Stress-unloading and gas migration improvement mechanism in the soft and hard interbedded coal seam using directional hydraulic flushing technology

To enhance gas drainage in the soft and hard interbedded(SHI)coal seam,it’s necessary to unload the insitu stress and improve its gas migration performance.In this research,a directional hydraulic flushing(DHF)technology was carried out.The stress-unloading and gas migration improvement mechanism was analyzed through numerical simulation,and systematic engineering tests were conducted to verify the gas drainage effect.The results show that the improvement of gas migration performance in the SHI coal seam is caused by a combined effect of seepage-improving and diffusion-improving.After DHF,stress-unloading and plastic failure could be achieved both in the soft coal(SC)sublayer and in the hard coal(HC)sublayer.However,the gas diffusion capacity improves significantly in the SC sublayer,while the gas seepage capacity improves notably in the HC sublayer.Meanwhile,the stress-unloading and gas migration improvement effect improves with the flushing radius and the thickness of the SC sublayer.Besides,after adopting the DHF technology,the gas drainage effect improved markedly.The borehole number dropped by 49%,the gas drainage ratio increased from 26.0%to 48.2%,and the average coal roadway excavation speed increased from 2.4 to 5.6 m/d.

Coal Seam Permeability Improvement and CBM Production Enhancement by Enlarged Borehole: Mechanism and Application

The permeability is a key factor to determine the efficiency of coalbed methane(CBM)production.The borehole enlargement technology using hydraulic and mechanical measures to cut coal is an effective method to increase the coal seam permeability and improve the efficiency of gas drainage.Reasonable design of the layout of boreholes is the prerequisite for efficient and economical gas drainage.In this paper,based on the strain-softening model,the stress and permeability model of the coal seam around the enlarged borehole was built,and based on the dual-medium model,the gas migration model in the coal seam was established.Then the borehole enlargement gas drainage engineering of E9/10 coal seam in Pingdingshan No.8 coal mine was simulated by using COMSOL Multiphysics software.The distribution of stress and permeability in the coal seam around a borehole was analyzed,and the reasonable borehole radius of 0.25 m and reasonable borehole spacing of 6 m were determined.Finally,in Pingdingshan No.8 coal mine,field application was carried out in E9/10 coal seam-21070 working face from the high-level gas drainage roadway.The results show that the actual average coal slag discharge rate is 77.82%,which achieved borehole enlargement.The natural gas flow rate from an enlarged borehole is 2.3–7.3 times that of a normal borehole,and the influence range of enlarged boreholes is more than 6 m.The average gas drainage concentration of a group of enlarged boreholes is about 42%,and the average gas drainage amount is about 0.53 m3/min.After two months of gas extraction,the outburst risk in this area was eliminated,which provides a guarantee for safe coal mining.

The influence of borehole arrangement of soundless cracking demolition agents(SCDAs)on weakening the hard rock

The hard roof difficult to collapse easily causes gas accumulation,which threatens the production safety of coal mine.Therefore,roof pre-cracking is required.Although blasting and hydraulic fracturing can also crack the roof,blasting can easily induce rock bursts,whereas hydraulic fracturing needs complex equipment.In contrast,soundless cracking demolition agents(SCDAs)with noise-free,dust-free,and safe characteristics have obvious advantages.The main component of SCDA is calcium oxide,which reacts with water to produce higher expansion pressure.In this paper,focused on the angles of the borehole,the effect of SCDA is analyzed by numerical simulation based on Pingdingshan coal mine.The research results showed that the azimuthal angle a(between borehole projection and the roadway direction)does not significantly affect the efficacy of SCDAs,whereas the influence of borehole elevation angle b is far more significant than that of the azimuthal angle.Therefore,the angle b is a dominant factor influencing the effect of SCDAs.Based on different effects of SCDAs at different angle of boreholes,the weakening unit was established,so the SCDAs could give full play to roof fracturing.Moreover,field tests validated the importance of borehole angle on weakening the hard roofs.

