Study On Micro-seepage Characteristics Of Low Permeability Uranium Bearing Sandstone Based On Digital Core

The study of rock micro-seepage mechanism and its simulation is one of the front topics of the current geotechnical research,which is of great significance to the normal operation of rock infrastructure projects and the efficient exploitation of oil and metal mines.In order to study the micro-seepage characteristics of low permeability uranium bearing sandstone,mercury intrusion(MIP),nuclear magnetic resonance(NMR),X-ray diffraction(XRD),wavelength dispersive diffraction(XRF),energy dispersive scanning electron microscopy(EDS-SEM)and micro-CT scanning experiments with different resolutions were carried out on the low permeability uranium bearing sandstone in Xinjiang to comprehensively analyze the pore and mineral distribution characteristics.The feature-based image registration method was used to accurately match CT images with different resolutions,and the multi-scale and multi-mineral low permeability uranium bearing sandstone digital core was constructed by pore segmentation and skeleton segmentation of fused core images with different resolutions.The connected pores of digital cores were extracted for unstructured body grid division,and the pore scale seepage simulation of low permeability uranium bearing sandstone was carried out with the computational fluid dynamics(CFD)theory and method.Based on the three-dimensional percolation theory of porous media,the percolation model of low permeability uranium bearing sandstone was constructed,and the starting pressure in the percolation process of low permeability uranium bearing sandstone was analyzed.The main conclusions are as follows:(1)The pore throat distribution of low permeability uranium bearing sandstone in this study is generally between 50 nm and 300 ?m.The middle and small pore throats are the main seepage channels,which determine the permeability of sandstone to a large extent.Although the micro porous throats have small seepage capacity,they account for a large volume of pore throats,so the impact on porosity and throat flow capacity cannot be ignored.(2)The pore types of low permeability uranium bearing sandstone mainly include intergranular pore,dissolution pore,intergranular micropore and micro fracture.Among them,the space scale of intergranular pores is relatively large but the number is relatively small.The dissolution of feldspar particles,dolomite and clay minerals produces a large number of dissolution pores,which are widely developed.Intergranular micropores are mainly formed by clay minerals,which are usually used as the throat connecting other types of pores.Micro fractures mainly exist in carbonate minerals,which enhance the connectivity and permeability of reservoir pores.(3)The connectivity analysis of three-dimensional digital core of low permeability uranium bearing sandstone shows that the main seepage channel is formed by intergranular pores in the pore space,and the connectivity between secondary pores and other pores is poor.The high content of clay minerals and ferruginous dolomite cements are filled in the primary and secondary pore throat of the sample,which significantly reduce the porosity of movable fluid in low permeability uranium bearing sandstone,leading to the low overall permeability of the core.(4)The results of seepage numerical simulation show that the main flow line area is formed in the connecting part of the large pore throat in the low permeability uranium bearing sandstone core,and the streamline is relatively dense.In the corner of the core,the streamline is not swept and sparse,which is the area with poor liquor leaching reaction.With the increase of pressure gradient,the seepage velocity increases,but the seepage path changes little,and the primary of fluid seepage path is still concentrated on the connected part of large pore throat.(5)The results of non-linear seepage simulation show that the seepage process of low permeability uranium bearing sandstone has a starting pressure gradient.The velocity growth rate increases gradually when it is larger than the starting pressure gradient.The relationship between velocity and pressure gradient becomes a straight line when all pores participate in the flow.Because of the complexity of pore structure,the flow rate varies with the pressure gradient.(6)Before the percolation transformation,the connection and expansion speed of the largest pore group in the uranium bearing sandstone increase,which makes the fluid to converge.The results show that: when the porosity is less than 5%,the probability of percolation is small;when the porosity is between 5% and 15%,the slope of percolation probability curve increases,and the probability of percolation suddenly increases;when the porosity is more than 15%,the slope of percolation probability curve decreases and increases linearly with the increase of porosity.