Numerical investigating the low field NMR response of representative pores at different pulse sequence parameters

Abstract

We developed a numerical simulation algorithm to explore the nuclear magnetic resonance (NMR) response of the porous media based on the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence and the Bloch equation. The evolution of the magnetization vector of two representative pores at different pulse properties, including the excitation angle, the refocusing angle, the phase angle, as well as the pulse duration are simulate to understand the NMR relaxation signals. The result showed that the normalized magnetization is symmetrical with the excitation angle and positive with the T2 spectrum's amplitude when the excitation angle is less than 90°. In additional, the refocusing angle has no clear influence on the NMR response. The phase angle of the excitation pulse is inversely correlated with the echo amplitude and can be neglected when the value is lower than 15°. The phase angle of the refocusing pulse causes the zig-zag phenomenon, but the response of the even echoes is not disturbed. Moreover, the influence of the pulse duration should not be neglected at higher values, particularly for the mesopore. The simulation results are helpful for the design and optimization of the pulse sequence, and the data manipulation of the measured signals.