Wang Sipeng 1,2

(1)Henan University of Technology,Jiaozuo City,Henan Province,454003;

2 Henan Coalbed Methane Development and Utilization Co.,Ltd.,Zhengzhou City,Henan Province,450016)

The characteristics of the new hydraulic fracturing and permeability enhancement technology are introduced,and more than 250 industrial tests have been carried out in mining areas such as Pingdingshan.The results show that the new hydraulic fracturing and permeability enhancement technology can increase the permeability coefficient of coal seams by nearly a thousand times,and increase the gas extraction amount of single hole by hundreds of times,which is an effective way to achieve regional outburst elimination in outburst coal seams.

Key words:low permeability coal seam,increased permeability,gas extraction rate

Currently,there are more than 6000 high gas,coal and gas outburst mines in China,accounting for about half of the total number of coal mines in China.With the gradual increase in mining depth and intensity,the difficulty of gas control will become increasingly difficult.The"Provisions on the Prevention of Coal and Gas Outburst"requires that"the principle of regional outburst prevention measures being taken first and local outburst prevention measures being supplemented shall be adhered to in the outburst prevention work."Regional outburst prevention measures refer to the outburst prevention measures taken within a relatively large range of outburst coal seams before the mining of outburst coal seams.Regional outburst prevention measures include the mining of protective layers and the pre extraction of coal seam gas.".The technology of mining protective layers has been applied quite mature at present;For a single coal seam without a protective layer,how to improve the extraction rate to achieve the purpose of eliminating outburst has been a problem that coal mine technicians have been studying for many years.

Increasing the permeability of coal seams is the key to improving gas extraction efficiency

In China,82%of coal seams have a permeability of less than 1mD,and the Fushun coalfield with the best permeability in China also has only 0.54~3.8mD,which is 2-3 orders of magnitude lower than that of the United States.Over the years,in order to improve the gas extraction rate,scientific researchers and engineering technicians have adopted measures such as increasing the power of extraction equipment,increasing the extraction aperture,increasing the number of extraction holes,strictly sealing the holes,and adopting new technologies.However,the average gas extraction rate in China's coal mines is only 23%,while the extraction rate in major coal producing countries such as the United States and Australia is above 50%,despite differences in equipment and technology,However,the further low permeability of coal seams is the fundamental reason that affects the gas extraction rate.Therefore,the key to improving gas extraction rate is to develop practical techniques for increasing permeability in low permeability coal seams.

Overview of traditional antireflection technology

Since the 1970s,China has conducted exploratory experimental research on various gas drainage pressure relief and permeability enhancement technologies for coal seams with low permeability,difficult drainage,high gas content,and outburst risk.Based on the understanding of the outburst mechanism,the current prevention and control of gas disasters follow the basic principles of pressure relief,permeability enhancement,and enhanced gas extraction,improving the stability of coal bodies,and changing the properties of coal bodies.For single coal seams,measures such as hydraulic punching,hydraulic slitting,hydraulic extrusion,deep hole loosening blasting,and deep hole controlled blasting have been adopted to increase permeability and extraction rate.

These measures have achieved some results by using water as a wetting agent or power to transform coal bodies,but they have failed to fundamentally solve regional gas control problems,fail to relieve pressure and increase permeability as a whole,and significantly improve the gas drainage rate of coal seams.Hydraulic punching technology reduces ground stress within the scope of drilling and punching,increases the permeability of coal seams,promotes gas desorption and emission,and increases the strength and humidity of coal bodies,However,generally,only soft coal seams with an f value less than 0.5 can be used,with poor applicability and complex processes.They are mainly used for cross cut coal uncovering or soft stratification with strong self jetting capacity;Hydraulic slotting technology can improve the natural gas emission,play a role in enhancing permeability and pressure relief,but the slotting depth is insufficient,the protection distance is insufficient,and the workload is large;Hydraulic extrusion technology can cause coal displacement and provide a channel for gas migration,but the effect is not obvious.The important thing is that the above measures lead to"close contact"between the operators and the coal body,which is highly likely to cause major accidents due to the sudden eruption of a large amount of high pressure coal and gas from the orifice at the work site,with a high risk;Moreover,the pressure relief impact radius of these measures is not large,and the limitations are very obvious under the requirements of"regional outburst prevention measures first,local outburst prevention measures supplemented","no excavation of outburst heads,no mining of outburst faces".

New technology for hydraulic fracturing and permeability enhancement

Theoretical research and equipment development

The basic principle of the new technology of underground hydraulic directional fracturing and permeability enhancement is to utilize the fracturing action of high-pressure fluid on the dual medium body of coal seams,and through the coupling of gas,solid,and liquid multiphase fields,to open,expand,and extend the weak surface.On the one hand,the opening and expansion of primary pore fractures increases the porosity of coal bodies,and on the other hand,the extension of primary pore fractures increases the connectivity between fractures,thereby forming an interwoven multi fracture communication network,The increase in gas migration channels(see Figure 1)is precisely due to the formation of this fracture connectivity network,resulting in a significant increase in the permeability of the coal seam.The overall uniform pressure relief of the coal body is achieved,and the adsorption of gas is quickly analyzed,thereby increasing the amount of gas extraction and reducing the risk of coal and gas outburst.

On the basis of theoretical research,inspired by surface drilling,fracturing,and positive pressure drainage of coalbed methane technology,combined with the negative pressure gas extraction system in coal mines and the migration law of surrounding rock under the influence of mining,a complete set of equipment suitable for drilling,sealing,fracturing,and extraction in the underground environment has been developed;The technological process has been designed,and technical standards and operating procedures have been formulated;In order to improve the gas extraction rate while increasing the effectiveness of the technology in preventing coal dust and spontaneous combustion,we have developed a series of fracturing fluids for different coal seams;In order to ensure safety,we have developed an operation command vehicle with remote video function for rotating cups,which is remote from the fracturing site and achieves remote operation at the second operation site.See Figure 2 and Figure 3.

