Improve Gas Drainage Research Articles

Experimental study of precursory features of CO2 blasting-induced coal rock fracture based on grayscale and texture analysis.

CO2 blasting has been identified as a potent method for enhancing the permeability of coal seams and improving gas drainage efficiency. This study is focused on elucidating the deformation and fracture mechanisms of coal and rock during CO2 blasting and on identifying the precursor characteristics of these processes. To this end, a CO2 blasting-induced coal rock fracture pressure model and a gas pressure distribution model were developed. The research utilized a self-developed CO2 blasting test platform along with a non-contact full-strain field measurement analysis system. Briquette samples were subjected to CO2 blasting tests under controlled experimental conditions, which included an axial pressure of 1.0 MPa and variable gas pressures of 0.5, 1.0, and 1.5 MPa. This methodology enabled the capture of the principal strain field on the surface of the samples. The Gray Level Co-occurrence Matrix (GLCM) was employed to extract and analyze the grayscale and texture features of the strain cloud maps, facilitating a quantitative assessment of their evolution. The aim was to pinpoint the precursor characteristics associated with coal rock cracking and crack propagation. The results revealed that: (1) During the cracking and subsequent propagation of samples, the strain field's grayscale histogram underwent a transformation from a "broad and low" to a "narrow and high" configuration, with a consistent increase in peak frequency. Specifically, at 3 ms, a primary crack was observed in the sample, evidenced by a grayscale peak frequency of 0.0846. By 9 ms, as the crack propagated, the grayscale peak frequency escalated to 0.1626. (2) The texture feature parameters experienced their initial abrupt change at 3ms. Correlation with the gas pressure distribution model indicated that this was the crack initiation moment in the sample. (3) A secondary abrupt shift in the texture feature parameters occurred at 9ms, in conjunction with experimental phenomena, was identified as the crack propagation phase. Monitoring the grayscale and texture features of the principal strain field on the coal rock surface proved effective in recognizing the precursor characteristics of crack initiation and propagation. This research has the potential to reduce blasting costs in coal mines, optimize blasting effects, and provided theoretical guidance for enhancing gas extraction efficiency from deep and low permeability coal seams.

Numerical simulation and experimental study on improving gas drainage by hydraulic fracturing

ABSTRACT To address the difficult problem of coal seam gas control that has arisen as a result of the recent development of deep coal mining, hydraulic fracturing technology is applied to the coal seam, the dual porous medium theory is used, and a multi-scale flow model of coal gas is constructed under certain assumptions. The dynamic evolution law of coal porosity and permeability is studied according to the gas multi-scale diffusion equation, gas-bearing coal seepage equation, and other theories. To study the variation law of important parameters such as coal seam pressure and drainage influence range in the process of coal and gas drainage, numerical simulation software is used, and industrial tests are supplemented to verify energy efficiency. The study found that using hydraulic fracturing technology to enhance the permeability of coal seams can significantly improve the average gas extraction concentration, and the maximum increase in the extraction net flow after fracturing is about 60% compared with that no fracturing, and can effectively suppress the attenuation of the extraction flow due to the extension of the extraction time, effectively improving the gas extraction effect of the test borehole. Simultaneously, if other auxiliary measures are implemented, the permeability enhancement effect may be more stable.

Fracture Evolution of Overburden Strata and Determination of Gas Drainage Area Induced by Mining Disturbance

Overburden strata fracture evolution is critical to dynamic disaster prevention and gas-relief drainage, so it is important to accurately determine the evolution relationships with mining disturbance. In this paper, experiments and numerical simulation were adopted jointly to characterize the time-varying fracture area of overlying strata. The experimental results showed that the roof strata gradually broke and collapsed with coal mining, which indicated the fractures of overburden strata developed in an upward direction. The fracture development causes were explained by numerical simulation, which showed that stress increase exceeded the strength of coal and rock strata, and fractures were formed and expanded. Both experiments and numerical simulation results showed the two sides and the top of fracture areas provided channels and spaces for gas migration and reservoir, respectively. In addition, the breaking angle of overburden strata and the height of fracture areas were analyzed quantitatively. Through microseismic monitoring at the mining site, the fracture scales and ranges of overburden strata were verified by the energy and frequency of microseismic events, which were consistent with the support of maximum resistance. The position of drainage boreholes was considered based on the results of overburden strata fracture evolution. Our study is aimed at promoting coal mining in safety and improving gas drainage with a sustainable approach.

