Rock Fragmentation By Pulsed High Voltage Discharge And Drilling Equipment Development

With increasing depletion of land oil and gas resources, exploration and development of offshore oil and gas resources has becoming increasingly urgent. Recently, a drilling technology based on pulsed electrical discharge fragmentation (PEDF) of hard rocks has attracted widely attention. The energy per pulse of existing generators of PEDF is hundreds or even thousands of joules per pulse, which limits operating frequency, energy efficiency and lifetime of electrodes. With lowering energy per pulse and voltage amplitude, there may be to miniaturize the equipment and to increase the frequency rate, lifetime of the electrode, and the use of safety. In this paper, theoretical and experimental research on PEDF has been conducted with voltage amplitude of30-50kV and energy per pulse of10～20J.Electrohydraulic (EHD) and Electrical discharge (ED) are two main methods for the PEDF technology. The research on the electrohydraulic is a bit earlier and the relevant theory is relatively mature. This article mainly discusses the process and mechanism of the ED method. The effect of porosity on the electrical strength is analyzed. The process of the energy injected into the plasma channel is analyzed using the equivalent circuit method. The simulation result shows that the impedance of the plasma channel is smaller than1Ω. A model of rock fragmentation by plasma channel is established based on the momentum transfer, assuming rock as homogeneous, isotropic and incompressible fluid. The energy in the storage device quickly injects into the plasma channel and the channel expands rapidly to produce a specific impulse to the surrounding rock. The rock begins to move at a certain speed but the initial velocity obtained is not consistent, the relative movement of the displacement difference will make the rock deformation. When the deformation of the rock exceeds its stress intensity, fragmentation occurs.Electrical strength of rock is the core issue of electrical fragmentation of rock. In this research, rocks were placed between a needle high-voltage electrode and a plate grounded electrode in deionized water. The electrical strengths with50%breakdown possibility of yellow sandstone, black marble, white marble, black granite and white granite are70kV/cm,105kV/cm,160kV/cm,135kV/cm and120kV/cm respectively. The result verifies the relationship of rock electrical strength and its porosity. A rock with a higher porosity may have a lower electrical strength. Moreover, the electrical strength drops with increasing the thickness of rocks. Experiments on rock electrical discharge were taken in needle-to-plate electrodes with small energy Marx generator. Fragmentation can be realized on the condition that power supply should provide energy more than8J per pulse.Taking into account the conditions of actual industrial applications of PEDF technology, rock fragmentations with needle-to-needle electrode pulse discharge in tap water were carried out. Several factors effect rock fragmentation performance, such as electrode gap, voltage amplitude, energy per pulse and water conductivity. Higher voltage on the electrode, greater energy of pulse, smaller conductivity of the water, is more prone to destroy rock. Rock fragmentation can be realized on the condition that, electrode gap of2-4mm, water conductivity of300μS/cm, voltage amplitude of30～50kV and energy per pulse of10～20J.Based on theoretical and experimental research of PEDF, a setup of plasma drill has been designed and developed. The combined pulsed power network has been raised as the drill’s generator. The combined network combined linear capacitor and non-linear pulsed forming line, which can match with the impedance of the plasma channel. A LCR triggered, multi-gap and multi-channel spark switch has been developed. Test results shows that the switch has stable voltage output, little energy consumption. The rise time of output voltage is less than40ns. A low wave impedance transmission line with10Ω has been developed. The inlet water tube is installed in the center of the transmission line. This design has combined the water inlet tube and transmission line together. An drilling mechanism have been developed and the mechanism comprises of a drilling frame, a drilling bar, a gravity plate and guiding bars, et al. Several unites of drilling bars can connect together and realize enough mechanical strength. The length of the drilling bar can be adjusted depending on the drilling depth. The drilling mechanism can meet requirements of drilling holes.Experiments on rock drilling using the plasma drill have been carried out. A radially symmetric electrode design was used as the drill head. The drill head consistes of an internal high voltage disc electrode with diameter of44mm and an external grounded cup-like electrode with outer diameter of54mm. The annular inter-electrode gap is2mm. Rock cannot be destroyed with the low impedance transmission line for its operating voltage. When the cable of RG218is adopted as the transmission line, only a concave groove on the rock surface is formed after several discharges. The position of the groove is corresponding to the electrode gap. The rock under the high voltage electrode is undestroyed. Rock drilling with multi-cable electrodes has been carried out. The diameter of the multi-cables drill head is40mm and the gap between high voltage electrode and low voltage electrode is3-3.5mm and10pairs of electrodes are combined. Results show that specific energies for different kind of rocks under different operating voltage have great difference. The specific energy decreases as the operating voltage increases. For red brick, when the operating voltage is30kV, the specific energy is100J/cm3. The specific energy decreases to45J/cm3when the operating voltage is45kV. When the operating voltage is40～50kV, the specific energy of granite is about1000J/cm3.