on a third kind of injection site: deep saline Formations lacking petroleum. Ideal sites must be geologically stable – absent of transmissive faults and fractures – and contain a seal – usually mudstone, tight carbonate, or evaporite – for effective CO2 trapping.
“Saline reservoirs are vast in respect to storage volume,” Wethington said, adding that saline Formations can potentially store up to 10,000 gigatons of carbon dioxide – about 10 times more than the storage capacity in oil and gas reservoirs.
It is also advantageous that deep saline Formations are much more prevalent and not dependent on petroleum systems to effectively store CO2. The government also offers tax incentives for sequestration projects that are independent of EOR projects at $50 per ton of sequestered carbon.
Candidates in the Mid-Atlantic Seaboard
Rui Zhai, a doctoral candidate in geophysics at the University of Oklahoma, is participating in the CarbonSAFE initiative as the seismic interpreter for the OU Basin Analysis and Seismic Stratigraphy Research Group by researching Formations in the mid-Atlantic seaboard.
His research group is incorporating a regional, rather than localized, study that incorporates both basin analysis and seismic stratigraphy. Zhai is working to identify potential CO2 sequestration sites by using 2-D seismic data gathered in the late 1970s and early ‘80s to determine reservoir properties in zones where CO2 can be stored in liquid form.
“Unfortunately, in comparison to the Gulf of Mexico, the mid-Atlantic seaboard is relatively sparse and vintage in terms of well and seismic data,” he explained. However, the inversion of 2-D seismic data regionally constrained with petrophysics contributed greatly to determining the facies and lithologic characterization of several promising units, he said.
Seismic data provided by Fugro and Schlumberger helped Zhai to find three sand packages that revealed the most potential in terms of sandstone permeability in the Aptian, Albian and Eocene sections. The total storage for these sand packages is up to 1,000-feet thick, with 31 percent porosity and 1.6 darcys permeability. The sand packages are estimated to have 2.037 gigatons in storage capacity, Zhai said.
“There are good sedimentary sands offshore,” he said. “Carbon sequestration is feasible, but we need more data.”
For decades, the United States has been storing natural gas underground, often as a buffer for seasonal demands for residential heating and electricity generation near large metropolitan areas. These storage reservoirs provide a good analog for CCS project risks, Leetaru said.
Plunging into a CCS project without understanding the subsurface geology can increase project risk. “We need to understand the reservoir injectivity, capacity and the seal,” he said.
For example, an economic failure would occur if the subsurface reservoir is highly compartmentalized, creating limited capacity for storing gas. Just like in an oil and gas field, the presence of faults, fractures and/or low seal capacity may not be adequate for storage of gas, resulting in leakage into overlying strata or even to the surface, Leetaru explained.
“Some think the sequestration of carbon dioxide has a simple solution, that it’s an engineering problem, no need for geology. But that’s not the way it works,” Leetaru said. “You must involve geologists and you must do the geological characterization properly. Carbon dioxide is buoyant. It wants to move upward.”
Initiatives such as CarbonSAFE are prompting success in other parts of the nation, paving the road to fewer CO2 emissions.
As CCS projects become more prevalent, Wethington suspects they will become a boon to the field of geology.
“The national implementation of CO2 sequestering would create thousands of jobs for geologists and engineers,” he said. “It has the potential to become a huge industry.”