Fracture Evolution and Permeability-Relevant Behavior in Gondwana Coal of the Damodar Basin: Insights from Acoustic Emission Monitoring

Coalbed methane (CBM) and geological CO 2 sequestration (GCS) in deep coal seams are critically dependent on coal seam permeability, which is in turn governed by the evolution of fractures under variable stresses. Despite the significant CBM and GCS potential of the Gondwana coal in the Damodar Basin, their geomechanical properties and fracture dynamics have not yet been well documented. In this work, an integrated laboratory investigation of permeability-relevant fracture evolution in coal from the Jharia, Ramgarh, and West Bokaro coalfields, using strength tests, petrographic characterization, and acoustic emission (AE) monitoring, was undertaken. The uniaxial compressive strength (UCS) values of coal samples ranged from 3.28 to 22.59 MPa, while tensile strength (TS) values ranged from 0.65 to 2.25 MPa. The variation was found to be significantly dependent on maceral composition, the vitrinite/inertinite ratio, and the cleat structure. The results of the AE monitoring indicate a systematic transition from tensile-dominated microcracking at low stress levels to increasing shear-event proportions at advanced loading stages, along with a progressive increase in cumulative AE energy and b-value reduction (by 75–90%) before peak stress. AE-monitored fracture mode transitions reflect permeability-indicative fracture behavior as tensile cracking occurs due to cleat widening and improved flow pathway formation, while shear deformations occur through compaction and closure of flow pathways. Higher inertinite content and heterogeneous composition caused Jharia coal to exhibit early failure initiation and a higher percentage of shear cracks, whereas vitrinite-rich Ramgarh coal demonstrated progressive failure behavior characterized by predominant tensile cracking. West Bokaro coal displayed an intermediate behavior due to the presence of different macerals in these coal seams. These findings provide mechanistic insights into how geological heterogeneity governs fracture behavior in the Damodar Basin coal.