Pore Effectiveness Of Marine And Continental Transitional Shale And Its Effect On Shale Gas Diffusion

As the main reservoir and migration space of natural gas in shale,microscopic pores(fractures)play an important role in the diffusion of natural gas in shale.Previous researchers have gained a lot of knowledge about shale pore structure,but few literature talked about the on pore effectiveness and its influence on shale gas diffusion behavior.In this paper,the pore effectiveness and gas diffusion behavior of the shale are studied by means of qualitative observation,experimental test,fitting calculation and correlation analysis.The microscopic pores of the Permian transitional shale in the Southern North China Basin are composed of intraparticle pores,interparticle pores,organic matter(OM)-hostd pores and fractures.The interparticle pores can be divided into two types:angular support of particles and marginal support of coarse particles.The intraparticle pores are mainly developed in calcite,pyrite,clay minerals and rutile.Rutile has a porous structure and has a high contribution to the Permian shale in the Southern North China Basin.Macropores and mesopores in OM of transitional shale are developed in OM with strong plasticity,which is controlled by OM type and dissoluble minerals.Micropores are developed in all types of OM.Through the comparative observation of different damage samples,the structural characteristics of natural fractures are summarized.Compared with the experimental data,it is found that the micropore volume is less affected by artificial damage,while the mesopore volume increases with the degree of damage.And the macropore volume is characterized by irregular changes.Combined with the fitting analysis of the main control factors,the empirical formula for calculating the effective micropores of shale is proposed.It is pointed out that the micropores in the transitional shale are mainly controlled by OM and clay minerals,and the mesopores are mainly controlled by illite and illite/smectite mixed layer.The effect of different damage degree on the effective mesopores analysis is explained.The diffusion rate of shale gas in transitional shale is mainly affected by OM and clay minerals.The unipore diffusion model model and the two combined first-order rate(TCFOR)dynamic model are used to fit the desorption data at room temperature.It is found that most of the transitional shale desorption at room temperature includes two stages:rapid diffusion stage and slow diffusion stage.OM with large specific surface area and pore filling characteristics will reduce the shale gas diffusion rate in desorption experiment.Kaolinite will reduce the shale gas diffusion rate in slow diffusion stage,and illite/smectite mixed layer can promote the rapid diffusion of natural gas in core samples.The effect of shale components on the diffusion rate of natural gas is essentially the control of pore structure in different components on natural gas.In core samples,micropores restrict the diffusion of natural gas in both rapid and slow diffusion stages,while mesopores provide channels for the migration of natural gas and promote the rapid diffusion.Based on the analysis of the influence of environmental change on pore structure,the idea of calculating the lost gas content in surface condition based on normal temperature desorption data and in well based on underground temperature desorption data is put forward.By comparing the principle and results of USBM method based on unipore diffusion model and TCFOR dynamic model,it is found that USBM method seriously underestimates the lost gas content due to considering only the normal temperature desorption data.It is a more reasonable method to calculate the lost gas content on the surface and in the well by using the TCFOR dynamic model.When mud and wellbore are not introduced,the lost gas content of transitional shale mainly occurs in the process of coring.However,this method does not take into account the impact of mud and wellbore,resulting in lost gas content will be greater than the actual.Although there are some shortcomings,this calculation method is more reasonable than the previous calculation method for lost gas.The research is of scientific and practical significance to the study of shale pore structure and shale gas diffusion behavior.