The content of carbon dioxide, hydrogen sulfied and hydrocarbon component related to calorific power in natural gas are important to weigh its quality. Currently, the main analyzing method for natural gas in our country is gas chromatographic method with standards of GB/T 13610, GB/T 27894.4, GB/T 27894.5 and GB/T 27894.6. It needs carrier gas and some chromatographic columns to conduct separating test, which needs more than ten minutes or tens of minutes, even the quickest with 100 seconds to 5 minutes. In recent years, Laser Raman scattering technique was developed for testing most components in natural gas with the advantages of fast test (10 seconds), continuous record and simple operation. That can be used as standard method of analyzing natural gas composition is a useful complement to gas chromatographic to rich natural gas composition measurement method.

The current method for testing natural gas composition is gas chromatographic method (GCM). Due to different analyzing purposes, there are several options when analyzing natural gas composition. The mainly analyzed routine items are nitrogen, carbon dioxide, methane to pentane and sometimes helium, hydrogen and C6+. For associated gas or oil field gas, sometimes it is needed to test the content of C5 or higher carbon number HC, which is called extended analysis. This standard consists of two series seen in table 1, one is series standard of ISO, such as ISO6974 (including 6 parts), the other is ASTM D1945 and GPA2261 of ASTM.

Table 1

Table 1

Table 1 Standards of natural gas composition analysis Standard Number Standard name GB/T 13610— 2014 ASTM D1945— 2014 Standard Test Method for Analysis of Natural Gas by Gas Chromatography GPA2261— 2013 Analysis for natural gas and similar gasous mixtures by Gas Chromatography GB/T27894.1— 2011/ ISO 6974— 1— 2012 Natural gas— — Determination of composition and associated uncertainty by gas chromatography— — Part 1: General guidelines and calculation of composition GB/T 27894.2— 2011/ ISO 6974— 2— 2012 Natural gas— — Determination of composition and associated uncertainty by gas chromatography— — Part 2: Uncertainty calculations GB/T 27894.3— 2011/ ISO 6974— 3— 2000 Natural gas— — Determination of composition with defined uncertainty by gas chromatography— — Part 3: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C8 using two packed columns ISO 6974— 4— 2000 Natural gas— — Determination of composition with defined uncertainty by gas chromatography— — Part 4: Determination of nitrogen, carbon dioxide and C1 to C5 and C6+ hydrocarbons for a laboratory and on-line measuring system using two columns ISO 6974— 5— 2014 Natural gas— — Determination of composition and associated uncertainty by gas chromatography— — Part 5: Isothermal method for nitrogen, carbon dioxide, C1 to C5 hydrocarbons and C6+ hydrocarbons ISO 6974— 6— 2002 Natural gas— — Determination of composition with defined uncertainty by gas chromatography— — Part 6: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and C1 to C8 hydrocarbons using three capillary columns GB/T 17281— 2016/ ISO 6975: 1997 Natural gas— — Extended analysis— — Gas-chromatographic method

Table 1 Standards of natural gas composition analysis

The GB/T 13610 of our standard is non-equivalent ASTM D1945. It set the rules of testing natural gas or similar gas composition with GCM. When GCM is in operation, it needs continuous and stable high purity carrier gas with many chromatographic columns and detectors to conduct complex separating detection, so, it needs a long time (more than ten or tens of minutes) to analyze HC, nitrogen, carbon dioxide and hydrogen composition, even the quickest on-line chromatography needs 100 seconds to 5 minutes for a completely natural gas composition analysis. In addition, the lower limit when using GCM to test hydrogen sulfied is 0.3% (mole fraction), so GCM is not used to test HC composition and content of hydrogen sulfide simultaneously. In China, quality test, assessment and control of natural gas often need combination of GCM, iodometry and cooling mirror hygrometer method with multiple instruments and equipment. What’ s more, the costing time, energy, and the cost are high.

