Study On Multi-Scale Structure Characterization And Microscopic Seepage Law Of Tight Reservoir

This paper is based on samples from the Yanchang Formation in Xunyi,Ordos Basin.X-ray diffraction analysis of minerals,combined high-pressure mercury injection and rate-controlled mercury injection experimental methods,micro-CT three-dimensional pore structure reconstruction,and atomic force microscopy were used to characterize the pore structure of tight reservoirs in multi-scale.The German DSA100liquid phase analyzer was used to test the wettability contact angle and interfacial tension of the reservoir surface with three kinds of ion solutions,and analyze the law of change and its relationship.Fourier transform infrared spectroscopy was used to identify crown energy groups on the surface of the reservoir,and then explore the type of action and chemical changes between microfluids and reservoir surfaces.An electric double layer model was used for electrochemical characterization of wettability and a flow model was established.The influence of p H and salinity of the injected fluid on the wettability of the reservoir surface was quantitatively studied by the Z potential.Quantum chemistry was used to analyze the adsorption energy and chemical bond strength of quartz and ions.The dissipative particle dynamics was used to simulate the microscopic flow of pore channels in tight reservoirs.The influence of wettability on the development effect is explored by establishing a phase permeability curve model considering wettability.The main understanding of this paper is as follows:The mineral composition of the tight sandstone in the Ordos Basin is dominated by quartz and feldspar,and there are a small amount of carbonate and iron minerals.The mineral composition has an influence on the physical properties of the reservoir.The number of nano-scale pores in the reservoir is the largest,the number of mesopores is second,and the micro-scale pore distribution is the least.The quartz and feldspar component corresponds to the intercrystalline pore type obtained by CT,while the clay mineral components are mostly dissolved pores and intracrystalline pores.The pore shape is round tubular and long tubular,and the isolated pores are mostly.Three-dimensional surface modeling of AFM at nanoscale was performed on isolated pores to evaluate nanoscale pore surface structure.The relationship between microscopic homogeneity coefficient,throat radius and physical properties in microscopic reservoir structural parameters is analyzed.It is found that the microscopic homogeneity coefficient has the largest linear correlation with physical properties.When the coefficient is larger,physical properties is better.The effects of the three solutions on the static and dynamic wetting contact angles of the rock surface are different.However,the static contact angle and the moving contact angle of the same solution are consistent.Compared with Na Cl and Mn Cl₂,Ca Cl₂solution can significantly reduce the surface tension value of the system.After high concentration in Mn Cl₂ solution,interfacial tension increases rapidly with increasing concentration.The surface tension value of the system under Na Cl solution is the largest among the three solutions.The interfacial tension curves of the three solutions deviate from the G value with the concentration curve.Infrared spectroscopy showed that there were 4 to 5 absorption bands in the range of 300-1000 cm^-1,among which CO₃^2-anti-symmetric stretching vibration was the most obvious,indicating that there are more CO₃^2-and hydroxyl-functional minerals in the tight sandstone.The types of interaction forces on the reservoir surface under microscopic conditions are summarized and analyzed.According to the Z-potential and electric double layer theory,the Z potential decreases with increasing concentration under the Na⁺system,and the Z potential decreases first and then increases with the concentration of the Ca²⁺system.Both solutions have a decrease in Z potential with increasing p H and a significant decrease in alkaline environment.The CASTEP module was used to establish,optimize and calculate the energy system of the quartz crystal system adsorbed by Na Cl/Ca Cl2 in the reservoir rock.It is concluded that the Ca Cl2-quartz system is more ionic.Using dissipative particles to simulate Oil Drop-Solution flow in the porosity of mesoscale quartz system and the process figure and velocity distribution of the solution flooding oil droplets are obtained.The effect of wettability on the relative permeability of the oil and water phase under strong water wet conditions was calculated.Combining the conclusions in Chapters 3and 4,lowering the Na Cl concentration will reduce the negative value of the Z potential and reduce the hydrophilicity,which can increase the fluidity of the oil phase.The intrinsic reason for the use of low salinity waterflooding is explained from a multi-layered perspective.In this paper,the tight reservoir structure is characterized from both pore structure and surface structure properties.The scale distribution ranges from a few nanometers to several hundred nanometers,from micrometers to macroscopic scales.This paper makes evaluation recommendations for actual development,and determines the large-scale development of tight reservoirs in China.