Multi-scale Characteristics Of Pore Structure And Seepage Characteristics Of GMZ Bentonite

As the preferred engineering barrier buffer/backfill materials of China's high-level nuclear waste deep geological repository project,the seepage characteristics of GMZ bentonite are directly related to the overall sealing performance of the repository,and then determine whether the repository can safely and effectively isolate high-level radioactive nuclear waste for a long period of time.In fact,the seepage characteristics of GMZ bentonite directly depend on its internal microscopic pore spatial structure.Therefore,it is of great practical significance to study the microscopic pore structure characteristics,fluid seepage characteristics and the relationship between them in GMZ bentonite.In this paper,3D digital model reconstruction theory,pore structure characterization theory,pore network model construction theory and pore-scale seepage simulation theory are comprehensively used to systematically study the multi-scale pore structure characteristics and seepage characteristics of GMZ bentonite from a microscopic perspective,and the following innovative results are obtained.(1)Comprehensively considering the accuracy,efficiency and economic cost factors of the reconstruction of 3D digital model of the pore structure with different scales of GMZ bentonite,in addition to directly using CT and FIB/SEM 3D scanning imaging technology to reconstruct nano-scale and micron-scale pore structure model,an improved simulated annealing numerical reconstruction algorithm based on incremental calculation to update statistical function is proposed,which is used to reconstruct the nano-scale and sub-micron pore structure models respectively.The improved simulated annealing algorithm has higher model reconstruction efficiency than the traditional algorithm,which makes the numerical reconstruction of large size samples possible.(2)Based on the gray gradient information of pixels in grayscale image,by analyzing the corresponding relationship between pixel gradient distribution histogram shape and pixel category,a fully automatic pore threshold determination and segmentation algorithm is proposed,which is suitable for images with unimodal pixel grayscale distribution.This algorithm is more robust than the existing algorithms,and successfully segment and extract the pore regions of the original grayscale images with different scales obtained by physical scanning,the results of which are in good agreement with the NMR test results.(3)According to the inherent logical correspondence and mutual conversion relationship between different-scale image pixels,a coarsening criterion that can convert high-resolution pixels to low-resolution pixels is proposed.Furthermore,a more reasonable fusion criterion is established,which can be used to superimpose images with different-scales to construct multi-scale models.Using this criterion,the superposition and fusion of pore structure models with different scales are successfully realized,thereby obtaining a multi-scale model that contains nano-submicron pores at the same time.(4)By introducing the maximum sphere definition of pore geometry in discrete space,an improved pore size distribution extraction algorithm based on the concept of continuous pore size is proposed.This algorithm has significantly improved the accuracy and calculation efficiency of the results compared with existing algorithms.Then,the algorithm is used to quantitatively extract and characterize the pore size distribution information of pore structure models with different scales.At the same time,different scale pore network models of GMZ bentonite are constructed,and all kinds of pore spatial information contained in GMZ bentonite are extracted and characterized quantitatively.The influence of different scales on the characteristic parameters of pore structure is analyzed,and it is proved that the multi-scale model obtained by fusion and superposition can represent the real pore structure characteristics inside the sample more accurately and comprehensively.(5)Using all kinds of pore structure characteristic parameters that have been obtained,the effect of them on the seepage characteristics of the GMZ bentonite sample was theoretically analyzed,and then the quantitative relationship between the pore structure characteristic parameters and permeability was established.The above relationship and other existing empirical relationships were used to complete the permeability prediction for GMZ bentonite samples with different scales.The comparison test results show that the above relationship is only used for the qualitative evaluation of the seepage characteristics of the samples.(6)Based on the real pore geometric structure model and corresponding pore network model of GMZ bentonite with different scales,the numerical simulation calculation of fluid flow transmission at the microscopic pore scale was carried out by using finite element method and single phase flow simulation method respectively,revealing the mechanism of fluid flow at pore scale.Compared with the experimental test results of permeability of macro-scale sample,it is found that the permeability values of the multi-scale model obtained by fusion superposition are in high agreement with the experimental data,which further proves the accuracy of multi-scale 3D digital model of pore structure,pore network model and the corresponding reconstructionsuperposition-extraction algorithm of GMZ bentonite samples used in this paper.There are 105 figures,12 tables and 275 references in this dissertation.