Influencing factors of rock surface relaxivity and pore size conversion method for overpressured reservoirs in Huangliu Formation of Dongfang area, Yinggehai Basin
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摘要:
莺歌海盆地东方区中新统黄流组超压储层具有岩性细、物性差、储集空间复杂且非均质性强的特征，导致核磁共振T2谱与毛管压力曲线的对应性差，弛豫时间与孔隙半径转换困难。为了明确该储层核磁共振T2谱与孔径的分布关系，解决弛豫时间与孔隙半径转换困难的问题，采用分段建模的方法，结合常规物性、X衍射、核磁共振及高压压汞等实验测试数据，建立了弛豫时间与孔喉半径之间的转换关系，并对不同弛豫区间内表面弛豫速率的主控因素进行了深入分析，采用四元回归分析方法构建了表面弛豫速率的动态表征模型，模型计算效果较好，长短弛豫组分平均相对误差分别为9.965%、2.227%。研究表明：该储层核磁共振T2谱与压汞孔径分布曲线在5.7 ms(对应孔喉半径约为0.1 μm)附近呈现明显分段特征，短弛豫组分(T2<5.7 ms)的表面弛豫速率显著低于长弛豫组分(T2>5.7 ms)，证实不同尺度孔隙的表面弛豫机制存在明显差异；表面弛豫速率受储层物性、矿物组分及孔隙结构共同影响，且短弛豫组分表面弛豫速率主要受黏土含量影响，长弛豫组分更依赖孔隙结构复杂程度。基于此，针对不同弛豫区间优选敏感因素建立了表面弛豫速率动态表征模型，实现了表面弛豫速率的定量预测，同时通过分段转换方法显著提升了弛豫时间与孔喉半径的对应精度，为基于核磁共振T2谱的孔隙半径准确转换提供了理论依据和方法支撑，对准确刻画超压储层孔隙结构、提升储层评价精度具有重要意义。
Abstract:The overpressured reservoirs in the Huangliu Formation of the Dongfang area, Yinggehai Basin, are characterized by fine lithology, poor physical properties, complex storage space, and strong heterogeneity, which lead to poor correspondence between nuclear magnetic resonance (NMR) T2 spectrum and capillary pressure curves, making the conversion between relaxation time and pore-throat radius challenging. To clarify the distribution relationship between NMR T2 spectrum and pore-throat radius in such reservoirs and solve the difficulty in converting relaxation time to pore-throat radius, a segmented modeling method was adopted. Combining conventional physical properties, X-ray diffraction, NMR, and high-pressure mercury injection experiments, the conversion relationship between relaxation time and pore-throat radius was developed. The main controlling factors of surface relaxivity in different relaxation intervals were thoroughly analyzed. A four-factor regression analysis method was used to build a dynamic characterization model for surface relaxivity. The model achieved a relatively good calculation performance, with average relative errors of long and short relaxation components being 9.965% and 2.227%, respectively. The results indicated that the NMR T2 spectrum and mercury injection-based pore-throat radius distribution curves of the reservoirs showed obvious segmented characteristics near 5.7 ms (corresponding to a pore-throat radius of about 0.1 μm). The surface relaxivity of short relaxation component (T2 < 5.7 ms) was significantly lower than that of long relaxation component (T2 > 5.7 ms), confirming notable differences in surface relaxation mechanisms across different pores sizes. Surface relaxivity was jointly influenced by reservoir physical properties, mineral composition, and pore structure. The surface relaxivity of short relaxation component was mainly affected by clay content, while the long relaxation component was mainly controlled by pore structure complexity. Based on this, a dynamic surface relaxivity characterization model was constructed through optimizing sensitive factors for different relaxation intervals, achieving quantitative prediction of surface relaxivity. Meanwhile, the correspondence accuracy between relaxation time and pore-throat radius was significantly improved through the segmented conversion method. This approach provides a theoretical basis and methodological support for the accurate conversion of pore-throat radius based on NMR T2 spectrum. The findings are significant for precisely characterizing the pore structure of overpressured reservoirs and improving the accuracy of reservoir evaluation.
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