Laser Raman spectroscopy is a kind of scattered spectroscopy, matter with different chemical structure has different Raman shift, and intensity of Raman peak is proportional to its concentration, so when using this technique to test natural gas quality and multiple components (including sulfuretted hydrogen and other acid gas), we don not need to separate every component in natural gas, it can reduce analyzing time, improve real-time and realize fast analysis. This technique can be used in composition analysis of natural gas in studies^{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]}. At present, the representative technique and instrument, analyzer of Atmosphere Recovery Inc (ARI), is based on US patent 4784486^[6] and China patent for invention (ZL201410584402.0). Among them, Laser Raman gas analyzer in RLGA series of ARI has been commercialized with a certain scale, which is commonly used in industrial process and environmental monitoring of metallurgy, petrochemical, chemical industry, natural gas, energy and aerospace. It is used to analyze marsh gas composition in Sunnyside Biogas Digester, Madison, USA; a Laser Raman analyzer installed on natural gas pipeline network of Emerald Park is used to analyze on-line natural gas composition; it has also been used in Southwest Oil and Gas Field Branch of CNPC, Shanxi Yanchang Petroleum LLC and Zhongyuan Oilfield Puguang branch of Sinopec. By now, only ASTM issued to use ASTM D7940— 2014 Standard Practice for Analysis of Liquefied Natural Gas (LNG) by Fiber-Coupled Raman Spectroscopy in 2014, this standard use Laser Raman to test LNG composition and analyze liquid sample.

The current representative Laser Raman natural analyzing technique is Laser Raman natural analyzing technique developed by 《Development and Application of Laser Raman Gas Analyzer》 of national major scientific instrument and equipment development special project of China in 2012 and united states patent (US patent 4784486). Because technique of national special project is not fully mature and the domestic equipment based on it has not been entered the market, the real tests of natural gas by Laser Raman are based on US patent 4784486. From the results of instrument 1 (RLGA-4800)^[10](seen in table 2 and 3), it shows that the measurements are in line with the indicated value of standard gas for the selected measuring components of CH₄, C₂H₆, C₃H₈, nC₄H₁₀, iC₄H₁₀ and H₂S under their range limits. The analyzed result is reliable. The RSD of 6 continuous testing results is from 0.02% to 1.66%, the range as repeatability is showed by xmax-xmin is from 0 to 0.04%(mole fraction).

Table 2

Table 2

Table 2 Reliability of instrument 1 composition Concentration of standard gas (Mole fraction/ %) Analyzed value of instrument (Mole fraction/ %) mean(Mole fraction/ %) Bias between analyzed value and the calibrated value (Mole fraction/ %) 1 2 3 CH4 1.000 0.985 1.017 1.007 1.000 0.003 9.940 9.940 9.944 9.942 9.940 0.002 99.900 99.945 99.995 99.997 99.980 0.079 C2H6 0.984 1.007 1.020 1.022 1.020 0.032 5.430 5.524 5.517 5.528 5.520 0.093 15.300 15.300 15.428 15.390 15.370 0.073 C3H8 0.501 0.505 0.504 0.500 0.500 0.002 1.980 1.979 1.981 1.964 1.970 0.005 4.100 4.090 4.106 4.102 4.100 0.001 nC4H10 0.520 0.521 0.520 0.521 0.520 0 0.984 1.015 1.012 1.023 1.020 0.033 1.490 1.556 1.546 1.548 1.550 0.060 iC4H10 0.515 0.489 0.483 0.483 0.480 0.030 1.010 0.956 0.949 0.950 0.950 0.059 1.510 1.526 1.524 1.513 1.520 0.011 H2S 0.020 4 0.024 0 0.017 0 0.017 0 0.019 3 0.001 0 0.004 9 0.004 9 0.005 0 0.005 0 0.005 0 0.000 1 0.010 9 0.011 7 0.011 9 0.011 0 0.012 0 0.001 0

Table 2 Reliability of instrument 1

Table 3

Table 3

Table 3 Reliability and repeatability of Instrument 1(Mole fraction/%) composition Concentration of standard gas Analyzed value of instrument (Mole fraction/ %) mean Bias RSD /% Xmax- Xmin 1 2 3 4 5 6 CH4 9.940 9.940 9.944 9.942 9.940 9.944 9.943 9.940 0.002 0.02 0.003 C2H6 5.430 5.524 5.517 5.528 5.540 5.536 5.554 5.530 0.103 0.23 0.037 C3H8 1.980 1.979 1.981 1.964 1.976 1.966 1.991 1.980 0.004 0.51 0.027 nC4H10 0.984 1.015 1.012 1.023 1.006 1.007 1.017 1.010 0.029 0.64 0.012 iC4H10 1.010 0.956 0.949 0.950 0.949 0.949 0.949 0.950 0.060 0.28 0.002 H2S 0.004 9 0.004 9 0.005 0 0.005 0 0.004 8 0.005 0 0.004 9 0.004 9 0 1.66 0.000 1

