Probing the pore space of geothermal reservoir sandstones by Nuclear Magnetic Resonance

Pulsed-Field-Gradient–Nuclear Magnetic Resonance (PFG–NMR) is an interesting method to determine microscopic but volumetrically averaged properties of pore space. In the present paper a number of sandstone samples, taken from drill cores of geothermal wells in North Germany, have been investigated. The time-dependent self-diffusion of water molecules in their confined geometry is used to probe the pore space. The short-time behaviour of the self-diffusion coefficient (anomalous diffusion) in the porous matrix allows the determination of the surface-to-pore volume ratio S/Vp. At long diffusion times, molecules scout the tortuosity of the interconnected pore space of the sandstones. The NMR results were compared with data from petrographic image analysis (PIA), adsorption experiments and electric conductivity measurements. The PFG–NMR measurements give surface-to-pore volume ratios S/Vp that are comparable to those estimated with the petrographic image analysis. The tortuosities match in most cases data from conductivity measurements, so the PFG–NMR is regarded as an appropriate tool to determine this quantity. The results are not influenced by the adherence of ‘scout-molecules’ to the pore walls. The surface-to-pore volume ratios and tortuosities were used to calculate permeabilities of the systems of interest, which were in good agreement with measured core-plug permeabilities. Results of additional NMR relaxation experiments are used to obtain adsorption isotherms for cations at active surface sites.