Effects of magnetic field intensity and gradient on measurement results of core nuclear magnetic resonance T2 spectrum
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摘要: 核磁共振测井仪器一般在共振频率为2 MHz的均匀场或共振频率小于1 MHz的梯度场中测量储层流体的核磁信号，而实验室核磁共振岩心分析仪的共振频率除了常用的2 MHz外，对于页岩等致密储层常会用到12 MHz或21 MHz的设备进行实验测量。为确定磁场强度和磁场梯度对核磁共振测量结果的影响程度，系统研究了饱和水状态下的砂砾岩、页岩核磁共振岩心实验对磁场强度和磁场梯度的敏感性，分析了不同岩样的T2谱形态、位置、核磁孔隙度、T2几何均值与磁场强度、磁场梯度的关系。实验结果显示，均匀场下，砂砾岩样品对磁场强度的变化非常敏感，而页岩样品对磁场强度变化的敏感性相对较弱；外部梯度场的存在会使砂砾岩和页岩的短弛豫信息缺失，导致核磁信号无法被完全测量。研究表明，利用核磁共振岩心实验刻度核磁共振测井解释参数时，若实验室核磁共振岩心分析仪与核磁共振测井仪的磁场强度或磁场梯度存在较大差异，需对实验测量结果进行校正。Abstract: Nuclear magnetic resonance(NMR) logging instruments generally measure the NMR signal of reservoir fluid in a uniform field with a resonance frequency of 2 MHz or a gradient field with a resonance frequency of less than 1 MHz. For laboratory NMR core analyzers, in addition to the commonly used 2 MHz, the resonance frequency of 12 MHz or 21 MHz is often used for experimental measurement of tight reservoirs such as shale. In order to determine the effects of magnetic field intensity and gradient on the NMR measurement results, the sensitivity of the magnetic field intensity and gradient of the core experiments of glutenite and shale cores under water-saturatedstate was systematically studied, and the relationship between the T2 spectrum shape, position, nuclear magnetic porosity, T2 geometric mean and the magnetic field intensity and gradient of different rock samples was analyzed. The experimental results show that under the uniform field, glutenite samples are very sensitive to the change of magnetic field intensity, while shale samples are less sensitive. The existence of external gradient field will make the short relaxation information of glutenite and shale missing, resulting in that the NMR signal cannot be measured completely. When the NMR core experiment is used to calibrate the interpretation parameters of NMR logging, the experimental measurement results must be corrected if there is a large difference in the magnetic field intensity or gradient between the laboratory NMR core analyzer and the NMR logging instrument.
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Key words:
- nuclear magnetic resonance /
- magnetic field intensity /
- magnetic field gradient /
- T2 spectrum /
- glutenite /
- shale
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图 3 核磁共振实验中4块饱水样品在不同磁场强度、梯度下的T2谱测量结果
Figure 3. T2 spectrum results of four water-saturated samples in different magnetic field intensities and gradients in NMR experiment
图 4 核磁共振实验样品的气测孔隙度与核磁视孔隙度对比
Figure 4. Comparison of gas porosity and nuclear magnetic apparent porosity of NMR experimental samples
图 5 核磁共振实验样品在不同磁场强度、梯度下的核磁视孔隙度对比
Figure 5. Nuclear magnetic apparent porosity comparison of NMR experimental samples in different magnetic field intensities and gradients
图 6 核磁共振实验样品在不同磁场强度、磁场梯度下的T2几何均值对比
Figure 6. T2 geometric mean comparison of NMR experimental samples in different magnetic field intensities and gradients
表 1 核磁共振实验岩心基础参数
Table 1. Basic parameters of NMR experimental core
岩心编号 岩性 长度/cm 直径/cm 取样深度/m 气测孔隙度/% 1号 砂砾岩 3.108 0 2.474 0 4 067.32 7.96 2号 砂砾岩 3.727 3 2.531 7 4 071.44 9.22 3号 页岩 2.301 3 2.489 0 3 034.20 5.76 4号 页岩 2.417 0 2.489 5 3 578.50 4.63 
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表 2 核磁共振实验采集参数
Table 2. Parameters acquired by NMR experiment
磁场环境 等待时间(TW)/ms 回波间隔(TE)/ms 回波个数(NECH) 扫描次数(SCAN) 1 MHz梯度场 6 000 0.2 1 600 2 048 1 MHz均匀场 6 000 0.2 1 600 256 2 MHz均匀场 6 000 0.2 1 600 256 21 MHz均匀场 6 000 0.2 1 600 32
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表 3 核磁共振实验样品在不同磁场强度、梯度下的T2谱形态
Table 3. T2 spectrum shape of NMR experimental samples in different magnetic field intensities and gradients
样品编号 1 MHz梯度场 1 MHz均匀场 2 MHz均匀场 21 MHz均匀场 1 双峰 双峰 双峰 梯形 2 双峰 三峰 双峰 梯形 3 双峰(左、右峰占比相当) 4 双峰(以左峰为主，右峰占比较小)
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表 4 核磁共振实验砂砾岩样品在不同磁场强度、梯度下的T2谱位置
Table 4. T2 spectrum positions of NMR experimental glutenite samples in different magnetic field intensities and gradients
ms 岩心编号 1 MHz梯度场 1 MHz均匀场 2 MHz均匀场 21 MHz均匀场 半弛豫时间 起始时间 终止时间 半弛豫时间 起始时间 终止时间 半弛豫时间 起始时间 终止时间 半弛豫时间 起始时间 终止时间 1 10.50 0.80 80.31 13.90 0.52 193.07 10.16 0.11 299.36 4.12 0.06 2 154.44 2 29.40 1.93 372.76 33.42 1.00 719.69 24.19 0.06 896.15 5.78 0.05 1 115.88
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表 5 核磁共振实验页岩样品在不同磁场强度、梯度下的T2谱谱峰位置
Table 5. T2 peak positions of NMR experimental shale samples in different magnetic field intensities and gradients
ms 岩心编号 1 MHz梯度场 1 MHz均匀场 2 MHz均匀场 21 MHz均匀场 左峰 右峰 左峰 右峰 左峰 右峰 左峰 右峰 3 1.93 80.3 0.80 100 0.42 100 0.33 26.8 4 1.55 193.1 1.25 155.1 1.0 193.1 0.80 299.4
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