Experimental Study of Pulsed Discharge Underwater Shock-Related Properties in Pressurized Liquid Water

Engineering background of hydraulic fracturing is applied to improve the permeability of unconventional gas wells, such as coal seams and shale gas wells, by a pulsed discharge mechanism. We studied the general relations between water shock wave properties (the maximum pressure, wave velocity, and energy conversion efficiency), the discharge voltage, and hydrostatic pressure during high-voltage pulsed discharge experiments in pressurized liquid water. +e following observations were made: (1) when the discharge voltage increased from 7 kV to 13 kV, the maximum pressure increased from 12.6MPa (hydrostatic pressure PH � 12MPa) to 40MPa (PH � 6MPa), wave velocity increased from 1418m/s (PH � 12MPa) to 1454m/s (PH � 6MPa), and energy conversion efficiency increased from 9% to 11%, and (2) when hydrostatic pressure increased from 0MPa to 12MPa, the maximum pressure and wave velocity augmented and then diminished slowly (the critical hydrostatic pressure occurs in the 3 to 6MPa range), whereas the change of energy conversion efficiency was not obvious. +eir properties are explained by the variation of electrical parameters during the pulsed discharge.


Introduction
e most direct and effective method to improve oil and gas production is reservoir fracturing, which can substantially improve reservoir permeability [1,2].Because of significant variations in the characteristics of these unconventional gas reservoirs (e.g., coal gas and shale gas) [3,4], the traditional fracturing techniques are not efficient to improve reservoir permeability.Problems such as low efficiency, poor degree of control, damage caused by the fracturing fluid to the reservoir and underground environment, and large water consumption are very common [5,6].erefore, the natural gas mining industry urgently needs a better fracturing technology for these unconventional types of extraction.
e phenomenon of high-voltage pulsed discharge in water (HVPD) has been applied to several domains, such as mechanical processing, lithotripsy, and waste water treatment [7][8][9].In recent years, there has been some investigation into the application of HVPD to remove oil and gas well plugs and for reservoir fracturing improvements [6,10].e water shock waves (WSWs) produced by HVPD are the main power source behind the fracturing of gas reservoirs, the coal and rocks of the gas reservoirs will exhibit different fracturing characteristics under different WSWs.[11].us, any research of this technology must consider the properties of WSW.Considering the depth of oil and gas wells may range from hundreds to thousands of meters, it is important to consider the effect of hydrostatic pressure (P H ) on plasma breakdown, WSW excitation, and transfer processes.
e results showed that P H increased the breakdown voltage of water dielectric and hindered the processes of HVPD and the formation of plasma channels.Schaefer's results showed that the larger the P H , the smaller the bubble maximum radius and the higher the peak frequency [23].Jeffrey conducted a theoretical study revealing that the bubble period and the minimum rarefaction pressure were dependent on P H , while the first peak pressures of WSW were unrelated to P H when P H < 0.9 MPa [24].Based on Jeffrey's results, Lu presented a new model including bound-bound transitions in the calculation of the thermal radiation power [25].His calculations agreed with the experimental results of Jeffrey.However, researchers determined the influence of law of P H on WSWs through theoretical study of the HVPD in pressurized liquid water.Moreover, comparison with experimental results was restricted to P H < 0.9 MPa.erefore, it is unclear if the first maximum pressure of the WSW is unrelated to P H values > 0.9 MPa [23].Investigating the relation between the pulsed discharge voltage U D , hydrostatic pressure P H , and water shock wave properties (e.g., the maximum pressure P M , wave velocity D W , and energy conversion efficiency η) is important to better understand the phenomenon of HVPD and characterize the impact load of WSWs. is can lead to improvements in the construction technologies for reservoir fracturing.
is study establishes the functional relations between P M , D W , and η and U D and P H during HVPD experiments.e characteristics of WSW were then controlled by adjusting U D and P H . Finally, we generated controlled and repeatable optimized shock loads that conformed to the requirements of fracturing rock mass.

