NMR-Based Wettability Index for Unconventional Rocks

Standardized and benchmarked wettability characterization and quantification workflows are lacking in unconventional reservoir development. Quantification of in-situ wettability and changes due to wettability alteration efforts can assist with completions decisions, and economic oil production. This manuscript summarizes the establishment and validation of an NMR wettability index (NWI) for unconventional rocks. The method builds upon Looyestijn’s NMR wettability analysis methods for conventional rocks and has been tested on core plug samples from a variety of major producing unconventional reservoirs. It will be of interest to readers to note the marked range of wettability values quantified among the aforementioned tight formations. Our NWI model is well suited for data sets featuring complex oil and water T2 spectra with multiple peaks, common features of unconventional rock spectra. The results were subjected to comprehensive experimental and analytical validation, including complementary 3D NMR imaging and replication of the experiments with D2O. The validation procedure and advantages of the approach over other NMR wettability models are discussed in this review. Finally, best practices are detailed so that the SCAL methodology can be deployed on a larger scale.