Pore characterization and clay bound water assessment in shale with a combination of NMR and low-pressure nitrogen gas adsorption

Pore size distribution (PSD) and the volume of clay bound water (CBW) are crucial parameters for gas shale reservoirs formation evaluation. Low-field nuclear magnetic resonance (LF-NMR) has been extensively applied to characterize petrophysical properties of reservoirs. However, limited understanding remains for unconventional shales. Defining NMR T₂ cutoff to differentiate CBW from free water is a challenge in shales since conventional approach, such as using centrifuge, is not feasible to completely remove free water in tight shales. Thermal treatment is therefore suggested for further extraction of movable pore water, however, the influence of temperature on nanoscale pore structure and clay mineralogical composition has been underestimated in previous studies and thus requires further investigation. This paper re-defines the critical dehydration temperature for accurate PSD interpretation in Permian Carynginia shale, Western Australia to determine T₂ cutoff for CBW. By using low-pressure N₂ gas adsorption (LP-N₂-GA) in parallel with LF-NMR, we identified a striking anomalous PSD consistency for critical temperature detection and verification. Our results shows that movable pore water can be maximally removed around 80 °C (75 °C), while the sensitive clay, CBW and microstructure are well-preserved for accurate petrophysical evaluation. Clay mineral conversion would occur when temperatures are higher than 80 °C, while temperatures lower than 75 °C would induce large misinterpretations for nanopore structure. Our recommended scheme could provide a potential adaptability for the formation evaluation of Permian Carynginia shale in the downhole practices.