A new technology to enhance gas drainage in the composite coal seam with tectonic coal sublayer

Composite coal seam is a kind of special coal seam that is formed after complex geological structure action, including a tectonic coal sublayer. Gas can't be drained easily from this coal seam, which cause coal and gas outburst accidents (CGOA) occur frequently. A new hydraulic flushing technology was proposed in this paper to improve gas drainage. Accordingly, a new set of mechanical equipment, technology process, roadway layout system and gas extraction monitoring system are provided by drawing lessons from the Guhanshan coal mine. The stress variation and damage failure were investigated by numerical simulation on the basis of flushed-out coal and borehole geometry. The gas drainage effect, coal seam permeability and outburst risk-sensitive indexes of the special coal seam were obtained by field measurements. According to the results, hydraulic flushing can enlarge the stress unloading and plastic damaged zone around boreholes dramatically, and the coal seam permeability can be increased largely.

Determination of crack closure stress under constant-fatigue loading based on damage variable evolution

Crack closure stress is a key stress threshold of rocks under various loading. The determination of which plays essential roles in deformation behavior and gas seepage characteristics of overlying strata. However, considering damage indicators evolution of rock under the constant-fatigue loading has rarely been studied under constant-fatigue unloading conditions. In this study, a series of uniaxial decreasing amplitude stress constant-fatigue tests were conducted on sandstone specimens to determine crack closure stress. The evolution of damage indicators depended strongly on the applied stress level. In addition, the influence of cycle numbers on the deformation and crack damage parameters was investigated. The results show that irreversible strain, elastic moduli, and load-unload response ratio show two-phase evolution characteristics. The energy evolution illustrates linear evolution when the stress level exceeds crack closure stress. However, when the stress level approaches crack closure stress, damage indicators show a three-phase evolution stage, and the energy evolution fluctuates greatly. Apart from those, the crack closure stress is a range rather than a specific value. It may be due to the progressive pre-existing or stress invasion cracks reopen. The proposed method can effectively complement existing methods to determine the crack closure stress and evaluate the stress relief extent, which is vital to improve gas drainage efficiency in overlying strata.

Borehole Protection Technology of Screen Pipes for Gas Drainage Boreholes in Soft Coal Seams

In this paper, research was carried out on borehole protection technology using screen pipes, aiming to address the problems of gas drainage borehole collapse and low gas drainage efficiency in soft coal seams, which have low gas permeability and high gas content. An experiment was conducted to study the compressive resistance and borehole protection effects of polyvinylchloride (PVC), polypropylene (PP), and acrylonitrile–butadiene–styrene (ABS) screen pipes with different sieve opening spacings. The optimal screen pipe material and sieve opening spacing were determined. The borehole protection effect and gas drainage efficiency using three borehole protection methods, namely, bare boreholes, traditional PVC screen pipes, and new ABS screen pipes, were compared and studied on site, and the influence of pipe diameter on borehole protection efficiency was revealed. The results show that the compressive resistance and borehole protection effects of the three types of screen pipes are in the following order: PVC < PP < ABS. ABS screen pipes can more effectively inhibit boreholes from collapse, and the best borehole protection is achieved with a sieve opening spacing of 50 mm. The field tests showed that the gas drainage concentration of the bare borehole in the creep deceleration stage was 113% higher than that of the borehole protected by the ABS screen pipe and 255% higher than that of the borehole with the PVC screen pipe. The screen pipe began to play a role in borehole protection at the beginning of the stable working stage, as the borehole wall began to deform and collapse. On the 30th day of gas drainage, the gas drainage concentration of the borehole installed with the ABS screen pipe was about 9 times that of the bare borehole due to the collapse and blockage of the bare borehole. The new ABS screen pipe with an outer diameter of 40 mm and a sieve opening spacing of 50 mm had a better borehole protection effect and gas drainage efficiency than the traditional PVC pipe. Proper sizing of the screen pipe diameter can improve borehole service life and gas drainage efficiency. The research results are of great significance for solving the problem of gas drainage borehole instability and for improving gas drainage efficiency in soft coal seams.

