Rheology of Coal at Particle Level Characterized by Nanoindentation

Rheological deformation of coal could severely jeopardize the extraction performance during deep coal mining and coalbed methane development by causing instability, shrinkage and collapse of drilling boreholes in the long run. Standard macroscale creep test could only reveal the bulk properties of coal while neglecting the high heterogeneity nature of coal mainly composed of various organic macerals. This study took a low-rank (0.39% R o ) humic coal as research object, and employed nanoindentation technique to study its mechanical and rheological heterogeneity from the particle level. The results showed that mechanical heterogeneity within the humic coal is manifested as inertinite being much harder and less likely to undergo creep than vitrinite. By focusing on vitrinite, both mechanical properties and creep resistance showed negative relationships with loading rate and indentation size. The relationship of creep displacement and time can be best depicted by the model with one spring and two Kelvin–Voigt bodies. To probe into the structure-dependent rheological properties of organic matter in geo-field, carbon stacking structure from X-ray diffraction, creep behavior via nanoindentation and stress distribution simulated by finite-element analysis of coal were compared with another natural carbon aggregation as solid bitumen. The results showed that the coal investigated exhibited a relatively weaker creep resistance than solid bitumen, which is a consequence of its looser stacking manner with small size of crystallites in plane but large layer space inside. The observations here signified the role of chemical structure in determining rheology of organic matter.