Pore Structure Analysis And Conductivity Modeling Of Shale Based On Digital Core

As an unconventional resource,shale gas has great potential for development.China pays more and more attention to the development of shale gas.Since 2018,the shale gas reservoirs in the Upper Ordovician Wufeng Forming-Lower Silurian Longmaxi Formation in Sichuan Basin,which are designated as the main shale gas reservoirs in the four shale gas pilot test areas,have made a lot of progress.There are two kinds of shale gas occurrence modes:adsorption state and free state.However,due to the complexity of pore microstructure and mineral composition of shale itself,the conductive mechanism of shale gas reservoir is unclear,and it is difficult to obtain gas saturation,which directly affects the reliability of free gas content evaluation.The current common practice is to convert rock resistivity to saturation with the help of the argillous sandstone evaluation method using Archie’s formula and its modified formula.Because the pore structure of shale is very different from that of sandstone,Archie formula is not applicable to shale.Therefore,the understanding of shale conductive mechanism and the construction of saturation model are the key points.Taking Wufeng-Longmaxi shale gas formation in southeastern Sichuan as the object,this paper studies the conductivity and gas saturation.The rock characteristics of shale strata in the study area are determined by thin section information.The shale formations in the study area are characterized by low porosity,low permeability and strong heterogeneity,in which the pores are mostly micropores and mesopores of nanometer to micron level,and the pore structure is complex.The pore structure of shale samples in the study area was studied by NMR and X-CT scanning.NMR method was used to determine the size and proportion of shale samples,and the obtained nuclear magnetic porosity was basically consistent with that obtained by laboratory saturation method.Combined with the characteristics of CT scanning,it was found that micropores were mainly found.The relationship between rock electrical parameters and pore structure parameters T_2lm and T_2cutoff can be established by conducting rock electrical experiments on 30shale samples and nuclear magnetic measurement on 20 shale samples.CT scan images are used to construct 3D shale digital cores.Through the CT image preprocessing（filtering and cutting）and the mineral content of the whole rock analysis as the basis,the mineral division and combination in the volume space of the rock sample are determined to determine the gray threshold.After the threshold segmentation,the digital core is formed.The electrical properties of 3D digital core are simulated by finite element method.After setting the salinity of formation water and the conductivity of each mineral component,and constructing digital cores with different water saturation,the voltage is applied in three directions respectively to obtain the conductivity results of digital core simulation in x,y and z directions.The rock electrical parameters including formation factor F,cementation index m and saturation index n in three directions are obtained through calculation.The accuracy of 3D digital shale cores in electrical simulation can be determined by comparing the electric parameters obtained from digital shale cores with laboratory experiments.The low resistivity phenomenon occurs in the Wufeng-Longmaxi shale strata in the study area.At this time,the simple Archie formula is no longer applicable,and the additional conductivity effect brought by clay and organic matter needs to be discussed.In addition,considering the wettability of organic matter and minerals,a new saturation model was constructed considering the effect of clay and organic matter conductivity.The new saturation model was applied to shale well interval of Pengye 1 in the study area to explore its application effect.The results show that the new saturation model is more accurate and accurate than Archie saturation model,Doll saturation model,Simandoux saturation model and Indonesian saturation model.In this paper,the pore structure and mineral composition of the shale of Wufeng-Longmaxi Formation in southeastern Sichuan are systematically analyzed,and a 3D digital shale core with certain applicability in the study area is constructed for electrical simulation.Finally,the applicability of the model in Wells is discussed by constructing a gas saturation model considering clay and organic matter.It provides a strong support for the study of gas saturation in shale reservoir.