Determination of initial gas saturation in tight gas formations with NMR relaxometry measurements

The NMR relaxometry measurements are designed and applied to determine the initial gas saturation in tight gas formations. After a full-diameter core obtained by sealed coring technique has been cut into two sections, one is weighed and then put into the NMR apparatus to measure its NMR T2 spectrum, while the other is used to obtain a core plug, and some clastic grains from its edge and center. Afterward these grains are weighed and their NMR T2 spectra are measured. Subsequently, the full-diameter section, grains, and the core plug are dried with their weights measured. Then they are saturated with water to measure their weights and NMR T2 spectra. It is found that the average initial gas saturations for three cores determined by weighting method are 41.87%, 38.59%, and 31.30%, respectively. From T2 spectra at the initial state and saturated-water state, the average initial gas saturations for these cores could be reproduced with a relative error of 0.29%, 1.37%, and 0.40%, respectively. If the cores are invaded by drilling fluid or not transported or preserved well, the initial gas saturations still could be determined by NMR T2 spectra of the cores at saturated-water state and T2cutoff with a relative error of 7.86%, 3.28%, and 8.16%, respectively.