Hydraulic Fracture Propagation Mechanism in Deep and Shallow Coal: A Comparative Study between Junggar and Qinshui Basin

Summary Hydraulic fracturing plays a vital role in enhancing the productivity of coalbed methane (CBM) reservoirs. However, stimulation strategies proven effective in shallow coal often fail to achieve similar success in deep formations. To investigate the reason, we conducted a series of true triaxial fracturing experiments and rock mechanics tests. Deep coal was collected from Baijiahai uplift, Junggar Basin, 2200 m, while shallow coal was collected from Zhengzhuang block, Qinshui Basin, 700 m. Results show that deep coal exhibits 79% higher uniaxial compressive strength (UCS), 157% higher elastic modulus, and 68% higher fracture toughness compared with shallow coal, while shallow coal displays a microfracture density 5.3 times greater and a leakoff coefficient 75% higher than those of deep coal. These differences lead to marked contrasts in hydraulic fracture morphology. Under the same injection rate, shallow coal develops complex, multidirectional fracture networks influenced by weak faces, whereas fractures in deep coal are mainly planar in geometry, as observed in the Jungger Basin under high in-situ stress and low weak faces density. We used a novel image-based method to quantify fracture complexity, revealing that shallow coal fracture networks are 1.86–2.89 times more complex. Importantly, decreasing injection rate increases fracture complexity in shallow coal but has a negligible impact on deep coal. Field data from CBM operations in the Junggar and Qinshui basins validate the experimental trends. Based on the results, we propose optimized fracturing strategies for deep coal reservoirs in Baijiahai uplift, Junggar Basin, recommending moderate fluid viscosity (10–25 mPa·s) and pump rates (2.0–5.5 m³/min per cluster) to balance fracture complexity and proppant transportation.