JOURNAL OF COAL SCIENCE & ENGINEERING (CHINA)

DOI 10.1007s12404-009-0313-4

pp 284–288

Vol.15 No.3

Sep. 2009

Dynamic effects of high-pressure pulsed water jet in low-permeability coal seams∗ LI Xiao-hong, ZHOU Dong-ping, LU Yi-yu, KANG Yong, ZHAO Yu, WANG Xiao-chuan ( Key Lab for the Exploitation of Southwestern Resources & Environmental Disaster Control Engineering, Chongqing University, Chongqing

400030, China )

Abstract Mine gas extraction in China is difficult due to the characteristics such as micro-porosity, low-permeability and high adsorption of coal seams. The pulsed mechanism of a high-pressure pulsed water jet was studied through theoretical analysis, experiment and field measurement. The results show that high-pressure pulsed water jet has three dynamic properties. What’s more, the three dynamic effects can be found in low-permeability coal seams. A new pulsed water jet with 200~1 000 Hz oscillation frequency and peak pressure 2.5 times than average pressure was introduced. During bubble collapsing, sound vibration and instantaneous high pressures over 100 MPa enhanced the cutting ability of the high-pressure jet. Through high-pressure pulsed water jet drilling and slotting, the exposure area of coal bodies was greatly enlarged and pressure of the coal seams rapidly decreased. Therefore, the permeability of coal seams was improved and gas absorption rate also decreased. Application results show that gas adsorption rate decreased by 30%~40% and the penetrability coefficient increased 100 times. This proves that highpressure pulsed water is more efficient than other conventional methods. Keywords high-pressure pulsed water jet, gas desorption, penetrability, dynamic effects

Introduction With the further development of coal mine production, the larger ground stress and the poorer coal seam permeability and osmosis, the current methane extraction theories and technologies cannot satisfy the requirement of safe extraction of coal bed gas and prevent gas outburst. This has become a problem of cosmopolitan science and technology. Due to low permeability, microporosity and high adsorption of coal seams in China, pre-pumping the coal bed gas is difficult to conduct, and extraction rate is low (Chongqing Coal Society, 2005; Feng, 2005; Ma and Chen, 2005; Hu et al., 2006). Therefore, it has become urgent to study how to improve methane extraction in low permeability coal seams to solve the key technical

problem. In methane extraction technology, the main problem to research is how to change adsorption gas into free methane and how the free methane can be extracted along a predetermined channel. To improve the permeability of a low permeability coal seam, methods such as Coal Bed Water Infusion, Hydraulic Fracturing, Deep Hole Blasting and Hydraulic Cutting Seam have been tested at home and abroad (Zhang, 2001). Coal Bed Water Infusion is based on the premise of the natural air permeability of coal. Low pressure water enters coal and rock mass under the seepage action and capillary pressure, and water replaces the original methane making adsorption gas become free methane. Then, the free methane outflows along the water permeating route. However, when the coal mine produc-

Received: 23 November 2008 ∗ Supported by the National Natural Science Foundation of China (50604019); the Innovation Team Foundation of China (50621403) Tel: 86-23-65106640, E-mail: [email protected]

LI Xiaohong, et al. Dynamic effects of high-pressure pulsed water

tion is further underground, the permeability of high gassy coal seams is much lower and it is difficult to improve the methane extraction rate using only Coal Bed Water Infusion. Hydraulic Fracturing is the method wherein high pressure water fractures the coal bed by the water wedge effect of the high pressure, and then the rate of gas permeability is improved. Unfortunately, cracks under this method are difficult to be maintained even if a propping agent has been used. Deep Hole Blasting includes the following two procedures. First, stress waves and shock waves appear in the coalbed after explosive fracture of the coal seam. After the stress wave passes, the blasting gas cause a quasi-static stress field to be wedged into cracks on the wall and further extend the cracks. However, these cracks often quickly close, and it is not easy to produce an effective methane seepage flow channel. In a word, for the different gas formations, the effects of the various methods are also quite the same, and the virtual application of these methods is far from the requirements of methane extraction nowadays. Based on the study of the self-excitation oscillation mechanism of a high-pressure pulsed water jet, the oscillation characteristics and effects produced in coal seams are studied and analyzed in this paper. A high-pressure pulsed water jet is used in the Songzao Coal Mine methane governance. The use of the highpressure pulsed water jet may effectively play a comprehensive role in Coal Bed Water Infusion, Hydraulic Fracturing and Shock Fracture, and can also greatly improve the methane extraction rate.

