A novel pulse sequence and inversion algorithm of three-dimensional low field NMR technique in unconventional resources

Schematic diagram of new 3D pulse sequence and the simulated echo trains. [Display omitted] •A tri-window pulse sequence to collect the full decaying information of porous media is proposed.•A joint inversion algorithm named ‘composite-data-processing’ to obtain the 3D correlation map is proposed.•A tight oil shale model containing four types of fluid is established and validated the proposed method. Compared with two-dimensional (2D) nuclear magnetic resonance (NMR) technique like correlations among the transversal relaxation time (T2), the longitudinal relaxation time (T1), and the diffusion coefficient correlation (D), three-dimensional (3D) NMR technique is superior with the complete measurement of T2, T1, and D simultaneously. It can solve the problem of overlaps in 2D correlation map and is helpful to characterize relaxation components in unconventional resources such as tight gas and oil shale. However, the existed 3D NMR technique is restricted due to the loss of short relaxation information and the inversion inaccuracy that caused by the incomplete measurement of the diffusion editing window. We developed a tri-window pulse sequence to collect the full decaying information of porous media. In the first window, the inversion-recovery pulse sequence is applied for T1 encoding. In the second window, D and T2 are encoded by an adjustable continuous pulse field gradient and echo spacing (TE). In the last window, CPMG with the shortest TE is used to acquire diffusion-free relaxation information. We then proposed a joint inversion algorithm named “composite-data-processing” to obtain the 3D correlation map. The algorithm adopts the dimension reduction technique and the truncated singular value decomposition (TSVD) to speed up the inversion process and enhance the inversion stability. Numerical simulations show that good estimations of the inversion results are obtained at different signal to noise ratios (SNRs). Our results suggest that the novel pulse sequence and inversion algorithm of 3D NMR can be effectively applied to the exploration of unconventional resources.