Research On The Correlation Between The Fractal Characteristics Of Sandstone Microstructure And Its Permeability

In this paper,the thick sandstone aquifer in the upper part of the main coal seam of Taiyuan and Shanxi formations in the North China type Permo Carboniferous coalfield is taken as the main research object.On the basis of understanding and being familiar with the geological background of the research area,a variety of analytical and testing technologies,such as physical property analysis,high-pressure mercury injection,centrifugal nuclear magnetic resonance,casting thin section,scanning electron microscope and CT three-dimensional reconstruction,are synthetically applied and combined with the present situation Based on the research results of porosity and permeability of dense sandstone in five sampling points(32 rock samples),the porosity and permeability tests of vertical and horizontal formations are carried out respectively.The comparison of the measured data and simulation results shows that:1.The microstructure is reflected by the organic matter and porosity of rock samples.The permeability and porosity of rock samples are directly related,and the porosity is determined by the sedimentary contact relationship of organic matter.In other words,the deposition of organic matter is the main controlling factor of porosity content,and the content of porosity affects the permeability to a certain extent.The pores of tight sandstone aquifer are characterized by complex structure,low permeability and weak connectivity between pores.Because of the complexity of pore structure,the process of measuring sandstone permeability in laboratory is tedious,and the accuracy and stability of measurement are poor,which need to be verified by many tests.In this paper,through collecting the existing data in the study area,using the method of field test and model analysis,it is found that the pore structure of tight sandstone is the main factor affecting the permeability,and the porosity of core sample also presents a certain fractal feature.With the increase of fractal dimension,it means that the roughness of sandstone is gradually increasing,and the water channel between connected pores will be more complex.2.In the field test method,after data arrangement,analysis,image and casting slice identification,it is found that there are some differences between the vertical and horizontal directions of rock samples.The porosity range of total pore is 0.35% ~ 6.17%,the permeability range is 0.00017mD~0.06953 mD,and the pore size range is 0.0000038)~18.542515 8).By comparing the test results of mercury injection method and nuclear magnetic method,it is found that the pore size range with the highest permeability contribution is 0.0238)~0.458)(that is,the pore connectivity of this part is excellent).However,due to the small proportion of pores in this part,the overall permeability of rock sample is weak.The reason can be found from the observation and statistical results of casting thin section and scanning electron microscope Card.The micro structure of the micro rock sample is mainly fine-grained lithic quartz sandstone,with a high content of lithic debris,followed by quartz,while a large number of feldspar is low due to alteration or dissolution,with a high content and uneven distribution of interstitial materials,mainly kaolin,siderite and clay minerals,a small amount of quartz in the rock sample increases the edge,biotite siderite is relatively common,and some rock samples contain siderite A small amount of heavy metal minerals(such as zircon,sphene,etc.).With the increase of burial depth,the clastic particles are obviously oriented,and the primary pores are gradually compressed,while the porosity and permeability are generally positively correlated.The main factors that cause this phenomenon are the joint action of compaction,cementation and dissolution of rock samples,resulting in the increase of sandstone compactness,the decrease of porosity,the complexity of pore channels,and the low permeability generally.Although the dissolution effect It is constructive for the increase of pore volume,but the location of dissolution is uncertain.It is also found by SEM that,with the increase of magnification,most of the cores exist in a closed and disconnected state after adjusting the contrast between light and dark.Therefore,based on the results of physical properties,mercury porosimetry and nuclear magnetic resonance,it can be further inferred that there are more isolated pores and less connected pores in the total pores,and the connected pores caused by the special phenomenon of dissolution are distributed in a local layered structure,resulting in the lack of connectivity between the pores,thus forming the permeability difference in the vertical direction and the horizontal direction.3.In the simulation test method,based on the deduction of the measured results,the x-micron CT scanning reconstruction project is carried out on the rock samples,and the 3D digital core model is obtained.The model can truly reflect the micro pore structure characteristics of the core.In the total pores,the isolated pores are dominant,the number of connected pores is small and locally layered,and there are few through pores at both ends.Among them,the maximum ball algorithm(ball and stick model),simple ball and stick,vividly reflects the relationship between connected pores and water conducting fractures,and the operation method is not only simple,but also can realize quantitative characterization of core micro pore structure parameters.The calculation results of the model are very consistent with the test data of mercury injection method and nuclear magnetic method.From the point of view of simulation,the inference of the measured results is demonstrated,which also makes Research is more scientific and rigorous.There are 118 figures and 15 tables in this paper91 Chapter.