Study On Fracture-permeability Of Tectonically Deformed Coal Under Multistage Reaming Conditions

Multistage reaming of horizontal wells is the basic technical guarantee and technological prerequisite for efficient development of surface wells for in-situ coalbed methane in tectonically deformed coal,tectonically deformed coal has gas-rich,low permeability,soft and other outburst characteristics.Studying the multistage reaming process of tectonically deformed coal horizontal wells,the damage characteristics of surrounding rock of tectonically deformed coal shaft wall and the meso-structure evolution have important practical and theoretical significance for in-situ pressure relief mining of tectonically deformed coal.On the basis of testing the basic physical and mechanical properties of tectonically deformed coal,using coal powder,gypsum,quartz sand and water as similar materials,the effects of the proportion of cementing agent and auxiliary materials in similar materials on the compressive strength,modulus of elasticity,Poisson's ratio,cohesion,angle of internal friction and porosity of tectonically deformed coal are studied by using the method of similarity ratio test and control variable;Under the condition that the similarity ratio of the above parameters is 1,the optimum mixing scheme of similar material for tectonically deformed coal is determined as follows:coal powder:gypsum:quartz sand:water=5:1.6:0.8:1.2.Then a reciprocating three-stage reaming drilling scheme is designed,and the permeability change rule of the surrounding rock of tectonically deformed coal under the condition of multistage reaming is studied by physical similarity simulation experiment;Discrete element simulation software PFC was used to study the structural change law of tectonically deformed coal under the condition of multistage reaming.Combining physical similarity simulation with numerical simulation,the following conclusions are drawn:(1)Multistage reaming has an important influence on the structure and permeability of the surrounding rock of tectonically deformed coal.After the first-stage reaming,there are 3023 cracks in the structure coal,and the radius of the crack penetration area is 0.05 m.No obvious penetration cracks are formed.After secondary reaming,there are 6154 cracks in the structure coal,and the radius of the crack penetration area is 0.08 m.Two obviously penetrating cracks are formed,which are X-shaped distribution.After three-stage hole enlargement,there are 9450 cracks in the structural coal,and the radius of the crack penetration area is 0.115 m,resulting inthree obvious penetration cracks.Moreover,under the action of vertical load,the vertical deformation and failure of the surrounding rock of the borehole section are more serious than the transverse deformation.(2)During each stage of reaming,with the increase of drilling length,the cracks in the surrounding rock of tectonically deformed coal expand and penetrate,and the range of the fracture penetration area gradually extends along the axial direction of the borehole,and the porosity increases linearly.After drilling,the range of fracture penetration area will become stable with time,which makes the permeability of tectonically deformed coal increase with the length of drilling footage,but the growth trend will be smaller and smaller.(3)After each stage of reaming borehole is completed,the deformation of the surrounding rock of tectonically deformed coal does not end,but is in the stage of residual deformation.With time,the new stress equilibrium state is gradually reached.The permeability of tectonically deformed coal in the residual deformation state has the characteristics of gradually increasing with time and finally tending to a stable value.Moreover,with the larger reaming radius,the time required for the permeability of the surrounding rock of tectonically deformed coal to reach stability is also increased.(4)With the increase of enlargement radius,the radius of fracture penetration area gradually increases and 2-3 obvious penetration cracks are formed,showing X-shaped distribution.The porosity of tectonically deformed coal increases linearly with the increase of radius,and the permeability of tectonically deformed coal wall rocks increases exponentially.The permeability of tectonically deformed coal is 4.25 times of the initial permeability after first-stage reaming,17.5 times of the initial permeability after second-stage reaming,and 31.5 times of the initial permeability after third-stage reaming.(5)Tectonically deformed coal fracture-permeability change mechanism:In the process of multistage reaming,the surrounding rock of tectonically deformed coal undergoes multiple stress releases,resulting in stress redistribution and shear failure of the surrounding rock of tectonically deformed coal.The internal cracks of tectonic coal expand and penetrate,forming obvious fracture penetration area around the borehole,accompanied by the formation of penetrating cracks.The porosity of tectonically deformed coal rises linearly,which induces the sudden change of the permeability of tectonically deformed coal.After drilling,the surrounding rock oftectonically deformed coal is in the stage of residual deformation.With time,the range of the fracture penetration area of tectonically deformed coal gradually tends to be stable,which results in the permeability of tectonically deformed coal increasing first and then tending to be stable.