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High voltage spark discharge (HVSD) could generate strong pressure waves that can be combined with a rotary drill bit to improve the penetration rate in unconventional oil and gas drilling. However, there has been little investigation of the effect of electrical conductivity on rock damage and the fragmentation mechanism caused by HVSD. Therefore, we conducted experiments to destroy cement mortar, a rock-like material, in water with five conductivity levels, from 0.5 mS/cm to 20 mS/cm. We measured the discharge parameters, such as breakdown voltage, breakdown delay time, and electrical energy loss, and investigated the damage mechanism from stress waves propagation using X-ray computed tomography. Our study then analyzed the influence of conductivity on the surface damage of the sample by the pore size distribution and the cumulative pore area, as well as studied the dependence of internal damage on conductivity by through-transmission ultrasonic inspection technique. The results indicated that the increase in electrical conductivity decreased the breakdown voltage and breakdown delay time and increased the energy loss, which led to a reduction in the magnitude of the pressure wave and, ultimately, reduced the sample damage. It is worth mentioning that the relationship between the sample damage and electrical conductivity is non-linear, showing a two-stage pattern. The findings suggest that stress waves induced by the pressure waves play a significant role in sample damage where pores and two types of tensile cracks are the main failure features. Compressive stresses close horizontal cracks inside the sample and propagate vertical cracks, forming the tensile cracks-I. Tensile stresses generated at the sample–water interface due to the reflection of stress waves produce the tensile cracks-II. Our study is the first to investigate the relationship between rock damage and electrical conductivity, providing insights to guide the design of drilling tools based on HVSD.

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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.

Plasma shock wave (PSW) has been proven to be able to destroy rocks and improve the ROP for conventional rotary drilling. The aim of this paper is quantify the damage caused by PSW generated by different discharge energies, and to study the mechanism and characteristics of PSW damage to rocks. The PSW has been extensively used in many areas, such as alternative fracturing. In the tests, the PSW reactor generated the electrohydraulic effect to convert electrical energy into mechanical energy, which was sufficient to destroy the hardest rock. Firstly, the experiment was carried out using concretes instead of real rocks. Secondly, three types of discharge energy were used to generate PSW to damage samples. Finally, the amplitude attenuation coefficient (AAC), the damage variable and the drillability were used to quantitatively analyze the damage of the samples. Experimental results showed that samples were impacted to produce many circular holes and cracks and the damage to samples increased as the energy increased. However, when the energy rose to a certain extent, there was a gradually decrease in the growth rate of damage. In addition, it was found that the AAC measured at different locations was different for the same sample, which indicated that the internal damage of a concrete sample produced by the PSW was anisotropic. Finally, the average drillability was improved after the sample was subjected to PSW impact. The findings gained will help understand the mechanism by which the PSW destroys rocks It also provide some methods to quantitatively evaluate damage of rock.