Experimental research on hydraulic fracture propagation in group of thin coal seams

The commingled production technology has been applied in group of thin coal seams exploitation. In order to explore the mechanism of fracture extension of multi-layer combined fracturing in thin coal seam groups with different perforating location, fracturing fluid displacement and fracturing fluid viscosity, this paper firstly adopted sand/coal (single and two-layer) indoor experiment to explore the basic law of fracture propagation and effective fracturing methods in cataclastic coal. After that, the multilayers experiment was carried out, and the fracture propagation law was studied by two types of (Model A: 3 layers sandstone and 2 layers coal) and (Model B: 3 layers coal and 2 layers sandstone). The experimental study of coal/sandstone (single and two-layer model) showed that the fracture morphology of primary structure coal was relatively simple. When the fracturing fluid was injected into the cataclastic coal, it was difficult to form effective fractures and the scale of the fracture was small. Therefore, the perforating location in the cataclastic coal should be avoided. In the secondary fracturing of the cataclastic coal seam, the fiber fracturing fluid could seal the fracture system of the primary fracturing of cataclastic coal, which provided technical reference for secondary temporary plugging fracturing. In addition, the law of fracture propagation in multi-layer coal measure strata was studied on the basis of the single and two-layer model, and the fracture propagation was evaluated from the aspects of fracture propagation morphology, fracture scale and fracture penetration. Multi-layers experiments showed that whether hydraulic fracture could achieve the penetration effect was mainly affected by the perforating location. If the perforating location was in the sandstone layer, the facture was easy to penetrate coal seam. When the fracture penetrated the interface between two layers, the generated pressure pulse activated the weak surface of developed fractures in coal seam, prompting the formation of complex fracture. In addition, the larger the fracturing fluid displacement and the lower fracturing fluid viscosity, the easier it was for hydraulic fractures to form branch fractures or activate natural fractures, increasing the fracture propagation volume to maximize hydraulic fracture complexity in the formation. In this experiment, the fracture propagation law of hydraulic fracturing in thin coal seam groups was studied under different engineering factors from multiple perspectives. The research results could provide reference for the fracturing construction design in thin coal seam groups.