A method to quantitatively determine water content in unconsolidated sediments using low-field NMR
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摘要:
核磁共振通过测定体系中氢质子弛豫的T2谱来确定液态水的含量。采用低场核磁共振技术定量测定松散沉积物体系中的含水量，探讨了沉积物粒径、黏土矿物种类与含量、孔隙水盐度、温度及气体压力等因素对测试结果的影响。由于不同介质体系中孔隙水表面弛豫机制不同，导致低场核磁测定松散沉积物中的含水量偏小。引入校正系数Cm对水量测试值进行了修正，结果表明：沉积物及孔隙水介质本身特性对水量测试结果几乎无影响，相对误差<0.5%，测试精密度<0.20%；温度变化对测试结果影响较大且呈负相关，温度从25 ℃降至1.7 ℃，水量测试值增加了10.71%；压力变化对测试结果的影响与充注气体是否含氢密切相关，不含氢气体的压力变化对测试结果没有影响，而对于含氢气体如甲烷，水量测试结果随压力变化线性增加，甲烷增加到5.05 MPa时，测试结果增加了12.15%。因此，在采用该法测量甲烷水合物生成分解过程中沉积物孔隙水的变化时，必须考虑体系的温度、压力对测试结果的影响，恒温恒压条件下监测的含水量变化能够准确指示甲烷水合物生成分解的微观过程，可望在海洋天然气水合物生成分解微观动力学研究方面得到广泛应用。
Abstract:The low-field nuclear magnetic resonance (LF-NMR) determines the content of liquid water by measuring the T2 spectrum of hydrogen.The water content was measured in unconsolidated sediments by the LF-NMR technique. The effects of particle size, mineral type and content, water salinity, temperature and gas pressure on the measured water content were investigated. Results show that the water content measured by LF-NMR was smaller than the actual value due to the surface relaxation of water in unconsolidated sediments. The correction factor Cm was introduced to correct the measured content of water. The characteristics of sediment and pore water have little influence on the measured content of water, with a relative error of <0.5% in precision of <0.20%. In contrast, temperature was negatively correlated with the measured content of water. The measured content of water increased by 10.71% when the temperature dropped from 25 to 1.7 ℃. The effect of pressure on the measured content of water depends on whether the gas contains hydrogen. The pressure change caused by the hydrogen-free gas had no effect on the measured content of water. For hydrogen gas, such as methane, the measured content of water increased linearly with the increase of pressure. The measured content of the water increased by 12.15% when the pressure increased from 0.10 to 5.05 MPa. Therefore, the effects of temperature and pressure on the measured content of water shall be considered when the change of water content is analyzed by LF-NMR during methane hydrate phase transition in sediment. The change in water content, which is monitored by LF-NMR under constant temperature and pressure, could accurately indicate the microscopic process of methane hydrate phase transition. Therefore, the method is expected to be widely used to study the microscopic kinetics of the formation and decomposition of methane hydrate.
