The Evolution Of Sandstone Pore Size Under Different Pressures Based On NMR

The pore size structure of rock mass is closely related to its physical and mechanical properties and is also a determining factor of the permeability of rock mass.It is of great significance to study the pore size structure and distribution in rock for rock grouting.In this paper,with the fine-yellow sandstone as the experimental rock sample,the evolution law of the pore size of sandstone under different loads and the dynamic diffusion characteristics of water in the process of online displacement are studied in combination with cyclic loading and nuclear magnetic resonance（NMR）experiments,so as to provide reference for the research of permeability of rock mass and grouting engineering technology.The main research achievements of this paper are as follows:（1）the relaxation spectrum peak and pore size distribution of sandstone T₂spectrum were studied.Under the loading of 0¹0MPa,the spectrum peak of T₂spectrum relaxation in middle sandstone decreases as the load increases,and the porosity of sandstone decreases.The peak area of the middle relaxation spectrum increases with the load increasing under 10²0MPa,and the peak of the long relaxation spectrum appears.Under the load of 0¹0MPa,the spectral peak of large aperture moves to the left with the load increasing and the spectral peak area decreases,and the porosity of sandstone decreases.Under the pressure of 10²0MPa,the spectral peak of the large and medium aperture moves to the right and the area increases with the increase of the load.（2）the variation law of pore throat distribution with load was studied.Increase in the number of small pore throat with the increase of load,the number of large and medium pore throat decreases with the increase of load,under pressure from 0 to 15mpa fissure（>1 microns）quantity is reduced,closed fracture by compaction under 20mpa load increase in the number of joints,sandstone aperture linking degree aggravate,pore number and size increases,continue to increase pressure sandstone pore number,crack extension to the damage.（3）the variation rules of porosity and NMR images of sandstone with load were studied.Under the load of 0¹0MPa,the water absorption and porosity of saturated sandstone gradually decrease,and the hue brightness of sandstone nuclear magnetic resonance image becomes darker with the load increasing;under the load of 10²0MPa,the hue brightness of saturated sandstone image becomes brighter with the load increasing.（4）the dynamic law of water diffusion during displacement of sandstone under constant displacement pressure（1.5mpa）was studied.When no confining pressure and displacement pressure are applied to dry sandstone,the signal amplitude of T₂ spectrum curve is low and the area is small.Displacement increases with time T₂ spectrum signal amplitude increase,T₂ spectrum curve area of the curve,but the T₂ spectrum curve signal amplitude,area increment decreases with the increase of displacement time,displacement process of T₂ spectrum curve of the last three samples of the same shape,area difference is not big,is the main process of displacement in the increase of the short relaxed（<10 ms）and the relaxation（10¹00 ms）.（5）the dynamic law of water diffusion during displacement of sandstone under constant confining pressure（5MPa）was studied.With the increase of displacement time,the signal amplitude and area of T₂ spectral curve gradually increase,and the semaphore in the core increases.The main additions during displacement are short relaxation and medium relaxation.The increase in long relaxation（greater than 100ms）is relatively small.The total displacement time decreases with the increase of displacement pressure.There are 48 figures,14 tables and 101 references in this paper.