Applying Fast-Field Cycling Nuclear Magnetic Relaxation to Petroleum Tight Sandstone Rocks

The fast-field cycling nuclear magnetic relaxation technique (NMRD) offers opportunities on multiple scales of both time and distance for characterizing the molecular dynamics and transport properties of complex liquids in bulk or in confined environments. Therefore, NMRD can characterize fundamental properties such as surface correlation times, diffusion coefficients, and dynamical surface affinity (NMR wettability) for confined liquids in porous materials. This paper presents the applications of NMRD to petroleum tight sandstone rocks for giving new information on pore-size distribution (PSD) and the surface wettability of brine confined within pores. The bimodal PSD in tight sandstones is extended to much smaller pores compared with the quasi monomodal PSD of conventional sandstones that is known to be centered on very large pores of several tens of μm. The interpretation of the NMRD profiles of tight sandstones has shown a water surface diffusion about one-third of the bulk diffusion. We evidenced for the first time a pore-size dependence of the water wettability in tight sandstones that qualifies the FFC-NMRD technique for probing (in a single day experiment) the molecular dynamics and wettability of these important unconventional sandstone reservoir rocks. On the basis of the promising results obtained, we believe that NMRD experiments will be a critical tool for investigating in situ the molecular dynamics and wettability of petroleum unconventional reservoirs.