CO₂ Wettability of Shales and Coals as a Function of Pressure, Temperature and Rank: Implications for CO₂ Sequestration and Enhanced Methane Recovery

The underground geological CO₂ storage into oil and gas reservoirs and/or saline aquifers is a promosing technique to reduce anthropogenic greenhouse gas emissions which thus ensures clean environment. CO₂ can also be injected into coal beds and shale formations where it gets trapped by means of adsorption trapping with additional benefits of enhanced methane recovery. In this context, wettability of CO₂/coal/brine and CO₂/shale/brine systems plays an important role in governing the suitable storage conditions. Wettability of a given system is a function of injection pressure, reservoir temperature and type of the coal or shale. Despite the vital benefits, relatively less attention has been given to CO₂ injection in coals and shales for storage and enhanced methane recovery purposes. Therefore, in order to access the storage potential in coals and shales, we experimentally tested CO₂-wettability by advancing and receding contact angles measurement using a drop-pendant titled plate technique for coals of high, medium and low ranks (data taken from previous work) and three organic-rich shale samples of varying TOC at in-situ pressure and temparture conditions. We found that both advancing and receding contact angles increased with increase in pressure and decreased with increase in temperature irrespective of the type of sample analysed. Moreover, at any given pressure and temperature, high rank coals exhibited higher values of contact angles (more CO₂-wet surfaces) in comparison to low rank coals. Similarly, high-TOC shales were more non-wetting as compared to low TOC shales. In summary, higher the organic carbon content of coal/shale, higher were the CO₂-wettability of the system under investigation. The increased CO₂-wettability of coals and shales implies that the injected CO₂ will be distributed rather uniformly in organic material of the coal or shale formation thus resulting in better adsorption of CO₂ into the micropores. Moreover, since it is experimentally proven by previous studies that adsorption capacity of CO₂ is up to ~9 times higher than that of CH4, therefore, higher CO₂-wettablity will result in improved displacement of methane towards production wells due to preferential adsorption of CO₂ over CH4. We finally conclude that high rank coals and high-TOC shales are better for CO₂ storage and methane gas prodcution due to better CO₂-wetting and that the benefits are further improved if formation temperature is low and injection pressure is high. The results of this study, therefore, lead to a guideline for optimum coal and shale formation selection for CO₂ injection.