Evaluating method of gas content of coalbed methane based on nuclear magnetic resonance technology
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摘要:
我国煤层气资源储量十分丰富，是良好的天然气后备资源，开发利用煤层气具有多重价值。煤层含气量是评价煤层的关键参数，也是决定煤层气勘探开发选区与产能潜力大小的重要因素，利用测井技术评价含气量可以低成本获得地下煤层气关键参数，准确获得含气量对煤层气勘探开发具有极为重要的影响。为了规避煤层中天然气不同赋存状态导致的密度不同和Langmuir单层吸附模型造成的含气量低估，提出了一种基于核磁共振技术评价煤层含气量的新方法，采用经过校准的核磁共振仪器测量煤层中的核磁共振信号，通过分离煤层中吸附态天然气的核磁共振信号，进而计算单位体积地层中天然气分子的摩尔数，综合体积密度测井换算为标准状态下煤层含气量。为验证方法的有效性，设计开展了实际煤样甲烷等温吸附与核磁共振联测试验，试验结果表明，利用不同的T2（横向弛豫时间）截止值可以区分煤层中不同赋存状态的甲烷气体，基于核磁共振新方法计算的与试验测量的煤层含气量符合很好，证明了新方法的有效性。实际测井资料处理解释结果表明，基于核磁测井新方法比常规测井方法计算煤层含气量更加有效。
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关键词:
- 煤层气 /
- 核磁共振 /
- T2截止值 /
- 含气量 /
- Langmuir吸附
Abstract:China has abundant coalbed methane (CBM) resources, which is a good reserve of natural gas. The development and utilization of CBM has multiple values. Gas content of coal is the key to evaluate coal seam, and it is also an important factor to determine the exploration and development area and productivity potential of CBM. Using logging data to evaluate gas content is an important means in CBM exploration and development. In order to avoid the density difference caused by different methane states and the underestimation of gas content caused by Langmuir monolayer adsorption model in coal seam, a new method is adopted to separate the NMR signal of adsorbed natural gas in coal seam, calculate the molecules mole number of natural gas in unit volume formation, and convert into the gas content in standard state by density logging. The experimental results of methane isothermal adsorption and NMR show that differentT2 cut-off values can be used to distinguish methane states in coal seam. The adsorpted methane content calculated by using new method based on NMR is in good agreement with the measurement of coal sample, which proves the effectiveness of the method. The actual logging data processing and interpretation results show that the new method based on NMR logging is more effective than the method based on conventional logging in calculating gas content of coal seam.
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Keywords:
- Coalbed methane /
- nuclear magnetic resonance /
- T2 cutoff /
- gas content /
- Langmui absorption
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图 1 煤层甲烷等温吸附与核磁共振联测试验流程
Figure 1. Experimental flow of isothermal adsorption and NMR of coal sample
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图 2 不同平衡压力下SM-17-10煤样中甲烷的核磁共振T2谱
Figure 2. T2 spectrum of SM-17-10 coal sample with the continuous methane injection at different pressures
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图 3 不同平衡压力SM-17-10煤样中吸附态、游离态和自由态甲烷的T2谱信号强度变化
Figure 3. NMR T2 amplitudes changes of adsorbed, free pore and free methane in sm-17-10 coal sample at different pressures
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图 4 SM-17-10煤样等温吸附试验与核磁共振计算兰格缪尔曲线比较
Figure 4. Comparison of Langmuir curves between NMR calculation and experimental measurement in sm-17-10 coal sample
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图 5 A井常规测井、核磁测井计算煤层含气量与实际取心分析含气量对比
（注：第1道红色曲线为自然伽马、蓝色曲线为自然电位、黑色曲线为井经；第2道为测量深度；第3道红色、黑色和蓝色曲线分别为深、中、浅阵列感应电阻率；第4道红色曲线为密度、蓝色曲线为中子孔隙度、棕色曲线为纵波时差和黑色曲线为光电俘获截面指数曲线；第5道为核磁共振测井T2谱；第6道红色曲线为核磁法含气量、蓝色曲线为常规法含气量和黑色点为岩心分析含气量）
Figure 5. Comparison of gas content calculated by conventional logging, nuclear magnetic logging and actual coring analysis in A well
表 1 等温吸附试验与核磁共振计算吸附甲烷含量及兰格缪尔方程对比
Table 1 Comparison of adsorbed methane content and Langmuir equation between NMR calculation and experimental measurement
样品号 工业分析/% 平衡压力/
MPa等温吸附法 核磁共振法 吸附气含
量相对
误差/%镜质体
反射率/%水分 挥发分 灰分 固定碳 吸附气
含量/
(cm3·g−1)Langmuir
体积/
(cm3·g−1)Langmuir
压力/
MPa吸附气
含量/
(cm3·g−1)Langmuir
体积/
(cm3·g−1)Langmuir
压力/
MPaSM-17-10 1.15 23.04 43.60 32.21 0 0 10.29 5.88 0 10.56 5.89 0 1.11 1.249 1.80 1.77 1.8 2.193 2.79 2.98 6.7 4.322 4.36 4.47 2.6 5.211 4.83 4.95 2.3 6.068 5.22 5.42 3.7 8.078 5.95 6.08 2.1 SM-14-17 1.10 12.75 34.45 51.70 0 0 13.01 3.98 0 12.89 3.68 0 1.30 2.510 5.03 5.24 3.7 3.608 6.19 6.33 2.1 4.555 6.94 7.19 3.0 5.623 7.62 7.60 0.2 6.573 8.10 8.42 3.4 7.578 8.73 8.88 1.5 8.755 8.94 9.06 1.1 SM-4-29 1.44 25.21 30.57 42.78 0 0 12.13 5.43 0 11.59 5.01 0 1.28 2.316 3.63 3.76 3.8 3.262 4.55 4.45 2.3 4.158 5.26 5.10 3.1 5.419 6.06 6.14 1.3 6.622 6.66 6.77 1.7 7.515 7.04 6.88 2.4 SM-10-40 0.96 10.19 28.24 60.61 0 0 10.29 2.94 0 10.91 3.52 0 1.27 1.355 3.26 3.07 6.0 3.168 5.33 5.11 4.2 5.285 6.60 6.67 1.1 7.298 7.31 7.15 2.3 9.183 7.78 7.98 2.6 11.128 8.16 8.32 2.0 
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