NMR characterizing mixed wettability under intermediate-wet condition

Applying the concept of effective relaxivity to characterize wettability is based on the configuration of fluid distributions in porous media. However, in mixed-wet porous media with intermediate-wet patches (homogeneous wetting region), effective surface relaxivity cannot fully characterize wettability because fluid distributions are not directly corresponding to wetting patch distributions. Patches with different wettability interact with the same fluid differently, which leads to different surface relaxivity. The distribution of this kind surface relaxivity from porous media saturated with single fluid matches mixed wettability distribution. Here, we apply decay due to diffusion in internal field plus Carr-Purcell-Meiboom-Gill (DDIF-CPMG) method to obtain T₂ and the pore size distribution correlation. The variation of surface relaxivity obtained from the correlation map is used to characterize wettability distributions of mixed-wet porous media. In this paper, we also redefine a parameter based on surface relaxivity distribution to evaluate mixed wettability under intermediate-wet condition. The experiment results with limestones show that, after wettability alteration of the sample, the distribution of the surface relaxivity is changed and closely correlated with pore size distribution, which demonstrates the features of mixed wettability under intermediate-wet conditions.