Quantifying the risk of leakage of CO2 from subsurface storage sites

Rationale and Aims: Carbon capture and storage (CCS) is one of the mitigation strategies with the greatest potential to reduce global CO2 emissions this century. The UK is well placed to implement this technology because of its existing oil and gas infrastructure, and its wealth of geologically suitable storage sites. A draft EU directive on CCS sets out strict conditions for potential storage sites regarding environmental impact assessment and monitoring requirements. However, at present, there is no internationally agreed protocol to quantify the risk that once emplaced, the CO2 will leak back to the surface or pollute groundwater resources. One major obstacle to such a protocol being agreed is the limited understanding of leakage mechanisms that might occur within the sealing sequences located above the storage reservoir. By analogy, similar mechanisms that are classically invoked for leakage from oil and gas accumulations might also be expected to apply to CO2 leakage. These comprise two end members: membrane or capillary leakage and hydraulic leakage. In addition, it has recently been recognised that the sealing sequences above a reservoir might be compromised by seal bypass systems that allow large fluid fluxes to bypass the pore or fracture networks. Identification of such bypass systems is thus a key requirement for any seal risking protocol, along with physical property characterisation of the sealing lithologies themselves. Bypass systems can be identified above any potential storage sites using 3D seismic data, but are limited by the spatial resolution of the seismic method. One possible novel approach to the identification of seal bypass, is by identification of seismic amplitude anomalies that can be shown to be associated with methane in the pore space of layers within the sealing sequence. By defining the spatial distribution of these methane anomalies, it would be possible to model the likeliest flow routes taking the methane through the seal, and hence define the leakage pathways for the vertically migrating methane. In this way, the spatial analysis of methane anomalies could act as a tracer for potential CO2 leakage, and so help define the risk of leakage. The aims of this PhD research are to develop workflows and methodologies of using 3D seismic data to track potential leakage routes of CO2 through likely sealing lithologies. The main thrust of this research will be to quantify the leakage flux of methane via contrasting leakage routes to yield a relative risk-based ranking of bypass systems and other leakage mechanisms for a range of potential storage sites. The hypothesis to be tested is that seal bypass systems can be uniquely identified by their impact on methane leakage. By placing this geospatial analysis of the leakage pathways in a context of the geological history, standard risk analysis techniques used by the petroleum industry can then be applied to risk of seal failure and leakage for CO2. The development of a workflow for evidence-based risk analysis of seal failure will be generic, and should ultimately be applied more widely to the global problem of assessing the viability of CO2 storage sites. The primary method will be 3D seismic interpretation and geospatial analysis of seal bypass systems and methane-related seismic anomalies above potential CO2 storage sites. Workstation interpretation and visualisation facilities in the Royal Society/Wolfson Laboratory for CCS in Cardiff and in Statoil’s CCS Research Laboratory in Trondheim will be available. Methane-related seismic anomalies will be identified through rock physics modelling (Hampson-Russell) and wireline log calibration in petroleum boreholes using standard petrophysical interpretation software in Trondheim (TerraLog). The project will mainly exploit Statoil’s comprehensive seismic/well database in the North Sea, one of the primary targets for shallow CCS in Europe, and where Statoil has pioneered CCS in the Sleipner Project.