Advanced Technology of Drilling and Hydraulic Loosening in Coal Bed Methane Using a Cavitation Hydrovibrator – Experience and Prospects

Abstract

Modern progressive technologies use static fluid injection into seams for safe and cost-effective operation of coal seams. However, the deterioration of mining and geological conditions leads to a significant decrease in the efficiency of the process of methane sequestration from coal seams in case of increase in the depth of development of gas-bearing coal seams. This deterioration is due to a change in the stress-strain state of deep rock massifs, their low permeability, strong anisotropy of soft coal, leading to an increase in dynamic manifestations of rock pressure in the form of sudden outbursts of coal and gas, and rock destructions with catastrophic consequences. An advanced technology for hydraulic loosening and recovery of methane from gas-bearing coal seams, based on the creation of hydrodynamic impulses in a well surface and their transformation into mechanical vibration loading to coal seam, was developed. Such impact to the coal mass leads to the development of a system of cracks. As a result, the efficiency of coalbed hydraulic loosening increases, the zones of moistening and unloading of the formation increase, the gas emission of methane is intensified, the level of dust formation and the resistance of coal to cutting during its destruction are reduced.

1. Introduction

The main problem of safe coal mining from gas-bearing coal seams is methane contained in the sorbed (bound) state in the system of natural coal cracks. The gas content of methane in dangerous concentrations in coal seams leads to sudden releases of methane, as well as to its explosions and coal dust, leading to catastrophic consequences and numerous deaths. In the main coal-producing countries of the world (China, United States, Russia, Australia, Ukraine, etc.), the development of highly gas-bearing coal seams is carried out using surface and underground drilling methods as well methods for extracting methane.

However, as the world experience in solving the problems of mine safety shows, the existing technological solutions for the extraction of methane in the development of gas-bearing coal seams are not always effective.

Thus, in [1], a quantitative analysis of the death of personnel in the mines of China for the period from 2006 to 2010 was carried out. Gas accidents account for a significant portion of the death toll in Chinese coal mining has been claimed. The causes of these accidents and the influence of mine ownership on death frequency are considered.

The state of methane safety of coal mines in Russia is constantly considered at the international symposia “Miner’s Week.” In particular, in [2], the conceptual approaches to the problem of coal mine methane in Russia are considered. It is shown that out of the 105 operating mines, 78% of the mines are dangerous in terms of the maximum methane content, and 45% of the mines are especially dangerous. Kuzbass (Russia) is the main region of the country, where problematic methane-unsafe coal mines are concentrated.

In 2006 and 2007, 256 miners died from coal methane problems due to the lack of reliable degassing systems in Russian mines. At the same time, degassing systems are widely and effectively used in such developed coal-producing countries, as the United States, Australia, etc. In these countries, coal mining with a methane content of more than 9 m³/ton is legally prohibited.

Ukraine is one of the countries with the highest (in the world) number of miner’s deaths per 1 million ton coal. So, in 2007, in the Donets Basin, at the “Zasiadko” mine, as a result of a gas (methane) explosion, 106 miners and rescuers died. In 2015, methane exploded at the same mine at a depth of 1230 m. Since 1991, 107 people have died in one of the most dangerous in the Donbas mine (the “Sukhodolskaya-Vostochnaya” mine) due to sudden outbursts of methane gas.

According to the authors of this study, the main reason for the mass death of miners is the reluctance of the coal enterprises owners to invest in a direct solution to the problem of methane safety, including the development of new technologies for extracting and capturing methane with its isolated removal to the surface or into mine workings, with dilution to its safe concentrations.

1.1 Analysis of recent research and publications on drilling wells for degassing gas-bearing coal seams

Currently, ensuring the methane safety of coal mining is carried out by two ways.

The first way is the land-based method of extracting methane. It provides early degassing of mine fields through wells drilled from the surface and increases the efficiency of methane extraction due to cavern formation or hydraulic fracturing. In Russia, such work was initiated by Academician A.A. Skochinsky about 60 years ago in the Karaganda basin. More than 50 million tons of coal reserves were processed on 10 mine fields through 140 wells. To date, more than 30 million tons of coal have been mined in the early degassing zones.

