Study On NMR Logging Inversion And Application

The logging evaluation technology of tight sandstone gas reservoir is one of the key technologies for efficient exploration and development of tight oil and gas reservoir,and the NMR logging technology is the key technology of logging evaluation technology of tight sandstone gas reservoir.For a long time,NMR logging has been highly applicable in reservoir fluid identification,porosity and permeability calculation,especially in the pore structure of tight oil and gas reservoirs,it has unparalleled advantages in other logging technologies.However,there are often failures in the application of NMR logging in tight sandstone gas reservoirs.For example,due to the complex pore structure and gas content of tight sandstone reservoirs in Hangjinqi area,the NMR differential spectrum signal often appears in the water layer in this area,and the calculated values of nuclear magnetic porosity and permeability are too small.These problems not only restrict the progress of exploration and development in the Hangjinqi area,but also pose new challenges to the effective application of nuclear magnetic resonance logging in tight sandstone and other complex reservoirs.Therefore,it is necessary to carry out NMR inversion and application research on tight sandstone gas reservoirs to solve some new problems in the practical application of NMR logging in the current complex reservoirs.This paper focuses on the inversion and application research of NMR logging.First,it is mainly based on nuclear magnetic resonance logging data,supplemented by conventional logging data,and combined with core experimental data to carry out reservoir characteristics analysis.Summarize the lithology,physical properties,electrical properties and gas-bearing characteristics of different reservoirs.Based on the analysis of logging response characteristics of typical gas and water reservoirs,the ten-component distribution characteristics of NMR T₂ spectrum of gas and water reservoirs are summarized.Then,according to the characteristics of tight sandstone gas reservoirs in Hangjinqi area,the clay bound water signal was added to the two-dimensional inversion of NMR.The inversion by TSVD method shows that the two-dimensional T₂-T₁ spectrum is better than the two-dimensional T₂-D spectrum in identifying the four signals in gas-bearing reservoirs when considering the clay-bound water signal.For T₂-T₁ spectral inversion,maintaining a long waiting time is beneficial to the stability of medium and long relaxation signal inversion,and keeping a low short waiting time is beneficial to focusing of the short relaxation signal.A large echo number is beneficial to enhance the focusing of the fluid signal of the long relaxation component,and a small echo number is favorable to the enhancement of the fluid signal of the short relaxation component.At the same time,a method of divergence correction for inversion of clay-bound water signals based on the intersection location method is established.This method is based on spectral peak positioning and spectral width calculation,and then reconstructs the two-dimensional spectrum signal of clay-bound water,which can solve the problem of inversion and divergence of clay-bound water signals except the quality of the original echo string.Finally,the application research of nuclear magnetic resonance logging was carried out in combination with the results of inversion research.In terms of identification and application of reservoir fluid properties,the relaxation mechanism of tight sandstone gas layer and water layer is analyzed,the T₁ and T₂ spectrum distribution range characteristics of gas and water layer in Hangjinqi area are calculated,and the T₂-T₁ two-dimensional spectrum interpretation template in Hangjinqi area is established.Through the application of two-dimensional NMR logging,the problem of overlapping the signals of various fluids in one-dimensional NMR logging field has been effectively solved,and the recognition rate of reservoir fluid properties has increased from 78%of the one-dimensional nuclear magnetic logging to 91%of two-dimensional NMR logging.In terms of porosity calculation,a method of calculating the time difference between NMR and sonic waves was established,which offset some of the gas-bearing effects,and effectively alleviated the problem of generally small porosity in tight sandstone gas layers calculated by NMR logging data.In terms of permeability calculation,a new nuclear magnetic permeability calculation model based on variable parameters p,q,h,and i is established.This model is based on the principle that the permeability of tight sandstone reservoirs is determined by their porosity and pore structure.It combines the advantages of core experiment data and nuclear magnetic logging data in permeability calculation.The example proves that the correlation coefficient between the permeability calculated by the new permeability model and the core analysis permeability reaches 0.94,which further improves the permeability calculation accuracy of tight sandstone gas reservoirs.In the aspect of pore structure identification,the nuclear magnetic ten component pore size classification method is established to identify pore structure,which can not only identify the fine changes of reservoir pore structure under continuous depth,but also further build a new pseudo capillary pressure calculation model based on the lateral conversion coefficient established by mud displacement fluid technology.It is verified by an example that the coincidence rate between the pseudo capillary pressure curve of the sandstone reservoir constructed by the model and that of the mercury injection capillary pressure curve of the core is more than 80%.