PORE STRUCTURE CHARACTERIZATION OF SHALES USING NMR CRYOPOROMETRY

Characterizing nano-pore structure is one of the most important factors in understanding gas storage and transport in shale reservoirs. Many techniques such as Scanning Electron Microscopy (SEM), Mercury Injection Capillary Pressure (MICP) and Nitrogen Adsorption Method (NAM), X-Ray Computed Tomography (XCT) have been used to measure pore size distribution (PSD) of shales, but each of them has a limited measuring range and could not cover the entire nanometer-range. This work reported nano-pore characterization of shale rock using Nuclear Magnetic Resonance Cryoporometry (NMRC), a novel and emerging technique which can probe PSD from nano- to micro- scales. NMRC 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 NMRC. Calibration experiments with the two fluids demonstrate that the PSDs for shale characterized by MICP, NAM and NMR-C are comparable. Due to its larger molecular size and corresponding large K_GT, NMRC-OMCTS is able to characterize pores to 2 microns but misses pores smaller than 5 nm. Meanwhile, NMRC-OMCTS images a broader PSD than that by NMRC-Water due to the propensity of OMCTS to imbibe into the organic matter, relative to water. NMRC-OMCTS shows the superiority and potential due to the higher signal/noise (S/N) ratio and wider measurement range up to 2 microns. We also combine the benefits of FIB-SEM and NMRC to characterize the nano-pore structure of lower Paleozoic shales from Chongqing, southwestern China. Mineral composition was qualified through SEM-EDS. PSD was measured via pore network model (PNM) and compared to the PSD recovered from NMRC. Results indicate that the small organic matter pores have favorable potential for storing adsorbed gas because of their apparent interconnectivity and higher porosity; conversely, InterP and IntraP pores are mostly isolated and with low porosity. Comparison of the PSD from PNM and NMRC also shows that NMRC has a higher sensitivity and accuracy in detecting nano-pores. Combining FIB-SEM and NMRC is a promising technique in detecting and characterizing the nano-porosity of shales.