Study on mechanism and application of liquid nitrogen injection combined with fracturing to enhance permeability in CBM wells

In view of the characteristics of “low pressure, low permeability and low saturation” of coal reservoirs in Pingdingshan mining area, and in order to improve the effect of surface coalbed methane reservoir reconstruction and increase the production of single well of coalbed methane, it was proposed to use liquid nitrogen with injection technology to fracturing reservoirs. Based on the theory of fracture mechanics, the mechanism of increasing production of liquid nitrogen with fracturing was systematically analyzed: the thermal stress caused by the vaporization of liquid nitrogen causes the temperature stress caused by the cooling of coal rock and the pore pressure generated by volume expansion to cause damage to the coal and rock mass.A large number of micro-fissures generated so as to improve the permeability of coal reservoirs and the permeability of the layer reaches the purpose of transforming the coal reservoir. The use of liquid nitrogenfracturing can significantly increase the construction sand ratio, reduce the fluid loss of the fracturing fluid, and reduce the damage to the reservoir. After fracturing, the formation energy can be effectively increased, the critical desorption pressure can be increased, and the gas acquisition cycle of CBM wells can be shortened. Field tests showed that the maximum daily gas production per well reached 1 708 m3, and the cumulative gas production was 620 000 m3, achieving stable and continuous gas production in the tectonic coal development area.