Hydraulic fracture initiation and propagation mechanisms in deep coalbed methane reservoirs based on Computed Tomography image reconstruction

• Large-scale coal samples were employed for hydraulic fracturing experiments simulating deep CBM reservoirs. • Effects of burial depth (key focus), injection rate, and in-situ stress difference coefficient on initiation and propagation mechanism of hydraulic fracturing were systematically. • investigated. • CT 3D reconstruction technology was applied to visualize fracture initiation and propagation patterns. • Distinct fracture propagation modes in deep coal seams under high-stress conditions were identified and categorized. • Key strategies for enhancing deep CBM stimulation efficiency were proposed based on experimental findings. China has great potential for deep coalbed methane (CBM) resources. Nevertheless, due to the "three high" geological conditions, traditional fracturing techniques for shallow reservoirs face adaptability bottlenecks. This study, utilizing hydraulic fracturing physical experiments in combination with Computed Tomography (CT) image reconstruction, reveals the mechanisms of fracture initiation and propagation in deep coal reservoirs under the influence of burial depth, in-situ stress difference coefficient, and fluid injection rate. The results demonstrate that: the fracturing pressure and fracturing time of the deep coal reservoirs are significantly higher than those of the shallow one. Hydraulic fractures in deep coal reservoirs are primarily micro-fractures with minimal aperture, significantly limiting the stimulation volume and range. The stimulation measures of deep coal reservoirs relies more on dense micro-fractures than the typical the macro-fractures in shallow coal reservoirs. High fluid injection rates is helpful to reduce fracturing time, increase the fracture volume and improve fracture complexity. The fracturing pressure and fracturing time decrease with the increasing in-site stress difference coefficient K. At low K values, fractures are more easily influenced by weak structural planes, while at high K values, the maximum horizontal principal stress dominates the fracture direction. In the field of hydraulic fracturing in deep CBM, it is suggested to increase the injection rate, increase the number of perforation clusters, and reduce the proppant particle size, thus to realize the fracture network with comprehensive coverage and densely connection.