Carbon Capture and Storage
Carbon capture and storage (CCS) is a developed technology to minimize CO2 emissions and reduce global climate change. Currently, shale gas formations are considered as a suitable target for CO2 sequestration projects predominantly due to their wide availability. However, the injected CO2 causes possible geochemical interactions with the shale formation during storage applications. The CO2/shale interaction is a key factor for the efficiency of CCS in shales, as it can significantly alter the shale properties. This paper reviews the current knowledge of the CO2/shale interactions and describes the results achieved to date, to gain an in-depth understanding of the impact of CO2/shale interaction on shale properties. With this evolving technology, further studies are needed to include various shale formations and identify how different shales’ mineralogy could affect the CO2 storage capacity in the long-term.
The development of carbon capture and storage (CCS) stands as a suitable technology to reduce the massive increase in CO2 emissions in recent decades, as global climate change is becoming a serious concern to the public environment and economic growth . CO2 geological sequestration was proposed as a reliable technique to mitigate the emissions of greenhouse gas from fossil fuels into the atmosphere, by injecting CO2 for long-term storage and enhancing gas recovery . The success in developing shale formations in recent decades has shifted attention towards shale reservoirs, and considered them as promising candidates to store CO2 for extended periods , mainly because shales with their ultralow permeability play a major role as barriers or seals in a petroleum reservoir system, and also due to their wide availability worldwide . CO2 is a relatively reactive substance; once injected into the shale formation, it will be trapped in the adsorbed phase. In the long-term, formation brine will dissolve the injected CO2 and causes reactions with the shale rock, leading to mineral precipitation and dissolution which may affect the shale storage capacity . The CO2/shale interaction is a key factor for the efficiency of CCS in shale formations; it can significantly alter the shale properties, which in return affect the rock geometry, fluid transportation, and storage capacity . This paper focuses on reviewing the existing knowledge of CO2/shale interactions and describing the results achieved to date. It provides a comprehensive-systematic review on the alteration of the physical, chemical, and mechanical properties of shale caused by CO2 exposure. It also highlights the topics on Life Cycle Assessment (LCA) and the economic viability of CCS applications in shales.
2. Environmental Evaluation of CCS
The environmental consequences of CCS are often evaluated through Life Cycle Assessment (LCA) studies. LCA is proven to provide a complete analysis of all environmental effects of applying CCS to power plants. Such studies are detailed and time-consuming and vary in scope, methodology, and outcomes, but they provide a suitable assessment of many environmental effects, including global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), photochemical ozone creation potential (POCP) and cumulative energy demand (CED) . Generally, there are three main factors incorporated to influence the environmental effects from the CCS systems : (1) efficiency energy penalty, (2) purity and capture efficiency of CO2, and (3) origin and composition of the fuel. However, CO2 capturing is out of the scope of the current study. Energy penalties are associated with the capture technology, generally, pre-combustion processes produce lower energy penalties compared to pre-combustion and oxyfuel processes . For instance, 29.6% of post-combustion thermal efficiency for hard coal was reported by Schreiber et al. , while 48% of thermal efficiency was reported with the pre-combustion process . This variation can be attributed to the different types of fuel composition (natural gas, coal), assumptions in time scale, and the different energy sources (gas, hard coal, bituminous, lignite) . For an electricity production process, CO2 is produced in different purities and captured by the different systems. Therefore by minimizing the consumption of electricity for the CO2 capture, the energy penalty is reduced, and thus reduces the environmental effects from the CCS system . Hard coal is considered as a valuable and wide available fuel to capture CO2; one LCA study shows that the power generation from hard coal has significantly reduced the GWP, indicating about 13% contribution to the total GWP for post-combustion . Similarly, the power generation from lignite power plant reduces the GWP, with lower share to the global total GWP compared to hard coal , due to the production of mono-ethanolamine during the capture process. However, natural gas implies higher efficiency in capturing process compared to hard coal and lignite, with a reported thermal efficiency of 49.6% and 44.7% in post-combustion and oxyfuel processes, respectively . This results in lowering the GWP of power plants and increases the efficiency in CO2 capture.
