Changes

Saline formations are deep [[sedimentary rock]]s saturated with formation waters that are unsuitable for human consumption or agriculture. They have been identified by many studies as one of the best potential options for CO₂ geological storage (e.g., <ref name=Bachu_2000>Bachu, S., 2000, Sequestration of CO₂ in geological media: Criteria and approach for site selection in response to climate change: Energy Conversion and Management, v. 41, no. 9, p. 953–970.</ref> and <ref name=Bradshawetal_2002 />). Possible drawbacks of saline formations are that the containment potential of the seal is commonly untested and limited amounts of data are commonly available for site characterization. However, their main advantages are that they are distributed widely over the world and their potential storage capacity is large.<ref name=Koideetal_1992>Koide, H., Y. Tazaki, Y. Noguchi, S. Nakayama, M. Iijima, K. Ito, and Y. Shindo, 1992, Subterranean containment and long-term storage of carbon dioxide in unused aquifers and in depleted natural gas reservoirs: Energy Conversion and Management, v. 33, no. 5–8, p. 619–626.</ref><ref name=Hendriksandblok_1993>Hendriks, C. A., and K. Blok, 1993, Underground storage of carbon dioxide: Energy Conversion and Management, v. 34, no. 9–11, p. 949–957.</ref><ref name=Riggetal_2001>Rigg, A. J., G. Allinson, J. Bradshaw, J. Ennis-King, C. M. Gibson-Poole, R. R. Hillis, S. C. Lang, and J. E. Streit, 2001, The search for sites for geological sequestration of CO₂ in Australia: A progress report on GEODISC: The Australian Petroleum Production and Exploration Association (APPEA) Journal, v. 41, no. 1, p. 711–725.</ref><ref name=IPCC_2005 />

Saline formations are deep [[sedimentary rock]]s saturated with formation waters that are unsuitable for human consumption or agriculture. They have been identified by many studies as one of the best potential options for CO₂ geological storage (e.g., <ref name=Bachu_2000>Bachu, S., 2000, Sequestration of CO₂ in geological media: Criteria and approach for site selection in response to climate change: Energy Conversion and Management, v. 41, no. 9, p. 953–970.</ref> and <ref name=Bradshawetal_2002 />). Possible drawbacks of saline formations are that the containment potential of the seal is commonly untested and limited amounts of data are commonly available for site characterization. However, their main advantages are that they are distributed widely over the world and their potential storage capacity is large.<ref name=Koideetal_1992>Koide, H., Y. Tazaki, Y. Noguchi, S. Nakayama, M. Iijima, K. Ito, and Y. Shindo, 1992, Subterranean containment and long-term storage of carbon dioxide in unused aquifers and in depleted natural gas reservoirs: Energy Conversion and Management, v. 33, no. 5–8, p. 619–626.</ref><ref name=Hendriksandblok_1993>Hendriks, C. A., and K. Blok, 1993, Underground storage of carbon dioxide: Energy Conversion and Management, v. 34, no. 9–11, p. 949–957.</ref><ref name=Riggetal_2001>Rigg, A. J., G. Allinson, J. Bradshaw, J. Ennis-King, C. M. Gibson-Poole, R. R. Hillis, S. C. Lang, and J. E. Streit, 2001, The search for sites for geological sequestration of CO₂ in Australia: A progress report on GEODISC: The Australian Petroleum Production and Exploration Association (APPEA) Journal, v. 41, no. 1, p. 711–725.</ref><ref name=IPCC_2005 />

