Experimental Investigation and Mechanism Analysis on Rock Damage by High Voltage Spark Discharge in Water: Effect of Electrical Conductivity

Zhixiang Cai,Yufei Chen,Qing Yu,Hui Zhang,Kerou Liu

doi:10.3390/en13205432

Zhixiang Cai, Yufei Chen + Show 3 more

Open Access

https://doi.org/10.3390/en13205432

Copy DOI

Journal: Energies	Publication Date: Oct 18, 2020
Citations: 14	License type: CC BY 4.0

Affiliation: China University of Petroleum, Beijing

Abstract

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.

Highlights

Improving the rate of penetration (ROP) is the focus of research in petroleum industries, becauseROP is inversely proportional to drilling cost in unconventional reservoirs, which is typically 30%to 40% of the total well costs
This research aims to investigate the effect of electrical conductivity on the discharge characteristics of high voltage spark discharge, the magnitude of pressure waves and rock damage, and to analyze the fragmentation mechanism from stress wave propagation
12b–f show the typical surface whereFigure it is reflected and into a tensile wave, since the acoustic impedance of the sample macroscopic damage characteristics produced by pressure waves at different conductivities

Summary

Introduction

Improving the rate of penetration (ROP) is the focus of research in petroleum industries, becauseROP is inversely proportional to drilling cost in unconventional reservoirs, which is typically 30%to 40% of the total well costs. ROP is inversely proportional to drilling cost in unconventional reservoirs, which is typically 30%. Over the past few decades, researchers and engineers have proposed some unconventional drilling techniques based on a different rock damage mechanism rather than using the drill bit’s mechanical force to cut the rock. Energies 2020, 13, 5432 the rock; researchers in the Gas Technology Institute have determined its technical feasibility and investigated the effects of specific laser energy on various rock types [1,2,3]. Electrical plasma drilling uses high thermal loads at thousands of degrees Celsius to spall, melt, and vaporize rock, where the thermal conductivity of the rock is a critical factor in breaking the rock [4,5]. Lasers and plasma can generate high temperatures in downhole, disabling sensors near the drill bit that measure drilling parameters and formation characteristics, which are essential for directional drilling and risk analysis [6,7]

Objectives

Results

Discussion

Conclusion