1

Self-excitation oscillation mechanism of high-pressure pulsed water jet

The modulation mechanism of a high-pressure pulsed water jet can be divided into pressure wave modulation and shear wave rectification. Low pressure pulse modulation of jet flow is the incentive process wherein the variation of jet transient velocity impels jet flow to produce pressure wave. Shear wave rectification is the incentive method wherein the disturbance with certain modal and frequency urges the unstable shear wave with related frequency to develop, associate and finally form a large eddy. The former modulating pressure wave belongs to longitudinal wave rectification, which can produce relative stationary frequency pressure wave. The latter modulating eddy current wave belongs to the category of wave and vortex coupling rectification, and it has characteristics such as: static pressure changing little, velocity or dynamic pressure wave with limited range amplitude and

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frequency bandwidth, and nozzle completing self-excitation oscillation. Research shows that (Karadogan, 1983; Staubli, 1987) the free shear layer instability to amplify disturbances is independent of originality disturbance amplitude value. Reynolds number and Mach number only depend on Strouhal number being defined:

Sd = fl u0 where, f is the disturbance frequency; l is the chamber length. When disturbance frequency meets the above formula, shear layer instability can amplify the disturbance in such range. The nozzle structural model and parameters of self-excited pulsed oscillation jet (Fig.1) has been described in the References (Li et al., 2000; Yang et al., 2001) by the author.

2 2.1

Dynamic characteristics of high-pressure pulsed water jet

Experimental devices Besides the data collection system, the experimental system is composed of a self-excited pulsed oscillation nozzle, motorized target, multistage centrifugal pump, flow rate flowmeter, and the inlet and outlet pipeline, return pipe, valves. Designing condition flow rate and pressure is 0~100 L/min and 0~ 35 MPa, respectively. To clarify the characteristics of high-pressure pulsed water jet and the structure size of self-excited pulsed oscillation nozzle, self-excited oscillation pulsed water jets have been researched at different conditions. Including the jet characteristics underwater and in air, the experiment focused on the research and analysis of the relationship between nozzle ratio d2/d1=0~1.8, chamber diameter ratio L/d1=0~6.5, and five kinds of impacting wall (Fig.1). 2.2 Experiment results and analyses (1) Under constant system pressure, flow rate and nozzle parameters, if nozzle ratio d2/d1 = 0.8 or 1.0, it is difficult to resolve and determine if it is self-excited pulsed oscillation or turbulent motion, just because of the low water jet pulsation amplitude; yet, if nozzle ratio d2/d1=1.2, 1.4 or 1.8, the nozzle produces a strong self-excited oscillation with high pulsation amplitude. The pressure seamark of the water jet when d2/d1=1.4, shown in Fig.2, and the oscillation effect is optimum at this condition. (2) As for self-excited pulsed oscillation water jet, jet pulsation amplitude will be lower if the downstream nozzle impacting wall is planar or has an internal spherical surface. It will be higher with stronger self-excited oscillation if the downstream nozzle im-

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pacting wall is a truncated cone surface or external spherical surface. The jet pulsation amplitude and frequency is similar with that under truncated cone surface if the downstream nozzle impacting wall is con-

cave shape, with the biggest pulsation amplitude corresponding to different chamber length. The concave shaped impacting wall more easily produces jet vortex and matched eddy.

Fig.1 The practicality pictures and sketch maps of the five kinds impacting wall

tive target distance range. The change relation between the water jet axis dynamic root mean square pressure value Prms and oscillating chamber length was tested, and change curves between the dimensionless value Prms/P0 and another dimensionless value L/d1 is shown in Fig.3.

Fig.2 The pressure seamark of water jet when d2/d1 = 1.4

(3) As a key parameter to produce a self-excited pulsed oscillation water jet, chamber length relates to shear layer unstable development and disturbance feedback. When it is too short, the shear layer is rarely stable, and insensitive to that range. When it is too long, disturbance frequency composition is so much that new vorticity cannot occur at effective incentive separation zone. Consequently, optimum chamber length should exist. Furthermore, it depends on the variation of beat power. When the optimum nozzle ratio d2/d1 = 1.4, the best ratio, the chamber length is best in submerged (the optimum chamber to diameter ratio L/d1 = 3) or in air (the optimum chamber to diameter ratio L/d1 = 2). Thus, high-pressure pulsed water jet would produce high speed pulsation with maximum oscillation frequency of 200~1 000 Hz. At a certain chamber length range, when the length of the oscillating chamber increases, pulse amplitude will increase with oscillation frequency decreasing. High pressure pulse peak pressure can reach 2.5 times average pressure at an effec-

Fig.3 The change curves between the dimensionless value Prms/P0 and another dimensionless value L/d1

3 3.1

Dynamic effects of pulsed oscillation water jet in low permeability coal seam

Principle and device of the test Pulsed oscillation water jet slotting is the primary cutting process in coal seams with low permeability and high original gas content. There are two kinds of application for this process. First, in mining-coal bed, along the drilled deep-long holes every 80~120 cm, the pulsed oscillation water jet slotting radially cuts a seam and produces a disk slot at a vertical direction. Second, in rock cross-cut coal uncovering, similarly, the pulsed oscillation water jet slotting radially cuts a seam and produces a disk slot at a vertical direction