Method for designing the optimal sealing depth in methane drainage boreholes to realize efficient drainage

The purpose of underground methane drainage technology is to prevent methane disasters and enable the efficient use of coal mine methane(CMM),and the sealing depth is a key factor that affects the performance of underground methane drainage.In this work,the layouts of in-seam and crossing boreholes are considered to analyze the stress distribution and failure characteristics of roadway surrounding rock through a numerical simulation and field stress investigation to determine a reasonable sealing depth.The results show that the depths of the plastic and elastic zones in two experimental coal mines are 16 and 20 m respectively.Borehole sealing minimizes the air leakage through the fractures around the roadway when the sealing material covers the failure and plastic zones,and the field test results for CMM drainage at different sealing depths indicate that the CMM drainage efficiency increases with increasing sealing depth but does not change once the sealing depth exceeds the plastic zone.Moreover,sealing in the high-permeability roadway surrounding rock does not have a strong influence on the borehole sealing performance.Considering these findings,a new CMM drainage system for key sealing in the low-permeability zone was developed that is effective for improving the CMM drainage efficiency and prolonging the high-concentration CMM drainage period.The proposed approach offers a valuable quantitative analysis method for selecting the optimum sealing parameters for underground methane drainage,thereby improving considerably the drainage and utilization rates of CMM.

A wireless-communication-based measurement of gas pressure with capsule-slime sealing device

To solve the existing problems of imperfect sealing and the inaccurate measurement of gas pressure in traditional sealing,the present study builds a new model of capsule-slime sealing device based on wireless pressure gauge.The new sealing device is mainly composed of two sets of capsules,a capsule connecting piece and a slime part,measures the pressure through the wireless communication technology,and seals through chemical reaction in the capsule,which generates gas to expand the capsule and extrude the pre-stored slime.Two methods of extruding pre-stored slime are proposed:in the first method,transverse force is generated by the expansion of the capsules at both ends,pushing the capsules toward the middle through the chute and squeezing the pre-stored slime out;in the second,high-pressure gas generated in the capsules is led into the expansion tube,which is inserted into the storage tube,squeezing it and letting the slime out to complete the sealing process.Then the research studies the effect of sealing under the condition of drilling with the structure of slime storage tube.The results indicate that the maximum standing time of slime wrapped in metal net is 2 h,instead of the expected 10 d,failing to meet the sealing requirements.When the slime viscosity is 1200–3000 m Paás in the structure of slime storage tube,the best viscosity of slime is achieved and can simulate sealing the drilling hole(5–4,4–2.36,2.36–1.18,1.18–0.15 mm in diameter),as well as the gap caused by mixed sand accumulation.

Experimental study on f-ω regulation model under abnormal methane emission

In view of the difficulty of automatic adjustment, the recovery lag and the major accident potential of the mine ventilation system, an experimental model of the pipe net was established according to the typical one mine and one working face ventilation system of Daliuta coal mine. Using the best uniform approximation method of Chebyshev interpolation to fit the fan performance curve, we experimentally determined fan characteristics with different frequencies and establish the data base for the curves. Based on ventilation network monitoring theory, we designed a monitoring system for ventilation network parameter monitoring and fan operating frequency automatic control. Using the absolute methane emission quantity to predict the air quantity requirement of branch and fan frequency, we established a f-ω regulation model based on fan frequency and absolute methane emission quantity. After analysing methane emission and distribution characteristics, using CO_2 to simulate the methane emission characteristics from a working face, we verified the correctness and rationality of the f-ω regulation model. The fan operation frequency is adjusted by the method of air adjustment change with methane emission quantity and the curve searching method after determining air quantity requirements. The results show that the air quantity in a branch strictly changes according to the f-ω regulation model, in the airincreasing dilution by fan frequency regulation, the CO_2 concentration is limited to the set threshold value. The paper verifies the practicability of a frequency regulation system and the feasibility of the frequency adjustment scheme and provides guidance for the construction of automatic frequency conversion control system in coal mine ventilation networks.