Study On Microscopic Pore Structure And Micro Seepage Characteristics Of Tight Sandstone

The pore structure of tight sandstone is difficult to be characterized accurately due to the small pore,complex pore structure,low permeability and low porosity characteristics of tight sandstone reservoirs.The pore structure of reservoir rock is usually studied by capillary pressure method,casting thin film observation method and scanning electron microscope,but the pore structure of tight sandstone is mostly micro nano scale and strong heterogeneity,it is difficult to accurately characterize the micro pore of tight sandstone by conventional methods.In this paper,the microscopic pore structure of tight sandstone is studied by scanning electron microscopy(SEM)mineral quantitative analysis,back scattering two-dimensional large area imaging and micro CT scanning experiments on the basis of conventional methods.According to the experimental results,the numerical core modeling and micro percolation simulation are carried out on the tight sandstone.The microscopic pore structure and micro percolation characteristics of tight sandstone are qualitatively and quantitatively analyzed.The results and conclusions of the research are as follows:(1)The USM sharpening image processing method is introduced during the process of core micro CT scanning and grayscale image processing on the earlier stage of digital core modeling.It can be found from the micro CT scanning gray image after the USM sharpening that the boundary between the skeletons and the pores of rock are more obvious,which makes the segmentation precision of the later micro CT scanning gray image improved greatly,thus making the digital core more precise and accurate.(2)The results of back scattering two dimensional large area imaging experiments and micro CT scanning experiments are compared and analyzed,so as to supplement the inner structure of the rock in the region with unclear image in the micro CT scan gray image.The results show that some high light pollution areas in the black pores of the micro CT gray image are mainly the Huang Tie minerals in the pores,and the gray matter inside the interior of the micro CT scan gray image are mainly the authigenic clay minerals(mainly chlorite and illite).The detail supplements to the micro CT scanning image provide a reference for the detail processing of the later digital core modeling process and the evaluation of micro pore structure based on the micro CT scanning image.(3)The method of calculating the effective crack rate is established through the extraction of effective cracks from the seepage simulation results,thus the evaluation of reservoir fracture effectiveness and fracture development degree is completed.The results show that the axial and radial effective fracture rate of the core are both less than 50%,indicating that the micro cracks in the tight sandstone are not fully developed and the utilization rate of the cracks is not high,which leads to the low oil and gas production rate.(4)The tight pores of reservoirs mainly consist of intergranular pores and microfractures,which contain a small amount of micropores and cleavage cracks.Most of the pores contain chlorite and illite(content of 7%~15%),and some of the pores contain pyrite.Therefore,the reservoir rocks have obvious water sensitivity,velocity sensitivity and certain acid sensitivity,and the microfissures are mainly shear fractures,and the specific forms are conjugated shear fractures and along grain sizes.The distribution of pore diameter is uneven,and the main diameter is 15 m-35 m,accounting for about 60% of the total porosity.