NMR Cryoporometry: An Alternate Non-Destructive Technique for the Measurement of Pore Size Distribution in Shales (Presented by W. Sassi)

Abstract

The characterization of shales is challenging due to their very small pore sizes. In many previous works, we have shown that Nuclear Magnetic Resonance (NMR) relaxation techniques are well suited for detecting and quantifying water in nanopores. However, the usual T2 distribution of relaxation times does not necessarily represent the true distribution of pore sizes in nanoporous samples due to diffusive exchanges between pores, yielding apparent narrow pore size distribution. The NMR cryoporometry experiment relies on the shift of the melting temperature of the saturating liquid, which is itself a function of pore size according to Gibbs-Thomson theory. In practice, a sample (a cylinder of diameter 4mm and length 20 mm) saturated with water is rapidly frozen at about -30°C and then heated slowly while the amount of water melted at a given temperature is measured by NMR. Typically the range that can be explored lies between 2 nm and 1 micron, well suited for the study of shales. Importantly outside this range, the pore volume can be determined but a pore size cannot be associated with this volume. We used this technique on different shales from different origins and compared the results with other techniques such as NMR T2 distribution, high pressure mercury injection and nitrogen adsorption. The measured distributions can differ significantly and we discuss the various physical reasons behind. The NMR cryoporometry technique open new horizons for characterizing shales in their natural hydrated state.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016