A novel procedure to locally assess wettability reversal in carbonate rocks by magnetic resonance imaging

Wettability greatly influences fluid transport properties of rocks and consequently has been widely studied over the years in the oil industry. The native state core sample provides the most reliable wettability behaviour; however, it is difficult to preserve their original reservoir conditions. In order to have samples with controlled wettability, it is a common laboratory practice to change their wettability. In this study, wettability of a carbonate rock is reversed from water-wet to oil-wet by saturating the rock samples with Soltrol®170 mixed with oleic acid. The wettability of the samples has been measured by Amott method. In order to have a significant Amott wettability index and to use it to compare different treated samples, it is paramount to evaluate the spatial homogeneity of the wettability change. Nuclear Magnetic Resonance (NMR) of 1H nuclei is a non-invasive and non-destructive technique largely used both in laboratory and in well logging measurements for the analysis of reservoir rock properties. In this study Magnetic Resonance Imaging (MRI), in particular longitudinal relaxation time (T1) maps of 1H nuclei, is proposed to track the process and to assess the quality of the wettability reversal. T1 maps clearly show local changes of T1 over the time of acid diffusion inside the core. Relaxation times are shorter in the regions of the sample not yet reached by oleic acid. Maps obtained at different times show that the homogeneity of the acid adsorption is obtained slowly, from outer to inner regions of the cores, with a sharp front edge. This analysis provides the determination of the aging time required for the completion of the acid adsorption. The method might be applied, after extensive evaluation, to correlate the wettability index with local values of T1 in regions of any internal section of the sample.