During carbon dioxide (CO2) injection in oil shale reservoirs for enhanced oil recovery operations, a volume of the injected CO2 can be stored in the shale reservoir. This is highly advantageous in many ways, including the increase in oil recovery associated with the CO2 injection, and the ability to reduce CO2 emissions from the injection process by storing a volume of the injected CO2 downhole. This research studies the storage potential of a shale sample from the Kansas Basin for CO2 storage and the impact of some of the factors on the storage capacity. The adsorption was measured using the pressure differential method (volumetric) using a specially designed setup. The void space between the shale particles was measured initially and was then used in the adsorption calculation. Results showed that the shale sample had a reasonable CO2 adsorption capacity, at different thermodynamic conditions.
CO2 injection is an enhanced oil recovery (EOR) method used to increase oil recovery from oil reservoirs. This technology has been applied since the early forties in conventional oil reservoirs, however it has just recently been applied in unconventional shale reservoirs in an attempt to increase oil recovery for these reservoirs beyond the conventional 7–10% obtained from hydraulic fracturing (Heller, and Zobak, 2014; Fakher, et al. 2018; 2018b; 2018d; 2019a; 2019b; Fakher, S. 2019; 2020; Fakher and Imqam, 2020a). During its propagation through the reservoir, the CO2 can also adsorb to the shale formation. This adsorption can result in a storage of a volume of the injected CO2 in the reservoir, and thus aids in the reduction of the greenhouse emissions that occur due to the production from the shale reservoirs.
Adsorption can be defined as a bond deficiency that results in the adherence of the adsorbent onto the adsorbate. There are different types of adsorption depending on the strength of the bond between the adsorbate and the adsorbent (Jeldi et al. 2017). In chemical adsorption, also known as chemisorption, the bond strength is extremely high and thus the adsorption is very strong. This is highly advantageous due to the extremely small potential for desorption, loss of adsorption. The main drawback of chemical adsorption is the fact that it is a monolayered adsorption. This means that only one layer of adsorbent will adsorb to the adsorbate. This decreases the overall adsorption capacity greatly, and thus reduces the potential for storage. Luckily however, chemical adsorption is not the main adsorption type that occurs between CO2 and the shale. The main adsorption that occurs during CO2 injection in shale reservoirs is referred to as physical adsorption, also known as physisorption. Physical adsorption is a multilayer adsorption which indicates that multiple layers of CO2 can be stacked on each other to create a greater storage potential (Jin and Firoozabadi, 2016; Kang, et al., 2011; Khosrokhavari et al. 2014). The drawback of physical adsorption however is the fact that it is a weak adsorption that is held via Van Der Waals forces, and thus there is a large potential for desorption to occur.