Key Reports:

State-of-the-Art of Monitoring Methods to evaluate Storage Site Performance

1.2.2 Potential leakage pathways

Monitoring of "Health, Safety and Environment (HSEHealth, safety and environment) risks" is focussed on the Earth's surface or the shallow subsurface. The probability of negative effects on protected subjects is highest where pathways could facilitate the ascent of fluids from the storage reservoir to the surface (Fig. 1-2). Such pathways have to be detected and mapped, and their properties have to be determined during site characterisation. This information provides input to site-specific risk assessments, which, in turn, provide fundamental data for setting up site-specific monitoring plans that include monitoring of these pathways.

Fig. 1-2: Schematic representation of potential leakage pathways for CO2 injected into saline formations (not to scale; slightly modified after v. Goerne et al., 2010).

Potential leakage pathways may comprise:

Caprocks (a; Fig. 1-2): A central task of site characterisation is to demonstrate that thickness, strength, lateral distribution and sealing properties of caprocks facilitate safe and efficient storage of CO2Carbon dioxide. However, the presence of potentially weak spots of caprocks that could provide leakage pathways cannot be excluded. Indications for leakage through caprocks by such unknown pathways can be obtained by monitoring secondary containment formations. The selection of suitable sites and parameters is critical for the early detection of such, potentially diffuse, leakage. For example in anticlinal structures, the largest pressure differences across a caprock above a static gas column prevail at the top of a structure. Thus, this might be a strategic point for monitoring caprock integrity. The risks of undiscovered "gaps" in caprock can be further minimised by monitoring areas where general geological features indicate chances for pathways. Such indicators could be trends and variations of sedimentary facies or formation thickness.

Faults (b; Fig. 1-2): Permeable faults in caprocks and in the overburden of reservoirs may provide pathways for fluid ascent and hence imply potential HSEHealth, safety and environment risks. Older faults are often impermeable, sealed by mineralisation. Faults in neo-tectonic active regions may also provide barriers to fluid flow e.g. through fault gouge or clay in unconsolidated sediments. Within the reservoir these faults may act as barriers to fluid flow and limit injectivity and reduce storage capacity and, thus, pose economical risks to storage operators.

Fault zones often comprise networks of faults and fractures that are difficult to characterise in seismic images. Fault properties vary along fault planes. Hydraulic properties of faults can change due to pressures induced by CO2Carbon dioxide injection. Closed faults become permeable, when pressures exceed the fault strength (e.g. chiaramonte, 2008). Geochemical reactions between fluids and adjacent rock or precipitation of minerals from ascending fluids may lead to self-sealing of faults or dissolution of carbonate fracture fillings. Therefore, detection and prediction of possible fluid pathways along faults is rather uncertain, so that faults need to receive special attention in monitoring.

Boreholes (c; Fig. 1-2): Boreholes, especially from improperly installed and/or abandoned wells, may provide direct leakage pathways between reservoirs, groundwater, and the surface. Due to technical improvements in well cementation and logging, recently sealed boreholes are often considered safer than older, plugged ones, where less information on the well condition may be available. Thus, monitoring of older, plugged wells has to be considered in monitoring plans.

Spill points (d; Fig. 1-2) of structural traps are crucial areas for monitoring the movement of a buoyant CO2 plume in saline aquifers. The actual expansion of a gas plume in a reservoir may be different from simplified reservoir simulations. In addition, spill points may be difficult to map in gently inclined structures. Spill points may be the starting points of leakage pathways. If a CO2 plume expands beyond spill points, it has to be carefully monitored. The ascent of fluids may follow a combination of several of the pathways described above in case of leakage. An illustrative example for such a complex leakage path is provided by the incident at the Bad Lauchstädt gas storage (Katzung et al., 1996), where gas leaking from a cavern storage well at 110 m depth found its way via faults and secondary accumulations to the surface. Finally, gas burst to surface in several vents in a zone of 1.5 km length. Scenarios of such combined pathways have to be considered in risk assessments and for the positioning of monitoring instruments.