Characterising Oil and Water in Porous Media using Decay due to Diffusion in the Internal Field

In petroleum research it is important to characterise the different liquids which is present in reservoir rock samples, and how the confinement influence their dynamic behavior. When the material is saturated with multiple liquids (water and oil), it is challenging to obtain quantitative information from NMR measurements. In the method Decay due to Diffusion in the Internal Field (DDIF), the diffusion behaviour of water molecules in the internal magnetic field makes it possible to determine a distribution of pore sizes in a sample [1,2]. The DDIF experiment can also be extended to a DDIF-Carr-Purcell-Meiboom-Gill (DDIF-CPMG) experiment to measure correlations between the pore size and the transverse relaxation time, T₂ [3]. In this study we have for the first time applied the DDIF experiment and the DDIF-CPMG experiment to porous materials saturated with both water and oil. Because of the large difference in diffusion rates between water and oil molecules, the DDIF experiment will act as a filter for the signal from oil, and we are left with the DDIF-signal from water only. This has been verified in model systems consisting of glass beads immersed in separate layers of water and oil. The method has then been applied to a sandstone sample saturated with water and oil.

We have shown that the DDIF and DDIF-CPMG experiments enable us to determine the confining geometry of the water phase, and also how this geometry is correlated to the transverse relaxation time. The results demonstrate that when a sandstone rock core is saturated with water and oil, water is found in smaller pores and is more confined to the walls in the larger pores. Furthermore, the DDIF-CPMG experiments show that with the exception of the smallest pores there is no clear correlation between pore size and the relaxation time of water in such a sample. This has to be taken into consideration when interpreting results from measurements of relaxation times in samples saturated with water and oil.

References.
[1] Y.-Q. Song, Determining pore sizes using an internal magnetic field, J. Mag. Reson. 143 (2) (2000) 397-401.
[2] Y.-Q. Song, Using internal magnetic fields to obtain pore size distributions of porous media, Concepts Mag. Reson. 18A (2) (2003) 97-110.
[3] H. Liu, M. Nogueira d'Eurydice, S. Obruchkov, P. Galvosas, Determining pore length scales and pore surface relaxivity of rock cores by internal magnetic fields modulation at 2MHz NMR, J. Mag. Reson. 246 (2014) 110-118.