Table 3 Reliability and repeatability of Instrument 1(Mole fraction/%)

Instrument 2(RLGA-2811)was used to conduct test analysis for completely studying natural gas composition analyzing method by Laser Raman spectroscopy, its principle and performance are in line with instrument 1(RLGA-4800)^[10], the difference is that the tested components of CH₄, C₂H₆, C₃H₈, nC₄H₁₀, iC₄H₁₀ and H₂S are replaced with CH₄, C₂H₆, C₃H₈, CO₂, N₂, H₂S, H₂O and C4+. On the basis of testing range of Laser Raman gas analyzer, standard natural gases with different concentrations are used to make indicated value of standard gas by GCM. According to manual, turn on the analyzer, preheat the instrument for 10 minutes, adjusting the flow at 275 mL· min^-1, the pressure at 84.64 kPa and the temperature at (25± 0.5) ℃. First, use high-purity argon to set zero, and calibrate with standard gas; then, it’ s time to test, select 16 bottles of standard gas that have similar component and content with real natural gas as sample to conduct test, on basis of instrument characteristic, the tested eight components are CH₄, C₂H₆, C₃H₈, CO₂, N₂, H₂S, H₂O and C4+, H₂O is not calibrated and studied due to the lack of standards. The results are summarized in table 4 to 9 on basis of original data, which shows that for every tested component, the lowest limit of detection are as follows: C₂H₆, 0.005%(Mole fraction); C₃H₈, 0.003%(Mole fraction); H₂S, 15 ppm; N₂, 0.023%(Mole fraction); CO₂, 0.008 5%(Mole fraction). For the main composition of natural gas is methane, it is not needed to study the lowest limit of detection of methane. The consistency between the tested results and the indication of standard gas GC calibrated value is as follows. The relative error of CH₄ and C₃H₈ results is less than 1%, which shows good match with GC; when the concentration of C₂H₆, CO₂, N₂ and H₂S is less than 0.1%, the relative error of their results are high between 3% and 22%, while their absolute error is under the acceptable range. As a whole, the tested results are in line with the indication of standard gas, the analyzed results are reliable. The repeatability of CH₄ and C₃H₈ show good calculated whether by RSD or the absolute deviation of two consecutive measurements, the RSD of their 11 tested results is more than 1%(1%~26%) when concentration of C₂H₆, CO₂, N₂ and H₂S is less than 0.1%; Under low concentration and industry practice, the repeatability should be expressed by difference of two continuous tested results, the experimental result showed that xmax-xmin of 11 tested results (X_max-X_min is more than deviation of the two continuously tested values) are all under the acceptable limit. In short, Laser Raman spectroscopy can have good repeatability.

Table 4

Table 4

Table 4 Experimental results of C H4using instrument 2 composition Concentration of standard gas (Mole fraction/ %) mean (Mole fraction/ %) Relative error /% Absolute error (Mole fraction/ %) RSD/% Xmax-Xmin (Mole fraction/ %) CH4 73.030 72.994 0.05 0.036 0.03 0.068 79.841 79.832 0.01 0.009 0.04 0.119 83.486 83.442 0.05 0.044 0.01 0.039 88.604 88.589 0.02 0.014 0.03 0.065 91.593 91.511 0.09 0.082 0.06 0.179 94.009 93.983 0.03 0.026 0.02 0.048