Degassing of mine fields through wells drilled from the surface was also used in a number of mines in Ukraine. Such work was also carried out at the “Zasyadko” and “Sukhodolskaya-Vostochnaya” mines.

However, this method of degassing does not provide the necessary efficiency from the standpoint of ensuring outburst hazard. This is due to the fact that, for economic reasons, wells are drilled at a distance of 200 or 300 m from each other and between them there are non-degassed sections of mine fields.

Note that the ground-based method of extracting methane continues to improve. Thus, in China, new technologies are being developed for degassing deep-seated coal seams [3] and multilateral radial borehole hydraulic fracturing “initiating the development of a fracture, increasing the permeability of coal and increasing the volume of gas drainage in the opening zone” (i.e. the technology increases fractures, enhancing coal permeability and raising gas drainage volume in the uncovering area.) [4].

The second way is the degassing of a coal seam from underground workings by static injection of liquid into it was an effective addition to the first method of extracting methane and reduced the problem of outburst hazard.

An analysis of the methods and means of combating gas and dust factors in countries with the most developed coal industry shows that to date, injection of liquid into coal seams is the fundamental method. Due to the preliminary moistening of the coal mass, dust formation and the intensity of gas emission during the destruction of coal are reduced.

In the world practice of conducting mining operations in these areas, a large amount of scientific and technical research has been carried out, on the basis of which regulatory documents have been developed that regulate the conduct of work, their control and evaluation of efficiency. At the same time, an increase in the depth of field development, changes in mining and geological conditions and properties of the coal rock massif have led to a significant decrease in the effectiveness of preventive measures. Analysis of studies shows that under conditions of great depths, the possibilities of fluid injection in a static mode have been exhausted. The widely used methods of hydraulic loosening and hydraulic pressing of the marginal part of outburst-prone formations become insufficiently effective. When fluid is injected, there are cases of spontaneous water breakthrough into the mined-out space and premature hydraulic pressing of the edge part of the formation with the threat of provoking a gas-dynamic phenomenon.

Considering that the equipment and technology for the use of hydraulic loosening and hydraulic pressing are almost the same (they differ only in the parameters of holes or wells and the effect achieved), their main disadvantage can be considered an uncontrolled process of fracturing, which reduces the efficiency of fluid filtration throughout the thickness of the layers and interlayers that make up the coal seam. In addition, the increase in the efficiency of measures related to the injection of liquid into the coal rock mass is constrained by the mining and geological factor—the presence of rocks prone to soaking, collapse, and heaving, and the mining factor—the formation of unloading zones and increased rock pressure in front of the working face. The fluid permeability coefficient k in these zones has directly opposite values, from the free flow of fluid flow through fractures, to the virtual absence of fluid filtration at all (see Figure 1).

Figure 1.

Water permeability of coal k and the rock pressure P from the distance X of the working face.

There is a clear inverse relationship between the water permeability of coal seams and the value of rock pressure P. In case of increase in rock pressure, it decreases and, conversely, as the rock pressure decreases, the water permeability of the formation increases. In the marginal, unloaded zone of the formation, the permeability of coal has a maximum value. As the distance from the bottom of the working to the depth of the array, it increases and in the zone of maximum stresses it is practically equal to zero. Further, as the rock pressure decreases, the filtration properties of the formation increase and approach a constant natural value. As practice has shown, when the filtration chamber is located in the unloaded zone, the injected fluid is filtered through cracks into the mined-out space, and when the chamber is located in the zone with increased rock pressure, due to the low water permeability of coal, an uncontrolled process of hydraulic pressing occurs. In this case, the provoking of explosion of methane and coal dust is possible.

The solution to this problem became possible after a number of experimental studies of pulsed fluid injection. In recent years, significant results in this direction have been obtained at the Institute of Geotechnical Mechanics of the National Academy of Sciences of Ukraine (IGTM NASU) in the development of methods and hydropulse action means. The solution to this problem is based on the use of the phenomenon of a periodically stalled cavitation flow of the injected fluid passes through a cavitation hydraulic vibrator.

