Study Of Unsaturated Flow In Low Permeability Sandstones Using Neutron Radiography And X-ray CT

In recent years,the mining industry is facing new opportunities and challenges under the background of capacity-reduction and ecological protection in China,such as the protection of groundwater in ecologically fragile mining areas in western China,the safety control and maintenance of underground reservoir in coal mine,the development of unconventional resources like coal seam gas and the treatment and utilization of abandoned mines.Water flow in unsaturated sandstone widely exists in all kinds of mining engineering mentioned above,but it has not gained enough attention in the field of traditional mining engineering.The pore structure of low-permeability sandstone is more complicated than that of high-porosity and high-permeability sandstone,and the study of water flow in low-permeability sandstone is also more challenging.However,there is a wide range of needs for the investigation of this problem at engineering sites,such as uranium mining with leaching method in low-permeability sandstone,water flooding for developing coal seam gas and geological storage of carbon dioxide.The homogeneous silty sandstone and the heterogeneous coarse grained sandstone were selected as the research object in the presented work.The microstructure of pores and fractures of the sandstone samples were characterized using X-ray imaging,Mercury Intrusion Porosimetry(MIP),scanning electron microscope.The neutron radiography was employed to visualize water flow in different unsaturated sandstones with low permeability.With the experimental data and theoretical models such as the fractal model,water flow in unsaturated sandstones was investigated in quantification.The effects microstructure on water flow in unsaturated sandstone was focused in the presented thesis and the main research content and results include:(1)The engineering background involved water flow in unsaturated sandstone was firstly introduced in the presented work.The review was presented on three topics including the characterization of microstructure of sandstone,the application of neutron imaging in geotechnical experiment and the theoretical and experimental researches on unsaturated flow.Then the challenge and deficiency in related research fields were concluded.(2)In order to quantify water imbibition in sandstones which was normally unsaturated,three parameters including sorptivity,unsaturated diffusivity function and capillarity coefficient and related models were introduced in the theoretical chapter(second chapter)of this work.To further investigate the effects of microstructure on the sorptivity of sandstone,sorptivity model for tortuosity and geometrical shape of capillaries and a three-dimensional fractal sorptivity model were presented in the theoretical chapter.Moreover,the Matano's method and Meyer-Warwick model were introduced which were used to determine the unsaturated diffusivity function of the sandstone sample based on the dynamic water content profiles.A fractal unsaturated diffusivity function model was improved by charactering the microstructure of the sandstone.In the last part of the theoretical chapter,a model describing the relation between sorptivity and capillarity coefficient was introduced.(3)In the presented work,the high-resolution X-ray CT and MIP were combined to characterize the pore structure of the homogeneous silty sandstone and heterogeneous coarse grained sandstone specimens.Method was discussed for determining the optimal threshold to segment the pore volume based on the X-ray CT images.The three-dimensional volume fractal dimension of the pore structure,the tortuosity and the sphericity of the pores for the tested two types of sandstones were measured based on the segmented pore volume.The porosity and average pore diameter of the sandstone specimen were measured by MIP.Furthermore,the neuron radiography was employed to monitor one-dimensional water imbibition in the cylindrical silty sandstone and coarse grained sandstone samples in real time.Technical procedure of processing neutron images was introduced and a method was proposed to identify the wetting front based on the neutron images as well.By analyzing the neutron images acquired during water imbibition in the sandstone samples,it can be found that the wetting front advanced linearly with the square root of time during one-dimensional water imbibition in the silty sandstone and coarse grained sandstone samples.Although significant difference exists with respect to the pore structure and permeability between the silty sandstone and the coarse grained sandstones,the sorptivity of the tested two sandstone samples were similar.Results calculated by the capillaries model for sorptivity with the tortuosity determined based on the X-ray CT images and the average pore diameter measured by MIP show:?Capillaries model for sorptivity gave an accurate estimation of the sorptivity of the coarse grained sandstone;However,the sorptivity of the silty sandstone was overestimated by this model which may be contributed to the reduced capillary diameter and increased