Molecular Dynamics Study on CO₂ Storage in Water-Filled Kerogen Nanopores in Shale Reservoirs: Effects of Kerogen Maturity and Pore Size

Abstract

CO₂ sequestration in shale reservoirs is an economically viable option to alleviate carbon emission. Kerogen, a major component in the organic matter in shale, is associated with a large number of nanopores, which might be filled with water. However, the CO₂ storage mechanism and capacity in water-filled kerogen nanopores are poorly understood. Therefore, in this work, we use molecular dynamics simulation to study the effects of kerogen maturity and pore size on CO₂ storage mechanism and capacity in water-filled kerogen nanopores. Type II kerogen with different degrees of maturity (II-A, II-B, II-C, and II-D) is chosen, and three pore sizes (1, 2, and 4 nm) are designed. The results show that CO₂ storage mechanisms are different in the 1 nm pore and the larger ones. In 1 nm kerogen pores, water is completely displaced by CO₂ due to the strong interactions between kerogen and CO₂ as well as among CO₂. CO₂ storage capacity in 1 nm pores can be up to 1.5 times its bulk phase in a given volume. On the other hand, in 2 and 4 nm pores, while CO₂ is dissolved in the middle of the pore (away from the kerogen surface), in the vicinity of the kerogen surface, CO₂ can form nano-sized clusters. These CO₂ clusters would enhance the overall CO₂ storage capacity in the nanopores, while the enhancement becomes less significant as pore size increases. Kerogen maturity has minor influences on CO₂ storage capacity. Type II-A (immature) kerogen has the lowest storage capacity because of its high heteroatom surface density, which can form hydrogen bonds with water and reduce the available CO₂ storage space. The other three kerogens are comparable in terms of CO₂ storage capacity. This work should shed some light on CO₂ storage evaluation in shale reservoirs.