Abstract
Multistaged fracturing in the roof of outburst coal seam is an efficient and creative technology for coalbed methane (CBM) drainage, which can effectively improve the permeability of coal seam. To reveal its mechanism of permeability enhancement, the effect of coal-rock interface on multistaged fracturing in the roof of outburst coal seam was simulated and discussed in this paper. Firstly, the lithological difference between outburst coal seam and roof was compared, and the concept and significance of multistaged fracturing in the roof of outburst coal seam were explained. Then, the mechanical conditions of multiple fractures in the roof traversing coal-rock interface were analyzed. The effects of mechanical parameters on multiple fractures were numerically simulated. The results indicated that fracturing borehole in adjacent rocks of outburst coal seam is much easier to drill and maintain gas drainage. Considering gas drainage efficiency and avoiding being blocked by coal fines, multistaged fracturing borehole is generally drilled in the stable rock stratum of roof. Whether the multiple fractures in the roof can traverse coal-rock interface is related to mechanical parameters of coal and rock, friction factor of coal-rock interface, angle between horizontal profile and coal-rock interface, cementing strength of coal-rock interface, minimum horizontal stress, and other factors. Higher fracturing fluid pressure contributes to propagating from the reservoir with low elastic modulus to the one with high elastic modulus for hydraulic fracture. Hydraulic fracture is more likely to propagate in the rock stratum with high brittleness index. The research results can improve multistaged fracturing theory and provide technological support for field test.
Highlights
Coalbed methane (CBM) is a kind of unconventional natural gas that reserves in coal seam, which has been successfully produced in Ordos, Qinshui, and Junggar basins in China [1,2,3]
To solve the problem during gas drainage in outburst coal seam, improve gas control, and ensure safe mining, Geng et al proposed transforming the roof of outburst coal seam and enhancing its permeability for building CBM migration channel [13], they suggested to drill a long borehole parallel towards of coal seam in the roof, and hydraulic strengthening methods were conducted in the borehole, forming fracture network that was interlinked in the roof and coal seam. e fracture network
Hydraulic fracture firstly propagates under adjacent rock stratum at step 36-1; this phenomenon indicates that hydraulic fracture tends to propagate into the fracturing stratum with low elastic modulus
Summary
Coalbed methane (CBM) is a kind of unconventional natural gas that reserves in coal seam, which has been successfully produced in Ordos, Qinshui, and Junggar basins in China [1,2,3]. In China, most outburst coal seams currently achieve methane predrainage and safe mining by means of rock tunnel and cross-measure borehole [8,9,10]. This method has disadvantages of great investment, huge amount of engineering, long construction period, and other bad aspects [11, 12]. Erefore, the research emphasis on mechanism of permeability enhancement by multistaged fracturing in the roof of outburst coal seam mainly focuses on whether the multiple fractures in the roof can form and traverse the coal-rock interface [18]. The effects of coal-rock interface on multistaged fracturing in the roof of outburst coal seam were theoretically analyzed and numerically simulated. e research results can improve multistaged fracturing theory and provide technological support for field test
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