tortuosity caused by the expansion of wetted clay minerals.?Compared with the porosity,the pore size and its distribution,tortuosity and pore shape have a greater effect on the sorptivity.The closure and reduction of pores in the low permeability sandstone in the stratum will cause a poor connectivity of the pore volume and further a larger tortuosity which can be taken the reason for the low sorptivity of the tested sandstone samples.In addition,the maximum capillary diameters of the tested sandstone samples were estimated based on the fractal model for sorptivity with the measured fractal dimension,tortuosity and porosity.By comparing with the maximum pore diameter measured based on the X-ray CT images,it was found the fractal model for sorptivity gave an accurate estimation of the maximum capillary diameters of the silty sandstone whose pore size distribution was more concentrated.By analyzing the distribution of pore sphericity measured based on the X-ray CT images,it was concluded that the assumption of the pores as sphere and poor connectivity of the large pores could cause the differences in the results of the two methods for the coarse grained sandstone.(4)A calibration experiment was performed for the first time using neutron imaging facility at China Advanced Research Reactor(CARR)to correct the effect of neutron scattering and beam hardening.Based on the modified Lambert-Beer law,water attenuation coefficient and correction coefficient for neutron scattering and beam hardening effects were determined at different sample position for various ranges of water thicknesses.The results of the calibration experiment were applied to analyze the neutron images acquired during water imbibition in the silty sandstone and coarse grained sandstone samples and the dynamic water content profiles data sets were further determined.The unsaturated diffusivity functions of the sandstone samples were calculated using the Matano's method and Meyer-Warwick model based on the measured dynamic water content profiles data sets.Moreover,the unsaturated diffusivity functions of the tested low-permeability sandstone samples were determined using the improved fractal model with fractal dimension of the pore structure determined based on the X-ray CT images,the air-entry value measured by analyzing MIP data and the saturated hydraulic conductivity.Results of the Matano's method,Meyer-Warwick model and fractal model show:?The value of water diffusivity increased nonlinearly over several orders of magnitude(i.e.from 10-7?10 mm2/s)with the increase of volumetric water content for both tested sandstone samples.The three presented models gave a close estimation of the silty sandstone sample.?Meyer-Warwick model produced a higher diffusivity value than that determined by the other two methods at low water content range.?When the capillary saturated water content was employed in the fractal model and the residual water contents was assumed as zero,the fractal model agreed well with other two models.Especially when water content was relatively high(>0.04 mm3/mm3),the result of fractal model accorded well with that of the Meyer-Warwick model.Nevertheless,comparing with other two methods,the fractal model performs slightly differently for the coarse grained sandstone,where it overestimates the diffusivity with increasing water content.(5)Water imbibition in single smooth-walled and rough-walled fracture within cylindrical silty sandstone samples was visualized using neutron imaging facility at CARR.Especially,rapid water transport within the fractures was captured using the neutron radiography with high speed imaging mode(10 frames/second)at the early stage of water imbibition.By analyzing the neutron images,it was found:?The wetting front advanced linearly with the square root of imbibition time along the vertical smooth-walled and rough-walled fractures.Then the sorptivity was determined based on the acquired data.It was found the values of sorptivity varied at different imbibition stages.?Along the horizontal direction,wetting front also traveled linearly with the square root of diffusion time from the fracture to matrix.The sorptivity was also determined in this process.?The cumulative water volume absorbed in the sample with a single smooth-walled or rough-walled fracture increased linearly with the square root of imbibition time and the capillarity coefficients were further calculated.(6)Issues needed to be further investigated were concluded at the end of the presented work.For example,the number and types of samples investigated in this work may be not abundant enough due to the limited neutron beam time.In the presented thesis,theoretical analysis and laboratory experiments were combined to investigate water flow in the unsaturated sandstones samples.Based on the pore structures reconstructed by X-ray CT images and other parameters determined in this thesis,numerical simulation work can be further performed.Compared with the results of neutron imaging experiment,the verification and improvement of numerical simulation method was an important preliminary work for investigating water flow in engineering scale.