| The fissure-solution-pore network is ubiquitous in the Ordovician and carboniferous carbonate rocks in the North China coalfield.It is not only the medium space for underground water storage,but also an important channel for hazardous water inrush in coal mining.However,the study on the structure and permeability characteristics of the fissure-solution-pore network is still in the exploratory stage.In this paper,the morphology of dissolved pores was firstly classified and a morphology database was established to provide data support for its random generation.On this basis,based on the monte-carlo method and multi-factor directional split of rock mass technology generated random dissolved pore-fracture network model of fracture network and dissolved pore morphology classification output data,and establish the geometrical data sets and FLAC3 D numerical software interact,fissure solution pores composite network seepage numerical model is established,probe into its permeability characteristic and permeability REV change rule,The results are as follows:(1)Based on monte-Carlo method and MATLAB software,25 working condition models of fracture network with different densities and ductility were generated,and their fracture trace lengths were statistically verified.The relationship between the connectivity of fracture network and the density and ductility of fracture network is discussed,and it is concluded that the greater the density and ductility of fracture network,the higher the connectivity of fracture network.The six fracture networks of medium density-low ductility,low density-low ductility,low density-low to medium ductility,and low density-low to medium ductility have the worst connectivity or even no connectivity.(2)Based on the actual solution pore contour,the solution pore morphology was standardized and classified,and stored to establish a solution pore morphology library.Based on the multi-factor block directional cutting technology,the blocks with orientation and certain particle size distribution were generated as the storage space of solution pores.The solution pores were randomly selected from the shape library of solution pores and placed into the block space to realize the random generation of solution pores.Six kinds of dissolution porosity models were randomly generated according to the size of dissolution porosity,such as low,low,medium,high,high and ultra-high,and the dip Angle and long-axis ratio of dissolution porosity were statistically verified.(3)According to the output fracture network morphological data,the geometric data set of the fracture network was established by AUTOCAD software and the interaction with FLAC3 D finite difference software was carried out to conduct seepage numerical simulation of the fracture network and explore the variation rule of REV.It was concluded that under the same fracture density,the higher the ductibility of the fracture network,the smaller the REV size and the stronger the permeability.Under the same fracture extension,the greater its density,the greater the possibility of the existence of REV and the smaller its size.In the simulation scheme of this paper,the existence condition of REV of 5 types of fracture networks from low to high density is that the mean value of fracture trace length reaches the threshold value,and the threshold value of the average trace length is 15 m,5.0m,2.0m,0.5m and 0.5m respectively,namely,high,medium,low,low and low ductility.(4)Take the medium-density and medium-ductility fracture network as an example,superposition it with the random solution pore network to generate a fissure-solution pore composite network with different solution pore rates,and conduct FLAC3 D seepage numerical simulation for the fissure-solution pore composite network,discuss its seepage characteristics and REV variation law,and compare with the sample fracture network.It is concluded that the presence of dissolved pores increases the connectivity of the sample fracture network,and with the increase of dissolved pores,the permeability of the composite network increases gradually,and the REV size decreases.In the model studied in this paper,when the dissolution porosity is in the range of 0.05 to 0.20,the REV size of the composite network and the sample fracture network is the same,both 12m×12m,but the equivalent permeability coefficient increases from 3.93×10-5m/s to 4.73×10-5m/s.When the porosity reaches 0.25,the REV size is 8m×8m and the equivalent permeability coefficient is 4.90×10-5m/s;when the porosity reaches 0.35,the REV size is 4m×4m and the equivalent permeability coefficient is 5.17×10-5m/s,indicating that the larger the porosity,the smaller the REV size of the composite network is,the larger the equivalent permeability coefficient is.Figure [164] table [37] reference [95]... |
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