Experimental Instruments and Design
2.1.Principles of Experiments.Our studies were based on the fundamental principle of the electrohydraulic effect.A conventional 220 V single-phase alternating current will charge the pulsed energy storage capacitor bank after filtering, boosting, and rectifying by corresponding circuits.
e capacitance of the capacitor bank was fixed, and the stored energy was controlled by U D .During the HVPD experiments, a trigger signal was generated by the control system to close the discharge switch.e energy stored in the capacitor bank was loaded instantaneously into the water body located between the discharge electrodes.A high-energy density plasma channel was generated and expanded rapidly from the inside to the outside.e excited pulse was transmitted through water in a given direction forming a shock wave.e relations between U D , P H , and water shock wave properties were investigated by monitoring and analyzing the voltage, current, and WSW waveforms.

Structure of the Experimental Instruments.
e experimental equipment was composed of a HVPD system, a pressure-bearing pipeline system, and a measuring system (Figure 1).e capacitance of the capacitor bank in the pulsed discharge cabinet was set at C � 60 μF, and its rated operational voltage was 15 kV.e discharge voltage of two MFM30-15 pulse capacitors was continuously adjusted within the prescribed range.A coaxial copper configuration was set in the electrode to make the WSW propagate along the pipeline horizontal direction and avoid electrode highvoltage ablation.
is set up ensures the stability of the HVPD and meets the strength of the electrode.e electrode was 300 mm long, and the high-and low-voltage electrode spacing (l) was 5 mm.e discharge chamber presents a tubular structure (Figure 2).e axle sleeve and electrode structure were sealed and fixed at one end of the discharge chamber by a threaded sleeve and clamping cap.
e flange plate was located at the other end of the discharge chamber, which was connected to the pressurebearing pipeline of the WSW. Figure 3  2 Advances in Materials Science and Engineering system was supplied by a BD-4DSB-16 pressure testing pump producing a maximum pressure of 16 MPa.Two pressure sensors, numbered 1 and 2, were installed on the pipeline wall.e CY400 high-frequency dynamic piezoresistive pressure sensor was selected for data acquisition.e sensor provided a range between 0 and 50 MPa with a 150% overload capacity.e maximum sampling frequency was 1 MHz.e bandwidth and response time of the pressure sensor are 1/(400 kHz) and 0.3 μs, respectively.e signals collected by the sensors were amplified and processed using a TST6250 instantaneous signal recorder.e measured precision was 0.01 MPa for dynamic pressure and 0.1 m/s for velocity.
e external trigger interface of the instantaneous signal recorder was connected to a discharge control box.e current signal was monitored by a self-integrating Rogowski coil with a sensitivity of 38 kA/mV.e voltage signal was detected using a Tektronix P6015A high-voltage probe.e current and voltage signals were loaded into an Agilent DSO6014A digital oscilloscope.

Anti-Interference Measures of the Experimental
Instruments.A significant energy conversion resulted from the breakdown of the water gap by HVPD. is was accompanied by strong electromagnetic radiations and a voltage/current break in the loop of the instruments [26].We thus applied the following anti-interference measures: (1) Power supply: a single-phase alternating current (220 V, 50 Hz) was supplied to the HVPD system.Internal direct current rechargeable batteries were used for the measuring system, that is, the signal recorder and computer, which was sensitive to interference signals.erefore, a conduction coupling interference formed by the power supply was avoided.
(2) Grounding measures: the pressure-bearing pipeline was connected firmly to the pulsed capacitor grounding point, and the measuring system is insulated at this grounding point.e chargedischarge control box, digital oscilloscope, and highvoltage probe must be grounded separately.Floating was used in the case of the signal recorder [27].
(3) Electromagnetic shielding measures: we used piezoresistive pressure sensors with metal shielding providing a clear and strong resistance to interference.We also selected an instantaneous signal recorder with a complete metal shell.e instantaneous signal Advances in Materials Science and Engineering recorder and the Rogowski coil were placed in an aluminum shielding box.( 4) Wires: we kept the wires in the experiment as short as possible.e two wires located between the capacitor and the electrode are coaxial cables.Radiation coupling and electrical loop inductance were reduced as much as possible.

