Study Of NMR Logging Forward And Inverse Methods And Its Applications In The Southern Ordos Basin

The target layer,Chang 8 formaiton in the Southern Ordos Basin,is a typical tight sandstone reservoir.In the reservoir,the complex pore structure,a large number of micro fractures and strong heterogeneity affect the Nuclear Magnetic Resonance(NMR)logging response and lead to the fuzzy chacracteristics.There is not a suitable overall optimization of the T2 spectral inversion in the study area.There are still some problems in the reservoir evaluation using NMR logging,especially the permeability evaluation in fractured reservoir is difficult.Therefore,further research is needed to solve these problems.The petrological and physical characteristics in study area are summarized.There are mainly developed high angle fractures with fracture width less than 0.2 mm.The characteristics of MRIL-Prime logging method and instrument are summarized.Based on the influencing factors in the study area,the NMR logging forward model and method are established and simulated.It is found as follows: the better change of the pore structure tends to make the T2 spectrum changes to long relaxation and wider spectrum;the T2 spectrum in the study area commonly can indicate the boundary and formation characteristics;with the hydrocarbon saturation increases,the T2 spectrum is similar when the reservoir is full of single-phase fluid,the T2 spectrum changes gradually from single water peak to double-peak and single oil peak in oil-water two-phase,and the amplitude of T2 spectrum decreased obviously in gas-water two-phase;the influence of carbonate cements and fractures on T2 spectrum can be neglected in the study area.For T2 spectrum inversion,the common algorithms are realized,and the influence factors are analyzed.The overall optimation scheme which is suitable for the study area is established.Firstly,the optimal stationing scheme which is suitable for different T2 spectrum characteristics is established.The cutoff value is selected based on the BG theory and the least value criterion.Then using the singular value decomposition(SVD)method to obtain The T2 spectrum,which is used as the initial value of the joint iterative reconstruction algorithm(SIRT),and the final T2 spectrum is iterated.Combined with the knowledge of NMR logging and the methods,a suitable fluid identification method,evaluation methods of fluid saturation and pore size distribution are optimized after the comparison.The permeability evaluation scheme of the fracture-matrix dual porosity systems in fractured tight sandstone reservoirs is deeply studed.In non-fracture tight sands,based on the HFU and FZI theory,the NMR logging permeability model is estabilished in classified formations.In fractured tight sands,two novel methods of permeability evaluation in the fracture-matrix dual porosity systems are proposed.The first one is the fast estimation method(FEM).It is the sum of the fracture permeability calculated by fracture medol of dual lateral logging(DLL)logging and the matrix permeability calculated by NMR logging permeability model in classified formations based on the HFU(Hydraulic Flow Unit)and FZI(Flow Zone Indicator)theory.The second one is the new fracture response equation(FRE)and new permeability theory model(NPTM).Firstly,the FRE of the DLL in the fracture-matrix dual porosity systems is established.The virtual core scheme is proposed to establish the calculation models of the fracture cementation exponent and matrix cementation exponent.Then the nonline equation of fracture parameters is established,as well as the NPTM.The characteristics of NMR logging responses are clarified.The numerical experiments indicates that,the T2 spectrum inversion efficiency and accuracy of the overall optimation scheme is comprehensively best by comparing with other common cutoff values methods and inversion method,and the optimal stationing scheme can improve the inversion accuracy of other methods.In application,the differential spectrum method of fluid indentification and the T2 cutoff value method of fluid saturation are preferenced,and the suitable evaluation model of reservoir pore size distribution is calibrated.They correspond well with logging or experimental results.Permeability evaluation applications in the fractured tight sands indicate that,the FEM has the highest accuracy,and can be used as an accurate permeability evaluation method in fracture-matrix dual porosity systems;and the second is the FRE and NPTM,which is suitable for field fast analysis.In summary,based on the NMR logging responses characteristics,the suitable overall optimation scheme of T2 spectral inversion,evaluation model and the fast and accurate methods for permeability evaluation in fractured tight sands are established.They can improve the accuracy and efficiency of the reservoir evaluation in the study area.They can be used as reference for the logging evaluation in tight sands,and can be extended to similar evaluation.