Table 4 Experimental results of C H4using instrument 2

Table 5

Table 5

Table 5 Experimental results of C2H6 using instrument 2 composition Concentration of standard gas (Mole fraction/ %) mean (Mole fraction/ %) Relative error /% Absolute error (Mole fraction/ %) RSD/% Xmax-Xmin (Mole fraction/ %) C2H6 0.005 0.004 22.15 0.001 25.49 0.003 0.012 0.012 0.21 0.000 0.53 0.000 0.031 0.031 0.53 0.000 0.25 0.000 0.048 0.049 2.9 0.001 0.12 0.000 0.091 0.091 0.14 0.000 0.12 0.000 0.195 0.195 0.16 0.000 0.19 0.001 0.472 0.473 0.21 0.001 0.3 0.005 1.010 1.005 0.46 0.005 0.08 0.003 2.020 2.021 0.06 0.001 0.16 0.009 2.110 2.116 0.28 0.006 0.12 0.009 4.970 4.969 0.01 0.001 0.07 0.011 9.670 9.671 0.01 0.001 0.02 0.007 18.470 18.472 0.01 0.002 0.03 0.023

Table 5 Experimental results of C2H6 using instrument 2

Table 6

Table 6

Table 6 Experimental results of C3 H8using instrument 2 composition Concentration of standard gas (Mole fraction/ %) mean (Mole fraction/ %) Relative error /% Absolute error (Mole fraction/ %) RSD/% Xmax-Xmin (Mole fraction/ %) C3H8 0.003 0.003 0.39 0.000 0.68 0.000 0.004 0.004 0.64 0.000 0.45 0.000 0.028 0.028 0.64 0.000 0.29 0.000 0.086 0.085 1 0.001 0.86 0.002 0.201 0.202 0.26 0.001 0.3 0.002 0.401 0.401 0.02 0.000 0.04 0.001 0.454 0.455 0.12 0.001 0.08 0.001 0.878 0.878 0.06 0.001 0.05 0.001 1.070 1.069 0.12 0.001 0.05 0.002 2.290 2.292 0.06 0.002 0.03 0.003 2.650 2.652 0.09 0.002 0.04 0.003 4.530 4.528 0.04 0.002 0.02 0.003 9.220 9.230 0.11 0.010 0.04 0.013

Table 6 Experimental results of C3 H8using instrument 2

Table 7

Table 7

Table 7 Experimental results of H2Susing instrument 2 composition Concentration of standard gas (Mole fraction/ %) mean (Mole fraction/ %) Relative error /% Absolute error (Mole fraction/ %) RSD/% Xmax-Xmin (Mole fraction/ %) H2S 0.001 5 0.001 7 10.56 0.000 2 15.61 0.000 9 0.007 4 0.007 2 3.29 0.000 2 6.86 0.001 6 0.111 0 0.111 0 0.01 0.000 0 0.37 0.001 3 0.413 0 0.395 7 4.18 0.017 3 1.13 0.017 4 0.845 0 0.847 2 0.26 0.002 2 0.24 0.006 5 5.790 0 5.791 4 0.02 0.001 4 0.03 0.004 7 9.120 0 9.122 3 0.03 0.002 3 0.01 0.003 9

Table 7 Experimental results of H2Susing instrument 2

Table 8

Table 8

Table 8 Experimental results of N2using instrument 2 composition Concentration of standard gas (Mole fraction/ %) mean (Mole fraction/ %) Relative error /% Absolute error (Mole fraction/ %) RSD/% Xmax-Xmin (Mole fraction/ %) N2 0.023 0.026 14.62 0.003 4.23 0.004 0.039 0.038 3.02 0.001 2.43 0.003 0.094 0.097 3.08 0.003 3.03 0.010 0.151 0.151 0.29 0.000 1.33 0.006 0.457 0.456 0.21 0.001 0.16 0.003 0.723 0.710 1.81 0.013 0.12 0.003 0.856 0.855 0.15 0.001 0.23 0.005 1.340 1.342 0.17 0.002 0.05 0.002 1.890 1.889 0.05 0.001 0.04 0.003 2.740 2.742 0.07 0.002 0.08 0.006 4.410 4.410 0 0.000 0.06 0.008 7.580 7.604 0.31 0.024 0.1 0.025 9.240 9.252 0.13 0.012 0.04 0.012