Experimental Process
. U D values of 7, 9, 11, and 13 kV were used, respectively.P H was increased in 1 MPa steps in the 0 to 4 MPa range and in 2 MPa steps in the 4 to 12 MPa range.e distance between No. 1 sensor and the electrode, and between Nos. 1 and 2 sensors, is 1 m.Several tests were repeated for each U D and P H to eliminate random system errors [20,28].Water in the pipeline was replaced before each test to ensure constant temperature (T � 20 °C) and conductivity (ρ � 340 μS/cm) [29,30].e voltage, current, and WSW pressure waveforms were monitored by the measuring system.Note that the shock wave peak pressure measured by the sensor included P H .In the following discussion, P M does not include P H .

Analysis of Experimental Data
e voltage, current, and WSW pressure waveforms from a typical HVPD process are illustrated in Figure 4. e transfer process of the WSW in the pipeline is monitored by two pressure sensors on the pipeline wall.Typical pressure waveforms are illustrated in Figure 5.
WSWs are generated after the discharge gap breakdown of the electrode and propagated from the end of the electrode to the bottom of the pipeline.e pressure was measured by the No. 1 sensor (Time-consuming T 1 ).After transmission on 1 m, the signal was captured by No. 2 sensor (Time-consuming T 2 ).e WSW is propagated and reaches the bottom of the pipeline after transmission on 1 m.en, it is reflected by the end flange and successively propagated to No. 2 (Time-consuming T 3 ) and No. 1 sensor (Time-consuming T 4 ).

Relationship between U D and WSW Properties.
e voltage and current waveforms were analyzed to determine the values of breakdown voltage U B , discharge breakdown time delay T B , loop resistance R L , peak current I P (the first half-wave), and peak power P P [31].
e plots reveal T B gradually decreasing with increasing U D values when P H remained constant (Figure 6).eoretically, increasing the U D values will enhance the field strength difference between the two ends of the gap,  4 Advances in Materials Science and Engineering accelerating the water ionization and gasification velocity [32].en, an ionization avalanche is started, increasing the growth speed of the main plasma channel and decreasing the T B values during the discharge process.e energy input into the plasma channel after the breakdown was enhanced by increasing the U D values.As a result, the plasma temperature grew, inducing a higher ionization rate and conductivity.e channel pressure also augmented due to the higher plasma temperature, accelerating the expansion of the channel, enhancing the conductive cross section, and finally leading to a decrease in channel resistance [33].e plasma is the energy-transforming component of the HVPD.Augmenting the U D values reduced the resistance of the plasma, which increased the I P and P P values in the plasma channel.P M and D W increased with increasing U D when the P H values were fixed.is is explained by the empirical equation of the water shock wave peak pressure (P M ) [34]: where k is a constant related to the experimental environment, Q B is the breakdown energy, and α forms the 7,000 V 9,000 V 11,000 V 13,000 V (a) 7,000 V 9,000 V 11,000 V 13,000 V Advances in Materials Science and Engineering attenuation coefficient of the water shock wave associated with the electrode structure.erefore, the P M value is enhanced by increasing the U D value when all other parameters are constant.For a weak water shock wave (P M < 0.1 GPa), the transfer process is isentropic, producing the following relation between wave velocity (D W ) and P M [35] such as where c 0 is the sound velocity of water before perturbation, n � 7.15, and B � 299 MPa. Figure 7 indicates similar increasing trends of D W and P M values with the augmentation in U D .
e experience formula for computing the energy of WSW is as follows [36,37]: where E S is the energy of WSW in J, S is the wavefront area in m 2 , ρ is the density of water in kg/m 3 , v is the velocity of WSW in m/s, and P is the pressure of WSW in Pa. e ratio between the WSW energy (E S ) to the electrical energy stored in the capacitors (E) is defined as the energy conversion efficiency (η):   From Figure 8, we observed a η value is roughly around 10% increasing slightly with the U D value when the P H values were fixed.is is principally related to the increase in U D that reduces T B and then lowers the leakage loss of energy and improves the energy conversion efficiency.

