Shuailong Feng, Mingyang Li, Junjian Zhang, Guangwei Xu, Veerle Vandeginste, Pengfei Zhang, Wei Ju
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Abstract
Pore-fracture structure distribution heterogeneity (PFSH) affects dynamic variation of porosity-permeability of tight sandstone reservoirs, then restricting gas production performance. A fractal model by low-field nuclear magnetic resonance technology (LF-NMR) has been used in the quantitative characterization of PFSH. Among some literature, PFSH was studied by using a saturated T 2 spectrum. However, there are few studies on fractal characteristics of T 2 spectral morphology in a centrifugal state and its influence on porosity-permeability parameters. In this paper, 30 tight sandstone samples were collected from Taiyuan Formation in Qinshui Basin. Then LF-NMR technology was used to analyze PFSH, and sample types were divided by using T 2 spectra difference under saturated and centrifugal conditions. Meanwhile, single (model 1 and 2) and multi-fractal model are adopted to calculate fractal parameters of saturated and centrifugal T 2 spectra, and then a difference in fractal parameters under different water conditions was compared. Correlation between different fractal parameters, pore structure and T 2 cut-off value are studied, and a mathematical prediction model for T 2 cut-off value by using fractal and pore structure parameters are established. The results are as follows. (1) All the samples are divided into four types A/B/C/D. For example, the type A sample is characterized by a single peak of T 2 spectrum and T 2 value is less than 10 ms, which indicates that this type belongs a smaller-pore developed. Type B sample is characterized by a single peak of T 2 spectrum and T 2 value is10–100 ms, which indicates that this type belongs to mesopore developed. (2) In saturated state (D S ), PFSH of type A sample by using model 1 and 2 is stronger than that of type B, followed by type C and D. Then the multifractal model shows that PFSH of type B sample is stronger than that of other sample types. Correlation between fractal dimension calculated by using single fractal and pore structure parameters is stronger than that of multifractal dimension. (3) T 2 spectrum in centrifugal state has fractal characteristics (D i ), and there are certain correlation D i with D s . Therefore, a mathematical prediction model for T 2 cut-off value by using fractal and pore structure parameters is established. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.
利用离心分离机 T2 频谱曲线分析致密砂岩的单分形和多分形维度变化
孔隙-裂缝结构分布异质性(PFSH)影响致密砂岩储层孔隙度-渗透率的动态变化,进而制约天然气生产性能。低场核磁共振技术(LF-NMR)分形模型被用于 PFSH 的定量表征。一些文献利用饱和 T2 光谱对 PFSH 进行了研究。然而,关于离心状态下 T2 光谱形态的分形特征及其对孔隙度-渗透性参数影响的研究却很少。本文从沁水盆地太原地层中采集了 30 个致密砂岩样品。采用 LF-NMR 技术对 PFSH 进行分析,并利用饱和与离心条件下的 T2 光谱差异对样品类型进行了划分。同时,采用单分形(模型 1 和 2)和多分形模型计算饱和和离心 T2 光谱的分形参数,比较不同水条件下分形参数的差异。研究了不同分形参数、孔隙结构与 T2 截止值之间的相关性,建立了利用分形参数和孔隙结构参数预测 T2 截止值的数学模型。结果如下(1) 所有样品分为 A/B/C/D 四种类型。例如,A 型样品的特征是 T2 光谱的单峰且 T2 值小于 10 ms,这表明该类型属于较小孔隙发育。类型 B 样品的特征是 T2 光谱的单峰且 T2 值为 10-100 ms,这表明该类型属于中孔发育型。(2)在饱和状态(DS)下,利用模型 1 和 2,A 型样品的 PFSH 强于 B 型样品,其次是 C 型和 D 型样品。利用单分形计算的分形维数与孔隙结构参数之间的相关性强于多分形维数。(3)离心状态下的 T2 光谱具有分形特征(Di),且 Di 与 Ds 存在一定的相关性。因此,利用分形和孔结构参数建立了 T2 临界值的数学预测模型。© 2023 化学工业协会和 John Wiley & Sons, Ltd. 保留所有权利。
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