Table 8 Experimental results of N2using instrument 2

Table 9

Table 9

Table 9 Experimental results of C O2using instrument 2 composition Concentration of standard gas (Mole fraction/ %) mean (Mole fraction/ %) Relative error /% Absolute error (Mole fraction/ %) RSD/% Xmax-Xmin (Mole fraction/ %) CO2 0.009 0.008 11.3 0.001 5.93 0.001 0.017 0.013 24.42 0.004 14.48 0.006 0.032 0.030 7.16 0.002 4.1 0.004 0.041 0.039 5.41 0.002 1.62 0.002 0.059 0.062 4.3 0.003 1.42 0.003 0.104 0.103 0.91 0.001 0.42 0.002 0.396 0.395 0.29 0.001 0.29 0.003 0.477 0.479 0.4 0.002 0.51 0.007 0.914 0.916 0.17 0.002 0.09 0.003 2.140 2.141 0.02 0.001 0.03 0.002 2.570 2.568 0.06 0.002 0.03 0.003 4.770 4.773 0.06 0.003 0.04 0.005 9.390 9.390 0 0.000 0.03 0.009

Table 9 Experimental results of C O2using instrument 2

Based on the above experimental results and afetr analyzing the datum, we make the following conclusions:

(1) The tested composition by Laser Raman spectroscopy consists of CO₂, N₂, H₂S, CH₄, C₂H₆, C₃H₈, nC₄H₁₀ and iC₄H₁₀.

(2) Range of every component concentration in natural gas tested by Laser Raman spectroscopy is as follows. Methane is from 1% to 99.9%, C₂H₆ 0.005 2% to 18.47%, C₃H₈ 0.002 7% to 9.22%, H₂S 0.001 5% to 9.12%, N₂ 0.023% to 9.24%, CO₂ 0.008 5% to 9.39%, nC₄H₁₀ 0.520% to 0.984% and iC₄H₁₀ 0.515% to 1.51%(the above concentration are all mole fraction).

(3) Repeatability of experimental results

The total 71 repeatable data obtained from every concentration point of all components in the above experiments is summarized as follows.

① There are 7 repeatable datum with concentrations less than 0.01%(mole fraction) as figure 1, the difference between the two tested results is less than 0.003.

Fig.1

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	Fig.1 Repeatability statistics with concentrations less than 0.01% (mole fraction)

② There are 14 repeatable datum with concentrations between 0.01% and 0.1%(mole fraction)as figure 2, the difference between the two tested results is less than 0.01.

Fig.2

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	Fig.2 Repeatability statistics with concentration between 0.01% and 0.1% (mole fraction)

③ There are 18 repeatable datum with concentrations between 0.1% and 1%(mole fraction)as figure 3, the difference between the two tested results is less than 0.02.

Fig.3

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	Fig.3 Repeatability statistics with concentration between 0.1% and 1% (mole fraction)

④ There are 18 repeatable datum with concentrations between 1% and 10%(mole fraction)as figure 4, the difference between the two tested results is less than 0.04.

Fig.4

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	Fig.4 Repeatability statistics with concentration between 1% and 10% (mole fraction)

⑤ There are 7 repeatable datum with concentrations more than 10%(mole fraction)as figure 5, the difference between the two tested results is less than 0.2.

Based on figure 1 to figure 5, obtain repeatability of natural gas analysis by Laser Raman as table 10.

Fig.5

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	Fig.5 Repeatability statistics for concentrations greater than 10% (mole fraction)

Table 10

Table 10

Table 10 Repeatability of gas analysis by Laser Raman (mole fraction/%) Range of concentration repeatability 0~0.01 0.003 0.01~0.1 0.01 0.1~1.0 0.02 1.0~10 0.04 > 10 0.20

Table 10 Repeatability of gas analysis by Laser Raman (mole fraction/%)

