| Nuclear magnetic resonance logging is a n important formation detection,which is widely used in the petroleum industry,such as formation pore structure and detectio n and identification of internal fluids.Conventional NMR characterization methods are based on the assumption that the porous medium is in a fast diffusion condition.In complex formations,non-fast diffusion regimes?intermediate diffusion and slow diffusion?are also present.For example,macropore or large surface roughness of solid particles in the pores makes it difficult to obtain pore size information by conventional methods.In this paper,a new method for characterizing porous media with non-fast diffusion regime and uniform pore size distribution by T2-T2 pulse sequence is proposed.First,the T2-T2 signals of three simple pores in different diffusion regimes are calculated according to the eigenmode theory,and the T2-T2 signal coefficient matrix is obtained.The pore size-surface relaxivity correlation maps of the three simple pores in different diffusion regimes are obtained by non-negative least squares inversion,and the pore characterization results in intermediate diffusion regime is the best.Then,in order to characterize the more complex three-dimensional porous media,the T2-T2 signals of single sphere pore,periodic stacked pore and randomly stacked pore models in intermediate diffusion regime are numerically simulated by the random wa lk algorithm,and the pore size-surface relaxivity maps are respectively inverted,in which the characterization results are all good.Finally,the pore size and surface relaxivity of the sample are estimated by inversion after measuring the T2-T2 signal of the uniform particle size glass bead samples. |
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