Research On The Characteristic Of Gas Flow In Mining-induced Discrete Fracture System And Its Application

Overlying rock mass three zones division was formed when the lower protective layer was mining. In the middle, the fracture zone as the join point connected the protected layer and the mining workface, was the main channel of the pressure-relief gas flow. Lots of pressure relief gas drainage project like the high rock roadway and the long borehole were constructed in this area. The fracture forms and the regularity of its distribution decided the effect of gas management. In this paper, based on the theory of rock mechanics, mining rock mass mechanics, fluid mechanics, percolation mechanics, by using theoretical analysis, experimental research, numerical calculation and on-site validation, the discrete aperture network model of fracture zone was established. The seepage characteristics of pressure-relief gas flow was described. Based on the established model of pressure-relief gas flow, the case of high rock roadway extraction in Yangquan Coalfield was calculated. The main conclusions was summarized as follows:(1) The layers in fractured zone were experienced three stages of the advanced support, mining induced pressure relief and the stress recovery. The scope of pressure relief area decreases with the increase of layer spacing and pressure relief ratio had the logarithmic relationship with the layer spacing. During the stress recovery stage, the gravity of rock mass over the workface was reloaded on the gob, the process could dimed as linear process. The aperture width would change caused by the elastic deformation (μσ1/Eai) of breaking rock and the compressional deformation (bmax[1-exp(-Aμ/(1-μ)σ1)]) of apertures. The coupling relationship of the stress and permeability was also deduced.(2) The failure characteristics, morphological and medium flow model was totally different in the overlying rock mass three zones. The result of the physical simulation show that the horizon level aperture and the vertical breaking aperture as the two main formation, occupied the 70.8% of the total apertures in fractured zone. The width of the apertures was rapidly with the increase of horizon height of the levels. The aperture which have small angle closure more clearly. The aperture with width less than 1 mm were dominant. The discrete aperture network model of fracture zone was established based on the distribution characteristics. Three main kind of parameters which defined the form of the model were given, such as the outer boundary control parameters, rock breaking scale parameters and the aperture width parameters.(3) During the low flow stage, the fissure flow had the linear relationship with the pressure. With the increasing of flow the differential linear relation was shown for the effect of the turbulence effects. Flow index changed continuously indicated the flow state developed gradually. Under the condition of the laboratory, the critical flow was about 261 mL/min. The corresponding critical flow of on-site fracture zone was about 39.2 m3/min.(4) The rock sample permeability range at 4.8-39.3×10-4 mD, the permeability of post-peak fracture rock sample range at 6.7-69.6 mD. Compared with the original permeability its increased by 4 orders of magnitude. During the reload process, the sample showed the elastic-class behavior. Based on the established strain-permeability model, the test data was fitted. The relevancy was above 0.95 verified the correctness of the model.(5) The gas flow collection characteristics was stated when upper adjacent layer was existed. By using element method the effect of high rock roadway relief gas drainage was simulation. The main factors were the level of the roadway and extraction time. By building the distribution matrix, the fracture permeability was converted to equivalent diameter and the equivalent discrete fracture network model was established.(6) Under the condition of constant pressure boundary, using the equivalent discrete fracture network model the gas flow field characteristics were analyzed. When the level of the high rock roadway change from 20 m lower than the upper adjacent layer to 60 m, the flow of gas fell by 25.0 m3/min, the concentration fell by 55.0%. The case of high rock roadway extraction in Yangquan Coalfield shown that when the roadway change from 60 m above the 15# mining coal seam to 40 m, the flow of gas fell by 18.8 mVmin, the concentration fell by 43.0%. The selection of high rock roadway had the significant impact on the drainage.