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-di€usion of water molecules in their con®ned geometry is used to probe the pore space. The short-time behaviour of the self-di€usion coecient (anomalous di€usion) in the porous matrix allows the determination of the surface-to-pore volume ratio S/Vp. At long di€usion times, molecules scout the tortu- osity 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 in¯uenced by the adherence of `scout-moleculesʹ 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. # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.