| Outburst mine with low permeability and soft and single seam, regional coal, gas outburst prevention and control have been a problem. For gas pre-drainage measure project amount is large, high cost, long time, poor effect, coal and gas outburst accident is difficult to stop and guarantee the safety in production. Through the study on the bedding of drilling hole mechanism research, the paper puts forward the theory named natural drilling and deep-hole boring " hole high pressure"; The study increases boring depth for the bedding drilling in soft coal seam with low air permeability and solves problem which gas regional outburst prevention measures cannot cover a section for the bedding hole pre-pumping section coal seam. Through the technology research on holes between the coal fracturing, computer simulation, field test, stress release of regional coal body, coal gas permeability increase, this study improves gas drainage rate, shortens the extraction time. The study opens up a new road with similar effects to mining protection layer for a single low permeability and soft coal seam with outburst.Because of its adaptation to single outburst coal mine site conditions,regional outburst prevention measures based on Bedding hole pre pumping gas has total cost low, flexible arrangement, more apparent effect, convenient drilling construction.It was listed as the priority choice of measures in outburst prevention provisions. However, this regional outburst prevention measures has not been effective used. Following problems are reasons:(1) Due to the single outburst coal seam poor permeability, low intensity, big gas pressure, severe nozzle hole and collapse hole, the drilling depth of not more than80meters, down the hole depth not more than60meters. Borehole coverage can not achieve a section.(3) Hole density based on Bedding pre-draining hole density is a dilemma. Large borehole spacing affects gas drainage and regional outburst elimination; Dense borehole layout close to or through the adjacent borehole gas jet area will cause drilling deviation and even discharge the borehole pressure air from adjacent the borehole by the drilling of coal body around the uneven stress.(3) Draining measures Only by natural drilling to eliminate stress, in tectonic stress concentrated area,its effect is not ideal. Crustal stress and structure stress in concentration zone effect coal seam permeability, so taking drainage measures will not still prominent accidents.(4)There are great defects for a simple high pressure hydraulic fracturing technology as regional outburst prevention measures. Firstly, hydraulic fracturing area can not be controlled artificially, unpredictable fracturing effect region of accurate range. Secondly, in the fracturing area to produce a pressure relief at the same time, as a result of coal body in the fracturing effect of horizontal displacement, there will be high stress concentration zone out of the fracturing effect area. Blindness exists in outburst prevention.In view of the above questions, this paper begins research about deep holes drilling and hole of coal body fracturing pressure relief in two key aspects.The first aspect:deep hole drilling in the form of the bedding drillingIn the actual drilling, drilling cuttings from drilling is far greater than the theoretical calculation of the slag amount. In the drilling process, as a result of ground stress, gas stress effect, it forms the plastic deformation zone around a borehole, and a large number of discharge gas and pulverized coal, is a kind of dynamic phenomenon in the hole. The main manifestations are holes collapse, hole drilling, card and so on. Hole collapse affected by ground stress, gas stress, and soft coal, the phenomenon of the collapse will appear on the wall of the hole. Jet hole belongs to dynamic phenomenon in the drilling similar to the coal and gas outburst which is mainly affected by high-pressure gas, stress concentration and soft coal and other factors. Along with the borehole drilling it will spew gas and pulverized coal. Because the jet hole mainly occurs at the bottom of the hole or holes near the bottom, spray hole is very easy to form the "Cavity", which causes the drill bit to lose constraint by holes wall and sweep hole. The Hole will be bigger and bigger until drilling hole is scrapped. Jet holes will cause the stress increases, deformation aggravates and get drill pipe stuck. Sticking and jet orifice has direct contact. Spray hole deforms the hole. With coal accumulation, the drill pipe and the drill bit will be hooped.1. Gas flow state during drilling hole by bedding pre-draining borehole. The form of gas flow and emission is different in the boring process. From the bottom of the hole to the orifice of the gas flowing state is divided into three categories:namely the ball to the unstable flow, radial unstable flow and radial stable flow.Due to the deep coal seam in the original rock stress state, borehole bottom in the coal body extending to deep coal, drill bit cutting coal Continuous, it is difficult to form a stable plastic zone in hole bottom and hole wall in a short period of time. Plastic zone is very thin, or even approximate to0. For serious outburst coal seam, with high gas content, large pressure, low permeability and fast gas adsorption and desorption, interaction of high pressure gas and stress is on the thin wall of the drilling holes bottom. Gas spherical flow in hole bottom causes the hole wall to exfoliate, and then develops to deep coal Continuously, so spray hole, hole collapse will develop into coal and gas outburst in the borehole. Therefore, the unstable spherical flow is the most