LI Xiaohong, et al. Dynamic effects of high-pressure pulsed water

every 50~80 cm along the recorded coal bed. The use of a high-pressure pulsed oscillation water jet enables the radial cutting in coal bed (producing a disk slot) and the water mixed with cinder flow out along the holes, which enhances the coal seam permeability and creates conditions for gas analysis and flow. The deep slots with pressure relief and gas discharge make the original stress of the coal seam redistributed, physical properties of coal mass changed, and permeability further increased. By using the pulsed oscillation water jet slotting, the are of the exposed coal body is increased and the internal layers are emancipated. The slots pressure relief creates a good condition for internal layer gas analysis and flow. There are two benefits obtained from the good condition. One is that coal seam permeability is enhanced due to more full pressure relief of the slotted coal body at a certain range. The other one is that the scope of pressure relief and Table 1 Measuring location K1 coal seam

Top plate

Adsorption constant a (m3/t)

b(MPa)

Moisture content (%)

Ash content (%)

Combustible content (m3/t)

Void volume K/(m3/t)

Gas pressure (MPa)

31.32

0.137

1.05

20.75

21.63

1.86

1.48

Lithology Microlite limestone

Bottom plate

gas discharge is enlarged due to the coal shift to the slots. The water replaces the methane under the coal bed by water infusion effect. Instantaneous high pressure and acoustic vibration created by bubble collapse are beneficial to further strengthening the adsorption gas analysis. 3.2 General geology In Songzao Coal Mine K1 coal seam No.+175N6 rock cross-cut coal uncovering, the K value of K1 coal seam is 27.6, which is higher than 10; K1 coal seam belongs to the transitional microbody structure with potential danger of coal/gas outburst. The majority of the coal bed is cloddy structure with the micropore index of 1.83. Due to potential danger of coal/gas outburst, outburst prevention measures must be taken before digging through the coal seam. The top and floor rock characteristics of K1 coal seam are respectively shown in Tables 1, 2.

Laboratory result of the K1 coal sample of No.+175N6 rock cross-cut

Table 2 Location

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Al mudstones

The top and bottom plate rock character of K1 coal seam Density (kg/m3) 2 731 2 703

According to the coal and gas outburst control detailed regulations, gas drainage holes were designed with a 50cm interval between radially arranged holes and the gas drainage time of 3 months. Without pulsed oscillation water jet slotting, 85 strata penetration gas drainage holes need to be drilled, while only 36 holes are needed with pulsed oscillation water jet slotting (Fig.4), among which Nos.18, 20, 22, 29, and 31 were used as slotting test detected holes.

Fig.4 The cutaway view of drilling holes

Voidage (%) 0.44 0.34

Permeability (m2)

Displacement pressure (MPa)

1.00×10

21

6.00

5.34×10

17

1.05

3.3 Application result analysis The working conditions of Oscillation Pulsed Water Jet Slotting were 10~18 MPa pump pressure, 1.5 m cutting radius in coal seam and 7~9 minutes cutting time in every disk slot. In the slitting process, in order to estimate whether coal/gas outburst is about to happen, in every five minutes, gas concentration is measured around the orifice by using a suspended gas detection instrument, and desorption index of drill-cuttings K1 is determined by an outstanding prediction instrument type WTC. According to the characteristics of real bore-hole data, the gas reserves in the extraction control field can be calculated (The Coal Ministry of the People’s Republic of China, 1995; Lu et al., 2002): Q=LBMρW=26×24×2.5×1.45×21.63=48 927.06 m3, where, L is the coal seam dip length in control field, m; B is the coal seam width in control field, m; M is the K1 coal seam depth, m; ρ is the density, t/ m3; W is the gas content in coal seam, m3/ t. After cutting the coal seam, holes must be sealed

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immediately for pumping methane. The change of gas concentrations and gas adsorptions before and after slit-cut coal samples were compared (Fig.5), and the permeability coefficients before and after slit-cut coal samples were respectively measured. The results show that the adsorbed quantity of gas decreased 30%~40% and the permeability coefficient reduced 100 times by using high pressure oscillation pulsed water jet slotting.

Fig.5

4

The gas isothermal adsorption seamark under oscillating water-jet

Conclusions

According to the effects of high pressure oscillation pulsed water jet in low permeability, the following three dynamic effects can be put forward definitely: ① increasing outflow surface of free gas and helping coal bed release pressure; ② dynamic fracturing and crack connecting; ③ strengthening the release of adsorbed gas. Under submerged 20 MPa pump pressure, the pressure of instantaneous high pressure can reach at least 100 MPa during bubble collapsing. The application in Songzao Coal Mine K1 coal seam No.+175N6 rock cross-cut coal uncovering showed that we can more quickly pass through rock cross-cut coal with less drilling footage by Oscillation Pulsed Water Jet Slotting.

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