Linking outcrop analogue with flow simulation to reduce uncertainty in sub-surface carbon capture and storage: an example from the Sherwood Sandstone Group of the Wessex Basin, UK

Modelling the behaviour of carbon dioxide (CO2) injected into sub-surface reservoirs as part of carbon capture and storage (CCS) strategies is often performed using models that incorporate very limited geological detail, particularly at the subseismic (metre to decametre) scale. Those modelling studies that incorporate varying degrees of geological realism show the inherent risks and uncertainties that can result from neglecting heterogeneity and reservoir–caprock topography along the migration path of an injected CO2 plume. A key problem is that detailed geological data are often not available for the relatively deep saline aquifers that are an important target for CCS. Deep saline aquifers fall between the relatively data-rich environments of shallow freshwater aquifers and hydrocarbon reservoirs and it is in these settings that outcrop analogues may play an important part in reducing the risks and uncertainties associated with CCS. This study uses an example from the Sherwood Sandstone Group (Otter Sandstone Formation) of the Wessex Basin to show how an outcrop study can impart a much greater understanding of heterogeneity in critical reservoir–caprock zones. Here the transition from the Sherwood Sandstone Group (fluvial sandstone reservoir) to the Mercia Mudstone Group (playa lacustrine mudstone seal) is not simple, but includes a major change in fluvial style that introduces considerable heterogeneity at the top of the reservoir. The study shows how laser-scanned outcrop can be used to rapidly construct static geological models that are taken through to flow simulations. In combination, the use of appropriate outcrop analogues and flow modelling can reduce the risks and uncertainties associated with CCS.