**2. Bench test for hydropulse coal seam loosening technology using a cavitation generator of fluid pressure oscillations**

The main results on the experimental study of the physical processes occurring in the hydrovibrator flow channel and their characteristic features are presented in [8]. It is shown that the most developed fluid oscillations are observed for a hydraulic system with the Venturi tube diffuser angle *β*, greater than 16°.

The object of this study is the dynamic characteristics of the hydraulic vibrator and their correspondence to the optimal modes of impulse action during the loosening of outburst-prone gas-rich coal seams with methane extraction from them. The layout of such hydraulic vibrator is shown in **Figure 2**.

The cavitation generator 1 with a post-diffuser hydraulic tube 5 was called a hydraulic vibrator. The presence of such a hydraulic tube is especially important to eliminate the loss of pulsed energy for case of the generator operates in a flooded liquid jet. The geometric parameters of the cavitation hydraulic vibrator were determined taking into account the implementation of the level of hydraulic pulsating loads for effective loosening coal seams. At the same time, the coal physical and mechanical properties and the characteristics of serial pumping units used in mines were taken into account [10].

The geometric parameters of the cavitation hydraulic vibrator designed for loosening outburst hazardous gas-rich coal seams are given in **Table 1**.

#### **Figure 2.**

*Simplified schematic of the hydraulic vibrator structure. 1 is Venturi tube; 2 is critical section; 3 is diffuser with β = 20°; 4 is input tube; 5 is post-diffuser hydraulic tube, 6 is feed pipeline.*


#### **Table 1.**

*The main geometrical parameters of the tested hydraulic vibrator.*

The hydraulic vibrator with a set geometry is characterized by the regime parameters—the total inlet pressure *Р*1, and the head pressure *Р*<sup>2</sup> and fluid flow rate *Q* through hydraulic vibrator and the main dynamic parameters—the fluid pressure selfoscillations range Δ*Р* (peak to peak values) and the frequency *f* of fluid selfoscillations in the hydraulic tube of the hydraulic vibrator.

The study of the parameters of high-pressure fluid injection through boreholes or wells into coal seams during preventive measures to prevent gas-dynamic phenomena in the faces of development workings is a complex experimental task. The injection parameters are determined by the pressure drop and the volume of the fluid injected into the reservoir, while the stress state and the mass discharge zone are determined by the parameters of the seismoacoustic signal and the well sealing depth.

A stand for modeling the operating conditions of the cavitation hydraulic vibrator in a well should ensure the reliability of instrumental measurements of its characteristics, taking into account the mining and geological conditions of occurrence of outburst hazardous coal seams and their properties. When developing such stand, the possibilities of using the technical means and serial equipment available at the mines should be taken into account.

For physical modeling of the cavitation hydrovibrator operation in a well, the Institute of Geotechnical Mechanics of National Academy of Sciences of Ukraine (IGTM NASU) developed the special stand installation [11]. The parameters of commercially available mining equipment and the physical and mechanical properties of coal in its place of occurrence are the initial data for the development of the stand and testing of the hydraulic vibrator. In view of the foregoing, the stand should provide the fluid pressure at the inlet to the hydraulic vibrator *Pp* from 5 to 32 MPa at the volumetric flow rate *Q* from 20 to 120 *l*/min (according to the parameters of pumping units), according to the properties of the coal seam (foremost, gas pressure in the seam), the water pump pressure head must be from 1.0 to 20.0 MPa.

A schematic diagram of the developed stand design for hydraulic testing and determining the efficiency of the cavitation hydrovibrator in the well is shown in **Figure 3**.

A system of pressure sensors and visual equipment is connected to the developed bench installation. The system registers the operating parameters (the bench fluid flow rates and pressures) with the supply of signals through an amplifier to a personal computer. Along the pumps there is an inlet manifold 4, which has a nipple connection with the inlet pipe of each pump. The outlet pump nozzles are connected in parallel to the high pressure pipeline 8, which goes to the site of the test object. The tank is connected via valve 10 through filter 11 to the inlet of the make-up pump with fluid flow rate of *Q* = 160 *l*/min and discharge pressure *Pbp* = 7 MPa, which ensures uninterrupted operation of high-pressure pumps 5, 6, and 7. The system is powered from switchboard 15. For ensuring the required water flow through the test object, the pump control panel allows researchers to turn on from one to three pumping units at the same time.

