An NMR-based clay content evaluation method for tight oil reservoirs

An effective assessment of tight oil reservoirs requires accurate calculation of their clay content. Nuclear magnetic resonance (NMR) logs play an important role in the evaluation process because of the abnormally high values of gamma-ray (GR) logs and the poor resolution of spontaneous potential (SP) logs in source reservoirs. In this study, an NMR method was proposed to determine the clay content in tight oil reservoirs. We analysed the NMR characteristics of the sandstone samples, which contained different types of main clay minerals, through various experiments. The experimental results demonstrate that the transverse relaxation time (T-2) cutoff value of clay-bound water varies with the type of clay. This indicates that different T-2 cutoff values will be determined according to NMR logging while calculating the clay-bound water saturation in a specific area. Based on the cation-exchange capacity equation and the clay water saturation, a certain relation could be observed between the clay content and the remaining three parameters, including the clay water saturation, density logs and neutron logs. Further, we used the least-squares method for calculating the clay content based on this relation. The calculation result obtained from the field examples agreed well with the experimental measurements, thereby demonstrating the accuracy and usefulness of the proposed method.