Abstract
The Draupne shale is a rock formation functioning as overburden for gas reservoirs in the Norwegian Sea and potentially as caprock for future CO2 storage locations as well. In this paper, the Draupne shale was exposed to several fluids: CO2 gas, supercritical CO2, CO2 gas dissolved in brine, supercritical CO2 dissolved in brine, as well as brine and dry air. The motivation for the exposure tests was to investigate whether injected CO2 in a reservoir coming into contact with the caprock could change the caprock’s mechanical properties and increase the risk for leakage of the stored CO2. In addition, a systematic exposure study will provide more insight into the various processes susceptible of altering the shale’s shear strength and acoustic velocity, such as clay hydration, mineral dissolution, and capillary forces. Due to the low permeability of the shale, experiments were conducted on mm-sized disk samples, reducing fluid diffusion into the shale, and allowing for many repeated tests on disks close by in the original core. The punch method, where a small circle is punched out of the shale, was used to assess shear strength, while continuous wave technique was used to assess ultrasonic velocity. Results show that the shale is not noticeably sensitive to CO2, in the sense that no additional weakening is observed in the presence of CO2 as compared to brine exposure. This last weakening effect is probably due to poor matching between pore fluid salinity and exposure brine strength.
Highlights
Carbon dioxide (CO2) capture and storage is considered an indispensable mitigation action to reduce the atmospheric emissions of CO2 from human activities, if the Paris agreement goals are to be reached (Masson-Delmotte et al 2018; Rubin and De Coninck 2005)
Scanning Electron Microscopy (SEM) imagining was done on the same sample before and after exposure tests to observe the change in chemical properties on the sample surface
A change in chemical composition is only observed for the sample exposed to the combination of brine and C O2
Summary
Carbon dioxide (CO2) capture and storage is considered an indispensable mitigation action to reduce the atmospheric emissions of CO2 from human activities, if the Paris agreement goals are to be reached (Masson-Delmotte et al 2018; Rubin and De Coninck 2005). Loss of containment of the stored CO2 is connected to the integrity of the sealing caprock; integrity can be lost as a consequence of too high increase in the reservoir pore pressure and accompanying stress changes in the caprock, or stress concentration reactivating. The processes occurring during exposure to dry CO2 are drying, precipitation of minerals, adsorption of minerals, dissolution of organic matter, swelling and shrinkage of clay. Exposure to brine saturated CO2 solution can result in dissolution of minerals, precipitation and reprecipitation of minerals and swelling/shrinkage of clays. These processes can lead to a change in composition, microstructure, and mechanical properties of the shale
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