NMR-based petrophysical parameter calculation with sparse inversion in tight reservoirs

Porosity, permeability, and saturation are important parameters reflecting the petrophysical properties of cores. The petrophysical parameters can be calculated from the inverted nuclear magnetic resonance (NMR) spectra. However, the continuous relaxation time distribution in the traditional NMR spectra is inconsistent with the sparse NMR relaxation signatures of the pore fluid. In this paper, a sparse inversion method was proposed to process NMR data in tight reservoirs, and the petrophysical parameters were calculated from the sparse NMR spectra. In the sparse inversion process, the relaxation time range of the core was first determined, only one new relaxation time was added to the sparse inversion in each iteration, and then the insignificant signal was eliminated. The signal amplitudes of sparse NMR spectra are constrained by the amplitude of the first echo to avoid the influence of noise. Compared with the traditional NMR spectra, the relaxation time is sparse and unfixed in the sparse NMR spectra. The T2 data of tight sandstone and T1-T2 data of shale were processed, and petrophysical parameters were evaluated to verify the effectiveness of the proposed method. The results indicate that the sparse NMR spectra can be better used to accurately calculate the petrophysical parameters in tight reservoirs, including porosity, permeability, and saturation.