Study Of Magnetic Resonance Relaxation Mechanism And Measurement Method On Shale

Until recently,the unconventional shale resources exploration have become a hot topic in global.However,the current understanding of shale is still not enough.The difficulty of evaluating shale is because low permeability and porosity.The permeability of shale is very low,the application of traditional core analysis method is difficult for shale.This makes nuclear magnetic resonance（NMR）a better way to measure shale.There are different microstructures,mineral components,geochemical parameters in shale,which are different from conventional reservoirs.These features determine their NMR response characteristics.NMR has been used extensively to characterize porous materials.In particular,NMR has been used by the petroleum industry since shortly after its discovery to assess fluid-saturated porosity and pore size distributions,and evaluate the pore fluid distribution and saturation.NMR result of Shale reservoirs is affected by factors such as nanoscale pore,complex mineral composition,special pore structure,higher organic matter content,ultra-low permeability and internal gradient and restricted diffusion.The interpretation of the NMR relaxation results of shales is more complex than conventional reservoirs.The surface relaxation in conventional reservoirs is mainly caused by the interaction of fluid molecules with paramagnetic impurities on the surface of the pores.However,in shale,the combination of nano-pores and the presence of hydrogen-rich organic matter can lead to more interactions.We discusses the different relaxation mechanisms that may exist in shale.The surface relaxation in organic matter may be caused by the coupling of the same nucleus,and thus will have a significant temperature dependence.Due to the small aperture in the shale,the effect of the internal gradient on the signal is negligible in almost all cases.The diffusion coupling between shale pores will affect the pore size distribution obtained by NMR.There are a lot of organic matters such as kerogen in unconventional shale resources.We proposed a new multi-magic echo sequence for the T₁ and T₁-T₂ measurement.The magic echo and standard T₁ and T₂ measurement technologies are combined to provide a more complete evaluation on hydrogen-containing shale sample components.Additional signal is collected on shale samples by the magic echo measurement compared to previous measurement.The additional signal is contributed by the homonuclear dipolar coupling in organic matters.Partial least-squares regression（PLSR）method is used to analyze the relationships between organic geochemical properties and T₁-T₂ correlation data.The results presented here show that the T₁-T₂ peak distributions provided by magic-echo agree well with the organic geochemical properties on shales.This method makes the NMR measurment on shales a common technique to detect organic matters.In the micro-cracks in the shale of crude oil is generally non-wetting phase,only the bulk relaxation,reflecting the inherent relaxation of crude oil properties.The correlation between T₁ and T₂ relaxation behavior can be used to detect the rotational correlation time distribution and to probe molecular dynamics include the slow motion and aggregation of molecules into supermolecular structures.We illustrate this with relaxation-relaxation correlation experiments and rotational correlation time distributions on a series of hydrocarbon mixtures.