important factor of drilling a hole into a hole.2. the hole pressure theoryWhen drilling holes and artificially elongating length of radial unstable flow, curbing abnormal gas dynamic phenomenon effectively in the borehole bottom is an important way to improve borehole depth as pre-draining gas in the coal seam. According to Darcy’s law, power source of the gas flow in coal seams is caused by the pressure differential with the original gas pressure of P0, and pressure P1in a drilled hole. If we artificially create a high pressure environment in the drilled hole, Pressure difference between borehole pressure and the original gas decreases, adsorption desorption intensity of gas in the coal seam can reduce. With the continuous drilling borehole, gas emission becomes into radial unstable emission. The gas pressure becomes slow gradually so as to ensure the quality of drilling hole.3. the application of drilling parametersSome ways would be used to improve pressure of pre-draining borehole gas in the coal seam to adapt to technology and the requirements of high voltage for drilling a hole, namely, increasing working pressure of air compressor and flow rate of compressed air, reducing pressure loss in the pipeline, optimizing the proportion of boring hole to diameter of drilling-pipe, increasing the diameter of the drill-pipe, reducing the compressed air flow resistance of drilling-pipe, Properly reducing the cross section of discharge channel in the drilling-pipe, Increasing wind pressure and speed of discharge channel.According to practice of many years in the Jiaozuo mining area, diameter of the hole is made89mm, diameter ratio of a borehole and drilling-pipe is optimized and the diameter73mm of a drilling-pipe is selected. According to original gas pressure of the Jiaozuo mining area, considering safety of the use of compressed air reasonable air pressure is selected for0.7-0.8Mpa4. drilling tools(1) Reaming bits (Patent number:ZL201020124318.8)The bigger is drill diameter, the more intense is the uneven gas emission from of the hole bottom; conversely, the smaller drilling-bit diameter is the smaller Influence drilling-bits cause on the bottom of the hole orifice. Based on this principle, the concept is put forward namely a second counter bore and multiple pressure relief. The drilling-bit is composed of two parts, the distance between two part is800-1000mm. namely, the front end for43mm-diameter small drilling-bit and the rear89mm. The small diameter-drill constructs small-diameter bores, gas pressure is released and the stress gradient is formed around in small diameter hole. Then the rear drilling-bit develops the bore to89mm diameter. The "Equal diameter drill" is welded the drill wing to the smooth rod and discharge channel is located between the drill wing. Drill rod rotating stirs deposited dust in the drilling hole and presses the wind out. Eliminating the drill-cuttings deposition on the smooth surface and discharging friction heat". Diameter of drill-pipe with equal diameter is basically the same to the diameter of the drill bit which also solves the problem of directional drilling.4. field tests.It has been tested about this study in Guhanshan Coal Mine and China and Thailand Mining Company Limited in Coking Coal Group. The field test has been done from2009July to2009October in Guhanshan Coal Mine. A total construction of downward are493holes, hole depth of minimum depth of103meters (the other reason for the failure of individual drilling not counted)the deepest drilling to135meters, average depth of118meters. The field test has been done in China and Thailand Mining Company Limited from2010November to2011February test. The boring depth by a mean is34meters, up to72.8%holes more than90meters deep. The drilling depth all can reach above130metres in sixteenth mining area of Jiuli Hill Coal Mine. In order to make the coal crushing test more accurate between the holes, the hole depth designed is120meters in test site of the16161th transport tunnel.The second aspect:the coal in pore space fractured under pressure and release pressure to increase permeabilityThe hydraulic fracturing technology for the coal in pore space was discovered based on the analysis of theory for coal and gas outburst. The coal and gas outburst happens under the comprehensive effect of ground stress, gas stress and coal structure, on which should be focused when implementation regional outburst prevention measure. Take care of the poor permeability of outburst coal seam, the huge project and the long time limited of outburst prevention measure used in coal and gas areas, and the bad cutting down ground stress, The hydraulic fracturing technology for the coal in pore space, at the time coal natural fissure expanding, makes the fractured coal expand to the free surface formed by the free holes, unloads the ground stress and increases the plasticityThe hydraulic fracturing technology contains the following steps:1. According the theoretical calculated hole space to construct, drilling along the coal seams, the injection water holes and free holes constructed alternately.2. Take hole punching to free holes, expand the equivalent diameter. The methods are:after drilling the free holes, punch the orifice for10meters, sealing used the polyurethane with diameter100mm PVC pipes; change the drill bits and drill pipes with hydraulic drill to punch the free hole.3. Using the grouting hole technology to seal the holes, injection slurry18m length.4. Taking high pressure hydraulic to fracture the coal through injection water holes.5. Network all fractured holes and free holes to emission gas after fractured the coal.Researching