Table 11

Table 11

Table 11 Comparison of Raman spectroscopy, gas chromatography and iodometry in measuring product gas from purification plant Ⅰ composition GC (mole fraction /%) Measured value(mole fraction/%) mean (mole fraction /%) Relative fraction /% Absolute error (mole fraction/%) RSD /% Xmax-Xmin (mole fraction /%) 1 2 3 4 5 6 7 8 9 10 11 C2H6 0.220 0 0.199 4 0.198 0 0.197 6 0.196 5 0.195 7 0.195 1 0.194 3 0.194 8 0.194 5 0.194 1 0.194 0 0.195 8 10.998 4 0.024 2 0.88 0.005 4 H2S* 0.015 3 0.015 3 0.015 5 0.015 5 0.015 6 0.015 5 0.015 2 0.015 3 0.015 8 0.015 7 0.015 5 0.015 5 0.015 5 1.017 9 0.000 2 1.10 0.000 6 C3H8 0.010 0 0.010 0 0.009 6 0.009 4 0.009 2 0.008 7 0.008 5 0.008 3 0.008 0 0.007 7 0.007 5 0.007 5 0.008 6 14.038 4 0.001 4 9.49 0.002 4 N2 0.980 0 0.978 9 0.978 8 0.977 9 0.978 5 0.978 1 0.977 9 0.977 2 0.978 0 0.978 3 0.977 6 0.978 4 0.978 2 0.188 6 0.001 8 0.05 0.001 8 CO2 0.710 0 0.709 9 0.707 7 0.706 8 0.707 0 0.709 5 0.709 1 0.708 9 0.709 2 0.707 9 0.709 2 0.709 0 0.708 6 0.199 4 0.001 4 0.14 0.003 1 CH4 98.040 0 98.041 5 98.030 0 98.015 1 98.023 2 98.017 9 98.014 5 98.014 7 97.996 6 97.977 0 97.958 4 97.957 6 98.004 2 0.036 5 0.035 8 0.03 0.084 0 * Get content of H2S by iodometry

Table 11 Comparison of Raman spectroscopy, gas chromatography and iodometry in measuring product gas from purification plant Ⅰ

Table 12

Table 12

Table 12 Comparison of Raman spectroscopy, gas chromatography and iodometry in measuring product gas from purification plant Ⅱ composition GC (mole fraction /%) Measured value(mole fraction/%) mean (mole fraction /%) Relative fraction /% Absolute error (mole fraction/%) RSD /% Xmax-Xmin (mole fraction /%) 1 2 3 4 5 6 7 8 9 10 11 C2H6 7.340 0 7.343 1 7.340 9 7.343 8 7.345 8 7.342 5 7.343 7 7.344 6 7.346 9 7.347 0 7.352 0 7.349 4 7.345 4 0.074 0 0.005 4 0.04 0.011 1 H2S* 0.000 1 C3H8 2.450 0 2.450 7 2.451 4 2.451 7 2.452 6 2.452 5 2.453 7 2.453 9 2.452 8 2.454 6 2.454 0 2.454 2 2.452 9 0.118 7 0.002 9 0.05 0.004 0 N2 5.740 0 5.735 3 5.736 2 5.735 9 5.734 3 5.736 9 5.734 0 5.732 1 5.731 6 5.730 6 5.729 4 5.730 1 5.733 3 0.116 5 0.006 7 0.04 0.007 5 CO2 0.010 0 0.009 0 0.009 7 0.008 8 0.009 9 0.008 9 0.008 5 0.009 1 0.008 4 0.009 2 0.009 4 0.009 5 0.009 1 8.799 8 0.000 9 4.92 0.001 5 CH4 82.540 0 82.527 0 82.526 2 82.521 4 82.536 7 82.544 9 82.548 6 82.550 1 82.568 4 82.559 0 82.548 1 82.544 3 82.543 1 0.003 8 0.003 1 0.017 0.047 0 C4+ 1.680 0 1.695 0 1.695 7 1.698 5 1.680 7 1.674 4 1.671 4 1.670 2 1.651 7 1.659 6 1.667 1 1.672 5 1.676 1 0.233 5 0.003 9 0.86 0.046 7 * Get content of H2S by iodometry

Table 12 Comparison of Raman spectroscopy, gas chromatography and iodometry in measuring product gas from purification plant Ⅱ

