Changes

Risks surrounding CO₂ geosequestration are summarized by Bowden and Rigg.<ref name=Bowdenandrigg_2004>Bowden, A. R., and A. J. Rigg, 2004, Assessing risk in CO₂ storage projects: The Australian Petroleum Production and Exploration Association (APPEA) Journal, v. 44, no. 1, p. 677–702.</ref> Essentially, the main concerns relate to the potential for unanticipated CO₂ leakage either caused by unanticipated CO₂ movement up the wellbore or along an unexpected geological migration path or caused by induced or natural seismicity. The risks associated with CO₂ storage, although considered very low, are characterized by a greater degree of uncertainty than those connected with CO₂ transport and injection. This is because of the fact that once the CO2 enters the geological reservoir, its fate is transferred from mostly human control to a natural system. Although most of the existing knowledge of CO₂ behavior in the subsurface exists from the long history of CO₂ floods associated with EOR, the risks associated with large-scale storage are at a different scale. The quantities of CO₂ stored for EOR floods are smaller, and the CO₂ residence times are shorter than required for large-scale carbon geosequestration. For geosequestration of CO₂, the risk of leakage depends on not only the likelihood of existence of potential leakage pathways (such as wells, faults, permeable zones in the seal, etc.), but also the likelihood that these potential pathways will intersect CO₂ while it is in a mobile phase and finally the likelihood that the potential leakage pathway will leak.<ref name=Riggetal_2006>Rigg, A., A. Bowden, M. N. Watson, 2006, Risk assessment of long-term containment of CO2 in geological storage projects: Abstracts volume, AAPG International Conference and Exhibition, November 5–8, 2006, Perth, Australia, v. 16, p. 117.</ref> As many containment risk assessments are benchmarked against an impact of 1% leakage over 1000 years,<ref name=IPCC_2005 /> the frequency, duration, and volume of potential leakage events need to be assessed for this time frame.<ref name=Riggetal_2006 />

Risks surrounding CO₂ geosequestration are summarized by Bowden and Rigg.<ref name=Bowdenandrigg_2004>Bowden, A. R., and A. J. Rigg, 2004, Assessing risk in CO₂ storage projects: The Australian Petroleum Production and Exploration Association (APPEA) Journal, v. 44, no. 1, p. 677–702.</ref> Essentially, the main concerns relate to the potential for unanticipated CO₂ leakage either caused by unanticipated CO₂ movement up the wellbore or along an unexpected geological migration path or caused by induced or natural seismicity. The risks associated with CO₂ storage, although considered very low, are characterized by a greater degree of uncertainty than those connected with CO₂ transport and injection. This is because of the fact that once the CO2 enters the geological reservoir, its fate is transferred from mostly human control to a natural system. Although most of the existing knowledge of CO₂ behavior in the subsurface exists from the long history of CO₂ floods associated with EOR, the risks associated with large-scale storage are at a different scale. The quantities of CO₂ stored for EOR floods are smaller, and the CO₂ residence times are shorter than required for large-scale carbon geosequestration. For geosequestration of CO₂, the risk of leakage depends on not only the likelihood of existence of potential leakage pathways (such as wells, faults, permeable zones in the seal, etc.), but also the likelihood that these potential pathways will intersect CO₂ while it is in a mobile phase and finally the likelihood that the potential leakage pathway will leak.<ref name=Riggetal_2006>Rigg, A., A. Bowden, M. N. Watson, 2006, Risk assessment of long-term containment of CO2 in geological storage projects: Abstracts volume, AAPG International Conference and Exhibition, November 5–8, 2006, Perth, Australia, v. 16, p. 117.</ref> As many containment risk assessments are benchmarked against an impact of 1% leakage over 1000 years,<ref name=IPCC_2005 /> the frequency, duration, and volume of potential leakage events need to be assessed for this time frame.<ref name=Riggetal_2006 />

