Abstract
The heterogeneity of carbonate reservoirs strongly influences their petrophysical properties and oil recovery procedures. Among many reservoir characterization techniques, NMR stands as a unique tool for oil reservoir exploration. The basic advantage of the NMR method is that oil, gas, and water possess different NMR properties. Therefore, they can be distinguished quantitatively. In this work we have combined NMR Laboratory methods and pore-scale numerical simulations, with MRI techniques in order to elucidate the oil/water-rock interactions of mixed-wet carbonates at molecular scale and on the basis of this information to acquire a quantitative measure of wettability. T2, D and D-T2 NMR measurements were performed on limestone core. Water and two types of oil, with different viscosities, were used in the experiments. Pore-scale numerical simulations of magnetization decay on samples taken from both rock types in the presence of different spatial distribution of fluid saturations and wettability assumptions were performed using a random walk algorithm. A sensitivity analysis were performed on the impact of wettability on T2 distribution in the two rock types. The T2 and D distributions were extracted from measured magnetization decay by implementing a new non-negative Tikhonov regularization algorithm, while for the D-T2 experiments a 2-D version of this algorithm was developed and implemented. For the MRI experiments CPMG trains were implemented for calculated T2 Images, and Diffusion Tensor Imaging methods, in order to acquire spatially resolved T2 and D values. The results of numerical simulations were compared against laboratory experiments.