Numerical Simulation of Crack Initiation and Propagation Evolution Law of Hydraulic Fracturing Holes in Coal Seams Considering Permeability Anisotropy and Damage

Hydraulic fracturing has been widely used in practical engineering as an essential means to prevent coal seam gas outburst, increase coal seam permeability and improve gas drainage efficiency. Accurate prediction of fracture propagation law is an important basis for optimizing fracturing parameters to achieve high-efficiency gas drainage in coal seams. In this paper, a new seepage–stress–damage coupling model considering permeability anisotropy is first established and then used to study the evolution laws of crack initiation pressure (σci), fracture pressure (σcd), AE behavior and pore water pressure with the lateral pressure coefficient (ξ) and permeability anisotropy coefficient (λ) in the process of hydraulic fracturing. Finally, the influence of initial pore water pressure on σci is discussed, and an efficient gas drainage method is proposed. Research results indicate that: the in situ stress still plays a leading role in the approach of crack propagation whether the permeability is isotropic or anisotropic; the non-uniform pressure condition is favorable for the crack growth compared with uniform pressure under the isotropic permeability condition; when the direction of maximum permeability is consistent with the direction of maximum principal stress (ξ = 0.5, λ < 0), the coal seams are easily fractured; AE behavior of fracturing holes can be divided into three stages: initiation stage, fracture smooth expansion stage and the breakdown stage for any λ or ξ; and the more complex the crack distribution, the more the area of the gas pressure release zone (GPRZ) increases, which is very beneficial to achieve high-efficiency gas drainage. This study can provide a basis for optimizing fracturing parameters and technology in improving the efficiency of coal seam gas drainage using the hydraulic fracturing method.

Hydraulic Fracture Propagation and Permeability Evolution in the Composite Thin Coal Seam

Hydraulic fracturing can improve the permeability of composite thin coal seam. Recently, characterizing hydraulic fracture (HF) propagation inside the coal seam and evaluating the permeability enhancement with HF extension remain challenging and crucial. In this work, based on the geological characteristics of the coal seam in a coal mine of the southwest China, the RFPA2D-Flow software is employed to simulate the HF propagation and its permeability-increasing effect in the composite thin coal seam, and a couple of outcomes were obtained. (1) Continuous propagation of the hydraulic microcrack-band is the prominent characteristic of HF propagation. With the increment of the injection-water pressure, HF generation in the composite thin coal seam can be divided into three stages: stress accumulation, stable fracture propagation, and unstable fracture propagation. (2) The hydraulic microcrack-band propagates continuously driven by the fluid-injection pressure. The microcrack-band not only cracks the coal seam but also fractures the gangue sandwiched between the coal seams. (3) The permeability in the composite thin coal seam increases significantly with the propagation of hydraulic microcrack-band. The permeability increases by 1~2 magnitudes after hydraulic fracturing. This study provides references to the field applications of hydraulic fracturing in the composite thin coal seam, such as optimizing hydraulic fracturing parameters, improving gas drainage, and safe-efficient mining.

Influence of hard-roof on gas accumulation in overlying strata: A case study

Protective seam mining is one of the most commonly used technique to realize stress relief and permeability enhancement of reservoirs, so as to improve gas drainage efficiency. However, for coal reservoir with a hard roof, such as the tight sandstone, magmatic rocks, the distribution and evolution of destressed zone and gas field have not been fully understood, which leads to an inefficient gas control in the field. In this work, taking the geological background of Pingmei 12th Coal Mine as an example, we numerically investigated the evolution and distribution of stress, permeability and gas field in the hard-roof stope during protective seam mining. The results show that in the zone with a distance greater than 3.7 m, stress in the hard-roof stope is significantly higher than that in the conventional stope and the permeability enhancement ratio in the hard-roof stope is significantly lower than that in the conventional stope. In the overlying strata, the existence of the hard roof inhibits the upward extension of the fractured zone, and the range of the fractured zone in the hard-roof stope is obviously smaller than that in the conventional stope. Therefore, in the hard-roof stope, the relieved gas accumulates in the gob, instead of entering the overlying fractured zone, which may put a threat to the mining safety. To solve this problem, the presplitting blasting technique was applied to cut the hard roof. After the collapse of the hard roof, the overlying fractured zone is fully developed, and the gas accumulated in the gob enters the fractured zone driven by the concentration difference and buoyancy. This can not only prevent gas overrun, but improve gas drainage efficiency of the upward borehole. This practice could be used as a valuable example for other coal mines with similar geological conditions.

