A Review on the Influence of CO2/Shale Interaction on Shale Properties: Implications of CCS in Shales

Abstract

Carbon capture and storage (CCS) is a developed technology to minimize CO2 emissions and reduce global climate change. Currently, shale gas formations are considered as a suitable target for CO2 sequestration projects predominantly due to their wide availability. Compared to conventional geological formations including saline aquifers and coal seams, depleted shale formations provide larger storage potential due to the high adsorption capacity of CO2 compared to methane in the shale formation. However, the injected CO2 causes possible geochemical interactions with the shale formation during storage applications and CO2 enhanced shale gas recovery (ESGR) processes. The CO2/shale interaction is a key factor for the efficiency of CO2 storage in shale formations, as it can significantly alter the shale properties. The formation of carbonic acid from CO2 dissolution is the main cause for the alterations in the physical, chemical and mechanical properties of the shale, which in return affects the storage capacity, pore properties, and fluid transport. Therefore, in this paper, the effect of CO2 exposure on shale properties is comprehensively reviewed, to gain an in-depth understanding of the impact of CO2/shale interaction on shale properties. This paper reviews the current knowledge of the CO2/shale interactions and describes the results achieved to date. The pore structure is one of the most affected properties by CO2/shale interactions; several scholars indicated that the differences in mineral composition for shales would result in wide variations in pore structure system. A noticeable reduction in specific surface area of shales was observed after CO2 treatment, which in the long-term could decrease CO2 adsorption capacity, affecting the CO2 storage efficiency. Other factors including shale sedimentary, pressure and temperature can also alter the pore system and decrease the shale “caprock” seal efficiency. Similarly, the alteration in shales’ surface chemistry and functional species after CO2 treatment may increase the adsorption capacity of CO2, impacting the overall storage potential in shales. Furthermore, the injection of CO2 into shales may also influence the wetting behavior. Surface wettability is mainly affected by the presented minerals in shale, and less affected by brine salinity, temperature, organic content, and thermal maturity. Mainly, shales have strong water-wetting behavior in the presence of hydrocarbons, however, the alteration in shale’s wettability towards CO2-wet will significantly minimize CO2 storage capacities, and affect the sealing efficiency of caprock. The CO2/shale interactions were also found to cause noticeable degradation in shales’ mechanical properties. CO2 injection can weaken shale, decrease its brittleness and increases its plasticity and toughness. Various reductions in tri-axial compressive strength, tensile strength, and the elastic modulus of shales were observed after CO2 injection, due to the dissolution effect and adsorption strain within the pores. Based on this review, we conclude that CO2/shale interaction is a significant factor for the efficiency of CCS. However, due to the heterogeneity of shales, further studies are needed to include various shale formations and identify how different shales’ mineralogy could affect the CO2 storage capacity in the long-term.

Linking multi-scale 3D microstructure to potential enhanced natural gas recovery and subsurface CO2storage for Bowland shale, UK

“…Oxygen-containing groups including −COOH, −H and −OH bonds, have been shown to have a severe impact on the adsorption of CO 2 and CH 4 on organic matter and mineral surface at reservoir conditions [50]. CO 2 injection causes a reduction in the intensity of C-O and C-H bonds within organic matter in shales [54], leading to an increase in hydrocarbon recovery from organic matter [55]. In practice, clay minerals have a significant impact on gas adsorption on shale [56], although in the presence of water the organic content reveals a stronger correlation with gas adsorption than clay minerals [57].…”

Revisiting Geologic Storage Potential in Unconventional Formations Is Key to Proactive Decision Making on CCS in India

“…It is also noteworthy that pilot-scale trials have not been carried out for CO 2 injection in shales as opposed to coal seams. However, based on laboratory studies, some potential constraints in caprock seal efficiency, porosity reduction and reduced tensile strength have been noted (Fatah et al, 2020).…”

On the Effect of CO2 on Seismic and Ultrasonic Properties: A Novel Shale Experiment

“…Dewhurst et al [18] reiterated that shale dehydration may alter the rock properties being measured: strength and elastic properties (Young's modulus and Poisson's ratio) from triaxial testing are particularly impacted by pore fluid loss. Bhuiyan et al [31], Fatah et al [36] neatly summarized CO 2 -shale interactions in terms of CCS implications.…”