Experimental Study On Self-sealing Performance Of Uplink Boreholes By Drilling Cuttings For Gas Extraction

Exploiting protective layer is the most effective regional prevention measure for controlling coal and gas outburst.It usually need to drill cross-seam boreholes from bed plate tunnel to coal seam for gas drainage.And the number of this upstream drillings is huge.Whether the upstream drillings are sealed effectively is the key of gas drainage with boreholes in coal mines.And sealing materials and methods have a major impact on the efficiency of gas extraction.This paper is aimed to find a new low-cost and high-efficiency method of upward boreholes sealing.The main research results are as follows:The stress field around borehole and the distribution of plastic zone were simulated on FLAC 3D.The leakage mode in sealing section of upstream drillings was analyzed.The failure mechanism of sealing on upstream drillings was determined: There exist 3 patterns in process of upstream borehole sealing,which included leakages on material,interface and fractures.How to plug this 3 kinds of the leak paths effectively is the key to prevent failure of upward boreholes sealing.So the method of sealing on upward boreholes with cuttings is proposed.By this way,the cuttings made by drilling will stopping holes automatically.Finally,it can achieve the integration of drilling and sealing.An experimental system for simulating the self-sealing of upward boreholes by drillings was then adopted to investigate the critical parameters,including the particle size distribution and movement of the drillings.Experimental results showed that most particles exhibited small voids between them after being deposited,and were therefore compacted and had poor breathability.Drilled particles could enter fractures in the fractured and plastic zones in the coal mass around the boreholes at a high speed,thus filling the fracture space.This prevented air from the roadway from flowing into the boreholes through these fractures.During the accumulation of the drillings,the angle of repose tended to increase until the borehole cross-sections were covered.Fine particles,which occupied a high proportion of the total,were more likely to be in contact with the borehole walls,thereby preventing leakage at this interface.This method can effectively prevent air leakage channels in upward boreholes and ensures sealing efficiency.The promotion that Mix water and drilling cuttings particle motion law sealed the boreholes in the condition of water drillings was studied.And it turns out that the particles with water can touch large cross-sectional drilling borehole wall in all directions and then went into the when borehole of the coal or rock,during the space motion of drilling process after jet drilling.When water crumbs mixture migrated in the borehole of the wall,water had the trend of leaking in the fractures around the borehole,consequently guided drilling cuttings into the cracks and filled the space,plugged the fracture leakage channels.During the process of drilling,water crumbs mixture accumulated continuously in the drilling holes.The water carrying the drilling cuttings moved from the surface of the deposit to the inclined top which would lead to the increase of the accumulation deposit angle until the cross section was encumbered,when two phase flow met the deposit of water crumbs mixture.After the hole-sealing,a solid-liquid-solid coupling steady state was formed which can inhibited the leakage with part of residual water filling in drilling cuttings and borehole wall interface.Two phase flow,solid and liquid,with larger energy moved downward and shock the accumulation in drilling hole sealing water crumbs,which would form the shape of drilling cuttings accumulation,promoted the cuttings density and then inhibit the air leakage of drilling cuttings material itself.NMR test system was used to test the air permeability of the consolidated mixture of water and cuttings and its influencing factors : The larger the particle size of the drilling cuttings,the higher proportion of the mesopores,macropores and cracks in the mixture,the greater total porosity(there is the smaller porosity of the micropores and mesopores),the greater permeability,the lower compactness,and the better air permeability.With the increase of curing time,the porosity and the permeability become lower.So the sealing effecr will be enhancedStudy the factors for sealing effect of self sealing cuttings under the working condition of dry and water drilling.Under the dry drilling condition,the bigger angle of upward boreholes,the faster drilling and the larger borehole diameters,the sealing effect will be better.Under the water drilling condition,the bigger angle of upward boreholes,the higher speed of water flow and the the faster drilling,the sealing effect will be better.The sealing effects of two kinds of working conditions were compared under the same and different influencing factors: Under the water drilling condition,the negative pressure of closed chamber in upward boreholes by self sealing cuttings is greater and the negative pressure decay rate is lower,so the sealing effect is better than dry drilling condition.The research results have some scientific value and practical significance to reduce the cost of drilling and sealing boreholes,improve the gas drainage efficiency and promote the protective coal seam mining.