Research on Reasonable Drilling Parameters of Hydraulic Reaming in Soft and Thick Coal Seam and Its Application in Field

In view of the fact that the outburst disaster of the soft 3# coal seam in the Sangshuping Coal Mine is mainly caused by in-situ stress, it is proposed to use hydraulic reaming, pressure relief and anti-reflection technology to remove some of the high geostress in the coal seam and improve gas drainage efficiency. Numerical simulation software was used to analyze the distribution of plastic failure and stress distribution of the coal body after reaming under the three-hole spacings of 4m, 6m, and 8m. The results show that the radius of the plastic failure zone around the borehole is about 2.2m when the rate of expanded coal chips is 3%; the width of the pressure relief area of a single reamed hole is about 3.2m. When the drilling hole spacing is 6m, it has a good pressure relief effect and reasonable amount of drilling work. After the reaming, the drainage concentration and flow rate are both improved. After reaming, the extraction concentration and flow rate have been improved. The hydraulic reaming technology can effectively solve the problem of outburst prevention in the thick seam of soft outburst dominated by ground stress in Sangshuping Coal Mine.

Influence of Deformation and Instability of Borehole on Gas Extraction in Deep Mining Soft Coal Seam

To study the effects of the three deformation instability modes of gas drainage borehole on gas drainage, the deformation instability mechanism of soft coal seams is analyzed, three deformation instability modes are proposed for soft coal seams, namely, complete holes, collapse holes, and plug holes, and a solid‐fluid coupling model incorporating dynamic change of borehole suction pressure is established. The results of the study show the following. (1) When there is no borehole deformation (i.e., complete borehole), the suction pressure loss of drainage system in the borehole is very small, whose effect on gas drainage can be neglected. (2) In case of borehole collapse, the suction pressure loss is big at the collapse segment, and the total suction pressure loss of the drainage system in the borehole is bigger than that in the complete hole. However, it is smaller than the suction pressure of the drainage system and exerts limited effect on gas drainage. As the borehole collapse deteriorates, the effective drainage section of the borehole becomes smaller, while the suction pressure loss in the borehole increases continuously; thus, the gas drainage effect continuously worsens. (3) In case of plug hole, a continuous medium forms between the plug segment coal body and the surrounding coal seam, the plug segment drainage pressure turns into coal‐bed gas pressure, and effective drainage length of the borehole shortens, seriously affecting the gas drainage effect. The study carries important theoretical guiding significance for improving gas drainage effect and effectively preventing gas disasters.

Study on Eliminating the Water Blocking Effect in Coal Seams Using Gas-Wetting Reversal Technology.

To eliminate or alleviate the water blocking effect (WBE) encountered while hydraulic fracturing of coal seams, the wettability of the coal body was changed, using gas-wetting reversal technology, thus improving gas drainage. After initial measurements of contact angles, surface tension, surface free energies, erosion resistance, self-absorption capacities, and self-absorption rates of coal samples, a hydrophobic and low-surface-tension gas surfactant was selected. After comparing the results of four candidate surfactants, two surfactants were selected, FC117 and FC134, which were both hydrophobic and exhibited low surface tension, to achieve the desired effect. Testing showed that with increased surfactant concentration contact angles increased and surface tension gradually decreased. After surfactant treatment, the coal surface energy decreased, self-absorption capacities and rates of coal samples decreased significantly, and their erosion resistance was good. After determination and verification of several evaluation parameters, the results showed that the two surfactants performed well and achieved the expected effect. By optimizing and evaluating gas-wetting reversal agents, our work has provided a theoretical basis and scientific guidelines for eliminating the WBE from coal seams, thereby facilitating gas drainage.

Study on crack propagation of the CO2 presplitting blasting empty hole effect in coal seam

AbstractCO2 presplitting blasting is an effective method to increase coal seams permeability and improve gas drainage efficiency. During the blasting process, the empty holes in the coal and rock mass have a significant effect on crack growth. In the experiment, a sample containing empty holes was used for CO2 presplitting blasting. The VIC‐3D was used to observe the change in the surface displacement of the sample; and the amplitude of the wave during the blasting process was collected using ultrasonic waves. Here, we calculated the attenuation coefficient α, analyzed the internal crack damage of the coal and rock mass, and compared the LS‐DYNA simulation results to analyze the empty hole effect on the crack growth of the coal and rock mass. The results show that: (a) radial tensile stress is generated due to the superposition of compression waves and reflected waves during blasting, resulting in an approximately straight main crack channel between the blast hole and the empty hole; (b) When both the stress and the attenuation coefficient reach their peaks, a main crack is formed and the most severe damage occurs inside the coal and rock mass; (c) the main crack channel was formed at 0.13 seconds, and the energy consumption reached the peak at 0.18 seconds, the negative linear correlation between energy and stress was obtained. Therefore, the empty hole effect provides a good guide for the crack propagation of the coal and rock mass and provides a new experimental scheme for increasing the permeability of coal seams.