The cavitation hydraulic vibrator, as an integral part of the drilling tool, was developed by the Institute of Technical Mechanics of the National Academy of Sciences of Ukraine (ITM NASU). This was a new direction in the development of well drilling technologies with submersible impact machines that create dynamic loads on the rock cutting tool using the effects of hydrodynamic cavitation [5]. The drilling tool with a cavitation hydraulic vibrator (for dynamic loads creation in the range of sound frequencies close to the natural frequency of the rock being destroyed) has undergone a full range of experimental studies on hydraulic and drilling stands [6]. An adequate mathematical model of the hydraulic vibrator dynamics has been developed. The model describes complex dynamic processes in the flow channel of the hydraulic vibrator and the vibrator interaction with the drill string structure [7]. The main results of the new technology with the use of the hydrovibrator are presented in [8].

The above studies allowed IGTM NASU in a short time to develop a new method and technology for hydropulse loosening of coal seams [9], as well as a device [10], which implements this method. A significant amount of laboratory research was carried out to substantiate the geometric and operating parameters of the hydrovibrator. At the enterprise of the “Krasnodonugol” PJSC, the hydropulse device was tested under industrial conditions. The new technological schemes and criteria for evaluating the effectiveness of fluid pulsed injection into outburst-hazardous coal seams were developed.

Below are the main results of research into the new technology effectiveness for drilling and loosening outburst hazardous coal seams in a special stand and in industrial coal mining.

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.

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.

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.

Geometrical parameter	Size
The diameter dcr of the generator critical (throat) section, mm	2.5
The length ℓcr of the generator critical section, mm	3.0
The diameter D of the generator diffuser at the output section, mm	10
The length ℓd of the generator diffuser, mm	21.3
The opening angle of the generator diffuser – β, °	20
The diameter D of the hydraulic vibrator tube, mm	10
The length ℓout of the hydraulic vibrator tube, mm	100
The diameter D1 of the inlet feed pipeline, mm	12
The length ℓen of the inlet feed pipeline, mm	120

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 Р₁, and the head pressure Р₂ and fluid flow rate Q through hydraulic vibrator and the main dynamic parameters—the fluid pressure self-oscillations range ΔР (peak to peak values) and the frequency f of fluid self-oscillations 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 P_p 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.

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 m³ 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.

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 P_bp = 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 P_p up to 32 MPa for fluid flow rates from 20 to 120 l/min and smoothly adjust the head pressure P_b 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 ΔР₁ and f) and by immersing it directly into the simulator pipeline (Figure 4b). At the same time, the various options of the 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).

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.

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 ΔР₂, ΔР₃, ΔР₄, and ΔР₅ 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.

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.

Figure 5.

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

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:

• carrier frequency is 40,000 Hz;

• measurement of pressure pulsations in the frequency range from 0 to 10,000 Hz;

• the unevenness of the amplitude-frequency characteristic in the frequency range up to 8000 Hz is no more than 10%, and at the frequency range from 8000 to 10,000 Hz is no more than 20%.

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 P_p = 5; 10; 20; 30 MPa. At each steady-state pressure at the inlet P_p, the outlet throttle changed discretely the outlet pressure P_b. 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 P_p = 10 MPa and pressure head P_b = 1.6 MPa.

Figure 6.

A fragment of the time recording the pressure p₁ at the outlet of the cavitation hydrovibrator with d_cr = 2.5 mm at P_p = 10 MPa and P_b = 1.6 MPa.

It can be seen from Figure 6 that periodic pressure oscillations p₁, 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 $f=n/t$. 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 $∆P_1$ at the experimental hydrovibrator outlet is the difference between the maximum $p_{1max}$ and minimum $p_{1min}$ pressure values in the pulse $∆P_1=p_{1max}-p_{1min}.$ By a similar way, the magnitudes of pulsations ΔР₂, ΔР₃, ΔР₄, ΔР₅ 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 ∆P₁ and pulse frequency f on the water pressure head P_b. In this case, the values of pressure at the inlet to the hydrovibrator and the volumetric flow rate of liquid through it were P_p = 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 ∆P₁ on the water pressure head P_p at various discharge pressures P_b 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%.

Figure 7.

Experimental and calculated dependences of the pressure oscillations range of ΔР₁ and the pulse frequency f on the outlet pressure P_b.

