Study On The Method Of T₂ Spectrum Inversion From Nmr Data

Nuclear magnetic resonance (NMR) logging is very important advance of logging technology. Combining with conventional log, it could show information which is independent with lithology such as porosity, aperture distribution, permeability, fluid saturation etc and could show great advantages in complex reservoir evaluation. The technology of T2 spectrum inversion is the basic and key to the applications of NMR data and its results have impact on the accuracy of reservoir parameter (such as porosity, permeability, aperture distribution and so on) calculation and fluid identification.T2 spectrum inversion with optimization algorithm and singular value decomposition (SVD) were showed by the way of researching fundamental of NMR and inversion algorism from NMR data. The method of T2 spectrum inversion with optimization algorithm transforms the linear mixed-determined problem of T2 spectrum inversion into the typical optimization problem of searching the minimum of objective function by building up the inversion objective function according to the basic idea of geophysics modeling. The optimization problem above is solved with the conjugate gradient algorithm that has quick convergence rate and quadratic termination. The other method uses singular value decomposition to solve the equations of mixed-determined and system of linear, adds damping factor to coefficient matrix that makes numeric of the algorism stable and uses iteration technology to make result positive.The realization process of these algorithms was discussed in detail. The noise free echo train generated from artificial spectrum, artificial echo trains with various signal-to-noise ratio and NMR experimental data of drilling core were progressed by the two methods and compared with the inversion results and artificial spectrum together with the results of NMR laboratory. Not only the advantages and disadvantages of two methods but also the influence factors of T2 spectrum inversion were analysed. The result shows that method of T2 spectrum inversion with optimization algorithm has strong anti-noise ability and doesn't require initial point highly so that it is suit for invert T2 spectrum of good continuity from NMR relaxation data such as core or theory echo with low signal-to-noise ratio although it takes long time. Method of T2 spectrum inversion with SVD has good ability of stable result and clipping inversion so that it is suit for NMR relaxation data with high signal-to-noise ratio. NMR T2 spectrum could be inversed more efficiently and accurately with the two methods combined to meet the demands of production and scientific research on NMR data-processing.