煤层气井液氮伴注压裂增透机制及应用研究

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

针对平顶山矿区煤储层“低压、低渗、低饱和度”的特点,为提高地面煤层气储层改造效果,增加煤层气单井产量,提出了采用液氮伴注技术对储层进行压裂改造。基于断裂力学理论,系统分析了液氮伴注压裂的增产机制:液氮汽化吸热致使煤岩降温所产生的温度应力和体积膨胀产生的孔隙压力两者作用下致使煤岩体发生破坏,生成大量微裂隙,提高煤储层渗透性,达到对煤储层改造的目的。采用液氮伴注压裂可显著提高施工砂比,降低压裂液滤失,减小对储层的伤害。压裂后能有效提高地层能量,提高临界解吸压力,缩短煤层气井排采见气周期。现场试验表明:单井最高日产气量达1708 m^3,累计产气量620000 m^3,在构造煤发育区实现了稳定、连续产气。

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 1708 m^3,and the cumulative gas production was 620000 m^3,achieving stable and continuous gas production in the tectonic coal development area.