Study On Mechanism Of Unsaturated Seepage Of Fractured Sandstone Based On Neutron Imaging

Unsaturated seepage phenomena are widely used in engineering applications and basic sciences.Spontaneous imbibition is the main macroscopic phenomenon,and its static and dynamic problems have received extensive attention and research.Coal is still China’s main energy source,and intensive green mining is the mainstream trend,with opportunities and challenges coexisting,such as the safety of groundwater reservoirs in the western mining areas of China,unconventional energy development and the disposal and utilization of abandoned mines.The above-mentioned various types of mining engineering problems involve the problem of unsaturated seepage of water in sandstone matrix and fissures,but have not received enough attention in traditional research.In addition,the problem also exists in the application of sand uranium mine in-situ leaching,coalbed methane water injection and other engineering applications.In this paper,high-resolution X-ray CT imaging,scanning electron microscopy and mercury intrusion method are used to accurately characterize the pore and fracture structure of high-permeability and low-permeability sandstone samples,and neutron imaging technology is used to observe the unsaturated seepage of different types of sandstone samples.The process is visualized.Based on experimental data and theoretical models such as fractals,the quantitative study on the unsaturated seepage mechanism of sandstone is carried out,focusing on the influence of sand pores and fracture structure on its unsaturated seepage law.The main research contents and achievements include:(1)In order to quantitatively study the typical unsaturated seepage phenomenon of sandstone matrix and fracture spontaneous imbibition,the sorptivity and unsaturated diffusion function are introduced in Chapter 2 to describe the unsaturated imbibition behavior of sandstone matrix and fracture structure.Based on the relevant parameters of sandstone pore(fracture)structure,using fractal medium mechanics,seepage mechanics and fractal theory,the fractal model of sandstone matrix and fracture structure sorptivity is established,and the sorptivity of sandstone pore(fracture)structure is quantitatively analyzed.The dynamic volumetric water content of sandstone samples is used to estimate the model of unsaturated diffusion function(eg generalized Fickian law,Lockington-Parlange(L-P)model and Meyer-Warwick(M-W)model).The characteristics and mechanism of unsaturated seepage in sandstone matrix and fracture are revealed.(2)The high-resolution X-ray CT imaging,mercury intrusion method and X-ray diffraction were used to fully characterize and determine the pore,fracture structure and clay mineral composition of different types of sandstone samples.The three-dimensional reconstruction of CT scan data was performed by Avizo 3D image analysis software to obtain the three-dimensional structure model of sandstone pores(fractures).The optimal threshold determination method for determining the pore structure data of the divided sandstones was discussed,and the sandstones were calculated according to the segmented pore structure data.The microstructure parameters are used to finely characterize sandstone pore(fracture)structures.The sandstone pore(fracture)structural parameters determined by the above method can lay a foundation for theoretical analysis of unsaturated seepage characteristics.(3)Perform a water calibration experiment on the cold neutron imaging spectrometer equipped with the China Advanced Research Reactor(CARR)and the Helmholtz Berlin Materials and Energy Research Center(HZB)to determine the water attenuation coefficient and its corresponding correction coefficient.The neutron two-dimensional imaging technique was used to visualize the unsaturated seepage process of sandstone matrix and fractures(such as smooth and rough fracture),and the neutron image processing method of imbibition was introduced.Based on the neutron image,the position of the leading edge of the wetting peak is measured with the change of the imbibition time and the dynamic distribution of the volumetric water content.The spontaneous imbibition behavior of sandstone matrix and fracture structure was quantitatively analyzed to determine the sorptivity and unsaturated diffusion function of matrix and fracture.(4)In order to study the influence of pore structure of different types of sandstone matrix on the sorptivity,the matrix sorptivity was estimated according to the neutron image of sandstone imbibition experiment.The results show that the movement of the wetting peak of the sandstone sample does not obey the classical imbibition behavior,but it has a linear relationship with the power exponential of the imbibition time.The model predicts the sorptivity and compares it with the experimental measurements.The pore microstructure plays a decisive role in determining the sorptivity of sandstone.The clay mineral may expand after interacting with water,which blocks the percolation channel,reduces the effective permeability radius of the pore,and increases the bending degree of the seepage channel.It has a significant effect on the sorptivity of sandstone.(5)In order to study the influence of different types of sandstone fissure structures(rough and smooth fracture)on the sorptivity,the neutron image of the fracture sandstone imbibition test was used to determine the position of the front edge of the wetting peak in the fracture and its two sides,and to estimate the water absorption.Coefficient of coefficient.The imbibition rate of water in the rough and smooth fissures of the high-permeability and low-permeability sandstone samples is significantly greater than that on both sides,and the imbibition process of smooth fissures is more complicated than the rough fissures.The coarse and smooth fissures and the wet peak movements of the matrix on both sides obey the classical imbibition behavior,but linearly with the power exponential of the imbibition time.The sorptivity of the fracture is significantly larger than that of the two sides of the matrix,and is larger than the intact sandstone matrix.It is known that the fracture structure can effectively enhance the water absorption capacity of the fractured sandstone.According to the model,the sorptivitys of the rough and smooth fracture and the substrates on both sides were predicted and compared with the experimental values.The results show that the prediction error of the water-splitting coefficient of the rough crack and its two sides is only 3%-7%,while the prediction error of the smooth crack and the water-absorption coefficient of the two sides is relatively large,but it is relative to the loss coefficient.The prediction error is much smaller.Pore and fracture structure parameters play an important role in predicting the sorptivity.In addition,the water exchange behavior between the smooth fracture of the high permeability sandstone and the matrix on both sides was analyzed based on the water vapor transmission neutron image.(6)The dynamic moisture content distribution data determined by the neutron imaging technique is used to extract the wet frontier position in each study area at different times.The generalized Fickian law and the Lockington-Parlange model considering the infiltration time index are derived.The Meyer-Warwick model is developed considering the non-Boltzmann variables,and the unsaturated diffusion function of the fractured fine sandstone is determined by the above model.The results show that the diffusion rate increases with increasing water content in several orders of magnitude.The diffusion rate calculated by the Meyer-Warwick model and the Lockington-Parlange model increases rapidly with volumetric moisture content from 0 to 0.20 mm3/mm3.When the volumetric water content is greater than 0.20 mm3/mm3,the increase in the diffusivity calculated by the model tends to be gentle.The Lockington-Parlange model significantly overestimates the diffusivity,probably due to the low clay mineral content of the sample being tested and the relatively stable pore space after water absorption.The Meyer-Warwick model considering non-Boltzmann variables can better describe the diffusion phenomenon at the non-Boltzmann scale,and provides a new method for quantitative analysis and modeling of diffusivity.