A Study Of Shale Pore Size Classification By Using Low Field Nuclear Magnetic Resonance Fluid Typing Method

Longmaxi Formation in Sichuan Basin belongs to typical unconventional reservoir with its source and reservoir in the same formation,whose micro-pore structure is extremely complex due to its special mineral composition especially the organic matter.Moreover,various pore fluids in the pore network of shale present a challenge for reservoir evaluation.Accurate understanding in shale pore structure is the basics to study shale reservoir characteristics such as reservoir quality,fluid migration,interaction between reservoir fluids and rocks,and so on.To make certain about the pore size distribution and pore fluids of shale,a combination of low field nuclear magnetic resonance?LF-NMR?,field emission scanning electron microscopy?FE-SEM?and low temperature gas adsorption method is used and a pore classification and pore fluid identification method by dual cutoffs is generated.The petrophysical parameters of seven shale outcrop samples collected from field sections show that the minerals are quartz,clay,feldspar,calcite and some magnetic mineral including ankerite and pyrite.The mean TOC value is 2.47%,which is low compared with core samples drilled from shale reservoir.The average value of helium porosity is 3.34%,and the mean pulse-decay permeability result is0.001684md.Low temperature gas adsorption?N₂ and CO₂?results indicated that the pore structure in shale is a mixture of open slit pore and double-ended cylinder holes and the micropores are mainly from the organic pores.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(T_2C1)and unrecoverable fluid cutoff(T_2C2)were derived from NMR centrifugal and heat-treated experiments to distinguish the unrecoverable fluid(T₂<T_2C2),capillary bound fluid(T_2C2<T₂<T_2C1)and movable fluid(T₂>T_2C1).Our results show that for the investigated shales,the T_2C2ranges from 0.09 ms to 0.36 ms,and T_2C1 has a wide range from 0.45 ms to 2.98 ms.The surface relaxivities??₂?range from 0.00426?m/ms to 0.02822?m/ms,and the shales having high silicate mineral contents commonly have low surface relaxivities.A conceptional model of SZS-6 sample 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.