A modified design of pulse sequence and inversion method for D-T2 two-dimensional NMR

In face of the increasingly complicated exploration targets, D-T-2 2D NMR technology cannot balance the range of diffusion coefficient and transverse relaxation resolution in practice. As the key technique of D-T-2 2D NMR, the property of pulse sequence affects its application. A modified design of bivariate D-T-2 pulse sequence based on pulsed field gradient is proposed after summarizing the technical characteristics of PFG, STE-PFG, BP-PFG, modified CPMG, "diffusion editing" and multi echo interval CPMG pulse sequences, which effectively integrates the advantages of D-T-2 pulse sequences based on pulsed field gradient and constant field gradient. In order to reduce the difficulties that the existing D-T-2 inversion methods cannot balance the inversion efficiency and accuracy, we propose a joint TSVD inversion method which sets the echos from the second window as the main part supplemented by the echos from the first window. Numerical simulation results for gas-water, oil-water, heavy oil and gas-oil-water models in different SNRs indicate that the D-T-2 modified pulse sequence can balance the diffusion coefficient measurement range and transverse relaxation resolution successfully, and the joint TSVD inversion method balances the inversion efficiency and accuracy successfully. Above all, the D-T-2 modified pulse sequence and the joint TSVD inversion method have wide applications in the exploration and formation evaluation for unconventional reservoir, and create favorable conditions for the development of D-T-2 2D NMR technology.