The rate of penetration is very low during the development of unconventional gas resources such as tight gas and marine shale gas, owing to high rock hardness and strength as well as heterogeneities at all scales. To improve the efficiency and reduce costs of developing unconventional gas resources, this paper proposed a new technology to assist drilling, Pulsed Arc Plasma Shockwave Technology (PAPST).
This technology converts electrical energy into mechanical energy to generate dynamic loads shockwave which can assist rock-breaking. Firstly, based on the fluid mechanics and bubble dynamics, the mechanism of shockwave generation was analyzed. Then, to verify the feasibility of PAPST technology, this paper conducted rock breaking experiment with shale samples from Longmaxi formation, China. Meanwhile, based on impact and damage mechanics, the mechanism of rock damage caused by dynamic load was analyzed.
The results show that shale samples were destroyed and there were cracks and collapse pits on shale samples after the impact of shockwave. Therefore, the application of PAPST technology to assist drilling is feasible, and the greater the discharge energy, the higher the efficiency of rock failure. Through theoretical analysis, it is found that the radial cracks of rock are caused by the tangential tensile stress, which is caused by the shockwave impacting the rock. The secant cracks are caused by the resultant force of the three component forces: the tangential and radial components of the force on the rock particle caused by the shockwave and the radial tensile force generated by the reflection of stress wave at the rock-water interface. The collapse pits are most likely caused by stress concentration.
For the first time, this paper proposed an idea of applying shockwave generated by PAPST to assist drilling for increasing the ROP in unconventional gas resources. And it also provided a theoretical basis for the application of PAPST in the field of oil drilling by analyzing the mechanism of shockwave generation in drilling fluids and the mechanism of rock breaking by shockwave.