Table 13

Table 13

Table 13 Comparison of Raman spectroscopy, gas chromatography and iodometry in measuring raw gas from a well composition GC (mole fraction /%) Measured value(mole fraction/%) mean (mole fraction /%) Relative fraction /% Absolute error (mole fraction/%) RSD /% Xmax-Xmin (mole fraction /%) 1 2 3 4 5 6 7 8 9 10 11 C2H6 0.140 0 0.139 0 0.139 5 0.140 3 0.140 0 0.140 1 0.139 4 0.139 1 0.139 4 0.140 0 0.138 3 0.139 6 0.318 4 0.000 4 0.41 0.002 0 H2S* 0.020 9 0.021 1 0.021 2 0.021 5 0.021 8 0.021 7 0.021 2 0.021 3 0.021 1 0.020 5 0.020 1 0.020 0 0.021 0 0.574 6 0.000 1 2.66 0.001 7 N2 0.760 0 0.760 2 0.759 4 0.759 4 0.759 6 0.759 7 0.759 6 0.760 2 0.760 4 0.760 1 0.761 5 0.761 3 0.760 1 0.017 5 0.000 1 0.09 0.002 1 CO2 1.070 0 1.070 3 1.072 0 1.071 4 1.073 1 1.073 0 1.073 1 1.071 5 1.070 9 1.069 5 1.068 7 1.069 7 1.071 2 0.111 4 0.001 2 0.14 0.004 4 CH4 98.010 0 98.013 3 98.012 7 98.015 4 98.017 8 98.032 6 98.044 0 98.040 2 98.053 0 98.048 0 98.060 8 98.059 3 98.036 1 0.026 6 0.026 1 0.02 0.048 1 C4+ 1.680 0 1.695 0 1.695 7 1.698 5 1.680 7 1.674 4 1.671 4 1.670 2 1.651 7 1.659 6 1.667 1 1.672 5 1.676 1 0.233 5 0.003 9 0.86 0.046 7

Table 13 Comparison of Raman spectroscopy, gas chromatography and iodometry in measuring raw gas from a well

* Get content of H₂S by iodometry

By using instrument 2 to continuously test content of nitrogen and oxygen in the air for the purpose of studying repeatability of continuous operation by this method, 8640 groups of data of nitrogen content in 24 hours (figure 6, interval: 10 seconds) were obtained. It is found that the tested nitrogen content is between 79.30% and 80.30%, oxygen is between 21.55% and 21.30%, and the respective repeatability error is 0.63% and 0.58%. So, there is a good repeatability when the method is in continuous operation.

Fig.6

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	Fig.6 Result fluctuation of nitrogen and oxygen in the air for 1 day measured by instrument 2

Use instrument 2 to analyze bottled natural gas sample got from the site, analyze one wellhead gas and product gases of two purification plant (Purification plant Ⅰ and Ⅱ ), compare them with GCM and iodometry, the results are seen in table 11 to table 13. It can be seen from the results that the analyzed results by Laser Raman, GCM and iodometry are in line with each other when concentration of real natural gas is more than 0.05%, and the repeatability meets the requirements of industry. When the concentration is less than 0.05%, the max relative error of analyzed results by Laser Raman, GCM and iodometry is up to 14%, the RSD of 11 continuous tested results is up to 9.49%. If using absolute error and absolute deviation of two tested values to study reliability and repeatability of this method, the result is acceptable.

(1) To sum up, analyzing technique by Laser Raman has improved a lot by now with an advantage of fast test (10 seconds), continuous record and simple operation which is suitable for rapid access to data of gas quality in the control process of analyzing raw gas quality for natural gas logging, well stations and gas-gathering station, as well as purifying natural gas. It is suggested to make standards of analyzing method by Laser Raman in order to offer service for the upstream area of exploration and development of natural gas and its process control.

(2) There has not been a standard about natural gas analysis by Laser Raman at home and abroad, and there is only ASTM D7940-2014 issued by ASTM for testing LNG components.

(3) It is suggested that the limited tested components in analysis standard of natural gas composition by Laser-Raman spectroscopy consist of CO₂, N₂, H₂S, CH₄, C₂H₆, C₃H₈, nC₄H₁₀ and iC₄H₁₀, the tested range is that methane is from 1% to 99.9%, C₂H₆ 0.005% to 20%, C₃H₈ 0.005% to 10%, H₂S 0.001 5% to 10%, N₂ 0.02% to 10%, CO₂ 0.01% to 10%, nC₄H₁₀ 0.5% to 2% and iC₄H₁₀ 0.5% to 2%(the above concentrations are all mole fraction), the repeatability is given from precision in part according to concentration, referring to table 10.