Effect of liquid nitrogen cycle treatment on seepage characteristics of coal samples with different water contents

The permeability of reservoir is generally low in China, so it is necessary to strengthen the exploitation of coalbed methane by anti-reflection technology.In recent years, the technology of liquid nitrogen cracking coal has become a hot spot in the research of coal seam anti-reflection technology due to its environmental protection, strong anti-reflection effect and so on.The frost heave of water-ice phase change is the main action mechanism of liquid nitrogen cracking water-bearing coal.In this paper, the coal samples with water content of 0%,2%,4% and 6% were prepared using a saturated water container, and were subjected to a liquid nitrogen cycle freeze-thaw cyclic treatment until the coal was broken or the five cycles of freeze-thaw treatment were completed.During this period, a camera was used to take photos at a fixed point for observing the evolution of the macroscopic cracks on the surface of the coal samples treated.The change law of coal sample permeability under different pressure points was obtained by nitrogen seepage experiment using the gas injection displacement test system of the coal bearing body.The experimental results show that① compared with the same number of fracture initiation, the higher water content of coal sample is, the more obvious the surface fracture development.② Liquid nitrogen cracking can't change the seepage characteristics of coal, but only improve it.In the axial seepage experiment, the permeability of coal sample under constant confining pressure presents a “U” shaped trend with the increase of gas pressure due to the Klinkenberg effect, effective stress and other influencing factors, which conforms to the quadratic function distributions.Under the influence of effective stress, permeability's average value decreases with the increase of confining pressure, and the relationship is negative exponential.③ The increases of the moisture content of coal sample and the number of cycles will enhance the permeability of coal samples.With the increase of the number of liquid nitrogen cycles, the permeability of coal sample is on the rise.With the same number of cycles, the permeability growth is directly proportional to the water content.Except for the permeability of coal sample after the third liquid nitrogen treatment with water content of 2%,the growth range of permeability of coal sample increases approximately by multiple times, and the growth rate of single average permeability decreases slightly.④ In the process of liquid nitrogen cracking coal seam, a good anti-reflection effect can be obtained by combining the increase of water content of coal seam and the appropriate increase of liquid nitrogen cycle cracking times.