3. Economic Viability of CCS in Shales Shale
Shale formations hold a promising potential to utilize CCS projects in terms of their technical feasibility. By combining ESGR operations with long-term CO2 storage applications, CH4 production can be maximized due to the strong adsorption capacity of CO2. However, the economic viability of CCS in shales has yet to be proven, as the related literature on this topic is limited. Considering the associated costs of CO2 capture, transport, and storage, together with infrastructure cost and petro-physical characteristics of shales could make CCS project costly . Mainly, there are two components of the cost of CO2 capture. First is the cost of removing CO2 from industrial emissions, as, currently, chemical adsorption of CO2 is believed to be the best available technology . Secondly, the cost of equipment and chemicals, as they increase the overall capture capital cost. CO2 capture is more of a technical factor, and innovative technologies are needed to reduce the costs of CO2 capture and deliver stable long-term benefits . The costs of CO2 injection and transportation are dominant factors affecting the economic viability of CCS in shales. These costs are controlled by the potential revenue form CH4 production and other factors including well spacing, CO2 separation, and bottom-hole pressure . The CO2 injection cost is related directly to the CO2 injectivity approach used, i.e., the applied huff-n-puff processes in the Big Sinking Field showed an increase in injection cost by USD 0.35/metric tonnes . Although CO2 injection is costly, integrated CCS systems in shales estimated a reduction of 30% on the average of the CO2 injection cost, with an average of USD 5–10/metric tonnes lower cost compared to saline aquifer . However, the added cost of CO2 transportation is large compared to injection and capture costs. A study  on Marcellus shales estimated a cost of USD 60–70/metric tonnes to transport CO2 from industrial source to the site, added to the USD 22.4/metric cost of CO2 injection. These results indicate that using shorter pipeline transport distances with smaller diameters could be a suitable method to reduce the transport cost, which eventually implies high incremental capital costs.
Apart from the consideration of the fixed costs, the application of CCS is derived by other factors, mainly related to the concerns regarding carbon price and carbon tax revenues . Addressing this topic is within the gaps between the economic theory and reality that prevents CCS to have an international breakthrough . Another concern about integrated CCS systems is how they can be utilized for large-scale fossil fuel power plants instead of refining industries only. However, reviewing and discussing these factors is out of the scope of this paper, yet it is reliable for generally highlighting these economic drivers and their impact on CCS deployment (Table 1). In summary, more studies are needed to provide clear assessments of the economic viability of CCS in shales. Although the application of CCS in shales is encouraging, the lack of available knowledge regarding storage capacity, reservoir data for best sequestration settings, and the effect of long-term CO2/shale interaction can affect its economic viability.
Table 1. Economic drivers for CCS projects 
The entry is from 10.3390/en13123200
- Metz,; Davidson, O.; de Coninck, H.; Loos, M.; Meyer, L. Carbon Dioxide Capture and Storage; Publ. by Cambridge Univ. Press. New York, 2005.
- de Silva, N.K.; Ranjith, P.G.; Choi, S.K. A study of methodologies for CO2 storage capacity estimation of coal. Fuel 2012, 91, 1–15.
- Blunt,; Fayers, F.J.; Orr, F.M. Carbon dioxide in enhanced oil recovery. Energy Convers. Manag. 1993, 34, 1197–1204.
- Lackner, S. A guide to CO2 sequestration. Science 2003, 300, 1677–1678.
- Kang, M.; Fathi, E.; Ambrose, R.J.; Akkutlu, I.Y.; Sigal, R.F. Carbon dioxide storage capacity of organic-rich shales. SPE J. 2011, 16, 842–855.
- Merey, ; Sinayuc, C. Analysis of carbon dioxide sequestration in shale gas reservoirs by using experimental adsorption data and adsorption models. J. Nat. Gas Sci. Eng. 2016, 36, 1087–1105.
- Liu, ; Li, Y.; Agarwal, R.K. Numerical simulation of long-term storage of CO2 in Yanchang shale reservoir of the Ordos basin in China. Chem. Geol. 2016, 440, 288–305.
- Zhou, ; Hu, N.; Xian, X.; Zhou, L.; Tang, J.; Kang, Y.; Wang, H. Supercritical CO2 fracking for enhanced shale gas recovery and CO2 sequestration: Results, status and future challenges. Adv. Geo-Energy Res. 2019, 3, 207–224.
- Guiltinan, J.; Cardenas, M.B.; Bennett, P.C.; Zhang, T.; Espinoza, D.N. The effect of organic matter and thermal maturity on the wettability of supercritical CO2 on organic shales. Int. J. Greenh. Gas Control 2017, 65, 15–22.
- Zhan, ; Soo, E.; Fogwill, A.; Cheng, S.; Cai, H.; Zhang, K.; Chen, Z. A systematic reservoir simulation study on assessing the feasibility of CO2 sequestration in shale gas reservoirs with potential enhanced gas recovery. Carbon Manag. Technol. Conf. C 2017 Glob. CCUS Innov. Nexus 2017, 1, 33–44.
- Hoffman, ; Gustaf, O.; Andreas, L. Shale Gas and Hydraulic Fracturing: Framing the Water Issue; Printing by Ineko, Stockholm, Sweden, 2014; Volume 34.
- Nuttall, ; Eble, C.; Drahovzal, J.; Bustin, R. Analysis of Devonian Black Shales in Kentucky for Potential Carbon Dioxide Sequestration and Enhanced Natural; Kentucky Geol. Surv.,Lexington, Kentucky; 2005.
- Rochelle, A.; Czernichowski-Lauriol, I.; Milodowski, A.E. The impact of chemical reactions on CO2 storage in geological formations: A brief review. Geol. Soc. Spec. Publ. 2004, 233, 87–106.
