Manika Prasad, Colorado School of Mines
The IPCC – AR6 report (AR6 Climate Change 2021: The Physical Science Basis — IPCC) shows that anthropomorphic CO₂ must be reduced to drastically to maintain reasonable temperature increases, and negative emissions are required to remain at warming levels of 1.5 C or lower. To this effect, the CCUS Taskforce formed by the Governor Polis of Colorado lists a goal to decrease greenhouse gas emissions by 50 percent below 2005 levels by 2030, and by 90 percent by 2050, as well as a target to generate 100 percent renewable electricity by 2040.
Currently, these targets can only be achieved by storing naturally occurring CO₂ as well as captured CO₂ trapped from various processes into subsurface geological formations. CO₂ can be stored in subsurface reservoirs in large quantities in free, dissolved, or adsorbed states. Major CO₂ trapping mechanisms include structural and stratigraphic trapping, capillary trapping, solubility trapping and mineral trapping.
I review some of our current efforts for CO₂ storage in the subsurface – ranging from designing CCS solutions with identified sequestration options, to improving geophysical imaging efforts to resolving the underlying physics of the trapping phenomena. These combined efforts not only allow us to design pathways to meeting the goals for emissions reduction, but they also help us develop and improve technologies to ensure storage security.
For example, CO₂-EOR, an initial step towards economic viability of CCS, has high potential for CO₂ usage in unconventional reservoirs. 4D changes in seismic attributes show an amplitude increase over time as well as 4D time shifts related to gas content over producing well locations. Such sensitivity to gas content can translate to CO₂ injection.
Similarly, various factors govern CO₂ mobility and trapping in saline aquifers. CO₂ reacts with specific minerals in shale – potentially altering seal properties. The interaction of minerals with CO₂ can take various forms: CO₂ sorption increases with increasing organic matter content, clay minerals can swell with CO₂ sorption, calcite can react with CO₂ depending on presence and amount of moisture present. Assessment of storage targets and storage security requires assessments of CO₂ storage capacity for CO₂ in the target formation as well as its seal – this can depend on the abundance of specific minerals in shales that govern the interaction with CO₂. This talk explores the insights from rock physics and simulations and their use for mapping and quantification of CO₂ storage and trapping in the subsurface with geophysical methods.