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 Р₁ oscillations occur, which are caused by the collapse of cavitation cavities in the hydraulic channel. At a fixed value of the water pressure head P_b, an increase in pressure P_p at the inlet to the experimental sample of the hydrovibrator leads to an increase in the oscillatory value of pressure ∆P₁. For case of the pressure P_b = 4 MPa with the increase in discharge pressure P_p from 10 to 30 MPa, the pressure pulse value ∆P₁ increases from about 10 to 38 MPa.

Dependences ∆P₁(P_b) for different values of pressure P_p have a maximum in the range of pressure head P_b from 0.14 and up to 4 MPa. As the discharge pressure P_p increases, the maximum pressure range value of ∆P₁ shifts toward higher P_b values. The maximum value of the pressure range ∆P₁ is approximately in from 1.3 to 2.5 times higher than the static pressure P_p at the hydrovibrator inlet. At the same time, as the discharge pressure P_p increases, the ratio ∆P₁/P_p 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 P_b [13] indicates their satisfactory convergence. The relative error does not exceed 10%. At a fixed value of the back pressure P_b, an increase in the pressure P_p at the inlet to the hydraulic vibrator leads to a decrease in the frequency f. Thus, at the value of the water pressure head P_b = 4 MPa with an increase in the discharge pressure P_p 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 P_p 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 P_p.

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 P_p = 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).

Figure 8.

The time recording of fluid pressure (∆P₂, ∆P₃, ∆P₄, ∆P₅) oscillations at different sections of the well simulator length.

It can be seen from Figure 8 that the water pressure head of P_b changes from 1 to 14 MPa, in all sections along the well simulator length, the periodic pressure of the ∆P₂, ∆P₃, ∆P₄, ∆P₅ oscillations are observed with a steep front of pressure rise and fall. The pressure ranges ΔР are symmetrical with respect to the average pressure value P_b.

Processing of time oscillograms with recording of fluid pressure fluctuations along the length of the well simulator of the ∆P₂, ∆P₃, ∆P₄, ∆P₅ (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.

Test data at Pp is 21 MPa and Q is 55 l/min “Sukhodolskaya-Vostochnaya”						Test data at Pp is 11 MPa and Q is 40 l/min “Molodogvardeyskaya”
Pb, MPa	f, Hz	ΔP2, MPa	ΔP3, MPa	ΔP4, MPa	ΔP5, MPa	Pb, MPa	f, Hz	ΔP2, MPa	ΔP3, MPa	ΔP4, MPa	ΔP5, MPa
1.1	1140	6.28	5.87	5.25	4.82	1.1	801	5.56	4.98	4.25	3.92
1.6	1216	6.83	5.99	5.41	4.98	1.6	1165	7.03	6.81	5.48	4.83
2.1	1426	7.19	6.29	5.79	5.08	2.1	1529	8.53	7.85	6.86	6.08
3.1	1619	8.17	6.79	5.98	5.30	2.6	1893	9.45	8.79	8.36	7.86
4.1	2224	11.0	9.79	8.92	8.58	3.1	2257	11.97	10.89	10.31	9.59
5.1	2695	13.8	10.3	8.97	7.81	3.6	2621	10.54	10.12	9.52	8.93
6.1	3055	8.99	7.74	6.91	7.08	4.1	2985	9.32	9.09	8.65	8.08
7.1	4102	7.11	6.86	6.01	5.18	4.6	3349	8.718	8.63	7.507	7.28
8.1	4585	5.99	4.97	4.76	4.46	5.1	3713	7.24	7.25	6.92	5.81
9.1	5280	5.58	4.81	4.10	4.19	5.6	4077	6.71	6.32	5.92	5.43
10.1	6187	12.2	7.99	6.89	5.51	6.1	4441	5.32	4.94	4.41	4.48
11.1	6506	15.6	8.84	7.59	6.35	6.6	4805	4.99	4.77	4.38	4.31
12.1	7226	10.2	6.94	4.87	3.67	7.1	5169	4.81	4.62	4.01	4.35
					4.82	7.8	5678	4.58	4.41	3.79	4.16

Table 2.

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

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

Figure 9.

Dependences of pressure oscillations ΔР ranges at various sections of the well simulator on the outlet pressure P_b at P_p = 21 MPa and Q = 55 l/min.

