Dynamic evolution characteristics of coalmacromolecules and micropores during CO2 injection: A molecular dynamics study
Jianxin Li, Songhang Zhang, Kai Wang, Shuheng Tang, Jienan Pan, Zhaodong Xi, Tengfei Jia
Abstract
The matrix deformation induced by CO 2 injection into coal reservoirs significantly impedes the successful implementation of CO 2 -ECBM technology. It is widely recognized that such deformation results from the changes in molecular structures, yet the characteristics of molecular rearrangement and the resulting evolution of micropore structures during CO 2 injection remain unquantified. Molecular dynamics simulations were employed to investigate the injection of CO 2 into coal macromolecules under varying pressures, quantitatively characterizing the dynamic responses of micropores and macromolecular structures. The results revealed that the expansion of micropore volume during CO 2 injection exhibited synchronous changes with macromolecular rearrangement. At injection pressures ranging from 4 MPa to 12 MPa of CO 2 , the experimental results indicated that micropore volume expanded by 6.07 %, 27.68 %, and 33.75 %, respectively, while the degree of orientational order of macromolecular units decreased by 2.88 %, 4.57 %, and 8.59 % relative to pristine coal macromolecules. The observed increase in micropore volume was attributed to the combined effects of open-pore expansion and closed-pore compression. Specifically, the proportion of open pores progressively rose from an initial 84.90 % to 86.95 %, 91.20 %, and 91.94 %, while the closed-pore fraction exhibited a corresponding decline from 15.10 % to 13.05 %, 8.80 %, and 8.06 %. The observed reduction in structural ordering resulted from macromolecular rearrangement, primarily driven by CO 2 -induced disruption of van der Waals interactions, which decreased the interaction energy by 0.58 %, 4.67 %, and 5.86 % compared to pristine coal macromolecules. The conformations of different chemical groups changed during CO 2 injection, with aliphatic and oxygen groups exhibiting significantly greater deformation than aromatic structures. With increasing coal metamorphism, a reduction in the levels of aliphatic and oxygen groups was observed, accompanied by a rise in the quantity of aromatic rings, which enhanced van der Waals interactions. Therefore, high-rank coal exhibits minimal molecular structural deformation during CO 2 injection, consistent with the swelling behavior observed in laboratory experiments. This study, for the first time, reveals the molecular-level mechanism of matrix deformation during CO 2 -ECBM, advancing knowledge of the theoretical usefulness for CO 2 -ECBM recovery and sequestration.