Geological CO2 storage and methane recovery in heterogeneous coal seams: Fractal descriptors for parameter upscaling

CO 2 -enhanced coalbed methane recovery (CO 2 -ECBM) combines geological CO 2 storage with methane recovery as part of the carbon capture and storage (CCS) system. CO 2 injection alters coal pore–fracture systems. Evaluating reservoir responses requires further parameterization of the structural information obtained from laboratory characterization. Fractal descriptors have been used to quantify pore–fracture heterogeneity and relate structural characteristics to reservoir properties, but their interpretation and application in CO 2 -ECBM models remain inconsistent. This review summarizes fractal approaches to characterizing structural changes in coal after CO 2 exposure and their incorporation into adsorption, diffusion, deformation and permeability models. Quantitative analysis shows that fractal dimension values decreased after CO 2 exposure in 69.6% of the compiled observations, although the direction and magnitude varied among studies because coal properties and treatment conditions differed. The results identify fractal parameter relationships supported by data and compare normalized local sensitivities of selected component relations. Distinguishing descriptors of pore size distributions, transport pathways and fracture connectivity across methods and scales help translate laboratory measurements into reservoir parameters. These parameters inform assessments of CO 2 injectivity, storage capacity and methane recovery. Current applications remain limited by inconsistent definitions, uncertain parameter transfer and insufficient validation under reservoir conditions. Future research should integrate in situ experiments, coupled modeling and field observations. The aim is to improve fractal parameter upscaling for CO 2 storage and methane recovery in heterogeneous coal seams.