The stand systems are able to maintain pressure at the inlet to the hydrovibrator *Pp* up to 32 MPa for fluid flow rates from 20 to 120 *l*/min and smoothly adjust the head pressure *Pb* in the range from 0.05 to 0.9 of the fluid feed pressure.

The technological capabilities of the hydraulic test bench and operation simulation of the cavitation hydrovibrator make it possible to carry out autonomous tests of the hydrovibrator (**Figure 4a**) to determine the parameters of the oscillatory process in the hydraulic channel of the hydrovibrator Δ*Р*<sup>1</sup> and *f*) and by immersing it directly into the simulator pipeline (**Figure 4b**). At the same time, the various options of the

*Advanced Technology of Drilling and Hydraulic Loosening in Coal Bed Methane Using… DOI: http://dx.doi.org/10.5772/intechopen.105812*

#### **Figure 3.**

*Schematic diagram of the stand for hydraulic testing and operation simulation of the cavitation hydrovibrator in a well condition. 1 is the tank with the 8 m<sup>3</sup> volume; 2 is make-up auxiliary pump; 4 is the pump inlet pipeline; 5–7 are the three high pressure pumps; 9 is the cavitation hydrovibrator; 10 is an intake valve; 11 is a filter; 12 is a fluid flow meter; 13 is the adjustable throttles for setting pressures at the inlet and outlet of the tested hydrovibrator; 14 is drain pipeline; 15 is electrical panel; 16 is water pipeline.*

hydrovibrator location in the well model [11] were tested. As a result of testing and comparing their results in terms of efficiency, the final version was chosen (**Figure 4b**).

The well model (well simulator) includes an inlet hydraulic channel 1, connected by means of a nipple to a flexible high-pressure stand hose, the cavitation hydraulic vibrator 2 with an outlet hydraulic tube 3, a well simulator pipeline 4 (with a diameter of 42 mm), at the outlet of which the booster throttle 5 is installed. The booster throttle is connected to the drain pipeline of stand 6.

The values of fluid pressure at the inlet to the hydrovibrator and the average volumetric fluid flow rate through hydrovibrator, as well as the ranges of the fluid pressure pulses in the well simulator pipeline Δ*Р*2, Δ*Р*3, Δ*Р*4, and Δ*Р*<sup>5</sup> at different distances from the inlet section to the well simulator (**Figure 4**) and oscillation frequency *f*, are the main physical quantities that characterize the hydropulse effect on the coal seam.

All measured parameters are distributed over the dynamic principle into two categories: the static parameters, which change with time at frequencies less, than 4 Hz, and dynamic parameters, which change with time at frequencies above 4 Hz. The dynamic parameters are the values of fluid pressure pulsations at the cavitation hydrovibrator outlet and in the pipeline simulator of the well and the fluid oscillation frequency.

#### **Figure 4.**

*Schematic diagram for the cavitation hydrovibrator autonomous tests in the well model. 1 is inlet hydraulic pipeline; 2 is cavitation hydraulic vibrator; 3 is adapter; 4 is well simulator pipeline; 5 is retaining throttle; 6 is stand drain pipeline.*

To measure static pressure parameters, technical pressure gauges with a reduced error of 0.6% are used, which makes it possible to determine the average pressure value at the pressure gauges' location. The flow rates of bulk fluid through the device were determined by a turbine fluid flow rate sensor with a reduced error of 1%. It is installed directly in the measured pipeline in front of the inlet pumps manifold.

The measurement of fluid dynamic pressure is based on the direct registration of the full pressure values using appropriate pressure sensors by converting a physical quantity into an electrical signal.

The block diagram of the system for measuring the dynamic parameters of the cavitation generator during bench tests is shown in **Figure 5**.

As primary transducers, inductive total pressure sensors of the DDI-20 type are used, which measure the pressure value up to *Р*max = 60 MPa per pulse. The sensor has a sensitive membrane, at a given distance from which an inductive coil is installed. The principle of operation of the sensor is based on the change in the inductance of the coil depending on the deflection of the membrane under the influence of static-dynamic pressure. The natural frequency of the membrane is not less than 20,000 Hz, the hysteresis is not more than 2%, the nonlinearity of the calibration characteristic in the pressure range from 0 to *Р*max is not more than 5%.

