Impacts of CO2 Injection on the Compaction Behaviour of Chalk Reservoirs

Abstract

The Danish council on climate change pointed at CO2 capture and storage (CCS) as crucial for reaching the 70 per cent target in reduction in greenhouse gas emissions by 2030. In this context and given the limited time frame, the storage potential and available infrastructures within chalk fields in the Danish North Sea cannot be ignored. However, to date, CCS initiatives have been limited to sandstone reservoirs because the high reactivity of calcium carbonate (dissolution, precipitation of new minerals) in contact with CO2 dissolved in the brine or present in a gas state was ranked as a risk. The onset of the Bifrost project, that aims at repurposing two depleted gas fields for CO2 storage, namely the Harald West sandstone field as the primary target and the neighboring Harald East chalk field as a potential upside, has nevertheless highlighted the current interest of industries and public authorities in chalk for CO2 storage. One of the challenges in assessing the feasibility of storing CO2 in chalk is the contradictory experimental results on the effects of supercritical CO2 (SC-CO2) on the mechanical properties of the rock. A wide range of deformation response reported on specimens flooded with SC-CO2 and spanning from a softening to strengthening of the elastic and plastic properties was reported, thereby making predictive models for short- to long-term reservoir behaviour difficult to implement. Therefore, a sensitivity study of the effects of SC-CO2 injection on the compaction of chalk reservoirs is here carried out. Based on published data, two end-member scenarios, a softening and strengthening of the elastic moduli and yield stresses of the rock were considered to cover a full spectrum of the potential deformation behaviour of the subsurface.

Keywords: Chalk reservoir, Compaction, Geomechanical simulation, North Sea