Insights into NMR response characteristics of shales and its application in shale gas reservoir evaluation

To elucidate the NMR response characteristics of gas-bearing shales, NMR T-2 and T-1-T-2 experiments were performed on 38 shale samples selected from the Longmaxi Formation in the southern Sichuan Basin in China. The NMR signals of the totally dried shales were basically distributed in a single left peak and could not be removed completely. The T-2 spectra of water-saturated shales presented twoor three-peak distributions. With the increase in depth, the T-2 peaks tended to be skewed to the right, indicating that the proportion of mesomacropores of the shale in the L1(l)(1) sublayer is the largest. The T-1-T-2 signal peaks of the dried shales are dominantly distributed in the region where T-1/T-2 ratio <100 and T-2 = 0.1 ms, and that of the saturated shales are 1 mainly distributed in the region where 1 < T-1/T-2 ratio <100 and 1 ms < T-2 <10 ms, respectively. A preliminary T-1-T-2 division method was proposed to differentiate hydrogen-bearing components in gas-bearing shales. Combining the T-1-T-2 maps and correlation analysis confirms that the NMR signal of dry shale is mainly from clay structural water. NMR porosity should be corrected by removing the NMR signal of dry shales. After correction, the error between NMR porosity and WIP (water immersion porosimetry) porosity was considerably reduced, and its mean absolute error was only 0.16%. Moreover, a parameter T-2g was proposed to evaluate the Longmaxi shale gas reservoir, which was increased in depth and reached the maximum in the L1(l)(1) sublayer. Moreover, the boundary value of T-2g was determined for reservoir evaluation and classification.