- Pan, ; Hui, D.; Luo, P.; Zhang, Y.; Sun, L.; Wang, K. Experimental Investigation of the Geochemical Interactions between Supercritical CO2 and Shale: Implications for CO2 Storage in Gas-Bearing Shale Formations. Energy Fuels 2018, 32, 1963–1978.
- Wan, ; Tokunaga, T.K.; Ashby, P.D.; Kim, Y.; Voltolini, M.; Gilbert, B.; DePaolo, D.J. Supercritical CO2 uptake by nonswelling phyllosilicates. Proc. Natl. Acad. Sci. USA 2018, 115, 873–878.
- Modahl, I.S.; Nyland, C.A.; Raadal, H.L.; Kårstad, O.; Torp, T.A.; Hagemann, R. Life cycle assessment of gas power with CCS—A study showing the environmental benefits of system integration. Energy Procedia 2011, 4, 2470–2477.
- Khoo, H.H.; Tan, R.B.H. Life cycle investigation of CO2 recovery and sequestration. Environ. Sci. Technol. 2006, 40, 4016–4024.
- Bauer, C.; Heck, T.; Dones, R.; Mayer-Spohn, O.; Blesl, M. NEEDS (New Energy Externalities Developments for Sustainability). In Final Report on Technical Data, Costs, and Life Cycle Inventories of Advanced Fossil Power Generation Systems; Paul Scherrer Institut (PSI): Villigen, Switzerland, 2009.
- Viebahn, P.; Nitsch, J.; Fischedick, M.; Esken, A.; Schüwer, D.; Supersberger, N.; Edenhofer, O. Comparison of carbon capture and storage with renewable energy technologies regarding structural, economic, and ecological aspects in Germany. Int. J. Greenhouse Gas Control 2007, 1, 121–133.
- Schreiber, A.; Zapp, P.; Kuckshinrichs, W. Environmental assessment of German electricity generation from coal-fired power plants with amine-based carbon capture. Int. J. Life Cycle Assess. 2009, 14, 547–559.
- Korre, A.; Nie, Z.; Durucan, S. Life cycle modelling of fossil fuel power generation with post-combustion CO2 capture. Int. J. Greenh. Gas Control 2010, 4, 289–300.
- Marx, J.; Schreiber, A.; Zapp, P.; Haines, M.; Hake, J.F.; Gale, J. Environmental evaluation of CCS using Life Cycle Assessment—A synthesis report. Energy Procedia 2011, 4, 2448–2456.
- IEA. Greenhouse Gas R&D Programme (IEA GHG): Environmental Impact of Solvent Scrubbing of CO2. 2006. Available online: https://ieaghg.org/docs/General_Docs/Reports/2006-14%20Environmental%20Impact%20of%20Solvent%20Scrubbing%20of%20CO2.pdf (accessed on 29 May 2020).
- Bradshaw, J.; Bachu, S.; Bonijoly, D.; Burruss, R.; Holloway, S.; Christensen, N.P.; Mathiassen, O.M. CO2 storage capacity estimation: Issues and development of standards. Int. J. Greenh. Gas Control 2007, 1, 62–68.
- Bachu, S.; Bonijoly, D.; Bradshaw, J.; Burruss, R.; Holloway, S.; Christensen, N.P.; Mathiassen, O.M. CO2 storage capacity estimation: Methodology and gaps. Int. J. Greenh. Gas Control 2007, 1, 430–443.
- Yun, Y. Recent Advances in Carbon Capture and Storage; Janeza Trdine 9, 51000; IntechOpen: London, UK, 2017.
- Tayari, F.; Blumsack, S.; Dilmore, R.; Mohaghegh, S.D. Techno-economic assessment of industrial CO2 storage in depleted shale gas reservoirs. J. Unconv. Oil Gas Resour. 2015, 11, 82–94.
- Jia, B.; Tsau, J.S.; Barati, R. A review of the current progress of CO2 injection EOR and carbon storage in shale oil reservoirs. Fuel 2019, 236, 404–427.
- Bielicki, J.M.; Langenfeld, J.K.; Tao, Z.; Middleton, R.S.; Menefee, A.H.; Clarens, A.F. The geospatial and economic viability of CO2 storage in hydrocarbon depleted fractured shale formations. Int. J. Greenh. Gas Control 2018, 75, 8–23.
- Hoel, M.; Jensen, S. Cutting costs of catching carbon-Intertemporal effects under imperfect climate policy. Resour. Energy Econ. 2012, 34, 680–695.
- Durmaz, T. The economics of CCS: Why have CCS technologies not had an international breakthrough? Renew. Sustain. Energy Rev. 2018, 95, 328–340.
- Smith, L.A.; Gupta, N.; Sass, B.M.; Bubenik, T.A.; Byrer, C.; Bergman, P. Engineering and Economic Assessment of Carbon Dioxide Sequestration in Saline Formations. In Proceedings of the National Conference on Carbon Sequestration, Washington, DC, USA, 15–17 May 2001.
- REN21. Renewables 2017 Global Status Report; Technical report; Renewable Energy Policy Network for the 21st Century: Paris, France, 2017.
- Please check and comment entries here.