| With sustained and rapid economic development,conventional oil and gas exploration of our country has not been able to meet the social needs,therefore it is necessary to increase the development and utilization of unconventional energy sources,such as coalbed methane.However,the porosity and permeability of coal reservoirs are low in our country,so for facilitating extraction of coalbed methane,artificial fracture or fractured technology should be used to transform the reservoir to increase permeability when exploiting.Based on the National Natural Science Foundation of mechanism of fracturing low permeability coal seam by injecting liquid nitrogen(Nos.51574139)and mechanical properties of saturating unsaturated rock post peak(Nos.51274079),this paper proposes originally the experiment about semi-soaking coal with liquid nitrogen based on low temperature interaction of liquid nitrogen and near wellbore coal.The effects of various factors such as water saturation,times of liquid nitrogen freezing and thawing,temperature gradient on the defect structure expansion of different types of coal with injecting liquid nitrogen were studied,and this paper obtained following conclusions by analysis:1.Compressional wave velocity of coal sample has positive correlation with coal rock density and negative correlation with porosity.The higher the strength and stiffness of coal rock,the denser the mineral particles,the smaller the influence of liquid nitrogen freezing.2.The level of liquid nitrogen freezing damage is positively correlated with water content and temperature gradient.3.There is a positive correlation between level of freezing damage and times of freezing.And the level of the second freezing damage has positive correlation with water content and has negative correlation with porosity.4.This paper introduce the concept of the crack tip stress intensity factors and fracture toughness etc.,while putting forward the crack fracture mechanism of coal rock. |
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