Any geological CO₂ storage project resulting in a catastrophic release of CO₂ is highly unlikely. This is because the pressure of CO₂ injected into a geological reservoir reduces as it moves away from the injection well and is diffused over large areas of the formation, thus avoiding large pressure buildups.<ref name=Streitandhillis_2002>Streit, J. E., and R. R. Hillis, 2002, [https://www.onepetro.org/conference-paper/SPE-78226-MS Estimating fluid pressures that can induce reservoir failure during hydrocarbon depletion]: Society of Petroleum Engineers/International Society for Rock Mechanics (ISRM) Rock Mechanics Conference, October 2002, Irving, Texas, SPE Paper No. 78226, 7 p.</ref> No record of a catastrophic CO₂ release from a natural CO₂ deposit to date (2009) is observed, and any potential release from a CO₂ storage project should be preventable through careful site selection, operation, and monitoring. The critical geological input to minimize the risk of such occurrence is seal analysis and geomechanics. The CO₂ column height calculations can be used to assess the safe storage volumes of CO₂ at any storage site,<ref name=Danielandkaldi_2009>Daniel, R. F., and J. G. Kaldi, 2009, [http://archives.datapages.com/data/specpubs/study59/CHAPTER18/CHAPTER18.HTM Evaluating seal capacity of cap rocks and intraformational barriers for CO₂ containment], in M. Grobe, J. C. Pashin, and R. L. Dodge, eds., Carbon dioxide sequestration in geological media—State of the science: AAPG Studies in Geology 59, p. 335–345.</ref> whereas geomechanical analysis of the faults can be used to assess the maximum sustainable pore fluid pressures at potential storage sites (Streit and Hillis, 2004). Understanding the rock fluid interaction potential between CO₂ and contacted minerals is also important. The CO₂ creates carbonic acid when it mixes with H₂O, and the risk of whether this could potentially result in the chemical erosion of some seal lithologies leading to leakage needs to be addressed. In most instances, because of their low permeability and capillary properties, CO₂ is unlikely to enter seals. Therefore, any potential reactions are likely to be limited to the base of the seal. In addition, because the pH buffering capabilities of the seal lithology are generally greater than the dissolution capabilities of carbonic acid, reactions are likely to be mineral precipitation instead of dissolution, thus leading to seal capacity enhancement instead of degradation.<ref name=Watsonetal_2005>Watson, M. N., R. F. Daniel, P. R. Tingate, and C. M. Gibson-Poole, 2005, CO₂-related seal capacity enhancement in mudstones: Evidence from the pine lodge natural CO₂ accumulation, Otway Basin, Australia, in M. Wilson, T. Morris, J. Gale, and K. Thambimuthu, eds., Greenhouse gas control technologies: Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies: Oxford, Elsevier, v. 2, part 2, p. 2313–2316.</ref>

Any geological CO₂ storage project resulting in a catastrophic release of CO₂ is highly unlikely. This is because the pressure of CO₂ injected into a geological reservoir reduces as it moves away from the injection well and is diffused over large areas of the formation, thus avoiding large pressure buildups.<ref name=Streitandhillis_2002>Streit, J. E., and R. R. Hillis, 2002, [https://www.onepetro.org/conference-paper/SPE-78226-MS Estimating fluid pressures that can induce reservoir failure during hydrocarbon depletion]: Society of Petroleum Engineers/International Society for Rock Mechanics (ISRM) Rock Mechanics Conference, October 2002, Irving, Texas, SPE Paper No. 78226, 7 p.</ref> No record of a catastrophic CO₂ release from a natural CO₂ deposit to date (2009) is observed, and any potential release from a CO₂ storage project should be preventable through careful site selection, operation, and monitoring. The critical geological input to minimize the risk of such occurrence is seal analysis and geomechanics. The CO₂ column height calculations can be used to assess the safe storage volumes of CO₂ at any storage site,<ref name=Danielandkaldi_2009>Daniel, R. F., and J. G. Kaldi, 2009, [http://archives.datapages.com/data/specpubs/study59/CHAPTER18/CHAPTER18.HTM Evaluating seal capacity of cap rocks and intraformational barriers for CO₂ containment], in M. Grobe, J. C. Pashin, and R. L. Dodge, eds., Carbon dioxide sequestration in geological media—State of the science: [http://store.aapg.org/detail.aspx?id=739 AAPG Studies in Geology 59], p. 335–345.</ref> whereas geomechanical analysis of the faults can be used to assess the maximum sustainable pore fluid pressures at potential storage sites (Streit and Hillis, 2004). Understanding the rock fluid interaction potential between CO₂ and contacted minerals is also important. The CO₂ creates carbonic acid when it mixes with H₂O, and the risk of whether this could potentially result in the chemical erosion of some seal lithologies leading to leakage needs to be addressed. In most instances, because of their low permeability and capillary properties, CO₂ is unlikely to enter seals. Therefore, any potential reactions are likely to be limited to the base of the seal. In addition, because the pH buffering capabilities of the seal lithology are generally greater than the dissolution capabilities of carbonic acid, reactions are likely to be mineral precipitation instead of dissolution, thus leading to seal capacity enhancement instead of degradation.<ref name=Watsonetal_2005>Watson, M. N., R. F. Daniel, P. R. Tingate, and C. M. Gibson-Poole, 2005, CO₂-related seal capacity enhancement in mudstones: Evidence from the pine lodge natural CO₂ accumulation, Otway Basin, Australia, in M. Wilson, T. Morris, J. Gale, and K. Thambimuthu, eds., Greenhouse gas control technologies: Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies: Oxford, Elsevier, v. 2, part 2, p. 2313–2316.</ref>