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Key words:
- unconsolidated sediment /
- low-field NMR /
- signal intensity /
- water content test /
- relative error
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表 1 核磁测量参数-1（NMR1）
Table 1. Nuclear magnetic measurement parameters-1（NMR1）
参数名称 参数值 参数单位 参数名称 参数值 参数单位 主频SF 21 MHz 射频延时RFD 0.080 ms 频率偏置O1 不定 Hz 模拟增益RG1 20 dB 90度脉宽P1 6.60 μs 数字增益DRG1 3 / 180度脉宽P2 12.48 μs 前置放大增益PRG 1 / 采样频率SW 250 kHz 累加采样次数NS 16 / 等待时间TW 8 000 ms 回波时间TE 0.10 ms 采样点数TD 630 026 / 回波个数NECH 18 000 / 注：每次测试前通过重置O1校正中心频率。 
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表 2 核磁测量参数-2（NMR2）
Table 2. Nuclear magnetic measurement parameters-2（NMR2）
参数名称 参数值 参数单位 参数名称 参数值 参数单位 主频SF 21 MHz 射频延时RFD 0.080 ms 频率偏置O1 不定 Hz 模拟增益RG1 20 dB 90度脉宽P1 14.20 μs 数字增益DRG1 3 / 180度脉宽P2 28.64 us 前置放大增益PRG 1 / 采样频率SW 250 kHz 累加采样次数NS 32 / 等待时间TW 5 000 ms 回波时间TE 0.18 ms 采样点数TD 990 036 / 回波个数NECH 18 000 / 注：每次测试前通过重置O1校正中心频率。
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表 3 核磁测量参数-3（NMR3）
Table 3. Nuclear magnetic measurement parameters-3 （NMR3）
参数名称 参数值 参数单位 参数名称 参数值 参数单位 主频SF 21 MHz 射频延时RFD 0.080 ms 频率偏置O1 不定 Hz 模拟增益RG1 20 dB 90度脉宽P1 13.20 μs 数字增益DRG1 3 / 180度脉宽P2 24.80 μs 前置放大增益PRG 1 / 采样频率SW 250 KHz 累加采样次数NS 32 / 等待时间TW 4500 ms 回波时间TE 0.18 ms 采样点数TD 810030 / 回波个数NECH 18000 / 注：每次测试前通过重置O1校正中心频率。
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表 4 精密度测试结果
Table 4. The test results of precision
水量测试值/g 水量测试平均值 
/g水量标准偏差S 相对标准偏差RSD/% 1.95 1.96 1.96 1.96 1.95 1.96 1.958 3.89×10−3 0.20 注：为减少计算误差，水量测试平均值 
保留3位小数。
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表 5 不同沉积物粒径的测试结果
Table 5. The test results of different particle sizes of sediments
沉积物粒径/μm 水量测试值/g 水量校正值/g 水量真实值/g 相对误差 45～53 1.93 1.969 2.00 −1.57% 58～75 1.95 1.992 2.00 −0.39% 80～120 1.95 1.995 2.00 −0.27% 120～180 1.95 1.997 2.00 −0.15% 250～380 1.99 2.031 2.03 0.07% 注：①水量校正值为水量测试值×校正系数Cm，为减少计算误差，保留3位小数；②校正系数Cm取值为1.022。
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表 6 不同黏土矿物及其含量的测试结果
Table 6. The test results of different clay minerals and their contents
黏土类型 黏土比例/% 水量测试值/g 水量校正值/g 水量真实值/g 相对误差 蒙脱石 2 1.98 2.027 2.02 0.36% 蒙脱石 5 1.99 2.034 2.03 0.20% 蒙脱石 10 1.98 2.027 2.03 −0.14% 蒙脱石 20 1.95 1.994 2.00 −0.28% 蒙脱石 40 1.94 1.985 2.00 −0.73% 高岭石 2 1.95 1.995 2.00 −0.26% 高岭石 5 1.95 1.992 2.00 −0.38% 高岭石 10 1.95 1.996 2.01 −0.72% 高岭石 20 1.94 1.977 2.00 −1.17% 高岭石 40 1.94 1.980 2.01 −1.49% 伊利石 2 1.99 2.028 2.02 0.40% 伊利石 5 1.98 2.020 2.02 0.02% 伊利石 10 1.95 1.993 2.00 −0.37% 伊利石 20 1.96 2.004 2.02 −0.77% 伊利石 40 1.94 1.983 2.00 −0.86% 注：①水量校正值为水量测试值×校正系数Cm，为减少计算误差，保留3位小数；②校正系数Cm取值为1.022。
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表 7 不同孔隙水盐度的测试结果
Table 7. The test results of different salinities of pore water
NaCl溶液浓度/（g/100mL） 水量测试值/g 水量校正值/g 水量真实值/g 相对误差 0.1 1.97 2.015 2.03 −0.72% 0.2 2.00 2.045 2.04 0.26% 0.4 1.99 2.029 2.03 −0.04% 0.6 1.97 2.009 2.00 0.43% 0.8 1.95 1.987 2.00 −0.63% 注：①水量校正值为水量测试值×校正系数Cm，为减少计算误差，保留3位小数；②校正系数Cm取值为1.022。
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