The sensor is included in one arm of the high-frequency inductive bridge, which is located in the IVP-2 converter. The inductive high-frequency two-channel transducer IVP-2 is a secondary transducer in the system for measuring rapidly changing pressures and is designed to convert the complex resistance of the sensor into electrical voltage. The change in the inductance of the sensor, due to the pressure acting on the membrane, turns into a deviation of the initial voltage of the IVP-2.

The brief description of IVP-2 converter is as follows:

*Advanced Technology of Drilling and Hydraulic Loosening in Coal Bed Methane Using… DOI: http://dx.doi.org/10.5772/intechopen.105812*

**Figure 5.** *Schematic diagram of the stand measuring system of dynamic parameters of hydraulic test.*


The signal from the DDI-20 sensor through the IVP-2 converter enters the multichannel analog information input board, converted into a digital form by the analog-to-digital converter, and fed to a PC. The total reduced error of pressure measurements by the DDI-20 sensor with the IVP-2 transducer is 5.19% [11].

Processing of test results is also divided into two types. Static parameters are calculated according to the developed template in the Excel package. For dynamic parameters, the primary records from the analog-to-digital converter are recalculated into physical values and presented in the form of a graph in the time domain for further analysis using Excel.

During studying the characteristics of the experimental sample of the hydrovibrator, the input throttle (see **Figure 3**) set the pressure at the inlet to the hydrovibrator *Pp* = 5; 10; 20; 30 MPa. At each steady-state pressure at the inlet *Pp*, the outlet throttle changed discretely the outlet pressure *Pb*. At steady-state values of pressure at the inlet and outlet, by the "Measurement" command, control and measurements of dynamic parameters are carried out with a registration time of at least 10 s at each frequency.

As an example of evaluating the application of the measurement technique and processing the research results, **Figure 6** shows the oscillogram recording the pressure value *p1* in time in the hydraulic channel of the hydrovibrator with *d*cr = 2.5 mm at inlet pressures *Pp* = 10 MPa and pressure head *Pb* = 1.6 MPa.

*A fragment of the time recording the pressure* p*<sup>1</sup> at the outlet of the cavitation hydrovibrator with* d*cr* = *2.5 mm at* Pp = *10 MPa and* Pb *= 1.6 MPa.*

It can be seen from **Figure 6** that periodic pressure oscillations *p*1, are observed at the outlet of the hydrovibrator, have a shock character with a steep front of pressure rise and fall. This type of oscillation in hydrodynamics is called pressure pulsation and is characterized by the frequency and range of pressure self-oscillations.

The frequency of pulsations of pressure self-oscillations at the outlet of the hydrovibrator is due to the occurrence of the periodic-stall cavitation mode and is determined from the oscillogram by the formula *f* ¼ *n=t*. Here *n* is the number of pulsation periods; *t* is the duration of *n* periods of pulsations in seconds.

The range of pressure self-oscillations Δ*P*<sup>1</sup> at the experimental hydrovibrator outlet is the difference between the maximum *p*1 max and minimum *p*1 min pressure values in the pulse Δ*P*<sup>1</sup> ¼ *p*1 max � *p*1 min *:* By a similar way, the magnitudes of pulsations Δ*Р*2*,* Δ*Р*3*,* Δ*Р*4, Δ*Р*<sup>5</sup> in the pipeline simulator of the well are determined.

The results of autonomous tests of the hydraulic vibrator are presented as dependences of the range of pressure self-oscillations Δ*P*<sup>1</sup> and pulse frequency *f* on the water pressure head *Pb*. In this case, the values of pressure at the inlet to the hydrovibrator and the volumetric flow rate of liquid through it were *Pp* = 5; 10; 20 and 30 MPa and *Q* = 0.45, 0.64, 0.91, and 1.11 *l*/s, respectively.

Note that the dependences of the range of pressure oscillations Δ*P*<sup>1</sup> on the water pressure head *Pp* at various discharge pressures *Pb* are in satisfactory agreement with similar theoretical dependences obtained by calculations using the refined linear model [12]. This is clearly seen from **Figure 7a**, which shows these dependencies. The relative error does not exceed 20%.

