Low pressure sorption analysis and CO2 adsorption measurements were carried out on shale samples from the Utica and Permian basins. To determine the impact of rock mineralogy on the adsorbed volume of CO2, the mineral composition of the studied samples varied. For example, the clay content of the studied samples ranged between 5% and 53%. The low pressure sorption analysis was conducted to estimate the pore-size distribution for the shale samples. Then, CO2 adsorption was measured using the volumetric method applied on an in-house experimental apparatus to generate the CO2 adsorption isotherms. The adsorption measurements were performed on crushed samples to speed up the equilibrium process. During the adsorption measurements, the temperature of the system was maintained at 40oC.
The adsorption measurements revealed that the adsorbed volume was controlled mainly by the clay content. A ~30 wt% increase in clay content resulted in an increase of the adsorbed volume by an order of magnitude. The adsorbed volume of CO2 ranged between 27 scf/ton and 152 scf/ton for the Utica shale samples and between 60 scf/ton and 640 scf/ton for the Permian samples. Based on pore-size distribution analysis, clay minerals (i.e. illite) provided the surface area for adsorption. SEM images using energy-dispersive X-rate spectroscopy (EDS) obtained for a clay-rich shale sample confirmed that the platy structure of the illite provided the surface area for increased adsorption capacity.
CO2-based technologies are emerging in the oil and gas industry for CO2 storage, stimulation and enhanced recovery. This study has shown that clay content significantly impacts how CO2 interact with the rock. Therefore, to optimally design the CO2-based technologies, rock mineralogy shouldn’t be ignored.
Unconventional reservoirs, such as shale formations, are known to hold abundant amounts of hydrocarbons. However, extremely low intrinsic permeability is a common characteristic of all unconventional shale reservoirs. Therefore, advanced technologies that enable operators to develop and produce hydrocarbons commercially from those low permeability reservoirs are necessary. Among the enabling technologies is hydraulic fracturing. During hydraulic fracturing operations, large volumes of water are commonly used. This becomes a challenge in areas where water resources are scarce. Hence, a proposed supplement for water in hydraulic fracturing of shale reservoirs is hydraulic fracturing with CO2.
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