An analysis of these experimental data shows that fluid pressure oscillations exist in the entire range of the P_b the water pressure head change (from 1 to 12 MPa) studied. The change in the range of pressure pulsations of the ∆P₂, ∆P₃, ∆P₄, ∆P₅ from the pressure head P_b (see Figure 9) is nonlinear. There are two pronounced maxima. The first one is at pressure head of P_b from 4.1 to 5.1 MPa and frequency f from 2200 to 2700 Hz. For these values of P_b and f pressure fluctuations are realized in different sections of the well simulator: ΔР₂ = 13.8 MPa, ΔР₃ = 10.2 MPa, ΔР₄ = 8.97 MPa, ΔР₅ = 8.58 MPa. The second one is at P_b ≈ 11.1 MPa and f ≈ 6500 Hz, at which the pressure rages are ΔР₂ = 15.6 MPa, ΔР₃ = 8.8 MPa, ΔР₄ = 7.6 MPa, and ΔР₅ = 6.4 MPa.

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

Figure 10.

Dependences of pressure oscillations ΔР ranges at various sections of the well simulator on the outlet pressure P_b at P_p = 11 MPa and Q = 40 l/min.

The change in the values pressure oscillations of ΔР₂, ΔР₃, ΔР₄, ΔР₅ on the water pressure head P_b is nonlinear with a pronounced maximum P_b (about 3.1 MPa) at the oscillations frequency f equal to 2260 Hz.

For the indicated values of P_b и f in different sections of the well simulator, the pressure pulsations ΔР₂ = 11.97 MPa, ΔР₃ = 10.89 MPa, ΔР₄ = 10.31 MPa и ΔР₅ = 9.59 MPa are realized. That is, the dynamic pressure range ΔР 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

$$\dot{\epsilon }=\frac{\mathit{d\epsilon }}{\mathit{dt}}=\frac{∆P·f}{E},$$E1

where $\epsilon$ is linear deformation of coal; $∆P$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⁻¹.

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

$$∆P=\frac{10E}{f}.$$E2

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) (for the values of the modulus of elasticity of coal along the bedding Е = 3·10²; 5·10² MPa; and in the vertical direction to the seam 2·10³ MPa).

Figure 11.

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

Figure 12.

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

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:

1. The values of pressure pulses generated by the hydraulic vibrator should reach from 6 to 16 MPa.

2. The fluid pressure pulse frequency must be in the oscillations range from 1500 to 7000 Hz.

For the “Molodogvardeyskaya” mine (Ukraine), testing of the hydraulic vibrator in the well simulator was carried out at P_p = 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:

1. The values of fluid pressure pulses generated by the hydraulic vibrator should reach from 4 to 10 MPa;

2. The fluid pressure pulse frequency must be in the range from 1500 to 6000 Hz.

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 Е from 3·10² to 5·10² 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 P_b ≥ 2 MPa, the generator dynamic parameters will automatically move into the range of values of ΔР 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].

3. Efficiency of hydraulic vibrator operating modes during pilot works on impulse loosening of outburst hazardous coal seams

For pilot testing the effectiveness of the hydraulic vibrator in loosening outburst hazardous coal seams by the manner prescribed by the rules [16], mining experimental sites were selected. The regulatory and technical documentation necessary for the work was developed, agreed, and approved. Mining and experimental studies of the hydraulic vibrator were carried out in difficult mining and geological conditions of the “Sukhodolskaya-Vostochnaya” and the “Molodogvardeiskaya” mines of “Krasnodonugol” Open Joint Stock Company. The schematic diagram of the hydraulic vibrator and its placement in the well is shown in Figure 13.

Figure 13.

Schematic diagram of the hydraulic vibrator for hydropulse loosening of coal seams. 1 is manometer; 2 is channel for measuring back pressure; 3 is fluid flow separator; 4 is sealer (l_s is hydrovibrator sealing length); 5 is tee; 6 is hydraulic vibrator; 8 is filtration part of the well.

The hydraulic vibrator for hydropulse loosening of coal seams works as follows. The high-pressure water flow through the fluid flow distributor 3 and the tee 5 enters the inlet of the hydraulic vibrator 6. It converts the stationary fluid flow into a pulsating flow, which propagates into the well filtration part 8 and is transferred to the coal mass. The pulse repetition rate lies in the range from 1 to 7 kHz. Impact fluid pressure self-oscillations, reducing the internal and contact coal friction, initiate the development of shear deformation and cracking in differently inclined planes.