Relation between P H and WSW Properties.
We cannot ignore P H when applying HVPD during unconventional gas well fracturing and in deep sea environments.Kao [38] and Korobeinikov [39] developed a bubble initiation theory to explain the plasma breakdown process, in which the environmental pressure significantly affected the breakdown field strength of a liquid.e influence of P H was determined by HVPD experiments in pressurized liquid water.During the pulsed discharge process, P H shows "inhibiting" and "enhancing" effects.e inhibiting effect is caused by the compression and hindering of the plasma channel expansion shock, since P H lowers the initial P M and D W . e enhancement is generated by a reduction of the attenuation coefficient during the transfer of the WSW through the water medium.When the P H values increase, both of these effects will change P M and D W starting by an augmentation followed by a decrease (Figure 9).e critical hydrostatic pressure (P CH ) occurs in the 3 to 6 MPa range.
e inhibiting mechanism works as follows: P H and the internal pressure within the channel and the pressure on the inner wall of the channel caused by the electric field were all applied during the channel breakdown process.e combination of these pressures causes a change in the channel radius, and P H hinders the channel expansion [40,41].
e experimental data (Figure 9) and theoretical models indicated that the compressive action of the external pressure on the plasma due to P H delayed the water breakdown and caused an increase in T B .erefore, when the U D value remains constant, the increase in T B with increasing P H values produces a lower energy input into the plasma channel.More energy is wasted as heat and infrared radiation before the plasma channel formed [42], and its electrical resistivity also increased.However, the plasma channel wall was extruded by a P H increase. e channel expansion was lowered, leading to a conductive cross section decrease relative to a situation without P H ; this also results in a growth in electrical resistivity and I P , whereas the channel P P declines accordingly (Figure 6).ese observations indicate that T B and P H increase simultaneously when U D is constant.
e maximum outward expansion pressure of the plasma also decreases.
e enhancing mechanism works as follows: e propagation of weak WSW (p ≤ 100 MPa) is isentropic.e propagation and characteristics are similar to that of sound waves in water.
e research of Saul and Wagner [43] showed that the shock wave maximum pressure and velocity generated by a similar source increased with increasing water hydrostatic pressure at room temperature.
P M and D W augmented with increasing P H when 0 ≤ P H ≤ P CH .Inhibiting and enhancing effects are present during this process, but the former is limited due to a low water pressure.In this case, the enhancing effect related to P H played a primary role.
P M and D W declined slowly with increasing P H when the inhibiting effect of P H played a dominant role and significantly weakened P M and D W .Although an increased pressure also enhanced the water shock wave transfer, P M and D W values began to slowly diminish when P CH ≤ P H because of the initial lowering of P M and D W .
Figure 8 also reveals that P H will reduce η when pressure is applied.However, due to the increase in instability of the discharge process with increasing P H , the change of η has no regularity.