For case of the hydraulic vibrator operates in the studied range of water pressure head changes, the regime of periodically stalled cavitation is realized in its flow part and fluid pressure *Р*<sup>1</sup> oscillations occur, which are caused by the collapse of cavitation cavities in the hydraulic channel. At a fixed value of the water pressure head *Pb*, an increase in pressure *Pp* at the inlet to the experimental sample of the hydrovibrator leads to an increase in the oscillatory value of pressure Δ*P*1. For case of the pressure *Pb* = 4 MPa with the increase in discharge pressure *Pp* from 10 to 30 MPa, the pressure pulse value Δ*P*<sup>1</sup> increases from about 10 to 38 MPa.

*Advanced Technology of Drilling and Hydraulic Loosening in Coal Bed Methane Using… DOI: http://dx.doi.org/10.5772/intechopen.105812*

#### **Figure 7.**

*Experimental and calculated dependences of the pressure oscillations range of Δ*Р*<sup>1</sup> and the pulse frequency* f *on the outlet pressure* Pb*.*

Dependences Δ*P*1(*Pb*) for different values of pressure *Pp* have a maximum in the range of pressure head *Pb* from 0.14 and up to 4 MPa. As the discharge pressure *Pp* increases, the maximum pressure range value of Δ*P*<sup>1</sup> shifts toward higher *Pb* values. The maximum value of the pressure range Δ*P*<sup>1</sup> is approximately in from 1.3 to 2.5 times higher than the static pressure *Pp* at the hydrovibrator inlet. At the same time, as the discharge pressure *Pp* increases, the ratio Δ*P*1/*Pp* decreases.

The location of the experimental points (see **Figure 7**) relative to the theoretical dependences of the frequency *f* of the fluid pressure cavitation self-oscillations in the hydrovibrator on the water pressure head *Pb* [13] indicates their satisfactory convergence. The relative error does not exceed 10%. At a fixed value of the back pressure *Pb*, an increase in the pressure *Pp* at the inlet to the hydraulic vibrator leads to a decrease in the frequency *f*. Thus, at the value of the water pressure head *Pb* = 4 MPa with an increase in the discharge pressure *Pp* from 5 to 30 MPa, the frequency of cavitation self-oscillations decreases from 5000 to 2100 Hz.

It should be noted that an increase in inlet pressure *Pp* of the hydraulic vibrator expands its operating range in terms of pressure head and is located in the operating range from 0.05 to 0.8 *Pp*.

Experimental modeling of hydraulic loosening of a coal seam by the cavitation hydraulic vibrator was performed on a well simulator in accordance with the schematic shown in **Figure 3**. The study of the hydrovibrator performance was carried out in modes in accordance with the mining and geological conditions of occurrence of outburst-hazardous seams of the "Sukhodolskaya-Vostochnaya" and "Molodogvardeiskaya" mines of the "Krasnodonugol" Association (Ukraine).

As an example, **Figure 8** shows an oscillogram of fluid pressure oscillations behind a hydraulic vibrator in various sections of a well simulator. The injection pressure in this case was *Pp* = 21 MPa at the fluid volume flow rate of *Q* = 55 *l*/min, which corresponds to the mining and geological conditions of the formation at a depth of 1300 m ("Sukhodolskaya-Vostochnaya" mine).

It can be seen from **Figure 8** that the water pressure head of *Pb* changes from 1 to 14 MPa, in all sections along the well simulator length, the periodic pressure of the Δ*P*2, Δ*P*3, Δ*P*4, Δ*P*<sup>5</sup> oscillations are observed with a steep front of pressure rise and fall. The pressure ranges Δ*Р* are symmetrical with respect to the average pressure value *Pb.*

#### **Figure 8.**

*The time recording of fluid pressure (Δ*P*2, Δ*P*3, Δ*P*4, Δ*P*5) oscillations at different sections of the well simulator length.*

Processing of time oscillograms with recording of fluid pressure fluctuations along the length of the well simulator of the Δ*P*2, Δ*P*3, Δ*P*4, Δ*P*<sup>5</sup> (averaged over eight measurements of their values) was performed by the standard "Oscilloscope" program and is shown in **Table 2**. It corresponds to the regimes of hydraulic loosening of the coal seam in mines.

