爆生气体驱动岩石裂缝动态扩展分析

Dynamic analysis for fracturing progress by detonation gas

爆燃产生的气体压力以脉冲荷载形式随裂缝扩展作用于裂缝表面,不仅使裂缝壁岩体产生动态响应,而且爆生气体随裂缝扩展而发生动态变化,进而对岩体的劈裂过程产生耦合影响,这一机理的数值仿真分析尚未见报道。模型中用特殊的裂缝单元模拟裂缝张开和闭合,用Newmark法求解反映接触问题的动态有限元方程,并采用与时间相关的爆生气体压力分布模型,模拟爆生气体驱动下裂缝扩展过程。分析结果指出:①爆生气体在压裂过程中起主要作用,弹性应力波的劈裂作用范围很小;②地应力值越大起裂越晚,且裂缝扩展速度越小;③初始裂缝越长裂缝起裂越早,裂缝扩展速度也越大;④钻井中气体升压越快起裂相对时刻越晚,但对裂缝扩展速度没有明显影响。而裂缝起裂时刻越早以及裂缝扩展速度越快,可以得到更长的裂缝长度。

When detonation gas pressure acts on crack surface of an oil well, the surrounding rock is subjected to dynamic response, and gas pressure varies with fracturing process, then the fracturing process is influenced by gas pressure variation： Special crack element was used in the proposed model to simulate the opening and closing process of crack. Such a dynamic FEM equation involving contact problem was solved by Newmark method, and the proposed gas distribution function was also considered. The numerical result shows： ①detonation gas had a major role in the fracturing rock comparing with elastic stress wave; ②crack growth velocity was smaller as the initial tectonic stress was greater; ③crack growth velocity was lager as the initial crack length was longer;④crack initiated relatively later as pressure rose fast, which meant a slowly rising gas pressure would produce a longer crack.