The main geological constraints for finding the right place to store CO₂ include a [[Porosity|porous]] and [[Permeability|permeable]] [[reservoir]] rock overlain by an impermeable [[cap rock]]. Because the stored CO₂ is less dense than the formation water, it will naturally rise to the top of the reservoir, and a trap is needed to ensure that it does not reach the surface. The CO₂ can be trapped by several different mechanisms (such as [[Structural trap system|structural]] or [[Stratigraphic trap|stratigraphic]], [[Hydrodynamic trap|hydrodynamic]], [[Residual gas trap|residual gas]], [[Solubility trap|solubility]], and [[Mineral trap|mineral trapping]]), with the exact mechanism depending on the specific geological conditions. Structural or stratigraphic trapping relates to the free-phase (immiscible) CO₂ that is not dissolved into [[formation water]]. When supercritical CO₂ rises upward by buoyancy, it can be physically trapped in a structural or stratigraphic trap in exactly the same manner as a [[hydrocarbon]] accumulation. The nature of a structural or stratigraphic trap depends on the geometric arrangement of the reservoir and seal units. The CO₂ can be hydrodynamically trapped in horizontal or dipping reservoirs with no defined structural closures when the dissolved and immiscible CO₂ travels with the formation water for very long residence (migration) times of the order of thousands to millions of years.<ref name=Bachuetal_1994>Bachu, S., W. D. Gunter, and E. H. Perkins, 1994, Aquifer disposal of CO₂: Hydrodynamic and mineral trapping: Energy Conversion and Management, v. 35, no. 4, p. 269–279.</ref> Residual gas trapping occurs when the saturation of CO₂ falls below a certain level and it becomes trapped in the pore spaces by capillary pressure forces and ceases to flow.<ref name=Enniskingandpaterson_2001>Ennis-King, J., and L. Paterson, 2001, Reservoir engineering issues in the geological disposal of carbon dioxide, in D. J. Williams, R. A. Durie, P. McMullan, C. A. J. Paulson, and A. Y. Smith, eds., Greenhouse gas control technologies: Proceedings of the 5th International Conference on Greenhouse Gas Control Technologies: Cairns, CSIRO Publishing, p. 290–295.</ref><ref name=Holtz_2002>Holtz, M. H., 2002, [https://www.onepetro.org/conference-paper/SPE-75502-MS Residual gas saturation to aquifer influx]: A calculation method for 3-D computer reservoir model construction: Society of Petroleum Engineers Gas Technology Symposium, April 30–May 2, 2002, Calgary, Alberta, Canada, SPE Paper No. 75502, 10 p.</ref><ref name=Flettetal_2005>Flett, M. A., R. M. Gurton, and I. J. Taggart, 2005, Heterogeneous saline formations: Long-term benefits for geo-sequestration of greenhouse gases, in E. S. Rubin, D. W. Keith, and C. F. Gilboy, eds., Greenhouse gas control technologies: Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies: Oxford, Elsevier, v. 1, p. 501–509.</ref> Solubility trapping relates to the CO₂ dissolved into the formation water.<ref name=Koideetal_1992 /> The time scale for complete dissolution is critically dependent on the vertical permeability and the geometry of the top seal but is predicted to occur on a scale of hundreds to thousands of years.<ref name=Enniskingandpaterson_2002>Ennis-King, J., and L. Paterson, 2002, [https://www.onepetro.org/conference-paper/SPE-77809-MS Engineering aspects of geological sequestration of carbon dioxide]: Society of Petroleum Engineers Asia Pacific Oil and Gas Conference and Exhibition, October 8–10, 2002, Melbourne, Australia, SPE Paper No. 77809, 13 p.</ref> Mineral trapping results from the precipitation of new carbonate minerals following the interaction of the CO2 with the in-situ formation water and the minerals of the host rock.<ref name=Gunteretal_1993>Gunter, W. D., E. H. Perkins, and T. J. McCann, 1993, Aquifer disposal of CO₂-rich gases: Reaction design for added capacity: Energy Conversion and Management, v. 34, no. 9–11, p. 941–948.</ref> This storage mechanism is the most permanent of the trapping types discussed because it renders the CO₂ immobile.<ref name=Bachuetal_1994 />

