The impact of water on gas storage in organic-rich rocks

Abstract

To achieve CO2 storage in organic-rich rocks such as coal and shale, the influence of H2O generated by organic matter and co-occurring CH4 and N2 cannot be neglected. Based on the molecular model of coal and kerogen established using Materials Studio, and verification of CH4 isothermal adsorption from 20 to 100 oC, the competitive adsorption between CH4、CO2、N2 and H2O is discussed. Both the coal and shale kerogen molecular models show similar relative adsorption capacities, namely H2O > CO2 > CH4 > N2. Competitive adsorption is more obvious in small pores with a greater apparent inhibiting effect of H2O on the gases. The adsorption of CO2, CH4 and N2 decreases to zero at 75 ℃ in the kerogen model. A positive correlation is shown between the charge transfer and the adsorption capacity, and the adsorption onto carboxyl group occurs indicate that H2O has the highest adsorption preference. The oxygen-containing functional group is the dominant adsorption site for H2O, with the carboxyl group being the best adsorption site. The hydroxyl group is the optimal site for all gases, while all lower than H2O. The adsorption heat released are of H2O>CO2 >CH4 >N2 sequences, which is consistent with the change in the amount adsorbed.

Keywords

• Coalbed methane
• Macromolecular model
• Competitive adsorption
• Molecule simulation
• Functional groups
• Density flooding theory