Research On Characteristics Of Microscopic Pore Throat And Classified Evaluation Of Chang7 Tight Sandstone Reservoir In Longdong Area,Ordos Basin

The potential of tight oil resources is huge.With the increase of exploration and development and the application of advanced technology,tight oil will gradually become an important alternative resource for oil and gas reserves and production in the future.However,the pore throat of tight reservoir is small,heterogeneous and complex.Its micro pore throat development characteristics not only affect the reservoir property and permeability of tight oil reservoir,but also are closely related to the later development.Systematic analysis of micro-pore throat characteristics,percolation characteristics of tight reservoirs and establishment of classification and evaluation criteria of tight reservoirs are helpful to provide a scientific basis for the decision-making of the next key objectives,which is of great significance for exploration and development of tight oil.Taking Chang 7 reservoir in Longdong area of Ordos Basin as the research object,the microscopic pore throat characteristics of tight sandstone reservoir are systematically characterized by various analysis and testing methods,the percolation characteristics and influencing factors of tight sandstone reservoir are further analyzed.Finally,the evaluation criteria of tight sandstone reservoir are established.The main conclusions are as follows:?1?The average porosity and permeability of Chang 7 tight sandstone reservoir are 9.4% and 0.182×10^-3?m²,respectively.The average plane porosity is 1.89%.The pore types are mainly intergranular pore and feldspar dissolved pore.The pore assemblage types are mainly micropore type,the average pore radius is 7.18?m,among them,the number of pore with radius less than 10?m is the largest,but the proportion of pore with radius greater than 20?m is the largest.The type of throat is mainly flake?slit flat?and open,the radius of throat is mainly distributed in the range of 0.05-1?m,the average radius of throat is 0.19?m,the average radius of mainstream throat is 0.23?m,and the lower limit of the average radius of connected throat is 0.175?m.?2?The average sorting coefficient is 1.06,the average variation coefficient is 0.1and the average mean value is 12.19.The sorting property of pore throat is negatively correlated with physical property.When sorting property is good,the distribution of pore throat is concentrated,but the whole is small,which makes the physical property worse.When sorting property is poor,the contribution rate of macro pore throat to porosity and permeability is larger,which makes the physical property better.The average displacement pressure is 3.07MPa,the average maximum mercury saturation is 73.09%,the average mercury removal efficiency is 24.96%,and the average pore throat volume ratio is 3.51.The displacement pressure and pore throat volume ratio are negatively correlated with physical properties,and the maximum mercury saturation and mercury removal efficiency are positively correlated with physical properties.The overall reservoir property is good,but the connectivity is poor.?3?A characterization method for characteristics of microscopic pore throat of tight sandstone reservoir is proposed:Firstly,the samples are selected for conventional experimental tests such as casting thin section identification,scanning electron microscopy,cathodoluminescence,physical property analysis,etc.to understand the petrological characteristics,physical properties and pore types of reservoirs from macroscopic perspective;On this basis,representative samples were selected for high pressure mercury injection and nitrogen adsorption tests,which were used to analyze the development characteristics of micron-submicron pore throats and nano pore throats respectively.The size and distribution of pore throats could be comprehensively characterized by the combination of the two methods;Thirdly,representative samples could be selected for CT scanning according to the need to characterize the structure characteristics of three-dimensional pore throats.?4?The average movable fluid saturation of Chang 7 reservoir in the study area is35.15%.The main types of water flooding are finger and mesh flooding,the residual oil is mainly cluster-like,supplemented by solitary and film-like,and the displacement efficiency is 24.7%⁴4.56%.It is concluded that the micro-seepage characteristics of tight sandstone are controlled by many factors.The basic reason is that the pore throat is small and the structure is complex,followed by the surface properties of rock particles.The more tight the reservoir is,the more obvious the influence of the surface adsorption and wettability of rock particles on oil-water seepage is.?5?The imbibition experiment shows that the overall imbibition rate decreases exponentially.Microscopically,the imbibition is characterized by displacement of oil from the fine pore throat to the larger pore throat.The displacement efficiency of imbibition is 15.4%²3.91%and it is mainly controlled by wettability and physical properties.?6?By selecting physical parameters,microscopic pore throat structure parameters and percolation characteristic parameters,and using comprehensive classification and evaluation method,the tight sandstone reservoirs are divided into four categories from good to differential,and the classification and evaluation criteria suitable for Chang 7 tight sandstone reservoirs in the study area are established.