NMR WETTABILITY INDEX MEASUREMENTS AND METHODS COMPARED ON A VARIETY OF UNCONVENTIONAL SAMPLES

Matrix wettability is a key driver in relative permeability and, hence, a critical factor controlling imbibition and drainage at UR fracture-matrix interfaces as well as enhanced oil recovery (EOR). In this study, we (1) adapt and apply the NMR-based wettability index (NWI) methodology of Looyestijn et al. (2006) to a variety of unconventional twin samples undergoing, respectively, spontaneous imbibition with oil-displacing-water and water-displacing-oil and (2) compare the robustness of this method among a variety of samples pairs and also to other NMR-based wettability methods. The samples analyzed cover a range of rock types, major formations, maturity and content of organic material. All displayed unique time-lapse wettability profiles and steady state NWI values. This work advances our previous works (Dick et al., 2019; Kelly et al., 2020) on this subject, where the viability of the methodology was established on end-member pilot samples, towards applicability as a UR SCAL method. The NWI methodology predicts T2 spectra using linear combinations (mixing) of “end-point” T2 spectra. The mixing ratios yielding the closest match to the measured spectra are then used to compute a wettability index. These mixing ratios were validated against (1) mass-balance calculations, (2) repeat experiments with heavy water (D2O) instead of H2O and (3) measured T1-T2 maps, enhancing confidence in the robustness of the method. Our comparisons show that alternative approaches representing the T2 spectra through a single mean T2 value or T2 peak-fit, fall short, especially in tight rocks where fast relaxation rate components tend to skew harmonic mean T2 values and also in samples where oil and water peaks are not clearly resolved. Full spectrum-based methods, akin to Looyestijn’s, appear more robust and stable over a much wider range of reservoir conditions. Repeated NMR acquisition throughout our long-term imbibition experiments shows that time-lapse NWI methodology probes the effects of rock properties, saturation changes, and injected fluid chemistry (enhanced oil recovery strategies) on wettability alteration. Additionally, this NWI study quantifies the wide variation in wettability among unconventional samples.