Permeability evolution of deep-buried coal based on NMR analysis: CO2 adsorption and water content effects

Abstract

Permeability evolution of deep-buried coal plays a crucial role in CO2 geological storage and coalbed methane (CBM) recovery. In this study, permeability and nuclear magnetic resonance (NMR) tests were conducted on coal specimens under triaxial conditions. Based on the evolution of relaxation time (T2), the effects of CO2 adsorption and water content on permeability, pore structure, and mechanical properties of coal were discussed. Experimental results show that CO2 injection causes adsorption-induced swelling, inhibits flow in the early stage, causes the “expansion” and “complexity” of pore structure, and generates new channels for fluid transport. The presence of water in coal inhibits the flow process, resulting in an overall reduction of permeability by 2 orders of magnitude. Softening occurs in coal specimens, as evidenced by plasticity and low brittle failure and an average strength loss of 29.3 %. Additionally, a complex CO2–H2O coupling effect in coal is observed, causing the dissolution of certain minerals and organic matter, which further amplifies the “weakening effect” of water. Finally, permeability evolution characteristics of coal affected by CO2 and H2O from multiple perspectives were discussed in consideration of NMR fractal analysis. This work provides valuable insights into deep CO2 geological sequestration and CBM recovery.