Data Fitting and Contrast
Data from the literature [20,34,40] and experimentations reveal that T B is a function of P H , or T B � h(P H ). us, the breakdown energy Q B is given by following equation: where k and α in the empirical formula (1) are parameters related to the experimental environment and electrode geometry [44,45].ese two parameters represent the energy conversion characteristics of the WSW in different experimental conditions [42].We then assume k and α are related to P H . e functions can be written as k � f(P H ) and α � g(P H ). Substituting ( 5) into (1), we produce an expression on P M : Finally, the empirical formula (7) representing P M in pressurized liquid water can be obtained by fitting of the Advances in Materials Science and Engineering experimental data.D W can also be obtained by substituting (7) into (2); thus, where P M is the water shock wave peak pressure in MPa, P H forms the hydrostatic pressure in MPa, U D is the discharge voltage in kV, C defines the capacitance of the capacitor bank in μF, and R gives the equivalent resistance of the water gap in kΩ.In our experiments, C � 60 μF and R � 8.5 kΩ. e functions f(P H ) and g(P H ) represent the energy conversion of the WSW in different experimental situations in (7).f(P H ) and g(P H ) are all linear increasing functions.
is indicates that the intensity of the WSW increases with P H , confirming the conclusions of Saul and Wagner [43] and Lu [46].e function h(P H ) represents the compression and delaying of the plasma channel expansion shock by P H . e expression reveals that the h(P H ) function increases regularly; the greater the h(P H ), the more compressed the plasma channel. is is also consistent with the conclusions of Jia et al. [20], Zhang et al. [21], and Liang et al. [22].
e experimental data and the fitting curves (Figure 9) indicate P CH is not constant.P CH increases when U D grows.
Figure 10 is a comparative diagram showing the experimental data, the fitted curve, and the data of Touya et al. [34] and Sun [42], when P H � 0 MPa and the distance d between the discharge electrode and pressure sensor is 1 m. e experimental data and fitted curve are very different from the data of Touya and Sun Bing, but their trends are e differences are explained principally by the settings of Touya's experiment, in which the distance of the electrode gap l is 10 mm, the curvature radius r is 15 mm, α � 0.35, and k � 9000/d (d being the distance between the discharge electrode and pressure sensor).e equation of the maximum pressure P T obtained by Touya is where P T is in bar, Q B is in kJ, and d is in mm. e pulsed discharge experiment of Touya was conducted in an open environment.One of the water boundaries was not restricted.is generates an energy loss during plasma expansion and impact on the free water surface, and the larger the distance d, the more prevalent is the drop in maximum pressure attributed to the loss.In this paper, the pulsed discharge experiments were carried out in an airtight pipeline which was fully filled with water, and d � 1 m (Touya's d values were 345-555 mm).
e different discharge environments produce large deviations in Touya's calculations relative to our experimental values.e Touya's empirical equation is no longer applicable for water with all restricted boundaries (have no free surface), such as oil and gas wells filled with water.
e pulsed discharge experiments of Sun Bing were carried out with rod-rod electrodes in an airtight pipeline (5 m in length and 300 mm in diameter) filled with water (310 μS/cm).e empirical formula of peak pressure P S by Sun Bing is where P S is in bar, Q D is in kJ, and d is in mm.
e maximum discharge energy of Sun Bing's experiment was 22.5 J (C � 50 nf and U D � 30 kV).However, the minimum discharge energy of our experiments is > 1 kJ (Q D ≈ 1.5-5 kJ).Furthermore, the peak pressure P S shows a linear correlation with the energy value Q D when the distance d is constant.
Figure 11 shows that the deviation in the calculation results of the two formulas is small when Q D < 50 J.However, if Q D > 50 J, the deviation is more pronounced.e main is led the efficiency of transforming the electrical energy into mechanical energy to be far higher than that of our experiment.For HVPD with larger energy (e.g., 1.5 kJ), the theoretical discharge voltage of Sun Bing was 200 kV (only 7 kV in our experiment).e calculations used Sun Bing's fitting formula (formula ( 9)), where the intensity of WSW was about 240 MPa and the energy conversion efficiency was about 180% (suppose the WSW width was 2 μs).erefore, the empirical formula for peak pressure calculation by Sun Bing does not apply for a pulsed discharge process with larger Q D .

