液相放电等离子体破岩室内实验与破岩机理

Experiments and mechanism of rock breaking by the plasma shock wave generated by underwater discharge

液相放电等离子体破岩是一种将电能转换为冲击波机械能的新型高效破岩技术,目前国内外对其在石油钻井中的应用研究较少。开展了液相放电等离子体破岩室内实验:首先使用3组不同厚度的页岩验证了等离子体冲击波破岩的可行性;然后使用损伤变量和声幅衰减系数作为岩样损伤表征量,定量分析了不同影响因素下,混凝土岩样受等离子体冲击波作用后的损伤程度;使用3种岩样(混凝土、页岩、砂岩),验证了该技术对不同抗压强度岩样,在不同地层中的适应性;最后借助CT扫描,观察冲击波作用前后的岩样,分析和揭示了冲击波作用下岩样的损伤破坏形态和机制。室内实验结果表明:冲击波峰值压力高达130~190 MPa;页岩被劈裂成多块,混凝土岩样的破坏深度为3~5 mm,破坏程度随放电电压和冲击次数的增加而增加;载荷相同时,岩样的破坏程度随抗压强度减小而增加,但页岩的层理结构有利于吸收冲击波能量,增加其破坏程度;岩样的破坏形式以径向裂纹、片落裂纹和冲蚀坑为主。液相放电等离子体破岩机理的研究结果表明:冲击波作用岩样,岩样表面受压被破坏,岩样内部产生切向拉伸应力,应力波从岩样内部往边界传播时,在岩-液界面反射并产生拉伸应力,当拉伸应力大于岩样抗拉强度时产生裂纹;冲击波的产生伴随着高速射流,射流的水楔作用加速裂纹的扩展,形成冲蚀坑。

Plasma shock wave is a novel and efficient rock breaking technology that converts electrical energy into the mechanical energy of shock wave. At present, its application research in oil drilling is not sufficient.We conducted experiments of rock breaking by plasma shock wave:First, the feasibility of plasma shock wave rock breaking technology was verified by using 3 sets of shale samples with different thicknesses;and then the damage variable and attenuation coefficient were used to quantitatively analyze the damage degree of concrete rock samples after being impacted by plasma shock waves under different influencing factors;the use of three types of rock samples(concrete, shale, sandstone)has verified the adaptability of the technology to different compressive strength rock samples in different formations. Finally, CT scans were used to observe the internal damage of rock samples before and after the shock wave, and the rock-breaking patterns and mechanism of the shock wave were analyzed.The experimental results showed that the peak pressure of the shock wave was as high as 130~190 MPa;the shale samples were split into multiple pieces, and the damage depth of the concrete rock sample was 3~5 mm. The damage degree increased with the increase of the discharge voltage and the number of impacts;the damage degree of rock samples increased with the decrease of compressive strength under the same shock load, but the bedding structure of shale was beneficial for absorbing shock wave energy and increasing the damage degree. The main failure modes of rock samples were radial cracks, chipping cracks and erosion pits. Further analysis on the mechanism of rock breaking by plasma shock wave showed that the surface of rock samples was damaged by compression due to the shock waves, and tangential tensile stress was generated inside the rock samples. When the stress wave propagated from the inside to the boundary, it was reflected on the rock-liquid interface and generated tensile stress;when the tensile stress was greater than the tensile strength of rock samples, cracks were generated. The generation of shock waves was accompanied by high-speed jets. The propagation of cracks was accelerated by the water wedge effect of the jets.