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With the development of coalbed methane production technology in Guizhou Province from single seam to multiseams combined, choosing appropriate hydraulic fracturing methods for adjacent coal seams has become a significant technical problem. To investigate the morphological and propagation mechanism of cracks in the process of hydraulic fracturing in the adjacent coal seams, laboratory hydraulic fracturing simulation tests were carried out on large-size artificial specimens containing adjacent coal seams by utilizing true triaxial hydraulic fracturing physical simulation equipment. Here, the crack expansion and morphology characteristics were analyzed by computerized tomography (CT) scanning, acoustic emission (AE) monitoring, and pump pressure monitoring technologies. The experimental results show that the increase in AE impact times and the fluctuation of the pump pressure curve can mainly reflect the propagation characteristics of hydraulic fracturing cracks. The existence of obvious cleavages in the coal seam can easily lead to the generation of vertical and cross-layer cracks. Furthermore, the fractal dimension statistical data indicate that the specimen with similar interlayer and coal seam tensile strengths has the greatest fractal dimension change rate at 111.34%, confirming its best fracturing effect. Hence, the interlayer tensile strength is regarded as a key factor for a favorable crack network. Additionally, the ratio of the interlayer to coal seam thickness also impacts crack propagation. According to the experimental results, in the process of hydraulic fracturing in the combined coal layers, the crack propagation in the adjacent coal seams can be divided into four different models. This study provides an important reference for coal seam selection and hydraulic fracturing design in adjacent coal seams in Guizhou or similar coalfields.
Qin et al. (Thu,) studied this question.