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Temperature Influence on NMR Relaxation Characteristics of Organic and Inorganic Pores in Shale

Nuclear magnetic resonance (NMR) technology is essential for evaluating fluid mobility and pore structure. However, the relaxation time of shale oil is significantly influenced by its fluid composition, pore types (organic and inorganic), and temperature. In most rock pore media, the surface relaxation and bulk relaxation of the fluid exert a significant or even dominant influence on the NMR response, with the relaxation mechanisms differing between various pore types. Consequently, investigating how temperature affects surface relaxation and bulk relaxation in different pore environments is essential for determining the temperature-dependent relaxation time of the fluid. This study employs variable temperature NMR experiments on shale samples to investigate the impact of temperature fluctuations from 30℃ to 90℃ on the NMR response characteristics of both organic and inorganic pores. This analysis established a model for characterizing the relationship between shale oil NMR relaxation time and temperature. The experimental results demonstrate that when bulk and surface relaxation dominate the NMR response, the T2 relaxation time and T2, LM of saturated crude oil shale increase with rising temperature. This increase is attributed to enhanced crude oil mobility; however, the rate of increase varies among different pore types, reflecting the differences between organic and inorganic pores in shale samples. In general, the variations in pore structure and crude oil composition lead to distinct thermodynamic processes in different environments. These processes can be broadly classified into two categories: (1) In samples with a high bitumen content and predominantly organic pores, the P1 signal peak decreases with rising temperature. In contrast, the P2 signal peak shifts rightward, broadens, and increases relaxation time, with T2, LM rising from 2.87ms to 12.71ms. (2) In samples with low bitumen content and primarily inorganic pores, the signal peaks shift rightward more rapidly with increasing temperature, broadening and showing an increase in relaxation time, with T2, LM increasing from 2.75ms to 20.43ms. According to the BPP model, the relaxation rate of the fluid is mainly related to the correlation time τc of molecular motion, which is strongly temperature- dependent. As the temperature increases, the viscosity η of the crude oil decreases, resulting in longer T2 relaxation times. The short relaxation component (P1 signal peak) in organic shale dominates and gradually decreases with increasing temperature due to the predominance of bulk relaxation in organic pores. In these pores, paramagnetic ions in the bitumen induce a restricted diffusion mechanism akin to surface relaxation, resulting in a rightward shift of the spectral peak. In inorganic shale, characterized by more extended relaxation components (P2 signal peak) and a higher proportion of inorganic pore structures, the reduction in crude oil viscosity η leads to increased T2 relaxation times. Based on these observations, a temperature correction model for the T2 spectrum was developed for downhole NMR logging temperature correction.
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