The equipment for hydroimpulse loosening of coal seams consists of: the pumping unit 4, the pulse loosening device, including the hydraulic vibrator 10, the Taurus-type sealer 9, the high-pressure hoses 7, and control equipment (the fluid flow meter, pressure gauges 7 and 8). The equipment was installed according to the scheme in Figure 14.

Figure 14.

Schematic diagram of equipment installation in the face preparatory development during hydraulic loosening of coal seams. 1 is water supply; 2 is valve; 3 is flow meter; 4 is pumping unit; 5 is control valve; 6, 8 are pressure gauges for measuring fluid pressure; 7 is high-pressure sleeve; 9 is sealer; 10 is hydraulic vibrator; 11 is well filtration part; 12 is mining face.

Mining and experimental work were carried out in two stages. At the first stage, the hydraulic loosening work was carried out by standard (static injection) [16]. The second stage of the hydraulic loosening work was made by the hydraulic impulse method. According to the results of the studies, the parameters of the two methods were compared and the effectiveness of hydroloosening was evaluated. The results of control and efficiency evaluation were documented in the protocol of mining and experimental work.

The formation hydraulic loosening according to the standard procedure was stopped after the pressure drop in the well was not less than 30% of the maximum pressure. The hydropulse action and evaluation of its effectiveness were carried out under the control of the system of sound catching equipment (ZUA - 98). The hydraulic loosening also stopped when the generator switched to “idle mode.” The scope of studies to evaluate hydroimpulse loosening compared to static loosening and to establish the rational working hydrodynamic parameters of pulse loosening of outburst-hazardous coal seams in “Krasnodonugol” mines of amounted to 34 hydroloosening cycles according to the standard mining methodology and 20 hydroloosening cycles in the operation mode of pulsed fluid injection.

As an example, Figure 15 shows the processes of change in time of the inlet average pressure of the hydraulic vibrator and the pressure in the well during loosening of the outburst-prone formation at a depth of 1300 m of the “Sukhodolskaya-Vostochnaya” mine (a). Fluid injection was carried out through the well with the diameter of 42 mm and the length of 7 m with sealing for the length of l_s = 5 m (see Figure 13).

Figure 15.

The time dependence of the inlet hydrovibrator average pressure P_p and pressure head at the filtration part of the well P_b at a depth of 1300 m (a) and of 617 m (b).

A similar loosening process is also presented here for outburst-prone layer at a depth of 617 m of the “Molodogvardeiskaya” mine (b). Fluid injection was carried out through a well with the diameter of 42 mm and the length of 6 m with sealing for the length of l_s = 4 m (see Figure 14).

Based on instrumental measurements of injection and water pressure head in the “Sukhodolskaya-Vostochnaya” mine, the following operation modes of hydropulse loosening of the coal seam were established:

• initial operation mode, complete filling of the well with water, and increase of injection pressure and water head in it (from 0 to 5 min);

• active operation mode of hydropulse loosening (see Figure 16) at a steady fluid pressure head in the well (from 5 to 30 min);

• the mode of the completion of the active operation process, with a drop in the pressure of the fluid pressure head in the filtration part of the well with from about 30% or 50% of nominal pressure head level (from 32 to 36 min).

Figure 16.

The seismogram at the active operation mode of coal seam hydropulse loosening. 1 is the coal seam crack development signal; 2 is the acoustic signal of the operation range of hydraulic impulse loosening: 3 is the acoustic signal of the pumping unit.

After the completion of the active operation process of cracking, hydropulse loosening becomes ineffective.

The seismogram at the active operation mode of hydropulse loosening of the reservoir by the ZUA-98 equipment is shown in Figure 16.

The nature of the time dependence of the acoustic signal of the pressure head oscillations in the well (see the curve 2 in Figure 16) during pulse loosening indicates operation of the hydrovibrator in the mode of periodically stalled cavitation. As a result, a system of cracks develops in the coal seam (see the curve 1 in Figure 16), which sharply reduces its hydraulic resistance upon completion of the active stage of hydropulse loosening.

