Characterization of hydraulic crack initiation of coal seams under the coupling effects of geostress difference and complexity of pre-existing natural fractures

Accurate characterization of the initiation of hydraulic fractures is crucial for understanding the formation of multiple hydraulic fracture networks in coal seams in coalbed methane extraction and disaster prevention. To this end, hydraulic fracturing has been extensively employed in the reservoir stimulation of shale and sandstone for decades. However, few studies have been conducted to provide accurate knowledge of the initiation of multiple crack networks in coal seams. This study reports an experimental investigation that incorporates triaxial hydrofracturing tests, computed tomography (CT) technology, and numerical tools to probe the coupling effects of horizontal geostress differences and natural fractures on crack initiation in coal samples. The cracking pattern of the hydraulic cracks was accurately specified by comparing the crack networks before and after the hydraulic fracturing experiments using the CT scan technique and a crack-identification algorithm. Fractal theory is also used to characterize the variation in the complexity of the crack network after hydraulic fracturing treatment in a coal sample, and the local stress perturbation induced by the complexity of natural fractures was quantitatively analyzed using fracture mechanics and finite element methods. The results show that the distribution of natural fractures plays a dominant role in the three-dimensional initiation of hydraulic cracks in the coal seam.