Shale pore size classification: An NMR fluid typing method

Shale reservoir is characterized by complex pore networks, within which there are various pore fluids including unrecoverable fluid, capillary bound fluid and movable fluid. Although considerable literature has investigated the pore structure of gas shale using different laboratory testing techniques, few papers have provided a quantified model to distinguish different types of fluids (corresponding to different pore types) in shale. In this study, seven shale core plugs from the Sichuan Basin were measured in a series of NMR experiments under full brine-saturated, centrifugal and heat-treated conditions to analyze the pore structure information and pore fluid transport during the processes of centrifuging and heating. For a typical T2 spectrum of 100% brine-saturated shale, the movable fluid T2 cutoff (T2C1) and unrecoverable fluid cutoff (T2C2) were derived from NMR centrifugal and heat-treated experiments to distinguish the unrecoverable fluid (T2 < T2C2), capillary bound fluid (T2C2  T2C1). Our results show that for the investigated shales, the T2C2 ranges from 0.09 ms to 0.36 ms, and T2C1 has a wide range from 0.45 ms to 2.98 ms. The surface relaxivities range from 0.00426 μm/ms to 0.02822 μm/ms, and the shales having high silicate mineral contents commonly have low surface relaxivities. A conceptional model based on the dual T2 cutoff method was constructed to illustrate the full-scale pore size distribution: unrecoverable fluid pores, capillary bound fluid pores and movable fluid pores. This study provides a new method of pore fluid typing and full-scale pore size distribution classification for shales.