田志
中国石油辽河油田分公司勘探开发研究院, 辽宁 盘锦 124010

基金项目: 国家科技重大专项"大型油气田及煤层气开发"子课题"渤海湾盆地北部油气富集规律与油气增储领域研究"（2016ZX05006-005）和中国石油天然气股份有限公司重大科技专项"辽河油田原油千万吨持续稳产关键技术研究"（2017E-16）联合资助


详细信息

作者简介: 田志, 男, 1992年生, 工程师, 主要从事岩石物理、油气地球物理勘探等方面研究. E-mail: tianzhicup@foxmail.com

中图分类号: P631

收稿日期:2020-03-05
修回日期:2020-12-31
上线日期:2021-03-10

NMR diffusional coupling of multiple-scale porous rock and its detection

TIAN Zhi
Research Institute of Exploration and Development, Liaohe Oilfield Company, Petrochina, Panjin Liaoning 124010, China



MSC: P631

--> Received Date: 05 March 2020
Revised Date: 31 December 2020
Available Online: 10 March 2021


摘要
摘要:油藏岩石的孔隙连通性是反映流体渗流难易程度的重要参数，对渗透率、有效孔隙度等岩石物理参数的评价具有重要作用.连通的孔隙中，核磁共振（NMR）弛豫的交换会产生扩散耦合现象，可作为孔隙连通性的表征和探测方法.本文提出利用横向弛豫T₂-T₂脉冲序列测量岩石的扩散耦合现象.运用随机游走方法模拟多孔岩石的核磁共振响应特征，分析扩散耦合的影响因素，推导表征扩散耦合强度的弛豫交换速率计算公式.结果表明：孔隙间的扩散耦合强度与T₂-T₂脉冲序列的混合时间呈正相关性，基于双孔弛豫交换模型推导的弛豫交换速率计算公式能够准确表征双尺度孔隙系统的扩散耦合强度.在孔隙尺寸不满足快扩散条件时，会出现与扩散耦合无关的非对角峰信号.针对含多类型孔隙的碳酸盐岩模型，随混合时间的增加，扩散耦合强度变大，一维T₂谱的形态畸变程度加重，在T₂-T₂二维谱中，代表微裂缝、粒间小孔、溶蚀大孔的信号能量变化趋势不同，反映不同类型孔隙间的连通性存在差异.本文的分析与讨论丰富了核磁共振弛豫在岩石物理性质评价中的应用方向，对利用核磁共振评价复杂孔隙岩石的孔隙结构和连通性提供了新思路和新方法.
关键词: 核磁共振/
扩散耦合/
碳酸盐岩/
孔隙连通性/
数字岩心

Abstract:The pore connectivity of reservoir rock is an important parameter to reflect the difficulty of fluid seepage, which plays an important role in evaluation of petrophysics parameters, such as permeability and effective porosity. In connected pores, the exchange of Nuclear Magnetic Resonance(NMR)relaxation can produce diffusional coupling, which can be used to characterize pore connectivity. In this paper, the T₂-T₂ pulse sequence is used to measure such diffusional coupling of porous rocks. The Monte Carlo random walk algorithm is adopted to simulate the NMR response characteristics of porous rocks under T₂-T₂ pulse sequence. Then, the influence of key parameters of T₂-T₂ pulse sequence on the diffusional coupling measurement is analyzed based on a dual-porosity rock model. And a relaxation exchange rate formula is derived to characterize the diffusional coupling strength based on an ideal two-site relaxation exchange model. The results show that the diffusional coupling strength is positively correlated with the mixing time. And the relaxation exchange rate formula can accurately describe the diffusional coupling strength of the dual-porosity system. When the pore size does not satisfy the fast diffusion limit, the non-diagonal peak signals independent of diffusional coupling will appear. Besides, the diffusional coupling among different pores in the carbonate rock can affect the distribution of T₂ spectra and reflect the connectivity of the pores. The analysis and discussion in this paper enrich the application direction of NMR transverse relaxation time T₂ in the evaluation of rock physical properties, and provide a new idea and method for the evaluation of pore structure and connectivity of the multiple-porosity system by NMR.
Key words:Nuclear Magnetic Resonance(NMR)/
Diffusional coupling/
Carbonate rock/
Pore connectivity/
Digital rock



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