hydraulic fracturing technology divides into the following aspects:1. The hydraulic fracturing technology should be researched by full investigation the present situation, research way and progress in home and abroad, theoretical analysis, numerical simulation, field test and comprehensive analysis.2. During be fractured, the coal destroyed by the high pressure water tensile around the borehole to form fissures, the high pressure water propped fracture, which make the fissures tips continue to prop, expand and extend. With the further injecting water, overcome the friction between coal layers, crush the coal to free holes. First, produce Ⅰ model (open type) fissures; then overcome the friction and cohesion between the seams, form Ⅱ model (sliding type) fissures; at last, part or whole coal body move to free ends, and stress release. The initial cracking pressure analyzed by theories is:Pb=0.29σ1+T0+P0+Pc。The injection pressure for Maintenance the crack is:3. According the vertical stress of rock stratum covered on the Jiuli Hill Mine, and the Poisson’s ratio of the coal seam in test areas, the injection water pressure for the coal fractured in pore space is more than14.78MPa, and less than20Mpa. According to Jiaozuo mining hydraulic hole equivalent diameter, calculating the equivalent section is0.6079m2. Draw lessons from Huainan Mining, mining the protective layer leads to protected layers deform4‰, the permeability coefficient increases300times, the coal between holes deform4‰, the space between holes is not exceed10.855m, given a certain coefficient, the space between holes is6m.4. The coal in hole space is in the asymmetric stress state, the vertical stress is σY, the horizontal direction is x and z, one side of the coal tunnel is not stressed, injection water holes are stressed by water in warp direction and bottom. Assumed analysis the coal dynamics properties, the coal in front of the working face is homogeneous, continuous, isotropic, assumed that the coal is linear elastic, followed Hooke’s law, the coal body is changed only by ground and water stress, ignore the gas pressure, don’t given the effect of stress, gas pressure, water coupling. On the basis of above analysis, establish the numerical simulation dynamics model.5. Take repeated trial to this model by software RFPA2D, contrast the actual and simulated parameters of stress in coal, displacement, elastic potential and so on. Determine the ultimate model, calculate the other models under different injection water parameters.6. Filed test is carried in Jiaozuo Coal Group Jiuli Hill Mine, which is coal and gas outburst mine. The absolute gas outburst rate is48.53m3/min, the relative gas outburst rate is24.17m3/t, the gas pressure in coal is average in above of0.75MPa, sometimes is up to2.08MPa, the gas storage in coal is22.05m3/t. Since the mine is built, the mine has happened65times coal and gas outburst, the highest occurred in October27,2011, in the16031returned air alley, the coal outburst quantity was3246t, the gas was291200m3.The field test site was in16161transport tunnel, which was layout along the roof of coal seam, cross sectional11.04m2, supported by anchor net and anchor cable. This tunnel used the forced ventilation, installed two2x30KW counter local fans, one work the other spare. The actual wind flow was550m3/min. Fixed test19drills, of which1,3,5,7,9,11,13,15,17,19,4,2were free holes,6,8,10,12,14,16,18were water injection holes, designed the hole120m deep, hole space6m., This test section was108m long, tilt wide120m.7. The field test results:from February15,2012to March8th, drilled along the layers, punched, February18th finished the fracturing and to extract the gas, to August26th, the total drainage gas was1820943.7m3, the extraction rate was67.8%, the gas content was drop to6.91m3/t. the actual test coal permeability coefficient was increase420timesCompared to preparatory work surface named the14121th area (in)(with the report of extraction to standard):coal reserve is255800T, gas content is4389500m3. The work surface extraction drills were949, the total holes’ depth was83239m. From May2010to March2012, extraction of gas volume was2483600m3, calculated the residual gas content was7.45m3/t.Compared with16161transport tunnel during pre-extract by dense drilling in coal tunnel: the regional gas content in evaluated area was21.46m3/t, average coal seam thickness was7m, coal reserves was29500T, gas reserves was633000m3, total drills’length was9687m, started to extract gas in March1st,2011, accumulation pumping capacity was322000m3, The gas pumping rate was50.8%, the residual gas content was10.5m3/t.From the above comparative analysis, the hydraulic fracturing technology has obvious advantage.8. During the field tests, along with injection water pressure increases, the coal body begins to crack, a large number of gas emitted around the free holes. The fact have proved that the effect of high pressure injection water to dispel the gas in coal, displace the gas adsorbed on coal exists. The water molecular structure is more suitable to absorption than gas, and has higher security contrast to CO2, N2and so on. The theory of dispelling and displacement of gas needs to further study.This research topic, has resolved the long term problem that single coal outburst mine how to prevent outburst, and the prevention measures used in gas content areas during drilling along coal layers is more flexible, less engineering, and drop the time to standard. This method will enhance the reliability of preventing outburst, and reduce the cost of preventing outburst, change the management predicament of outburst mines. |
PDF Full Text Request