The main geological constraints for finding the right place to store CO₂ include a [[Porosity|porous]] and [[Permeability|permeable]] [[reservoir]] rock overlain by an impermeable [[cap rock]]. Because the stored CO₂ is less dense than the formation water, it will naturally rise to the top of the reservoir, and a trap is needed to ensure that it does not reach the surface. The CO₂ can be trapped by several different mechanisms (such as [[Structural trap system|structural]] or [[Stratigraphic trap|stratigraphic]], [[Hydrodynamic influence on trapping|hydrodynamic]], [[Residual gas trap|residual gas]], [[Solubility trap|solubility]], and [[Mineral trap|mineral trapping]]), with the exact mechanism depending on the specific geological conditions. Structural or stratigraphic trapping relates to the free-phase (immiscible) CO₂ that is not dissolved into [[formation water]]. When [[Wikipedia:Supercritical carbon dioxide|supercritical CO₂]] rises upward by buoyancy, it can be physically trapped in a structural or stratigraphic trap in exactly the same manner as a [[hydrocarbon]] accumulation. The nature of a structural or stratigraphic trap depends on the geometric arrangement of the reservoir and seal units. The CO₂ can be hydrodynamically trapped in horizontal or dipping reservoirs with no defined structural closures when the dissolved and immiscible CO₂ travels with the formation water for very long residence ([[migration]]) times of the order of thousands to millions of years.<ref name=Bachuetal_1994>Bachu, S., W. D. Gunter, and E. H. Perkins, 1994, Aquifer disposal of CO₂: Hydrodynamic and mineral trapping: Energy Conversion and Management, v. 35, no. 4, p. 269–279.</ref> Residual gas trapping occurs when the saturation of CO₂ falls below a certain level and it becomes trapped in the pore spaces by capillary pressure forces and ceases to flow.<ref name=Enniskingandpaterson_2001>Ennis-King, J., and L. Paterson, 2001, Reservoir engineering issues in the geological disposal of carbon dioxide, in D. J. Williams, R. A. Durie, P. McMullan, C. A. J. Paulson, and A. Y. Smith, eds., Greenhouse gas control technologies: Proceedings of the 5th International Conference on Greenhouse Gas Control Technologies: Cairns, CSIRO Publishing, p. 290–295.</ref><ref name=Holtz_2002>Holtz, M. H., 2002, [https://www.onepetro.org/conference-paper/SPE-75502-MS Residual gas saturation to aquifer influx]: A calculation method for 3-D computer reservoir model construction: Society of Petroleum Engineers Gas Technology Symposium, April 30–May 2, 2002, Calgary, Alberta, Canada, SPE Paper No. 75502, 10 p.</ref><ref name=Flettetal_2005>Flett, M. A., R. M. Gurton, and I. J. Taggart, 2005, Heterogeneous saline formations: Long-term benefits for geo-sequestration of greenhouse gases, in E. S. Rubin, D. W. Keith, and C. F. Gilboy, eds., Greenhouse gas control technologies: Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies: Oxford, Elsevier, v. 1, p. 501–509.</ref> Solubility trapping relates to the CO₂ dissolved into the formation water.<ref name=Koideetal_1992 /> The time scale for complete dissolution is critically dependent on the vertical permeability and the geometry of the top seal but is predicted to occur on a scale of hundreds to thousands of years.<ref name=Enniskingandpaterson_2002>Ennis-King, J., and L. Paterson, 2002, [https://www.onepetro.org/conference-paper/SPE-77809-MS Engineering aspects of geological sequestration of carbon dioxide]: Society of Petroleum Engineers Asia Pacific Oil and Gas Conference and Exhibition, October 8–10, 2002, Melbourne, Australia, SPE Paper No. 77809, 13 p.</ref> Mineral trapping results from the precipitation of new carbonate minerals following the interaction of the CO2 with the in-situ formation water and the minerals of the host rock.<ref name=Gunteretal_1993>Gunter, W. D., E. H. Perkins, and T. J. McCann, 1993, Aquifer disposal of CO₂-rich gases: Reaction design for added capacity: Energy Conversion and Management, v. 34, no. 9–11, p. 941–948.</ref> This storage mechanism is the most permanent of the trapping types discussed because it renders the CO₂ immobile.<ref name=Bachuetal_1994 />

[[file:CO2TrappingMechanisms.JPG|thumb|400px|{{figure number|2}}Schematic representation of the change of dominant trapping mechanisms and increasing CO₂ storage security with time.<ref name=Kaldietal_2009 />]]

[[file:CO2TrappingMechanisms.JPG|thumb|400px|{{figure number|2}}Schematic representation of the change of dominant trapping mechanisms and increasing CO₂ storage security with time.<ref name=Kaldietal_2009 />]]

In any geological storage site, the injected CO₂ will ultimately be trapped by several of the mechanisms described above. The type of trapping that occurs, and when, is dependent on the dynamic [[flow behavior]] of the CO₂ and the time scale involved. With increasing time, the dominant storage mechanism will change and typically the storage security also increases. [[:file:CO2TrappingMechanisms.JPG|Figure 2]] shows how the initial storage mechanism will dominantly be physical structural and stratigraphic trapping of the immiscible-phase CO₂. With increasing time and migration, more CO₂ is trapped residually in the pore space or is dissolved in the formation water, increasing the storage security. Finally, mineral trapping may occur after the geochemical reaction of the dissolved CO₂ with the host rock mineralogy, permanently trapping the CO₂.

In any geological storage site, the injected CO₂ will ultimately be trapped by several of the mechanisms described above. The type of trapping that occurs, and when, is dependent on the dynamic [[Fundamentals of fluid flow|flow behavior]] of the CO₂ and the time scale involved. With increasing time, the dominant storage mechanism will change and typically the storage security also increases. [[:file:CO2TrappingMechanisms.JPG|Figure 2]] shows how the initial storage mechanism will dominantly be physical structural and stratigraphic trapping of the immiscible-phase CO₂. With increasing time and migration, more CO₂ is trapped residually in the pore space or is dissolved in the formation water, increasing the storage security. Finally, mineral trapping may occur after the geochemical reaction of the dissolved CO₂ with the host rock mineralogy, permanently trapping the CO₂.

==Site characterization==

==Site characterization==