Characterization Of Pore Structure Of Tight Sandstone Reservoirs

The fundamental reason that the early production,stage production and cumulative production in single well all show great differences in tight sandstone gas reservoir lies in the micro-and nano-scale complex characteristics of pore structure.However,in the practical research of the pore structure characteristics,we still use the techniques applied to the conventional reservoir because of the shortage of the experience studying tight sandstone reservoir,the result indicates that we not only can't get the accurate evaluation of pore structure,but also make a lot of waste.Therefore,the article takes the Cretaceous Bashijiqike formation tight sandstone reservoir in Keshen 2 block for example,and focus on the research of method and the characteristics of pore structure.Through the research and evaluation of the different techniques,we can find out the suitable means for the research of pore structure of tight sandstone,and then study the characteristics and the influencing factors of it.The main work carried out and the results obtained are as follows:(1)Using the conventional casting thin section and the common scanning electron microscope,combining the high precision laser scanning confocal microscopy and the field emission scanning electron microscopy,we find that the main pore types in the study area are residual intergranular pores,intergranular dissolution pores,intragranular dissolution pores,micro pores and cracks,which are dominated by intergranular pores.The throat type is mainly sheet or bent sheet,and the second one is cluster,and the connectivity is poor overall.(2)To tight sandstone reservoir,the high pressure mercury test is still very important for the study on the pore structure characteristics.The results of high pressure mercury test show great differences mainly in the experimental devices and process.By the contrast experiment,the differences focus on the samples' salt-cleaning and the distribution of pressure point,and the results show that the Corelab's are more in line with the actual.(3)The pore structure characteristics in study area are as follows:the throat is micro-scale,the curve of pore throat radius distribution frequency mainly shows a single peak,and minorly double peaks,the sorting of pore throat radius is better.The displacement pressure range by 0.3?3.18MPa,average by 1.45MPa;the median radius range by 0.022?0.37?m,average by 0.138?m;the pore throat average radius range by 0.051?0.46?m,average by 0.145?m;and the sorting coefficient range from 0.039 to 0.414,average by 0.122.The relatively large pore throat radius is mainly distributed from 0.1 to 0.5?m which also determines the permeability.(4)The correlation between the parameters of pore structure characteristics and porosity are always very poor,when permeability better.The parameters which are arranged from better to poor by the correlation with permeability are as follows:the main flowing pore throat radius whose correlation coefficient is 0.9221,the sorting coefficient 0.9008,the displacement pressure 0.8554,and the average pore throat radius 0.8238.The permeability in the study area is mainly controlled by the distribution of the pore throat which is more large than 0.1?m.According to the Q type cluster analysis,the pore structure of the reservoir in the study area can be divided into four types.(5)The main factor which causes the heterogeneity of pore structure of the reservoir in this study area is the diagenesis which includes the strong compaction,the quartz and feldspar's overgrowth,the carbonate's cementation and dissolution in varying degrees which determines the current densified and complex feature of the reservoir.