Discussion
e relationship between the WSW properties (P M , D W , and η) and U D and P H was studied by experimentation.Results show that an increase of U D can enhance the WSW properties (P M , D W , and η), but the processes of increasing P H on the WSW properties (P M , D W , and η) are more complicated.
e results obtained in this paper after fitting the experimental data are compared with that of other studies, and we explained discrepancies with the other published data.
rough this study, we understood the basic method of controlling P M and D W .As a result, we can specifically modify the U D and P H values and then adjust the WSW properties (P M and D W ) during the process of deep well fracturing.
In the process of HVPD, Jones found that the conductivity of water almost had no effect on the discharge process [28]. is is mainly because the discharge voltage of Jones' experiment was relatively higher (U D > 50 kV), which caused the discharge breakdown process to be in nanoseconds.us, the influence of the conductivity on breakdown time delay was negligible basically.But when the discharge voltage was lower (U D < 5 kV), the higher conductivity of water would lead to the shorter breakdown time delay [7,30].For the intensity of WSWs, Zhu et al. found that when U D � 4 kV, with the conductivity increasing from 0 mS/cm to 120 mS/cm, P M declined rapidly from 3.5 MPa to 0.62 MPa [29].While Zhuang Jiasheng found that when U D � 7 kV (HVPD was carried out in a pipe which had a parabolic reflector), with the conductivity increasing from 0.0177 mS/cm to 109.7 mS/cm, P M increased from 20 MPa to 100 MPa [47].us, under low discharge voltage, the influence of conductivity was huge and uncertain on intensity of WSWs. is means that a better fracturing effect of oil and gas wells can be achieved when using a fracturing fluid with suitable conductivity.So, the influence rules of conductivity on the intensity of WSWs need to be further studied.
Large capacity pulse capacitors were adopted in our experiment to store the electric energy.
e capacitance value cannot be modified.If it was adjustable, we would need multiple parallel capacitors of small capacitance.However, this would bring the discharge process of each capacitor out of sync, release the pulse energy separately, and reduce the intensity of the first pulse shock wave [27].
e rise time of WSW obtained in this paper is significantly larger relative to other studies, and the P H influence on the η value is unclear.is needs further research and analysis in the future.

Conclusions
When P H is constant, T B and R L decreased, and I P and P P all increased.Furthermore, the WSW properties (P M , D W , and η) augmented with increasing U D .A larger U D always strengthened the effect of HVPD and the WSW properties.
Moreover, when U D is constant, T B and R L increased and I P and P P all decreased with increasing P H .
e P H augmentation demonstrated its "inhibiting" effect on the HVPD process.However, increasing P H will partially reduce the attenuation coefficient of WSW and enhance the transfer of WSW. is demonstrates the "enhancing" effect on P M and D W . us, when U D is constant, P M and D W first grow and then diminish with increasing P H .
is is caused by the inhibiting and the enhancing effects.Moreover, the η value of HVPD in pressurized liquid water is slightly smaller to that in stress-free water, but the change of η has no regularity.
e experimental data fitting enabled a partial application of the empirical formula which would provide a basis for theoretical calculation to further improve the permeability using pulsed discharge hydraulic fracturing.

Figure 1 :
Figure 1: Schematic diagram of the experimental instruments.

Figure 2 :Figure 3 :
Figure 2: Schematic diagram of the discharge chamber structure.

Figure 6 :
Figure 6: Variation diagram of (a)T B , (b) R L , (c) I P , and (d) P P versus P H for a range of U D and P H values.

Figure 8 :
Figure 8: Variation diagram of η versus P H for a range of U D .

Figure 9 :
Figure 9: Binary diagrams showing the correlations between P H , P M , and D W under four different values of U D : (a) 7 kV, (b) 9 kV, (c) 11 kV, and (d) 13 kV.

Figure 10 :Figure 11 :
Figure 10: Comparative diagram showing the experimental U D versus the water shock peak pressure with the fitted curve and the data of Touya and Sun Bing, using P H of 0 MPa and d of 1 m.