基于CDEM的页岩甲烷原位燃爆压裂数值模拟

Numerical simulation on fracture propagation of methane in-situ explosion fracturing in shale gas reservoirs

甲烷原位燃爆压裂是一种利用原位解析的甲烷气体作为爆源的页岩气储层改造方法。基于连续-非连续单元法(CDEM),结合朗道爆源模型和线弹性拉剪复合断裂本构建立甲烷燃爆压裂数值模型。针对页岩气储层开展直井甲烷燃爆压裂数值模拟,分析燃爆峰值压力、增压速率、初始最小主应力、应力差以及天然裂缝参数对燃爆裂缝扩展的影响。结果表明:甲烷燃爆压裂在井周形成复杂裂缝,随着峰值压力和增压速率的增大以及初始最小主应力的减小,燃爆裂缝扩展范围会增加,其中峰值压力的影响最显著;在应力差的影响下燃爆裂缝范围偏向最大主应力方向,且应力差越大这种偏差越明显;天然裂缝能够增强燃爆压裂效果,其分布密度越大、缝长越长、与最大主应力方向夹角越小,燃爆裂缝扩展范围越大。

Fracturing viamethane explosion is proposed as a novel reservoir reconstruction method, in which in-situ resolved methane gas is used as explosion source for fracturing of shale gas reservoirs. In this study, based on the continuum-discontinuum element method(CDEM), a numerical simulation model of methane explosion fracturing was established by combining the Landau explosion source model and the linear elastic tensile-shear composite fracture constitutive model. The numerical simulations of the methane explosion fracturing in vertical wells for shale gas reservoirs were carried out. The effects of peak pressure, pressurization rate, initial minimum principal stress, initial stress difference and natural fractures on explosion fracture propagation were analyzed.The results show that methane explosion fracturing can generate complex fractures around the wellbore, and with the increase of peak pressure and pressurization rate, as well as the decrease of initial minimum principal stress, the propagation range of the explosion cracks can be increased, in which the peak pressure is the most significant factor. The fracture range in the direction of the maximum principal stress is larger due to the influence of stress difference, and larger stress difference can lead to a more obvious deviation of fracture range. The existence of natural fractures(NFs) can enhance the effect of explosion fracturing. A greater NFs density, a longer fracture length, and a smaller angle between the dip angle of NFs and the σhdirection can increase the range and area of the explosion fractures.