Low Field Cryoporometry NMR for Mesopores Distribution in Shale

Abstract A large number of micro–nanopores constitute the main reservoir space of shale oil and gas reservoirs. To evaluate a reservoir pore throat system's structure, it is of great significance to quantitatively characterize the pore distribution. There are two commonly used methods for characterizing the pore structure: mercury intrusion and gas adsorption analysis. The mercury intrusion method is to determine the pore size distribution by measuring the amount of mercury in the pores under different external pressure. The detection range of this method is pores with diameters of 3 nm–400 μm, and the optimal detection range is pores with diameters of 100 nm–100 μm. The gas adsorption analysis method is to measure the adsorption amount of nitrogen and other gases at the liquid nitrogen temperature. the optimal detection range of this method is pores with diameters of 0.4–100 nm. Because of the complex structure of porous materials, the results of different experimental methods are inconsistent, and the pore structure information provided by a single method is limited. Since different porous solid materials have different chemical and physical properties and different pore diameter range, it is necessary to select experimental methods for the target material. This paper introduces a new method of measuring pore distribution, which is a Low Field Cryoporometry NMR method. Based on the principle of the nanometer-confined effect, this method can measure mesoporous distribution of different porous materials and shale. The logarithmic pore diameter distribution and the pore diameter distribution histogram indicate that the shale sample's pore volume is mainly comprised of pores with a diameter of 80–500 nm, which is consistent with the pore statistical results of the SEM analysis. The porous size distribution (PSD) was obtained by Croporometry NMR (NMRC) for shale samples, which was compared with that from the measurements by Brunauer-Emmett-Teller Method (BET). This method enlarges the scope of the classical gas adsorption method, then can be used to study pore diameters from 10 nanometers to micron size. Furthermore, it provides an important reference to the study of the pore structure of shale oil and gas reservoirs.