One of the most significant problems facing the oil and gas industry nowadays is the environmental impact resulting from carbon dioxide (CO2) emissions. One of the most important initiative in the industry therefore has become the carbon capture, utilization and storage (CCUS) initiative. This research focuses on the carbon storage aspect of the CCUS. The research introduces the main mechanisms by which CO2 is injected into oil and gas reservoirs, and how the CO2 can be stored in the unconventional shale reservoirs. The main mechanism of storage, adsorption, is explained, and the isotherms used to model adsorption of CO2 to shale are listed and their advantages and limitations are described. Another main focus of this research is the capacity of CO2 that can be stored in shale via adsorption. The research therefore covers the adsorption capacity in scf/ton for different shale plays at different thermodynamic conditions. Following this, the research explains the main factors that will impact long terms storage of CO2 in shale reservoirs, since this is a paramount factor for safe and enduring CO2 disposal. Finally, the advantages and limitations of depending on adsorption as a main mechanism for CO2 storage are explained. This research can be viewed as a guideline to understating the mechanism of adsorption and its limitation and advantages for CO2 storage in unconventional hydrocarbon shale reservoirs.

Introduction

Carbon dioxide (CO2) emissions have resulted in severe environmental impacts and long lasting damage to the atmospheric balance. One of the prominently useful methods to reduce the carbon footprint of the oil and gas industry is the utilization of CO2 for increasing oil recovery and then storing the CO2 in the underground reservoirs (Fakher, S. 2019a; 2019b; 2020; Fakher, S. et al., 2018; 2019; 2020). CO2 can be injected into the reservoir in different methods depending on the formation, however, for unconventional shale reservoirs, many researchers have reported a high oil recovery rate when using the cyclic injection method, also referred to as huff-n-puff (Fakher, S. and Imqam, A., 2018; 2019; 2020; Sheng, J., 2017; Vega, B. et al., 2010). The mechanisms by which the CO2 is stored in these unconventional reservoirs are various, however the main mechanism for storage in the majority of the shale reservoirs is adsorption (Fakher, S. and Imqam, A., 2020d).

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