Liquid Nitrogen Submersion Coal On Fracturing And Increase Permeability Of Test Study

With the rapid development our country economy, for all kinds of energy are increasing, So it is necessary to increase the development and utilization of coal bed gas. But the coalbed methane reservoir is low porosity and low permeability, Therefore, in the exploitation of the need for artificial fracture or the use of fracturing technology to transform the reservoir to increase the permeability to facilitate mining.In this paper, supported by NSFC(51574139,51274079), the experiment of different saturated coal samples with liquid nitrogen leaching, Observation the coal before and after submersion, record of the change rate of the cracks’ s area ?, the change rate of the wave velocity? and other indicators. Analysis of the effect of different water saturation on the coal sample cracking caused by freezing in liquid nitrogen at low temperature. And the coal sample for two times freeze-thaw, The change of the indexes of 1 times of the coal sample in times of the submersion and the 2 times of the submersion. Analysis of results.(1) In the liquid nitrogen submersion, the primary cracks in the coal and there are new crack initiation.(2) The water content of coal samples has a great influence on the dissolution and leaching of liquid nitrogen. The change of the crack area and the wave velocity of saturated samples is more obvious than that of the two groups.(3) The coal sample was more soluble in two times than the first submersion solution, and some coal samples were cracked after two times submersion. This table by repeated freezing and thawing can change the internal structure of the coal sample increased.Fracture mechanics method is used to analyze the primary fracture propagation in liquid nitrogen. The results show that the fracture of coal is the same as the fracture toughness of coal in the process of cooling. If the stress intensity factor is not up to the fracture toughness, the crack is not extended. Application of FLAC software to build a model to analyze the evolution of permeability of the reservoir after the injection of the borehole coal into liquid nitrogen. It was found that the tensile failure zone of 0.16 m near the borehole after the injection of liquid nitrogen in the coal seam was simulated. In the tensile failure zone, the crack initiation and propagation, the permeability of the coal element increased significantly.