Study On Pore Structure Characterization Andconnectivity Of Shale Reservoir

As a self-storage petroleum,shale oil and gas has achieved a huge commercial success not only in North America,but also attracted extensive attention for the past few years in China.With the deepening of shale oil and gas exploration and exploitation,the core content of shale reservoir evaluation including internal pore structure and connectivity has become a hotspot of current research.Fully understanding the microporous structure and its connectivity is not only important to study the physical properties of mud rock reservoirs and the occurrence,migration of oil and gas,but also a vital factor to evaluate the shale oil and gas resources.In this paper,the pore structure of three formations of shales in different sedimentary environments are studied,which are from the Dongying Depression of the Paleogene,the upper Permian of the Lower Yangtze Area,and the lower Silurian of the lower Yangtze Area.The pore characteristics of different shale reservoirs were studied by argon ion polishing-field emission scanning electron microscopy based on TOC,Ro,XRD and some other geochemistry data.Also,the pore types were classified.By using small angle X-ray scattering,nuclear magnetic resonance freeze-thawing and nitrogen adsorption,shale porosity and pore size distribution were studied,and the differences of different methods were compared.The applicability of different porosity characterization methods and the pore developments of shale in different sedimentary environments was also discussed.Based on the observation of shales with different lithofacies in Dongying Depression,the classification scheme of shale porosity connectivity is proposed after three argon ion beams polishing technology and field emission scanning electron microscopy.Besides,with the ImageJ image rendering,PCAS pore quantitative characterization techniques,combining the actual oil production data,geological significance of the different types of shale connections in shale was comprehensive analyzed.The three-dimensional pore structure images of shale reservoirs in Dongying depression are obtained by FIB-SEM experiment and Avizo visualization software.The quantitative parameters of the seam feature are extracted,and the relationship between the pore joint and the contribution of different lithofacies shale porosity to reservoir connectivity were discussed.The main results are as follows:1.There are obvious differences in the development of shale porosity in different sedimentary environments.The heterogeneity of lacustrine shale in Dongying sag is the strongest,the pore type is diverse,and the pore structure is more complicated.Especially,the dissolution pore and the septum are the most developed.The heterogeneity of the marine facies shale in the Lower Yangtze area is relatively good,the pore type is simple,mainly intergranular pore,clay mineral interbedded porosity and organic matter pore.The pore development of the marine-terrigenous facies shale is between the two above.2.The maturity of marine shale has the highest maturity in the three shale formations in this study,and the organic matter pore is well developed,mostly stomatal with large pore size.The shale of sea-land transitional shale is affected by the difference of maturity and micro-composition,the pore development is uneven and the pore size is relatively small;Lake shale in the oil phase,almost has no organic pores.3.High porosity marine shale porosity characterization include a wide range of methods,including imaging methods,small angle X-ray scattering,gas adsorption,nuclear magnetic resonance and so on.The low-maturity lacustrine shale due to the filling of a large number of high-wax crude oil and so on,washing oil treatment is needed.As the concequence of that the nanopore wall waxy crude oil is difficult to remove,fluid injection method is not suitable for pores characterization of this kind of shale,physical methods such as imaging methods and small angle X scattering are the preferred methods for the characterization of low-maturity lacustrine shales.4.The porosity connectivity of low-maturity lacustrine shales are divided into three types:the main fracture-the seams-intralayer facture-the interstitial pores,the main fracture-the interstices-the interstitial pores,the interstices-the interstitial pores.It is concluded by the PCAS experimental data and actual production data that the crack-stitch-matrix porosity is the best connectivity,and wear cracks-matrix pore connectivity is the worst.The pore size distribution and fractal dimension of the shale have a certain influence on the connectivity of the pores.5.Through the three-dimensional PCAS quantitative data analysis,lamellar shale connectivity is better,and block mudstone connectivity is poor.The results of three-dimensional reconstruction show that the porosity of the layered shale is the highest,and the two-dimensional results are sirmilar to those of the PCAS.The proportion of the number of independent meshes in the block shale is the highest,and the macrophages of the lamellar shale are relatively high,which demonstrated that the connectivity of the layered shale is significantly better than that of the block shale.