Research On The Multi-scale Analysis And Model Of Shale

As a type of porous medium,shale is a diverse sedimentary rock that contains a variety of clay minerals,quartz,silicates and other non-clay minerals.The geological characteristics,mechanical properties and engineering factors on shale are key factors for the safe and efficient exploitation of shale oil and gas.The mechanical properties within different regions differentiate,because of anisotropic effects that cause wellbore instability,and they have a strong influence on the supporting effect of the proppant used in hydraulic fracturing.At present,the research on the physical and mechanical properties mainly addresses the established relationship between physical properties and mechanical properties.Macro-based measurements have trouble such as large sample volume,time-consuming experiments,and an unsatisfactory level of detail of the characterization of the target formation.Deformation mechanism of shale is difficult to be understood,due to the lack of awareness of the shale microscopic mechanical mechanism.To be able to analyze the microscopic mechanical properties of shale and the relationship between the micro and macro,the multi-scale mechanical analysis of shale is carried out.According to the study of the dissertation,the major knowledge and results are as follows:(1)The geological characteristics and multi-scale analysis model were evaluated.The geological characteristics of the changing-weiyuan zone for shale gas were analyzed in the Sichuan basin,according to the shale outcrop,logging data.The effective pore size distribution of shale was analyzed by mercury injection method,nitrogen adsorption study and nuclear magnetic resonance method.The mineral composition of shale was evaluated by using X-ray diffraction analysis.Based on the analysis of composition and microstructure,the multi-scale model of shale was established and scale parameters were determined.It laid the foundation for the coupling study of multi-scale component and mechanics.(2)A large number of the micro/nano-indentation test was carried out.Based on the researches of equipment,rock characteristics and parameter settings,the test parameters of indentation were determined.The effect of tip radius can be disregarded for micro-indentation,and have influence on the nano-indentation.Since the set of test parameters have large influence on the measurement results,test parameters need to be clear before the start of the test.Based on the indentation test,the method of measuring about the elastic modulus and hardness is analyzed.Contact stiffness-contact depth method can be used for the analysis of shale parameters.Material invariant properties of shale were evaluated by nano-indentation analysis.(3)Micro-scale mechanical characteristic of shale is analyzed.Based on the grid indentation,elastic modulus and hardness are more convergence at the nano-scale.At the micro-scale,elastic modulus and hardness are distributed in a certain range,its statistical average predict the macroscopic parameters.The component-mechanical coupling evaluation method is established at the micro/nano-scale.Micro-fracture toughness of type I was evaluated based on the fracture analysis of indentation test.(4)The homogeneous model for shale elastic multi-scale property was proposed.Based on dimensional analysis,the modulus-packing density model under the micro-scale was proposed,and the homogenized stiffness tensor of porous media was analyzed.Based on the theories of equivalent inclusion and Eshelby inclusion,equivalent matrix-inclusion model under the meso-scale was proposed.(5)The multi-scale strength homogenization of shale was proposed.Based on the Drucker-Prager criterion and dimensional analysis,hardness-strength model was established.The strain rate energy and the expression of nano-scale homogenization function II are computed by a variational approach and effective strain rate approach.Based on the same theory,the micro-scale homogenization function was proposed.(6)The finite element simulation of shale multi-scale indentation test was carried out by the component analysis of the porous clay of the shale.Elastic modulus-packing density model was carried out.Analyzed the relationship between hardness-strength(cohesion,friction coefficient)-packing density,and established the micro-scale hardness-strength model of the solution of four factors fitting.Based on the finite element simulation of shale micro-scale indentation,multi-factor model of the elastic modulus and strength under the micro-scale was proposed.The models were verified by comparing the calculated results and the analysis of the test data.Finally,the multi-scale evaluation method of shale was used to verify the rationality of the shale cuttings,which came from the horizontal well in Changning-Weiyuan zone in Sichuan basin,the analysis results were consistent with the actual situation.