Abstract
Due to the complicated coalbed methane (CBM) occurrence conditions and the diverse geological structures in China, the promotion and application of the coal and gas simultaneous extraction technology have been seriously restricted. In view of this, this paper chooses Qingdong Coal Mine protection layer mining and CBM extraction field practice as the research background. Firstly, based on the similar material simulation experiment that simulates coal mining, the dynamic changing pattern of a mining field’s overburdened strata and corresponding stress are obtained, the relationship between gas desorption and stress can then be clarified. Further, with the help of the fractal theory and box counting method, the fracture development characteristics of the overlying strata are quantitatively described on the basis of experimental images. Finally, by building a model for calculating the penetrability coefficient of coal seam based on fractal dimension of mining fissure and analyzing the relationship between fissure development and fractal dimension, the gas migration law and the fissure development areas of #7 and #8 overburden strata where CBM concentrates can be revealed and determined. According to the orientation of the area mentioned above, the location of the CBM pumping field in relation to the coal seam roof and the arrangement of CBM extraction boreholes can be optimized, which make CBM extraction efficient. Meanwhile, the risk of coal and gas outburst is significantly reduced when the CBM concentration is controlled within 0.2% to 0.6% outside the corner of the working face and 0.1% to 0.35% in return flow, which is lower than 0.8%, the threshold of CBM concentration.
Highlights
In recent years, with China’s increasing emphasis on coal mine safety especially on the safe production of coalbed methane (CBM), for different CBM reservoir characteristics, a series of theories and techniques for mine disaster prevention and efficient green resource exploitation have been proposed [1,2,3,4,5]
Among all coal mine disasters, coal and gas outbursts caused by gas pressure, ground stress, and other related factors are the biggest threat to coal mine safety [6, 7]
Even though in soft coal seam with high ground stress, due to high gas pressure, hole collapse and plugging are common during the construction of CBM drilling, which seriously restricts the efficiency of CBM extraction [9, 10]
Summary
With China’s increasing emphasis on coal mine safety especially on the safe production of coalbed methane (CBM), for different CBM reservoir characteristics, a series of theories and techniques for mine disaster prevention and efficient green resource exploitation have been proposed [1,2,3,4,5]. The mining of one layer contributes to the unloading of ground stress within adjacent areas and permeability increasing in nearby coal seams Speaking, this technology develops and correspondently creates fractures, thereby increasing the permeability of high-pressure and low-permeability CBM reservoirs, which ensures a better environment for CBM exploitation [21,22,23]. This technology relies on the find of a CBM-rich area, which is mainly based on the development of the surrounding coal and rock mass’ fissures and pores, along with the CBM flow mechanism during mining processes. This paper attempts to put forward a new quantitative analysis method for safe coal and gas simultaneous extraction in adjacent coal seams based on similar material simulation experiment and thereby provide a new perspective for the optimization of CBM extraction and permeability enhancement in coal seams featuring high-pressure and low-permeability
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