Time-lapse seismic feasibility studies for planning CO2 geological sequestration monitoring campaigns

Time-lapse seismic monitoring of CO2 geological sequestration activities is a crucial process to ascertain continued containment and conformance of CO2 within the subsurface storage site. Time-lapse seismic monitoring produces a 3D image of the injected CO2 plume within the subsurface, helping to identify CO2 migration pathways and determine if and where leakage has occurred. Planning a time-lapse seismic campaign is a site-specific process that involves a multidisciplinary effort in building geological computational models, reservoir fluid flow simulations and time-lapse seismic modelling. It is a common workflow in oil and gas production activities and can be redeployed for CO2 geological sequestration. However, CO2 storage involves fluid physics processes that are more complex than most oil and gas conditions, which requires the use of specific physics models to properly predict fluid flow and seismic monitoring behaviour. These complexities are mainly related to the fact that CO2 is highly miscible in formation water, oil and hydrocarbon gas. For this reason, compositional fluid flow simulations must be used to model CO2 injection, rather than the simpler black-oil fluid models more commonly used for oil and gas production. Current commercial reservoir simulators are well capable of such complex simulations. However, this dissolution process must also be modelled in detail in the seismic modelling workflow, and this is largely neglected in time-lapse seismic feasibility studies. We first present a workflow for taking CO2 dissolution processes into account in time-lapse seismic modelling. Then we present a full time-lapse feasibility workflow applied to two different North Sea reservoirs, a saline aquifer and a depleted hydrocarbon gas reservoir. We show the importance of taking such detailed physics processes into account in these two different storage situations. As well as show the importance of time-lapse seismic feasibility studies for planning CO2 monitoring campaigns, in order to achieve the desired objectives and necessary requirements to verify CO2 containment and conformance.