Application research of liquid CO2 fracturing in coal seam penetration

Aiming at the characteristics of high gas content and low gas permeability in coal seams, high-pressure gas generated by the liquid phase transition of liquid CO2 is applied to the coal rock mass to produce a cracking effect. By analyzing the cracking process of CO2, using linear elastic fracture mechanics to analyze the crack propagation conditions and the damage and failure criteria of the coal body under the action of liquid CO2 blasting, the crack expansion state at different times were numerically simulated by the finite difference software ANSYS_LS_DYNA3D. Field tests show that CO2 cracking can improve gas drainage efficiency in the mining face.

Fractal characteristics of gas migration channels at different mining heights

Fracture randomness and irregularities have a decisive effect on the gas reservoir and migration in overlying strata at different coal mining heights. To determine the boundary of a gas migration channel, its effect on gas storage, and migration in the overlying strata at different coal mining heights, this study integrated the two-dimensional (2D) physical similarity simulation of the calculated fractal dimensions to examine the results obtained using analytical techniques. With the increase of the mining height, the range of gas migration channels increased significantly. Moreover, we identified the primary parameters of the gas migration channel boundary in the mutant regions of bed separation and the broken fracture density. Apart from the increasing working distance, the fractal dimension increased when the fractal dimension of a working face side was larger than an open-off cut side. The fractal dimensions of a mine with a height of 6.0 m were 1.07 and 1.23 times of those for heights of 4.0 and 2.0 m, respectively. According to the variation principle of fractal characteristics and Reynolds number, the gas migration channel was divided into three regions, namely a gas active, gas circulation, and gas enrichment areas. On the basis of the fractal theory, an effective model was proposed to calculate the evolution characteristics of gas migration channels at different mining heights. Compared with the previous gas migration channel model, the proposed model could be appropriately estimated the position of high-level boreholes; the improved estimations can improve gas drainage efficiency and ensure safe and efficient mining.

Liquid carbon dioxide fracturing application and its effect on gas drainage in low permeability coal seams of underground coal mine

ABSTRACT Outbursts comprise the predominant natural hazards in Chinese underground coal mines. Extracting high content gas within coal seams is one of the most effective ways to reduce the risk of triggering outbursts. However, with the depth increase of coal mining production, conventional gas drainage might be failed due to the low permeability of coal seams. Additional permeability enhancement methods should be adopted to improve the gas drainage efficiency. In this study, we proposed the application of liquid carbon dioxide fracturing for enhancing gas drainage in a underground coal mine in China with low permeability coal seams. By drilling cross-seam boreholes, liquid carbon dioxide fracturing was simultaneously performed in the two drilled fracturing boreholes. The effective damage radius of liquid carbon dioxide fracturing was further identified by continuously monitoring pure gas flow rates in different observation boreholes. After liquid carbon dioxide fracturing, there was no significant decrease in pure gas flow rate of observation boreholes for a period of 2 weeks. A conceptual model of liquid carbon dioxide fracturing was proposed for illustrating the mechanisms of improving gas drainage efficiency. Some methods for further optimizing gas drainage with the operation of liquid carbon dioxide fracturing under various thickness of coal seams and geological conditions were discussed. These results suggest that the application of liquid carbon dioxide fracturing are effective for gas control and hazards prevention in underground coal mines.

Increasing Permeability of Coal Seam and Improving Gas Drainage Using a Liquid Carbon Dioxide Phase Transition Explosive Technology

