A Study On Micro Pore Structure And Pore-throat Connectivity In Tight Sandstone Reservoir

Tight sandstone oil and gas is an increasingly important resource.Pore structure and connectivity are key factors that determine reservoir quality in such oil and gas fields.However,compared with conventional reservoirs,heterogeneity and abundant nano-scale pore throats hinder investigations of tight sandstone oil and gas.Its single quantification method and unknown characterization parameters are difficult points in this field.In order to improve the characterization of pore structure and the connectivity,in this paper,new methods to investigate such reservoirs are applied to the analysis of a typical tight oil reservoir in China(i.e.,the Chang-7 reservoir in the Upper Triassic Yanchang Formation,Ordos Basin).A variety of methods,including nuclear magnetic resonance cryoporometry(NMRC),triple ion beam(TIB),Wood's metal impregnation(WMI),Nano-computed tomography(CT),focused ion beam-scanning electron microscope(FIB-SEM)and other nano technology and fluid impregnation technology,were applied for the first time to qualitatively assess the reservoir connectivity in two dimensions and quantitatively characterize it in three dimensions.Combined with logging data,thin section identification,physical property data,pores(particles)and cracks analysis system(PCAS),the pore structure and connectivity of different types of tight reservoir samples were studied.The main results of this study are as follows:(1)Using NMRC to quantitatively characterize the pore structure of different types of tight sandstone samples,this method realizes the fine characterization and full-scale characterization of the pore size distribution(PSD)of tight sandstones.The pore structure characteristic parameters of different types of sandstone are classified and compared by this method.Our data suggest that the method can effectively characterize the pores in the range of 4 nm-1400 nm,which basically meets the requirements of full-scale characterization of pore size distribution of tight sandstone;The different types of tight sandstones have different PSD characteristics,and the ratio of the pore volume contribution ratio from 125�375 nm to the pore volume contribution ratio from 4�11nm(C_MA/C_MI)increases with oiliness.The C_MA/C_MI value of type I sandstone is 0.71,the C_MA/C_MI value of type II sandstone is 1.44,the C_MA/C_MI value of type III sandstone is 1.65,the C_MA/C_MI value of type IV sandstone is 2.51.(1)Through WMI-TIB-SEM and a series of comparative experiments,the connectivity of tight sandstone reservoirs is qualitatively and finely described.The connectivity of the four types of sandstone within a fixed field of view can be divided into three types:poor,medium,and good connectivity.Areas with poor connectivity have abundant micro-cracks and interconnected grain-edge and intra-granular solution fractures and a large area of intergranular and intragranular dissolved pores that are separated by a small distance.Areas with poor connectivity have the pores are separated by a large distance and the pore area is small and the presence of grain edge and intragranular solution fractures with no large effective storage space around the fractures.The characteristics of the area with medium connectivity are between the above two.(3)Through the combination of TIB-SEM-PCAS technology,the form factor and probability entropy are quantitatively extracted,which can characterize the pore structure of reservoir.The average form factor of type I sandstone is 0.86,the average probability entropy of type I sandstone is 0.91,the average form factor of type II sandstone is 0.86,the average probability entropy of type II sandstone is 0.91,the average form factor of type?sandstone is 0.96,the average probability entropy of type?sandstone is 0.87,the average form factor of type?sandstone is 0.96,the average probability entropy of type?sandstone is 0.82.The pore orientation gradually becomes chaotic with increasing complexity of the pore edges.(4)Based on the previous studies in the field of three-dimensional reconstruction(nano CT and FIB),this paper proposes a new method to characterize the connectivity of pore throat network in tight sandstone reservoir based on complex network theory(CNT),and discusses the applicability of the theoretical model to characterize the connectivity of tight sandstone,which provides an application example for promoting the extensive application of complex network theory in tight reservoir.The results of different types of sandstones show that the connectivity entropy of type III sandstone is 4.65,and the connectivity reliability is 0.41.The connectivity entropy of type IV sandstone is 2.54 and the connectivity reliability is 0.67.It shows that the pore-throat connectivity and reliability of Type IV are better than Type III.(5)Our results show that the C_MA/C_MI values,pore morphology,and pore connectivity will have an effect on reservoir quality and oiliness.However,these effects will not be fully reflected by the physical properties.Using porosity and permeability as parameters to evaluate the quality of tight reservoirs may lead to an underestimation of oil and gas resources.Therefore,future exploration and development should not be limited to reservoir sections with a high macropore content and good physical properties,but also target intervals with high C_MA/C_MI values,pores with smooth edges,well-oriented pores,good pore connectivity,and high reliability.