Abstract
The permeability of coal seam is the main factor restricting the safe production of coal bed methane (CBM). In order to improve the extraction efficiency of CBM and reduce the probability of coal and gas outburst, it is of great significance to adopt artificial measures to enhance the connectivity of fracture network of coal reservoir. In this paper, the modification test of the fracture network in soft and low permeability coal seam is carried out by using the liquid CO2 phase-transition fracturing (LCPF) technology through cross-boreholes in bottom drainage roadway. The results showed that the diameter of the borehole is obviously increased, and the gas flow rate and gas concentration are greatly enhanced, which indicates that the pores and fractures of the coal reservoir are effectively connected, and the influence radius of gas extraction is about 10m. Although, at the later stage of gas drainage period, the pure gas quantity and gas concentration show a decay trend, they still maintained at a high level, which is favorable for CBM extraction. Compared with hydraulic fracturing, the LCPF technology has better effect on permeability enhancement of coal seam in the early drainage stage after fracturing. The LCPF technology not only enhance the CBM extraction efficiency, but also shorten the driving period of roadways.
Highlights
As we all known, the major fossil energy sources in the world are coal, oil and natural gas, while China's energy structure is characterized by "rich coal, poor oil and little gas"
In order to improve the efficiency of gas extraction, domestic and foreign experts devote themselves to reforming the fracture network of the original coal reservoir with different methods, including deep-hole pre-splitting blasting [1,2], hydraulic slotting [3,4,5], hydraulic fracturing [6,7,8,9] and gas fracturing technology [10,11], etc. so as to improve the permeability of coal seam
The average compression radius is 5.6m, the duration of compression effect is about 4-5h, and the influence radius of gas extraction is about 10m
Summary
The major fossil energy sources in the world are coal, oil and natural gas, while China's energy structure is characterized by "rich coal, poor oil and little gas". Domestic and foreign experts have proposed many effective methods to improve the permeability of coal seam. In order to improve the efficiency of gas extraction, domestic and foreign experts devote themselves to reforming the fracture network of the original coal reservoir with different methods, including deep-hole pre-splitting blasting [1,2], hydraulic slotting [3,4,5], hydraulic fracturing [6,7,8,9] and gas fracturing technology [10,11], etc. The liquid CO2 phase-transition fracturing (LCPF) technology is mainly used to fragment coal seam using the energy released from the instantaneous phase change of CO2 through fracturing boreholes. Due to its advantages of safety, high efficiency, low cost and simple operation, it has been well applied in coal mines, providing a new idea for the treatment of coal mine gas disaster
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