Shale Characterization with Low Field NMR Magic Echo

Unconventional shale resources may contain a significant amount of hydrogen in organic solids such as kerogen, but it is difficult to detect the solids component directly with NMR method. We applied a magic echo pulse sequence to a range of natural and experimentally-altered shale samples. Homonuclear dipolar couplings between pairs of spins in shales are refocused by solid echoes[1]. The multiple solid echo sequence will give a longer signal decay of the rigid samples and very viscous fluids because the homonuclear dipole-dipole interaction between pairs of spins is eliminated in the echo maximum. The dipole-dipole interaction between many spins in organic solid is eliminated by the magic echo [2] . So a multi-magic echo sequence might perform better than a multi-solid echo sequence. It has never been tried with a logging tool. The magic echo measurements produce additional signal in the oil shale samples compared to the standard methodologies, indicating the presence of components undergoing homonucleardipolar coupling. Combining these techniques with standard T1, T2and solid echo measurements provides a more complete assessment of the hydrogen-bearing constituents in shale samples. This suggests that magic echo is a potential method for determining solid organic content and kerogen-bitumen discrimination.

Fig.1 Magic Echo Train , (a) magic solid echo train (b) inversion recovery with magic echo train.