Nanomechanical properties of CH4-containing coal during CO2 storage under different injection pressures based on molecule dynamics

The mechanical properties of the storage medium are one of the key factors ensuring the safety of CO2 storage. The Grand canonical Monte Carlo and Molecular Dynamic were conducted in this paper to investigate the micromechanical characteristics of storage medium after CO2 storage in the coal molecular layer and the pore wall between the layers. Four different initial pressures (0.5, 1, 1.5, and 2 MPa) of CH4-containing coal molecular pore wall models were established, five different pressure ratios (1/1, 2/1, 3/1, 4/1, and 5/1) of CO2 were injected into the model, and the uniaxial compression of the storage medium was implemented. According to the stress-strain curve of the elastic phase, the mechanical parameters (i.e., Young's modulus, Poisson's ratio, and bulk modulus) and sensitivity index under different CO2 injection pressures were analyzed. The results showed that the storage characteristics of CH4 and CO2 in coal were both consistent with Langmuir characteristics, and the relative concentration of CO(2 )in the pore wall of model increased with the increase of CO(2 )injection pressure. The stress-strain curves of model and pore wall subjected to different storage pressure were different under the same external stress loading, that was, higher gas content made the model and pore wall easier to resist deformation caused by external stress. The van der Waals energy and electrostatic energy in the model gradually increased with the increasing gas content according to Lennard-Jones potential energy. The Young's modulus, Poisson's ratio and bulk modulus of the coal molecular model and pore wall all increased with the increasing gas content. When the CO(2 )injection pressure was larger than 3-4 times the CH(4 )pressure, the sensitivity index of the model's mechanical parameters significantly improved.