Shale Pore Characterization Using NMR Cryoporometry with Octamethylcyclotetrasiloxane as the Probe Liquid

Fluid flow and chemical transport within shale are determined by the pore size distribution and its connectivity. Because of both low porosity and small (nanometer) pore size, common characterization methods, such as mercury injection capillary pressure (MICP) and the nitrogen adsorption method (NAM), have limited resolution and applicability. Nuclear magnetic resonance cryoporometry (NMR-C) is a novel characterization method that exploits the Gibbs-Thomson effect and provides a complementary method of characterizing aggregate pore structure at fine resolution. We use water and octamethylcyclotetrasiloxane (OMCTS) as probe liquids for NMR-C on controlled porosity samples of SBA, CPG, and shale. The analysis accommodates the influence of melting temperature, K_GT, and surface layer thickness, ε, on the pore size distribution (PSD). Calibration experiments permeated with the two fluids demonstrate that OMCTS has a larger K_GT and that the PSD for different cryoporometric materials is not subject to different surface curvatures of pores. Furthermore, the PSDs for shale are characterized by MICP, NAM, and NMR-C, which give comparable results. Shale samples have heterogeneous pore distributions with peak pore diameters at ∼3 nm and mesopore diameters of 2-50 nm comprising the main storage volume. Because of its larger molecular size and correspondingly large K_GT, NMR-C-OMCTS is able to characterize pores to 2 μm but misses pores smaller than 5 nm. Meanwhile, NMR-C using OMCTS images a broader PSD than that by NMR-C-Water due to the propensity of OMCTS to imbibe into the organic matter relative to that of water. NMR-C-OMCTS shows the superiority and potential due to the higher signal/noise (S/N) ratio and wider measurement range up to 2 μm. With regard to shales, one insight is that 115 K nm is an appropriate K_GT value for measurements with the surface layer thickness of 2 nm. Moreover, the applications of NMR-C-OMCTS will come down to other rocks through further research.