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

Abstract

To elucidate the NMR response characteristics of gas-bearing shales, NMR T2 and T1–T2 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 T2 spectra of water-saturated shales presented two- or three-peak distributions. With the increase in depth, the T2 peaks tended to be skewed to the right, indicating that the proportion of meso-macropores of the shale in the L1l1 sublayer is the largest. The T1–T2 signal peaks of the dried shales are dominantly distributed in the region where T1/T2 ratio＜100 and T2 = 0.1 ms, and that of the saturated shales are 1 mainly distributed in the region where 1＜T1/T2 ratio＜100 and 1 ms＜T2＜10 ms, respectively. A preliminary T1–T2 division method was proposed to differentiate hydrogen-bearing components in gas-bearing shales. Combining the T1–T2 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 T2g was proposed to evaluate the Longmaxi shale gas reservoir, which was increased in depth and reached the maximum in the L1l1 sublayer. Moreover, the boundary value of T2g was determined for reservoir evaluation and classification.