The results of testing the device in the well simulator in the form of dependences of the values of pulsation ranges of Δ*Р* in various sections of the pipeline along its length (see **Figure 3**) on the water pressure head of *Pb* are shown in **Figure 8** at *Pp* = 21 MPa, *Q* = 55 *l*/min and **Figure 9** at *Pp* = 11 MPa, *Q* = 40 *l*/min.

An analysis of these experimental data shows that fluid pressure oscillations exist in the entire range of the *Pb* the water pressure head change (from 1 to 12 MPa) studied. The change in the range of pressure pulsations of the Δ*P*2, Δ*P*3, Δ*P*4, Δ*P*<sup>5</sup> from the pressure head *Pb* (see **Figure 9**) is nonlinear. There are two pronounced maxima. The first one is at pressure head of *Pb* from 4.1 to 5.1 MPa and frequency *f* from 2200 to 2700 Hz. For these values of *Pb* and *f* pressure fluctuations are realized in different sections of the well simulator: Δ*Р*<sup>2</sup> = 13.8 MPa, Δ*Р*<sup>3</sup> = 10.2 MPa, Δ*Р*<sup>4</sup> = 8.97 MPa, Δ*Р*<sup>5</sup> = 8.58 MPa. The second one is at *Pb* ≈ 11.1 MPa and *f* ≈ 6500 Hz, at which the pressure rages are Δ*Р*<sup>2</sup> = 15.6 MPa, Δ*Р*<sup>3</sup> = 8.8 MPa, Δ*Р*<sup>4</sup> = 7.6 MPa, and Δ*Р*<sup>5</sup> = 6.4 MPa.

The results of testing the hydraulic vibrator in the well simulator at *Pp* = 11 MPa, *Q* = 40 *l*/min (**Figure 10**) show that fluid pressure pulsations exist in the entire studied range of water pressure *Pb* from 1 to 8 MPa.

The change in the values pressure oscillations of Δ*Р*2, Δ*Р*3, Δ*Р*4, Δ*Р*<sup>5</sup> on the water pressure head *Pb* is nonlinear with a pronounced maximum *Pb* (about 3.1 MPa) at the oscillations frequency *f* equal to 2260 Hz.

For the indicated values of *Pb* и *f* in different sections of the well simulator, the pressure pulsations Δ*Р*<sup>2</sup> = 11.97 MPa, Δ*Р*<sup>3</sup> = 10.89 MPa, Δ*Р*<sup>4</sup> = 10.31 MPa и


*Advanced Technology of Drilling and Hydraulic Loosening in Coal Bed Methane Using… DOI: http://dx.doi.org/10.5772/intechopen.105812*

#### **Table 2.**

*The results of processing the test data of the hydraulic vibrator in the well simulator at* Pp *is 21 MPa,* Q *= 55 l/min and at* Pp *= 11 MPa,* Q *= 40 l/min.*

#### **Figure 9.**

*Dependences of pressure oscillations Δ*Р *ranges at various sections of the well simulator on the outlet pressure* Pb *at* Pp *= 21 MPa and Q = 55 l/min.*

Δ*Р*<sup>5</sup> = 9.59 MPa are realized. That is, the dynamic pressure range Δ*Р* along the length of the well simulator slightly decreases, the attenuation process occurs.

The theoretical substantiation of the hydrodynamic parameters of the impulse action was carried out using the data of P. M. Mokhnachev and V. V. Pristash [14], where the rate of coal deformation is expressed in the following form

#### **Figure 10.**

*Dependences of pressure oscillations Δ*Р *ranges at various sections of the well simulator on the outlet pressure* Pb *at* Pp *= 11 MPa and* Q *= 40 l/min.*

$$
\dot{\varepsilon} = \frac{d\varepsilon}{dt} = \frac{\Delta P \cdot f}{E},
\tag{1}
$$

where *ε* is linear deformation of coal; Δ*P* is impulse pressure; *f* is pulse frequency; *Е* is modulus of elasticity of coal.

Due to the urgency of the problems of rock bumps and sudden outbursts, VNIMI [15] investigated the properties of outburst coals. Here, at the first time, the drop in the coal strength was discovered. In this case, a particularly sharp decrease in strength is observed in the range of strain rates from 1 to 10 с�<sup>1</sup> .

