Investigation on the micro-mechanism of borehole instability in coalbed methane reservoirs by molecular simulation

Abstract

To investigate the micro-mechanism of borehole instability in coalbed methane reservoirs, diverse pore-structured reservoir models were constructed based on the structural characterization of demineralized coal. Subsequently, the behavior of coal-adsorbed water, adsorption energy and hydrogen bonding within adsorption system, and the mechanical properties of coal matrix with adsorbed water, were explored utilizing grand canonical ensemble Monte Carlo and molecular dynamics simulation. The findings demonstrate that the saturated adsorption capacity of reservoirs dominated by super-micropores is 2.04–99.5 times higher than that of ultra-micropores due to the fact that the adsorption sites coincide with the pore structure characteristics. And the adsorption potential energy distribution similar to the pore size distribution of coal matrix, highlighting the influence of pore structure on the interaction between water and coal. Furthermore, the average length of hydrogen bonds within reservoirs dominated by super-micropores is relatively short, indicating stronger intermolecular interactions within the adsorption system than that of ultra-micropores. Additionally, the coal-adsorbed water plays a pivotal role in borehole instability. For normal reservoirs, adsorbed water results in a decrease of 9.08–34.06 % in elastic modulus (E), weakening the mechanical properties of reservoir stratum during the drilling process. However, different internal stress status within coal seam led to distinct mechanisms of borehole instability, internal stress release within abnormal high-pressure reservoirs leads to coal matrix expansion, and changes in the pore structure cause a decrease of 31.24–75.97 % in E, substantial water filled in the loose pore structure of abnormal low-pressure reservoirs resists elastic deformation induced by external forces.