The low permeability of coal seams makes gas drainage difficult in lots of coal mines. This study presents a low‐temperature, safe, and efficient liquid carbon dioxide phase transition explosive technology (LCDPTET) to increase the permeability of coal, thereby improving the efficiency of gas drainage and eliminating the dangers of coal and gas outburst. Meanwhile, an integrated approach for experimental determination, numerical simulation, and field testing was applied to study the damage ranges of coal and to determine a reasonable spacing between the gas drainage hole and blast hole. A numerical simulation model of liquid carbon dioxide phase transition explosion (LCDPTE) was built, and the damage index M was introduced to analyze the degree and range of coal damage after explosion at different spacings between the blast hole and the gas drainage hole. Furthermore, another aim was the assessment of the permeability changes and comparison of the gas drainage effects of different borehole spacings. The results showed that as the borehole spacing became smaller, the degree of coal damage around the gas drainage hole increased, and the gas drainage effect improved. However, to avoid the collapse of the gas drainage hole, the gas drainage holes should not be located in the crushing zone caused by LCDPTE. Based on the numerical analysis conducted to guide the borehole arrangement of the field test, the latter was carried out to study the increasing ranges of permeability of coal and the drainage effect after explosion. The results indicated that LCDPTET could greatly improve the permeability of the coal seam and gas drainage efficiency. In addition, this new technology could not only improve the safety and efficiency of mine production but could also turn carbon dioxide into an effective energy source worthy of popularization and application.

Experimental investigation of N2 injection to enhance gas drainage in CO2-rich low permeable seam

Seam gas pre-drainage, is widely used as an effective method to control gas and coal outburst in underground coal mines. However, in CO2 abundant low permeable seams, this technology seems to be less efficient due to the CO2 sorption characteristics and the lower safe mining threshold limit for CO2 applied in many outburst risk management plans. This paper presents the experimental investigations of the applicability of nitrogen (N2) injection to improve gas drainage in CO2 rich seams. Core specimens were obtained from Bulli coal seam, Sydney Basin and N2 flood tests were conducted under different permeability conditions (0.3 mD and 0.06 mD). A triaxial permeability test rig equipped with a back pressure regulator was used to conduct the test. The variation of gas composition, gas outlet flow rate, N2 flushing efficiency, and permeability variation were analyzed. In addition, a comparative study between N2 flushing coal seam CO2 and N2-ECBM was conducted and it was observed that CO2 was much more difficult to drain out and with longer time compared with CH4. Based on the test results, a two-stage flushing mechanism was obtained. In the first stage, the original free phase coal seam gas accounted for the large percentage and this stage last for a shorter time. In the second stage, desorption time governed the flushing efficiency and desorbed gas was the primary gas. It took much longer time than the first stage. The final recovery rate in 0.3 mD and 0.06 mD scenarios were 90.5% and 87.7%, and the N2 consumption/CO2 production ratios were 15 and 11.2, respectively. Through these laboratory experiments, we concluded that N2 injection can significantly decrease coal seam CO2 content level and increase gas drainage efficiency.

A protective seam with nearly whole rock mining technology for controlling coal and gas outburst hazards: a case study

Coal seams with low gas permeability and high gas outburst hazards are becoming more serious as coal mines extend deeper, but there are no appropriate protective coal seams for this kind of coal seam in China. In this paper, mining technology using a protective seam with nearly whole rock (PSNWR) is used to improve gas drainage and ensure safety during production. The characteristics of the distribution and occurrence of PSNWRs and their mechanical properties are analyzed. A theoretical mechanics model and three-dimensional numerical model are established to study the controlling effect of PSNWR mining on pressure-relief gas drainage. In this context, the mining process, system and gas extraction design for PSNWRs are introduced. The results for Pingdingshan No. 12 Coal Mine show that mining with a PSNWR 2.0 m thick can effectively reduce the danger of coal and gas outbursts and improve gas drainage and utilization. The gas drainage rates are >80 %, which significantly increases the social, economic and environmental benefits of Pingdingshan No. 12 Coal Mine.

Improvement of drainage gas of steep gassy coal seam with underground hydraulic fracture stimulation: a case in Huainan, China

The paper introduced a new technique named underground hydraulic fracture stimulation to improve gas drainage efficiency. And the technique was successfully applied in a steep gassy coal seam in Huainan of China. Application results showed that water injection changed water/gas content distribution greatly and gas drainage efficiency was improved greatly. After the stimulation, central part of coal seam around fracturing borehole showed lower water content and much higher gas content than other parts. Average pure gas flow rate for single borehole of central part is 3~8 times higher than that of original part. The upper and below part showed higher water content and lower gas content than central part. The phenomenon is closely related to the steep structure of coal seam. In consideration of the wide distribution of steep gassy coal seam in China, the results have great significance for popularisation and application of the technique in Chinese coal mines. [Received: December 11, 2014; Accepted: July 4, 2015]