磨料气体射流冲蚀磨损岩石特征分析

Erosive wear characteristic of rock impacted by abrasive gas jet

磨料气体射流在辅助钻孔和卸压增透时,能够避免塌孔,从而提高采出率。明确磨料气体射流破岩特征是推广磨料气体射流应用的重要理论基础。但目前的磨料气体射流冲蚀磨损模型忽略了反射磨料的作用。为此,开展了不同磨料种类和不同靶距条件下,磨料气体射流冲蚀灰岩实验,采用电镜扫描冲蚀坑不同区域的冲蚀特征进行分析,结合离散元模型分析磨料在冲蚀灰岩过程中的运动轨迹,明确磨料气体射流冲蚀磨损特征以及反射磨料的作用。得出磨料气体射流冲蚀灰岩时,冲蚀坑的形态大致为倒圆锥形,在冲蚀坑底部存在环形平台,环形平台下面为类球体状的冲蚀坑底部。造成这种冲蚀坑形态的主要原因为磨料气体射流流场结构的特殊性,在磨料气体射流轴心和边界处存在环形区域,环形区域内没有磨料存在。轴心处入射磨料是形成类球体的主要因素,磨料反射后趋向环形区域运动,在反射过程中,扩大了类球体和冲蚀坑直径,并形成了环形平台。类球体底部是以入射磨料的冲击应力波导致岩石破坏,类球体侧面是反射磨料造成的多次塑性变形,环形平台处存在入射磨料导致的塑性变形和反射磨料导致的疲劳破坏,冲蚀坑侧面主要以反射磨料产生疲劳破坏为主。磨料硬度对煤岩体冲蚀磨损诱因和磨损破坏特征没有影响。但较硬磨料冲蚀煤岩表面具有更高的粗糙度,同时具有更高的破岩效率。

Assisted drilling with non-liquid technologies,such as supercritical CO2 jet,are more and more important in natural gas exploration,especially in coal-bed methane exploration.As a kind of non-liquid technologies,abrasive gas jet is widely used to drill metal and glass.It is also powerful enough to break coal and rock,which is the primary precondition of its usage in coal-bed methane exploration.To understand the rock erosion wear mechanism of abrasive gas jet is critical important to apply the technology preferably and widely.However,the existing erosion mechanism mostly neglects the effect of rebound abrasive.To this end,the experiments of limestone erosion by the abrasive gas jet under different kinds of abrasives and different stand-off distances were carried out.The experiment results show that the erosion pit has an approximate inverted cone-shape.The bottom of erosion pit is of hemispherical.There is an annular platform above the hemispherical bottom,which connects the bottom and side of erosion pit.The primary reason that induces the peculiar shape of erosion pit is the specific flow field of abrasive gas jet.There is an annular region between the axis and boundary of the abrasive gas jet,and there is no abrasive in the annular region.The abrasive around the axis induces the formation of hemispherical bottom.After rebounding,the abrasive around axis moves to the annular region.The rebound abrasive with high velocity enlarges both the diameters of hemispherical bottom and erosion pit and induces the formation of annular platform.The failure characteristics of different areas of the erosion pit were analyzed by Scanning Electron Microscopy(SEM).It can be concluded that the failure models of different spots are different.The failure is caused by the impact stress wave of the incident abrasive at the base of the hemispherical bottom.Plastic deformation is the primary failure model induced by the rebound abrasive at the side of hemispherical bottom.The plastic deformation induced by incident abrasive and fatigue failure induced by rebound abrasive are the primary failure model at the annular platform.Fatigue failure induced by rebound abrasive is the primary model failure at the side of erosion pit.The rock failure reason of abrasive with different hardness is the same,but the rock damaged by hard abrasive has rougher surface and higher rock breakage efficiency.