Characterization Of Pore Structure And Simulation Of Elasticity And Permeability Based On 3D Digital Cores

As one of the methods of rock physics research,digital core technology plays an increasingly important role in the study of rock micro properties.The microscopic pore structure of rock determines the macroscopic physical properties of rock,so the characterization of pore structure plays an important role in analyzing the pore space characteristics and carrying out the simulation of physical properties.Permeability is a parameter that characterizes the ability of rock to conduct fluid,and elastic modulus is a parameter that characterizes the elastic properties of rock,which is one of the most important physical properties of rock.The numerical simulation method is used to study the influence of microscopic factors on the elasticity and permeability properties of reservoir rocks,which makes up for the shortcomings of traditional petrophysical experiments and provides a bridge to study the relationship between permeability,elastic modulus and pore structure of reservoir rocks.The research is of great significance for petrophysical model construction and reservoir geophysical evaluation.Firstly,three-dimensional digital cores of CT scanning method and process-based method are constructed.The core constructed by CT method directly reflects the pore structure characteristics of the real core.The process-based method can simply,conveniently and systematically establish three-dimensional digital cores with different pore structures.Nine representative CT digital cores and six kinds of process-based core models with gradual change of pore structure are constructed as the research objects.Then,based on fractal theory and pore network model,the pore structure complexity and pore throat characteristics of digital core are studied,respectively.On the one hand,box counting algorithm is used to calculate the fractal dimensions of skeleton,pore and boundary of 3D digital core to characterize the complexity of pore structure of different core models.On the other hand,the maximum sphere method is used to establish the pore network model which has same topological properties with digital core,and the topological structure,pore throat size and shape of different pore network models are compared and analyzed.Finally,based on the quasi-static model and the finite element method,the characteristics of permeability and elasticity of digital core are studied respectively.The microscopic factors of rock affect the macroscopic physical properties,so the comprehensive and quantitative study on the influence of skeleton,pore structure,fluid properties and other factors on rock permeability and elasticity is carried out.Through quantitative analysis of permeability change multiple and elastic parameter change amplitude of multiple factors,the primary and secondary order of these influencing factors is judged,and the relationship model between rock permeability and elastic parameter and single variable factor is established.It is found that the combination of process-based method and mathematical morphology method can effectively establish three-dimensional digital cores with gradually changing pore structure,which is the basis of pore structure characterization and physical property simulation.When the box counting method is used to calculate the fractal dimension of digital core,the model size and the pixel size are unfavorable factors affecting the calculation error of fractal dimension,and the methods to reduce or even eliminate these effects are proposed.When the fractal dimension is used to characterize the complexity of rock,the fractal dimensions of skeleton and pore usually can not effectively reflect the complexity of pore structure,while the boundary fractal dimension has better distinguishing and characterization ability.Based on the relationship between porosity and fractal dimension in uniform cementation model C,a fractal reference model is proposed for evaluating the complexity of pore structure.Taking CT digital cores as an example,the complexity of pore structure can be evaluated by comparing the relative position relationship between fractal dimension and fractal reference model at the same porosity.The comprehensive comparative analysis of the two characterization methods based on fractal theory and pore network model can more effectively understand the pore structure characteristics of digital core.By analyzing the influence of pore throat parameters on permeability in representative cores,the primary and secondary influencing factors are ordered as follows: throat size > coordination number > throat shape > pore size > pore shape.By analyzing various process-based core models to study the influence of different factors on permeability,the relationship models between single variable factors and permeability are established,and the influence degree of these factors on permeability is judged as porosity > cementation type > skeleton property.Compared with permeability,elasticity is affected by more factors,which not only need to analyse the pore structure,but also need to consider the type and content of skeleton minerals,fluid properties and other factors.The process-based method can effectively establish multi-component mineral digital cores for the finite element elastic simulation.Through the comparison of change range of elastic parameters,the primary and secondary factors are analyzed: the most critical factor is porosity;the main factors include cementation type,skeleton mineral type,skeleton mineral content and cement mineral type;and the secondary factors include particle distribution and fluid saturation.