Experimental Investigation of Long-Term CO2 Exposure to Brine-Saturated Reservoir Chalk Core Material
Authors: Mokhtari R. , Talaei A. , Feilberg K. 
Published: October 2023

Abstract
Geological media have long been regarded as a viable method to mitigate the environmental impact of human-caused CO2 emissions from specific sources, by storing carbon in the subsurface. The depleted hydrocarbon chalk fields found in the Danish North Sea area hold promise as potential sites for carbon storage. These fields possess significant pore volumes and many are nearing the end of their production lifespan. To unlock Denmark's main storage potential and develop cost-effective, low-risk CO2 storage solutions in depleted former hydrocarbon chalk reservoirs, studies of the potential and robustness of these reservoirs for storage are needed. The potential for CO2 storage in carbonate reservoirs, such as limestones, dolomites, and chalks, is substantial, given that these types of hydrocarbon reservoirs are prevalent worldwide. However, it is crucial to carefully investigate the geochemical response of chalk and other fractured and layered carbonate rocks under consideration to CO2, as well as the resulting geomechanical consequences, in order to minimize the risks associated with storage. Laboratory experiments involving exposure of chalk samples under representative reservoir temperature and pressure conditions, geochemical analyses, and geomechanical tests conducted at in-situ conditions are presented here. The chemical processes that occur at the rock's surface when in contact with natural formation water and dissolved CO2 can lead to potentially significant changes within the reservoir, both during injection and over time. If CO2 is injected as supercritical CO2, it undergoes solvation when it comes into contact with the formation water forming carbonated water. Alternatively, CO2 can be injected in the form of carbonated water directly, leading to immediate acidification of the water in contact with the rock. The geochemical reactions between carbonate rock and dissolved CO2 depend on various factors, including thermodynamic conditions, water salinity, pressure, and they can potentially cause significant dissolution of the rock matrix, either near the wellbore or in the wider rock matrix. Other factors that influence the safety and efficiency of storage include the presence of residual hydrocarbons, specific lithography, and local formation water chemistry. Dissolution can result in general geomechanical weakening, subsidence, leakage, changes in permeability, and altered flow paths. The rate of dissolution can vary, occurring rapidly during injection or gradually over the longer storage period of hundreds of years. The experiments conducted within examine the short to medium-term responses of chalk to CO2 exposure, considering the relevant physical and chemical processes. The experiments presented here show the response of real reservoir chalk from the Danish North Sea to formation water and CO2 exposure under reservoir conditions and it is observed that the scenarios where the chalk rock is in contact with the water phase leads to measurable but limited dissolution of the chalk indicating that CO2 storage in chalk might be more viable than previously thought.