The results of pulse loosening of the outburst-prone coal seam of the “Molodogvardeiskaya” mine are shown in Figure 15b. As in the previous case, there are three modes of loosening the coal seam:

• the mode of filling the well with water with an increase in injection pressure and backwater in it (from 0 to 1 min);

• active stage, at a steady backwater pressure in the filtration part of the well (from 1 to 7 min);

• completion of the active stage, with a drop in the pressure of the liquid back-up in the filtration part at the level from 30–50% (i.e., from 8 to 36 min).

The results of monitoring the loosening regimes (initial, active, and final stages) of this coal seam are detailed in [17].

A pilot test of the effectiveness of a hydraulic vibrator when loosening outburst-hazardous coal seams showed that the pressure of fluid back-up in a well is a fundamental criterion for controlling the completion of the hydroloosening process. The unloading zone of the coal seam at the same time increased from 4 to 8 m. The transformation of the static fluid injection mode into a hydropulse effect on the formation made it possible to achieve:

• absence of formation leaks along layering cracks, at which static injection was completed;

• the appearance at the bottom of the working of a lot of seeps and abundant dripping, relatively evenly distributed over the area of the bottom around the well;

• the two-time increase in specific gas release through adjacent wells per meter of exposed coal surface [18] compared with static injection.

An analysis of the results of the study of the hydraulic loosening parameters under static and pulsed fluid injection modes at the “Sukhodolskaya-Vostochnaya” and “Molodogvardeiskaya” mines has made a comparative assessment of their effectiveness (see Table 3). In the static mode (without cavitation self-oscillations), the fluid injection pressure Р_p was determined within ranges from 0.75 to $1.0\mathit{\gamma H}$, where γ and H are the average overlying rocks specific gravity and the formation depth. The required volume of injected fluid was determined by calculation in accordance with the instruction [U]. Under hydropulse action, the pressure Р_p range is less than $\left(0.75\right)\mathit{\gamma H}$, and the injected fluid volume was measured by the flow meter upon completion of the hydraulic loosening process.

Fluid injection mode	Time injection, min	Pressure head Рp, MPa		Fluid volume, Q, m^3
		max	min
“Sukhodolskaya-Vostochnaya” mine
Static	92	27	20	1.72
Impulse	40	21	16	0.75
Decrease in time injection on:	56%			56%
“Molodogvardeiskaya” mine
Static*	15	16	10	1.28 (0.83*)
Impulse	10	11	10	0.50
Decrease in time injection on:	33%			61%

Table 3.

Comparative assessment of the efficiency of static and pulse modes of fluid injection (* is case of fluid broke through the working face along the coal seam).

Visual observations showed that for case of *(see Table 3.) during static injection of fluid at the “Molodogvardeiskaya” mine, after 15 min of hydraulic action, fluid broke through the working face along the coal seam. This did not allow to pump the standard fluid volume of 1.28 m³. For operation in hydrovibrator mode with pulsed fluid injection, no fluid released into the working face from the coal seam.

An analysis of the data given in the table shows that, in accordance with the accepted conditions of hydraulic loosening and evaluation of its effectiveness, the use of hydropulse action allowed, on average, to reduce the time of fluid injection into the reservoir and the volume of fluid injected into the reservoir by more than two times.

4. Conclusions

The advanced technology of drilling and coal seam hydraulic loosening by pulsed fluid injection using a cavitation hydrovibrator has a number of advantages compared with the technology of static fluid injection. The technology achievements consist in the possibility of discrete-pulse impact on the coal seam and lead to a significant increase in the permeability of coal, a decrease in its hydraulic resistance, and an increase in injectivity. This allows researchers to increase the rate of injection of fluid supplied to the coal mass while reducing time costs. As a result, the efficiency of hydraulic loosening increases, the zones of moistening and unloading of the formation increase, the gas emission of methane is intensified, the level of dust formation and the resistance of coal to cutting during its destruction are reduced.

The hydraulic vibrator has a number of advantages over other well-known impulse hydraulic means, such as:

• ease of manufacture, lack of moving parts, long service life, exclusion of the transmission of fluid vibrations to the pump, which increases its service life;

• the design of the generator is organically suited to the technology of hydraulic loosening of the coal massif, does not require modification of the equipment, and allows intensifying the processes of gas release at lower specific energy costs compared with traditional technologies.