Mechanism Of Pore Differential Evolution In Tight Sandstone Reservoir Of Jinci Formation In Central-Eastern Ordos Basin

The upper Carboniferous tight sandstone reservoir is an important replacement area for oil and gas exploration in Ordos Basin.However,due to the control of sedimentation and influence diagenesis,the reservoir mineral composition varies significantly,meanwhile,it shows complex pore throat structure and poor connectivity.The strong physical heterogeneity of the reservoir not only reduces the reservoir permeability,but also makes it difficult to be accuratly predicted.In addition,the strong stress sensitivity seriously affects the efficient of gas production.Therefore,on the basis of the previous research result,testing analysis technology and computer simulation means was introduced to investigate the mineral composition,micro pore throat characteristics,diagenetic pore evolution,seepage characteristics of Jinci sandstone reservoir,and the dynamic variation of reservoir porosity and permeability under stress,as well as its impact on the productivity of different quality reservoirs,which may provides theoretical basis for reservoir evaluation,favorable area prediction and gas reservoir development.Following conclusions were drawn:（1）The configuration mode of reservoir pore throat is the internal control factor of reservoir physical property.The type and distribution of reservoir pores determine the type of reservoir throat,which makes the reservoir form a unique pore throat configuration mode,determines the distribution of reservoir pore throat radius,and then controls the reservoir physical properties.The type I pore throat configuration reservoir（intergranular pore dominated type）has a variety of pores and throat types,resulting in a wide range of pore radius in the reservoir.Therefore,the reservoir shows good seepage capacity（the permeability is generally>0.6m D）.The pore system of type II pore throat configuration reservoir is mainly composed of intragranular dissolved pores and clay mineral intergranular pores,which determines that the reservoir throats are curved sheet and tube bundle throats characterized by small radius and concentrated distribution.Therefore,the reservoir permeability is poor（0.2～0.8m D）.The extremely high proportion of intergranular pores in typeⅢpore throat configuration reservoir makes the throat type in the reservoir single,which is dominated by pipe bundle throat with small radius.Therefore,the reservoir shows the worst seepage capacity（<0.1m D）.（2）The diagenesis controlling mechanism of pore differential evolution of Jinci sandstone reservoir is clarified and then the pore evolution patterns of Jinci sandstone reservoir was established.The content of ductile materials in the original sedimentary components and the early cementation determine the preservation of primary intergranular pores of reservoirs,thus affecting the intensity of dissolution and the contribution of physical properties of dissolution pores,and controlling the type of cementation associated with dissolution.leading to varied porosity increase and reduction caused by dissolution and and cementation,combined with the different degrees of subsequent cementation,The reservoir eventually show different porosity and presents different types of reservoir pore evolution patterns.Therefore,on the basis of defining the provenance and sedimentation characteristics of sedimentary sand bodies,combined with the reservoir pore evolution patterns in the study area,the pore throat configuration model and reservoir quality of sand body reservoirs can be predicted.（3）The seepage ability of Jinci sandstone reservoir is quantitatively evaluated.The multifractal parameters D_min-D_max,D_min/D_max of of the reservoir NMR T₂ spectrum and the reservoir flow zone index（FZI）shows significant correlation with T₂ cutoff value.On this basis,the prediction model of T₂ cut-off value of tight sandstone reservoir is established.Then the seepage ability of reservoirs with different types of pore throat configurations was quantitatively evaluated by using the parameters of NMR gas water displacement efficiency and T₂ cutoff value caculated from prediction model.From type I pore throat configuration reservoir to type III pore throat configuration reservoir,the percolation capacity of the reservoir decreases in turn,and the gas production capacity gets worse.（4）The characteristics of pore stress and strain of sandstone reservoir are revealed.The pore strain characteristics of different types of sandstone reservoirs are studied based on the DP-P experiment.Fromtype I to type III reservoirs,the permeability sensitivity of the reservoir is significantly increased.The pore compressibility of the reservoir is mainly controlled by the content of ductile materials such as phyllite,slate cuttings,mica and clay minerals in the reservoir.The pore compression of sandstone reservoir shows a"two stage"characteristic of rapid compression in the early stage of pressurization and reduced compressibility in the later stage.It is believed that the first stage of sandstone pore compression is dominated by viscoplastic failure deformation of plastic materials,and the second stage is dominated by small-scale non ideal elastic deformation of rigid quartz particles.The DP-NMR experiment was introduced to investigate the changing characteristics of stage pore volume and total pore volume of the reservoir under different effect of stress.Based on the stress and strain of pore volume,a calculation model for dynamic variation of reservoir permeability under the control of pore stress and strain is established.（5）The distribution of favorable reservoirs in the study area has been predicted,and the influence of pore stress and strain on the single well productivity of different types of reservoirs in the study area has been explored.Based on the material composition and diagenetic evolution model of the reservoir in the study area,the distribution of favorable reservoirs in the study area has been predicted.Supported by the calculation model for dynamic variation permeability,the natural gas productivity of single wells with different pore throat configurations under the conditions of stable permeability and dynamic change permeability was simulated,and the influence of dynamic change of reservoir permeability under stress on the productivity of different types of reservoir wells was clarified.This dissertation contains a total of 111 figures,31 tables and 296 references.