CO2 Storage Potential in the North Sea
Authors: Chamwudhiprecha Nalinee , Blunt Martin Julian 
Published: May 2016

Abstract
The feasibility of carbon dioxide (CO2) injection into an extensive aquifer in the North Sea is assessed. The impact of seal permeability, horizontal permeability, perforation interval, number of wells, aquifer size and cap rock size on the effectiveness of large-scale storage is assessed. A compositional numerical simulation is performed in the period of 50 years (30 years of injection and 20 years post-injection). We study volumes equivalent to the emissions of a large power station in the UK, 10 Mtonnes/year and find the necessary aquifer dimensions, cap rock size and horizontal permeability to allow safe storage assuming that the pressure increases by no more than 10%.
We find no impact of the number of wells on field-average pressure response as long as the total injection rate remains constant. However, using more wells enables CO2 to be trapped by immobilization and dissolution while giving poor sweep efficiency. In order to achieve the target injection rate with 5 injectors, two appropriate aquifer dimensions are proposed: an area of 3,850 km2 with thickness of 1,260 m; and an area of 11,550 km2 with thickness of 630 m. For the same aquifer volume, thickness plays a more important role on pressure response than area. Only deeper layers of the aquifer should be perforated in order to minimize pressure build-up and enhance CO2 displacement efficiency.

Introduction
CO2 emissions contribute towards the greenhouse effect and climate change (Ghanbari et al., 2006). The majority of anthropogenic CO2 comes from power and industry sectors, for example, fossil fuel combustion (IPCC, 2005). Carbon Capture and Storage or CCS has generated considerable interest because it is a way of reducing these emissions (Holloway et al., 2006). Various geological sites are considered suitable for storage in CCS, including depleted oil and gas reservoirs, deep saline aquifers and deep unminable coal seams (Gale, 2004). In the past, the main interest of CO2 injection relied on oil or gas reservoirs as an enhanced oil recovery (EOR) technique where CO2 and the remaining oil in place become miscible and the oil can therefore be extracted from the reservoir. However, saline aquifers are currently considered as potential sequestration sites since they have a large estimated capacity and wide distribution throughout the globe (Gale, 2004, Nicot, 2008). Saline aquifer sequestration was first mentioned in 1992 (Van der Meer, 1992) and currently, there are several successful projects of aquifer injection, for example, Sleipner (Norway), In Salah (Algeria), Ketzin (Germany), and K12B (Netherlands) which prove the feasibility of this emerging storage option. However, saline aquifer storage may cause several problems including migration into groundwater leading to contamination (Gale, 2004) and risk of overpressure, causing fracturing and possible leakage due to its shallow position. Pressure build-up is also one of the associated risks for aquifer storage, as large amounts of fluid are added. This could eventually induce fracturing or aquifer deformation which negatively impacts storage security.