Taking into account the above, the expression (1), for the ultimate case of the coal deformation rate equal to 10 с�<sup>1</sup> , is transformed as follows:

$$
\Delta P = \frac{10E}{f}.\tag{2}
$$

**Figures 11** and **12** show the theoretical dependences of the optimal values of pressure fluctuations on the frequency of their repetition, calculated by expression (2)

#### **Figure 11.**

*Theoretical dependences of the optimal values of pressure pulsations* Δ*P on their repetition rate f and experimental data of the fluid pulsed action implemented by the hydraulic vibrator in the well simulator at* Pp *= 21 MPa and* Q *= 55 l/min.*

*Advanced Technology of Drilling and Hydraulic Loosening in Coal Bed Methane Using… DOI: http://dx.doi.org/10.5772/intechopen.105812*

#### **Figure 12.**

*Theoretical dependences of the optimal values of pressure pulsations* Δ*P on repetition rate f and experimental data of the fluid pulsed action implemented by the hydraulic vibrator in the well simulator at* Pp *= 11 MPa and* Q *= 40 l/min.*

(for the values of the modulus of elasticity of coal along the bedding *<sup>Е</sup>* = 3<sup>10</sup><sup>2</sup> ; <sup>5</sup><sup>10</sup><sup>2</sup> MPa; and in the vertical direction to the seam 2<sup>10</sup><sup>3</sup> MPa).

It also presents experimental data on the pulsed action of the fluid implemented in the well simulator with a change in backwater pressure in the range from 2 to 12 MPa and an injection pressure of 21 MPa (**Figure 11**). This corresponds to the conditions of loosening outburst hazardous coal seams at the coal occurrence depth from 1100 to 1300 m ("Sukhodolskaya – Vostochnaya" mine, Ukraine). Experimental data were obtained by measuring pressure oscillations at section distances of 0.5, 1.0, 1.5, and 2.0 m from the hydrovibrator outlet (see **Figure 3**).

The given dependencies allowed to substantiate the optimal values of the parameters of hydrodynamic loosening of outburst-hazardous coal seams of the "Sukhodolskaya-Vostochnaya" mine. The optimal values of the hydraulic vibrator parameters must meet the following requirements:


For the "Molodogvardeyskaya" mine (Ukraine), testing of the hydraulic vibrator in the well simulator was carried out at *Pp* = 11 MPa with the fluid flow rate of *Q* = 40 *l*/min. This corresponds to coal seam occurrence depths from 700 to 800 m. The coal seam hydraulic resistance does not exceed 6 MPa.

An analysis of the given dependencies made it possible to substantiate the optimal dynamic parameters of loosening of outburst-hazardous coal seams of the "Molodogvardeiskaya" mine (Ukraine). The optimal dynamic parameters must meet the following requirements:


As can be seen from the presented data, the parameters of dynamic loading of the coal seam during pulse loosening with a hydraulic vibrator satisfy the conditions of theoretical dependences of the optimal values of pressure pulsations on their frequency. The operating points of the hydraulic vibrator in both cases (see **Figures 11** and **12**), as a rule, are higher than the corresponding theoretical dependencies. It should be noted that the indicated range was chosen for the values of the coal modulus *<sup>Е</sup>* from 3102 to <sup>5</sup>102 MPa during compression along the bedding, since it is in this direction that the dynamic action sets in motion cracks inclined toward the bedding, i.e., in the direction of low permeability of the coal seam in the natural state.

Considering the physical process of hydropulse loosening of the coal seam, it should be noted that at the beginning of the operation of the hydraulic vibrator in the range of low water pressure head in the seam, when the level of pulse values and their repetition rate are below the optimal values of the hydropulse action parameters, cracks in the coal seam will not develop. The water pressure head, due to pumping fluid into the coal seam, will increase. Upon reaching the value of *Pb* ≥ 2 MPa, the generator dynamic parameters will automatically move into the range of values of Δ*Р* and *f*, equal to or exceeding the optimal values of coal seam hydropulse loosening. That is, the hydraulic vibrator, as it were, switches to the self-regulation mode. This will allow effective loosening of the coal seam in the directions of compression along the bedding and along the normal to the seam.

The results of the presented bench tests and the conclusions obtained from them made it possible to proceed to work to evaluate the effectiveness of the hydraulic pulse loosening device in industrial (mines) conditions and compare the results obtained with the data of static fluid injection into the reservoir according to the standard procedure [16].
