Microscopic fracture distribution characteristics and control mechanism of deep coal body fractured by supercritical CO2 fracturing

WANG Xiangzeng; WU Jinqiao; ZHANG Fengsan; CAO Wenqi; YAN Jiwei; ZHU Dijie

doi:10.13225/j.cnki.jccs.2025.1075

Abstract：Deep coal seams are commonly characterized by high burial depth,dense coal-rock structures and relatively un-developed natural fractures,which together restrict permeability enhancement and efficient coalbed methane(CBM)ex-traction.Under such conditions effectiveness of conventional hydraulic fracturing is constrained by its tendency to gener-ate predominantly planar fractures.In contrast,supercritical CO2(ScCO2),owing to its small molecular size,low viscosity,and strong penetration capability can interact with coal micro-and nano scale pores and fractures in coal,thereby promot-ing the development of more complex fracture networks which facilitate additional pathways for gas permeation and mi-gration.This study integrates micro-computed tomography(micro-CT),acoustic emission instrument and morphology scanning to investigate the spatial distribution and propagation behavior of micro-fractures in deep coal from the Yan'an block following ScCO2 fracturing.Fracture spatial distribution characteristics under different stress constraints are system-atically examined.The controlling mechanism of fracture initiation and propagation associated with coal structure,in-situ stress conditions and ScCO2 characteristics in fracture initiation and propagation were analyzed.In addition,fracture mor-phology and heterogeneity are quantitatively characterized providing insight into governing mechanisms of fracture propagation and spatial distribution.The results indicate that the vertical stress difference coefficient(Kv)exerts a strong control on fracture initiation pressure.When Kv=0.4(sample 1-3),fracture pressures are approximately 25 MPa,whereas at is Kv=0.6(sample 4),the fracture pressure decreased to 16 MPa,indicating the increased Kv facilitates to the fracture ini-tiation.Following ScCO2 fracturing,extensive micro-fractures develop in coal,with complex geometries and enhanced connectivity,which is conducive to gas migration.ScCO2 preferentially accesses coal through relatively large pore-frac-ture channels and subsequently penetrates micro-nano scale pores and fractures.With increasing injected gas volume,the compressive effect of ScCO2 on the coal intensifies,leading initially to micro-fracturing;as ScCO2 accumulation pressure increases,fractures are progressively opened and coalesce into macroscopic failure.Owing to the ability of ScCO2 to pen-etrate coal across multiple scales and its relatively uniform distribution,the resulting fractures exhibit a wide spatial distri-bution.The fractal dimension of fracture surface roughness for samples 1-4 are 1.915,1.828,1.814 and 1.797 respectively.This indicates that the higher fracture pressures are associated with increased fracture surface complexity and stronger het-erogeneity.The coal structure,in-situ stresses and ScCO2 properties are the primary factors controlling fracture develop-ment and distribution in deep coal.These findings provide guidance for permeability enhancement for deep coal seams and coalbed methane reservoir transformation using ScCO2 fracturing.