# Investigation of Properties Alternation during Super-Critical CO\({}_{2}\) Injection in Shale
## 1 Introduction  
The boom of Bakken formation in North America is limited by the effective recovery method even with the recent advancement of hydraulic fracturing and horizontal drilling technology utilization [1]. Since then, the CO\({}_{2}\) flooding method has been proposed as the potential way to increase the recovery factor for the unconventional shale reservoir in the Bakken formation, especially combined with the advanced water-alternating-gas (CO\({}_{2}\)-WAG) and CO\({}_{2}\) huff and puff technologies [2; 3; 4; 5; 6; 7]. Meanwhile, the injection of the supercritical CO\({}_{2}\) into the shale and coal bed methane reservoir would assist the emission reduction of greenhouse gases [8; 9; 10]. However, the Sc-CO\({}_{2}\) injection process may change the mechanical properties such as the Young's modulus and hardness [11; 12; 13]. Meanwhile, the physical nature of the rock such as pore structure and pore size will alter with CO\({}_{2}\) adsorption [12], let alone the mineral components varied mainly due to the chemical reaction [14]. Therefore, it is critical to investigate and evaluate the properties' alteration of the Bakken shale which is being treated with Sc-CO\({}_{2}\) injection. To date, many researches had focused on the investigation of the effect of Sc-CO\({}_{2}\) to the shale. [PERSON] et al. [14] examined the shale from Gippslan Basin, Australia, and characterized the shale sealing efficiency as the caprock. They found that several chemical reactions between the rock-forming minerals and the brine-Sc-CO\({}_{2}\) system occurred. [PERSON] et al. [15] also investigated the shale from Australia and stated that the geophysical properties, such as the shale porosity, permeability and diffusion nature were significantly affected by the carbonate dissolution behavior by Sc-CO\({}_{2}\) adsorption. [PERSON] et al. [16] conducted several lab tests to investigate the mechanical properties alteration of a low-clay shale which acquired from Sichuan Basin, China. They illustrated that both the subcritical and supercritical CO\({}_{2}\) soaking processes can decrease the rock strength while enhancing the shale ductility. [PERSON] et al. [17] investigated the variation in pore structure of the shale treated with Sc-CO\({}_{2}\). They identified that the temperature will have the positive effect on the specific surface area increment of shale under the Sc-CO\({}_{2}\) environment. [PERSON] et al. [18] studied the structural and chemical properties of the shale by conducting four types of instrumental analysis after the Sc-CO\({}_{2}\) soaking procedure. They believed that the Sc-CO\({}_{2}\) treatment reduced the shale strength and altered the mineral content of the shale. [PERSON] et al. [19] demonstrated the specific surface area and porosity of the shale samples were changed with the Sc-CO\({}_{2}\) treatment, which relies heavily on the variation of pore structure and permeability caused by the Sc-CO\({}_{2}\) adsorption. To the best of our knowledge, research on the corrosion effect in Middle Bakken shale by Sc-CO\({}_{2}\) injection is very scarce, and the pore structure and strength of shale under Sc-CO\({}_{2}\) treatment is not well understood. Therefore, the main objective of this research was to assess mechanical, physical and mineralogical alteration of Middle Bakken shale samples exposed to Sc-CO\({}_{2}\) under various adsorption durations. Instead of the macroscale tests that most researchers applied to analyze the mechanical property changes during the Sc-CO\({}_{2}\) adsorption, in this study, the advanced nano-indentation test was applied to test the Young's Modulus. The low-temperature N\({}_{2}\) adsorption test was conducted to analysis the pore structure and pore size properties. X-ray diffraction (XRD) was utilized to investigate the mineral content of the samples. This study is significant for the CO\({}_{2}\) storage and migration after being injected into reservoir.