Application example

Since its implementation in September 2008,the technology has been implemented more than 250 industrial tests in mining areas such as Pingdingshan,Hebi,Zhengzhou,Jiaozuo,Yiluo,and Yuxian,basically covering different geological types of mining areas in Henan Province.The following is an example of Pingmei No.10 Mine and Hemei No.6 Mine to illustrate the test results.

Example 1.

Pingdingshan No.10 Coal Mine is located in the east of Pingdingshan Mining Area,with a gas content of 12.7-30 m3/t,a gas pressure of 1.5-2.4 MPa,and a permeability of 0.0013-0.0019 mD.It is extremely difficult to extract and belongs to an outburst coal seam.Currently,the mine emission accounts for 1/5 of the total emission of Pingdingshan Coal Group Company,ranking second in Henan Province.It is the most serious mine with gas disasters in Pingdingshan Mining Area.In history,more than 50 coal and gas outburst accidents have occurred,with a coal outburst volume of 2000 tons.In October 2008,directional fracturing and penetration enhancement were conducted in 24110 working face machine roadway,air roadway,and corresponding bottom pumping roadway.After enhanced pumping and effectiveness testing,the risk of outburst in the working face was eliminated.

The remaining sections of 24110 working face were mined,and 90 effectiveness inspection indicators were counted.A total of 4 indicators exceeded the standard,with an exceeding rate of 4.4%.The exceeding range of effectiveness inspection was significantly reduced,far less than the previous 13.4-16%exceeding rate of effectiveness inspection,significantly reducing the amount of outburst prevention measures and effectively improving the production efficiency of mining.

At the same time,before and after fracturing,dust content monitoring was conducted in various processes within the scope of impact of fracturing.After fracturing,dust content in coal cutting and drilling construction processes significantly decreased,with a decrease of 58.18%,effectively improving the working environment of the working face and reducing the incidence of occupational diseases.

Example 2.

The 2115 working face of Hemei No.6 Coal Mine is located in a dangerous area of coal and gas outburst,with high gas content and high pressure in the coal seam.During excavation,the effectiveness inspection indicators of coal and gas outburst hazards often exceed the standard,resulting in large quantities of outburst elimination works,long periods,poor results,and extremely low excavation speed.The total footage accumulated in 9 months is 110.5 m.At the end of October 2008,the application of downhole fracturing and antireflection technology was conducted head-on in the roadway at 110.5 m of the 2115 lower trough.

As of August 17,2009,2115 lower channel fracturing was conducted six times,with a total gas extraction volume of 720000 m3,achieving safe tunneling of more than 400 meters of roadway.The roadway excavation speed has been increased by 3-4 times,and a total of 142 effective inspections have been conducted after fracturing,of which 5 have exceeded the standard,with an effective inspection exceeding rate of 3.2%.Compared with the exceeding rate of 35%before fracturing,the exceeding rate of the outstanding hazard effectiveness inspection indicators has significantly decreased,greatly improving the excavation speed and effectively alleviating the tense situation of the replacement of the mining face.The effectiveness inspection situation is shown in Figure 4.

The hazard of flour dust during excavation is serious.Fracturing at the excavation face effectively reduces the dust concentration and improves the working environment.After fracturing and pumping in the 2115 lower trench,the concentration of coal dust after blasting decreased from 4.36 to 10.44 mg/m3 before fracturing to 0.78 to 2.68 mg/m3,with a dust reduction rate of 74.33 to 82.11%.

4 Conclusion

The new technology of underground directional hydraulic fracturing and permeability enhancement can equalize the fracturing of coal bodies and communicate existing fractures,provide channels for gas migration,increase the permeability of coal seams,and improve gas extraction efficiency.Through more than 300 successful underground fracturing operations under different geological conditions in Henan Coal Mine,the effectiveness,applicability,and safety of the technology have been demonstrated.Statistical data show that by implementing this technology,the permeability coefficient of coal seams can be increased by nearly a thousand times,the gas extraction amount of a single hole can be increased by hundreds of times,and the attenuation coefficient can be reduced by 7.45 times.

reference

[1]Coal Mine Safety Supervision Bureau"Regulations on the Prevention and Control of Coal and Gas Outburst"readership[M].Beijing:Coal Industry Press,2009

[2]Shi Biming,Cheng Xinlong,Ren Kebin.Comprehensive outburst prevention technology for rapid excavation of low permeability and high gas outburst coal seams[J].Coal Engineering,2002(11)

[4]Zhang Dejiang.Vigorously Promoting Gas Extraction and Utilization in Coal Mines[J].Qiushi,2009(24)

[5]Feng Zengchao.Theory and Application of Enhanced Gas Extraction in Low Permeability Coal Seams[M].Beijing:Science Press,2008

[6]Yu Bufan,Wang You'an.Technical Manual on Prevention and Utilization of Coal Mine Gas Disasters[M].Beijing:Coal Industry Press,2002

[7]State Administration of Work Safety,State Administration of Coal Mine Safety.Regulations on Prevention and Control of Coal and Gas Outburst[M].Beijing:Coal Industry Press,2009

[8]Research Report on Complete Set of Directional Fracturing,Permeability Enhancement and Outburst Elimination Technology in Coal Mines[R].Henan:Henan Coalbed Methane Development and Utilization Co.,Ltd.,2009

Author profile:Wang Sipeng(1963-),male,born in Tangyin,Henan Province,is a doctoral student at Henan University of Technology,a professor-level senior engineer,mainly engaged in research work in coal mine safety.Currently,he is the general manager of Henan Coalbed Methane Development and Utilization Company.