Executive Summary
The capture and storage of CO2 in deep underground rock formations is known as Geologic Carbon Sequestration. Geologic Sequestration is one of the technologies that will help reduce the amount of CO2 in the atmosphere as society continues to use fossil fuels. This sequestration technique is performed by injecting CO2 into a suitable rock formation or target zone. A suitable formation is one which has the capacity to hold large quantities of CO2 while preventing the CO2 from migrating out of its intended storage zone. A caprock or geologic sealing system directly above the storage formation is required to limit upward migration. Rock formations that can meet this type of criteria are mudstones, shales and some carbonates. Porosity and permeability attributes of sealing rocks are excellent barriers to the CO2, thereby trapping it in the storage formation. Thickness, burial depth and lateral continuity can also influence the integrity of the geologic sealing system. To determine if a site is suitable for a geologic sequestration well, a subsurface evaluation must be performed to analyze the presence of both storage and sealing formations.
CO2 and Greenhouse Gases
One of the greatest challenges of our times is climate change due to the rate at which CO2 along with other greenhouse gasses are being emitted into the atmosphere. An IEA study suggests that CO2 emissions from fossil fuels alone could increase by 130% by 2050, if there are no supply constraints or any new public policies put in place to curb this action (1). Carbon capture and storage (CCS) is one of the most promising options for curtailing CO2 emissions and geologic sequestration the most viable means of storing CO2 for very long periods of time. However, trapping the CO2 into a specific geologic formation requires having a caprock or sealing system that will not allow the CO2 to escape.
Solutions
A good sealing system is an absolute must for geologic sequestration in order to prevent the CO2 from migrating vertically and potentially escaping into the aquifer system or back into the atmosphere. To this migration, the sealing system must be made up of one or more low-permeability rock formations that overlie the rock unit(s) in which the CO2 is going to be stored. These low-permeability formations can consist of chalks, evaporites, mudstones, shales or well-cemented carbonates. In practice, mudstones, shales and well-cemented carbonates are the most common for sealing systems.
Mudstones are rocks that are comprised of mixtures of very fine-grained sediments such as sand, silt and clay-sized grains, terrestrial in origin. Shale rocks are similar to mudstones, but differ in that shales tend to be more clay-rich, ductile and laminated. On the other hand, carbonate rocks are very different. Carbonates are comprised of sediment grains that are marine in origin, calcium carbonate (CaCO3). The calcium carbonate can be dissolved and precipitated in a variety of surface and near surface conditions. Therefore, it is of great importance that carbonate rocks are very well CaCO3cemented to be considered part of the sealing system. All of these sealing system formations contain very low-permeability and porosity.
Porosity is defined as the percentage of void space in a rock or its holding capacity. Whereas, permeability is the measure of the ability of fluids to flow through the pores in rocks. The pores and pore throats in sealing formations are extremely small, thereby providing a capillary barrier that prevents the CO2 from penetrating them. Of all the potential sealing rock formations, shale is one of the best. The characteristics of thermal, chemical and inelastic deformation of shale minimizes the effect of CO2 due to high entry pressure during injection (2,3).
Besides lithology, permeability and ductility, there are other factors that can influence the integrity of a sealing system such as thickness, burial depth and lateral continuity (4). All of these factors must be taken into account when assessing a site for a potential geologic carbon sequestration well. A detailed subsurface analysis that includes regional mapping, along with coring (whole and/or sidewall) will be done on the formations of interest to determine if they meet all necessary criteria for a geologic sequestration well.
When you are considering a geologic sequestration project, it is essential that you select a company that has experience in not only subsurface analysis, but also in well design, drilling, completion, subsurface monitoring and well maintenance.
References
(1) CO2 capture and storage: a key carbon abatement option. IEA/OECD: Paris: 2008.
(2) Song, J. an D. Zhang, 2013. Comprehensive review of caprock-sealing mechanisms for geologic carbon sequestration. Env. Sci. & Tech. 47(1): 9-22.
(3) Bourg, I.C. 2015. Sealing shales versus brittle shales: a sharp threshold in the material properties and energy technology uses of fine-grained sedimentary rocks. Env. Sci. Tech. Lett. 2: 255-259.
(4) Allen, P.A., and Allen, J.R., 2005: Basin Analysis: Principles and Applications. 2nd ed., Malden, MA, Blackwell Pub., p. 549.
Sharon Minor is Director of Geosciences with AD3 Technologies. Sharon has a MS in Geology and a BS in Environmental Geology from The University of Oklahoma and is a registered Professional Geoscientist for the state of Texas. Her diverse background in Hydrology, Field Operations, Storage & Disposal Well Evaluations and Leadership brings a vast amount of knowledge and experience to our Technical Steering Committee.