State of the art review of CO2 Storage Site Selection and Characterisation Methods

 
1. INTRODUCTION
 
2. GEOLOGICAL CHARACTERISATION OF THE SITE
 
3. FLOW MODELLING
 
4 REACTIVE FLOW MODELLING
 
5. COUPLED GEOMECHANICAL AND FLOW MODELLING
 
6. ENVIRONMENTAL IMPACT AND RISK ASSESSMENT
 
7. ECONOMIC ANALYSIS
 
8. PUBLIC PERCEPTION AND ACCEPTANCE
 
9. CONCLUSIONS AND RECOMMENDATIONS
 
 
6.1 Health, safety and environmental risks and impacts
 
6.2 Risk analysis
 
6.3 Risk assessment tools for CGS projects in various field cases
 
6.4 Application of risk assessment activities in various field cases ...
 
 
6.1.1 Local environmental impacts and risks at offshore storage sites
 
» 6.1.2 Local environmental impacts and risks at onshore storage sites
 
6.1.3 Evaluation of consequences versus environmental criteria
 
 

6.1.2 Local environmental impacts and risks at onshore storage sites

Onshore pipeline routes and development of the storage site may cause some environmental disturbance and interfere with other interests (land owners, nature protection areas, military training, etc.). The risks of CO2 leakage during separation, transport and injection are well known and subject to health and safety regulations. Potential CO2 leakage during pipeline transport and injection are usually restricted to the immediate vicinity of the leak, but they might represent a threat to people, animals and biodiversity of ecosystems nearby (Oldenburg et al., 2003).

CO2 quickly dissipates into the atmosphere; however, since it is heavier than air, there are known fatalities associated with natural releases of CO2 (Lewicki et al., 2006; WRI, 2008). Risks associated to a diffuse subsurface CO2 leakage on human health and safety are minimal in many regions, because of atmospheric mixing that prevents high atmospheric CO2 concentrations from making contact with a potential receptor (Bogen et al., 2006; Lewicki et al., 2006). In the atmosphere, CO2 concentrations are actually likely to be diluted rapidly below critical levels due to ground-layer turbulence. This can be observed at natural CO2 emissions sites and has also been confirmed by a leaking storage scenario (Oldenburg et al., 2003). However whenever surface conditions allow leaking CO2 to locally accumulate in areas with poor ventilation, high concentrations might be reached in depressions and confined spaces (basements or shallow dips in the ground) and might then be hazardous to humans and other living organisms causing stress or even asphyxiation (Chadwick et al., 2008; WRI, 2008). In built-up areas for instance, CO2 might accumulate in underground rooms of buildings, where even small rates of seepage can lead to hazardous concentrations in case of badly ventilated rooms (Chadwick et al., 2008).

Slow seakages of CO2 are known to have detrimental effects on burrowing fauna and flora. Indeed, air being much less mixed in soils than at the surface, hazardous concentrations in the ground might result from CO2 fluxes far smaller than those required to produce harm to above-ground organisms (Benson et al., 2002; Saripalli et al., 2003). At organism level, tolerance thresholds related to increased CO2 concentrations vary between species. However, because of differences in sensitivity, it might be difficult to determine a well-defined threshold beyond which CO2 cannot be tolerated and a continuum of impacts on ecosystems is more likely to occur (Chadwick et al., 2008), such as acidification of soils and displacement of oxygen in soils (IPCC, 2005). Plants will be affected as soon as roots become saturated with CO2 (WRI, 2008). The main characteristic of long-term elevated CO2 zones at the surface is actually the lack of vegetation: CO2 releases into vegetated areas cause noticeable die-off. In areas where significant impacts on vegetation have occurred, CO2 makes up about 20-95% of the soil gas, whereas normal soil gas usually contains about 0.2-4% CO2. Carbon dioxide concentrations above 5% might be dangerous for vegetation and for concentration about 20%, CO2 becomes phytotoxic. Today there is no evidence of any terrestrial impact on vegetation from current CO2 storage projects. However it has to be noted that the effect of CO2 on subsurface microbial populations is not well studied (IPCC, 2005).

Impacts of CO2 leakage on potential deep subsurface ecosystems, in and around the reservoir, might be significant (e.g. on microbes in the deep subsurface), but they might be considered as acceptable from an environmental viewpoint.

Brines displaced from deep formations by injected CO2 can potentially migrate or leak through fractures or defective wells to shallow aquifers and contaminate shallower drinkable water formations by e.g. increasing their salinity. In the worst case, infiltration of saline water into groundwater or into the shallow subsurface could impact wildlife habitat, restrict or eliminate agricultural use of land and pollute surface waters (IPCC, 2005). Risks to groundwater quality also arise from the potential for CO2 to mobilize organic or inorganic compounds, acidification, and contamination by trace compounds in the CO2 stream.. Possible groundwater pollution from migrating CO2 will cause a decrease in pH in groundwater Aquifers and may cause dissolution and alteration of minerals from rocks and soils that could release elements such as heavy metals, potentially contaminating fresh water supplies (Chadwick et al., 2008). In carbonate aquifers, carbonate dissolution along localised fluid (water and CO2) paths could create larger voids that might create sinkholes at the surface. Rapid ascent of water in larger fault zones accelerated by rising and expanding gas-bubbles could cause vigorous eruptions and surface craters in soil and incompetent rocks. Similarly, in fine-clastic unconsolidated sediments, suspensions might form and cause mud-volcanism and mudflows. Foundations of buildings might be damaged by seepage of carbonated groundwater in shallow unconsolidated sediments and soils, for example, historical city centres, other heritage objects, or archaeological sites. Undetected accumulations of CO2-supersaturated water or gaseous CO2 in shallow traps might be a risk for future drilling. Long-term risks might result from the gravitational Sinking of dense CO2 saturated brines; if they come into contact with salt formations this could lead to a degassing of the formation water and the ascent of CO2 outside of the original closed storage structure.

Injection of CO2 deep underground causes changes in pore-fluid pressures and in the geomechanical stress fields that propagate far beyond the volume occupied by the injected fluid (IPCC, 2005). Geomechanical risks are not necessarily directly linked to CO2 leakage (Chadwick et al., 2008). Under some circumstances, injection of large fluid volumes can generate seismic activity (Wesson and Craig, 1987). In most cases, these effects will remain quite small, but in certain circumstances they might be quite large. Differential movements along reactivated fault-lines in the caprocks could cause seismicity (Chadwick et al., 2008). Fault re-activation depends primarily on the extent and magnitude of the pore-fluid-pressure perturbations and is thus related to the quantity and rate of fluid injected (IPCC, 2005). Injection of CO2 near a fault will thus not automatically trigger a large earthquake (WRI, 2008). Neotectonically active or volcanic areas should of course be avoided (Chadwick et al., 2008). Lastly, it must be kept in mind that, even without causing any damage, microseismicity induced by CO2 injection might result in public concern. Non-seismic displacements of the Earth's surface could also damage built infrastructure, comparable to the effects of subsidence in underground mining areas. Vertical uplift above large reservoirs could affect lake levels and shift streams in lowland areas with low topographic relief. The risk of initiating a mud diapir in unconsolidated (plastic, water-rich, undercompacted) reservoir and overburden strata, possibly including the entire reservoir, because of the buoyancy of stored CO2 has not yet been investigated (Chadwick et al., 2008).