| Tight gas reservoirs are the main areas of the development of unconventional natural gas,which has the characteristics ofpoor reservoir physical properties,intense diagenesis,strong heterogeneity,complex and diverse pore throat structure and so on,especially the presence of nano-scale pore throats.In the process of gas reservoir development,due to the lack of detailed research on the microscopic pore structure and few quantitative pore throat characteristic parameters,the pore structure is not accurately characterized.In addition,due to the limitation of research methods and data,the influence of microscopic characteristics on reservoir physical properties is ignored,reservoir classification and is not comprehensive enough.In this paper,tight reservoir sandstone of the He-8 member at Z30 wellblock in the eastern sulige is taken as the research object,the petrological characteristics,physical characteristics,diagenesis and pore-throat types of reservoir are studied by using the data of thin section,physical analysis,SEM analysis and CL analysis.The capillary pressure curve and effects of pore-throat characteristic parameters on reservoir physical properties were disc ussed by means of high pressure mercury injection and constant-velocity mercury intrusion,and fluid occurrence in pores and its influence on reservoir development were studied by NMR technique.0 n this basis,reservoir evaluation parameters are selected from aspects of accumulating space,seepage capacity and microscopic pore structure,and the reservoir is comprehensively evaluated by means of multivariate statistical analysis method and fuzzy mathematical evaluation method.which provide reliable geological basis for the optimization of high-quality reservoir.The reservoir of reservoir study in Z30 wellblock He-8 member shows that:lithic quartz sandstone and lithic sandstone are the mainly reservoir rock.with few quartz sandstone;The fillings are the muddy matrix and carbonate.siliceous and clay cementing minerals,Cementation type is pore cementation.The results of reservoir physical properties characteristics study in Z30 wellblock He-8 member show that:it is mainly composed of ultra-low porosity and ultra-low permeability reservoirs,the average porosity and permeability are 6.37%、0.29×10-3μm2.In terms of diagenesis,the reservoir in the study area mainly entered mesodiagenesis B.the diageniesis includes compaction(dissolution).cementation.metasomatism and dissolution.The pore types is dominated by intergranular pores.cuttings dissolved pores and intererystalline pores with eluster pore throat occurring as the main throat type.The capillary pressure curves of samples with high pressure mercury injection and constant-velocity mercury intrusion show three different curve shapes.The NMR study shows that most of the Ⅳ and V types of reservoirs are difficult to develop.The characteristic parameters of constant-velocity mercury injection that have significant influence on reservoir properties include mainstream throat radius,the effective throat volume per unit and the effective mumber of throats.The radius of pore throat and the connectivity of pore throat are the key factors to control reservoir storage capability and percolation capacity.After analyzing the actual geological conditions in the study area,optimizing the evaluation parameters from macro to micro,covering reservoir and sedimentation,these parameters including physical parameters,displacement pressure,the maximal radiuses of pore throats,the maximum merc ury saturation,gas saturation,median grain diameter,cement content,shale content and so on.Finally,two classification and evaluation systems of tight gas reservoirs are established.they are as following:(1)multivariate statistical comprehensive evaluation system:the reservoir is divided into three categories based on factor-cluster analysis,and then discriminant analysis is applied to establish the three categories discriminant function to completely divide the reservoir.(2)fuzzy mathematics comprehensive evaluation method:entropy weight method is used to determine the weight of evaluation index,and combined with fuzzy mathematics comprehensive evaluation to achieve quantitative evaluation of reservoir.Finally,by comparing the fuzzy mathematics evaluation method with the multivariate statistical evaluation results.The results show that the evaluation classification results have good consistency with the test gas results by comparing the fuzzy mathematics evaluation method with the multivariate statistical evaluation results. |