Induced seismicity is not expected to be a significant problem at geological CO₂ storage sites. Induced seismicity has been documented during hydrocarbon production, EOR, AGI, natural gas storage, and waste injection operations.<ref name=Wallrothetal_1996>Wallroth, T., A. J. Jupe, and R. H. Jones, 1996, Characterization of a fractured reservoir using microearthquakes induced by hydraulic injections: Marine and Petroleum Geology, v. 13, no. 4, p. 447–455.</ref><ref name=Maxwelletal_1998>Maxwell, S. C., R. P. Young, R. Bossu, A. J. Jupe, and J. Dangerfield, 1998, [https://www.onepetro.org/conference-paper/SPE-47276-MS Microseismic logging of the Ekofisk reservoir]: Proceedings of the 1998 Society of Petroleum Engineers/International Society for Rock Mechanics (ISRM) Eurock '98, July 8–10, Trondheim, Norway, SPE Paper No. 47276, 7 p.</ref><ref name=Jupeetal_2000>Jupe, A. J., R. Jones, S. Wilson, and J. Cowles, 2000, [https://www.onepetro.org/conference-paper/SPE-63131-MS The role of microearthquake monitoring in hydrocarbon reservoir management]: Society of Petroleum Engineers Annual Technical Conference and Exhibition, October 1–4, 2000, Dallas, SPE Paper No. 63131, 16 p.</ref> These induced seismic events have been caused by poor engineering practices such as the injection of the CO₂ at too high a pressure, which in turn can result in microfracturing of the reservoir rock and/or small movement along existing fracture lines. Note, however, that most of the recorded events have been of a very small magnitude and have caused no harm. Moreover, the risk of induced seismicity can be reduced through careful siting and placement of injection wells, adherence to proper pressure guidelines, and a sound understanding of the geomechanical properties of the storage reservoir. A range of technologies can be identified by a rigorous process of risk assessment and conformance to clearly identify performance criteria, which can be subsequently verified. These criteria are agreed in conjunction with the regulatory authorities to manage the project through all phases, addressing responsibilities and liabilities and providing assurance of safe storage to the satisfaction of the public at large.

Induced seismicity is not expected to be a significant problem at geological CO₂ storage sites. Induced seismicity has been documented during hydrocarbon production, EOR, AGI, natural gas storage, and waste injection operations.<ref name=Wallrothetal_1996>Wallroth, T., A. J. Jupe, and R. H. Jones, 1996, Characterization of a fractured reservoir using microearthquakes induced by hydraulic injections: Marine and Petroleum Geology, v. 13, no. 4, p. 447–455.</ref><ref name=Maxwelletal_1998>Maxwell, S. C., R. P. Young, R. Bossu, A. J. Jupe, and J. Dangerfield, 1998, [https://www.onepetro.org/conference-paper/SPE-47276-MS Microseismic logging of the Ekofisk reservoir]: Proceedings of the 1998 Society of Petroleum Engineers/International Society for Rock Mechanics (ISRM) Eurock '98, July 8–10, Trondheim, Norway, SPE Paper No. 47276, 7 p.</ref><ref name=Jupeetal_2000>Jupe, A. J., R. Jones, S. Wilson, and J. Cowles, 2000, [https://www.onepetro.org/conference-paper/SPE-63131-MS The role of microearthquake monitoring in hydrocarbon reservoir management]: Society of Petroleum Engineers Annual Technical Conference and Exhibition, October 1–4, 2000, Dallas, SPE Paper No. 63131, 16 p.</ref> These induced seismic events have been caused by poor engineering practices such as the injection of the CO₂ at too high a pressure, which in turn can result in microfracturing of the reservoir rock and/or small movement along existing fracture lines. Note, however, that most of the recorded events have been of a very small magnitude and have caused no harm. Moreover, the risk of induced seismicity can be reduced through careful siting and placement of injection wells, adherence to proper pressure guidelines, and a sound understanding of the geomechanical properties of the storage reservoir. A range of technologies can be identified by a rigorous process of risk assessment and conformance to clearly identify performance criteria, which can be subsequently verified. These criteria are agreed in conjunction with the regulatory authorities to manage the project through all phases, addressing responsibilities and liabilities and providing assurance of safe storage to the satisfaction of the public at large.