Method for obtaining paleopressure of CO2-CH4 mixed gas inclusion through Fourier transform infrared spectroscopy

Abstract

The invention discloses a method for acquiring paleopressure of a CO2-CH4 mixed gas inclusion through Fourier transform infrared spectroscopy. The method comprises the following steps: S1, preparing CO2-CH4 mixed gases with different components; s2, calibrating components of the mixed gas by gas chromatography; s3, Fourier transform infrared spectrograms of the mixed gas under different pressures are collected; s4, extracting a methane peak position, and establishing a quantitative model of the methane peak position, the pressure and the methane content; and S5, acquiring the Fourier transform infrared spectrogram of the natural CO2-CH4 mixed gas inclusion, extracting the methane peak position, calculating the methane content according to the peak area ratio, substituting the methane content and the methane peak position into the quantitative model, and calculating the paleopressure of the CO2-CH4 mixed gas inclusion. According to the method, the CH4-CO2 fluid inclusion quantitative model is established based on the Fourier transform infrared spectrum, the paleopressure of the related fluid inclusion can be determined, the method is also suitable for quantitative analysis in all laboratories, and errors caused by different laboratories are avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of geological fluid physics, and in particular to a method for obtaining the paleopressure of CO2-CH4 mixed gas inclusions by Fourier transform infrared spectroscopy. Background Art

[0002] Natural gas reservoirs contain multi-component inclusions such as methane and carbon dioxide (the most common is the CH4-CO2-H2O-NaCl system), which can usually be divided into two types: gas-rich inclusions (common gas inclusions contain trace amounts of water) and gas-rich water-brine inclusions. Determining the pressure of multi-component inclusions containing natural gas such as methane and carbon dioxide in natural gas reservoirs is an important way to reveal the history of natural gas charging. For simple binary or ternary system inclusions, microthermometry is usually used as the main means to obtain the phase transition temperature of the inclusion during heating or cooling (such as freezing point, homogenization temperature, melting temperature of clathrate hydrates, dissolution temperature of salt daughter minerals, etc.). Based on three assumptions (fluid homogeneous capture, unchanged composition after capture, and unchanged volume), the paleopressure of the inclusion is calculated by the state equation that characterizes the fluid PVTX relationship and phase equilibrium. To determine the paleopressure of a single inclusion in a gas-water-salt system (CH4-CO2-H2O-NaCl system) containing four components, three independent observation data are usually required, including phase transition temperature (microthermometry), carbon-rich phase gas composition (Raman analysis), and volume fraction (conventional microscope estimation or confocal scanning microscope slice area integration), and then the total composition of the inclusion is calculated through a thermodynamic model. However, the accuracy of obtaining the total composition of the inclusion is not only affected by the accuracy of these observational data, but also related to the accuracy of the thermodynamic model describing the system. The error is sometimes so large that the inclusion data loses its indicative significance.

[0003] For fluid inclusions in the CH4-CO2 system where multiple gas components and water coexist, there is currently a lack of experimental data such as quaternary phase equilibrium and PVT-x, there are not many available phase transition points, and the Raman quantification of composition and the estimation accuracy of phase volume fractions are not high, which restricts the accurate quantification of the composition and pressure of the system. At present, for the paleopressure recovery of fluid inclusions in the CH4-CO2-H2O-NaCl system, the laser Raman spectroscopy quantitative model is often used to calculate the paleopressure of inclusions. However, the problem with this method is that since the Raman quantitative parameters are greatly related to the instrument parameters and instrument environment, the Raman quantitative models fitted and established are different between different laboratories due to the differences in Raman parameters and humidity, temperature and other environments. Sample tests between different laboratories cannot guarantee high accuracy, consistency and minimum error. Therefore, it is urgent to provide a CH4-CO2 fluid inclusion paleopressure model that is suitable for quantitative analysis in all laboratories and eliminates the errors caused by different laboratories. Summary of the invention

[0004] The technical problem to be solved by the present invention is: to overcome the shortcomings of the prior art and provide a method for obtaining the paleopressure of CO2-CH4 mixed gas inclusions through Fourier transform infrared spectroscopy, and to establish a CH4-CO2 fluid inclusion quantitative model based on Fourier transform infrared spectroscopy, which can not only be used to determine the paleopressure of related fluid inclusions, but also be applicable to all laboratory quantitative analyses, eliminating the errors caused by different laboratories.

[0005] The technical solution of the present invention is:

[0006] The method for obtaining the paleopressure of CO2-CH4 mixed gas inclusions by Fourier transform infrared spectroscopy comprises the following steps:

[0007] S1 prepares CO2-CH4 mixed gas with different component contents;

[0008] S2 performs gas chromatography calibration on the prepared mixed gas components to determine the component contents of the prepared mixed gas;

[0009] S3 collects Fourier transform infrared spectra of various mixed gases under different pressures;

[0010] S4 extracts the methane peak position data in the Fourier transform infrared spectrum and establishes a quantitative model of the methane peak position and pressure, and the methane content in the mixed gas;

[0011] S5 collects the Fourier transform infrared spectrum of the natural CO2-CH4 mixed gas inclusion, extracts the methane peak position data in the Fourier transform infrared spectrum, and calculates the methane content in the CO2-CH4 mixed gas inclusion according to the peak area ratio, substitutes the methane content and methane peak position data into the quantitative model established in step S4, and calculates the paleopressure of the CO2-CH4 mixed gas inclusion.

[0012] Preferably, in step S3, when collecting the Fourier transform infrared spectra of each mixed gas, the pressure range is 0-60 MPa.

[0013] Preferably, in step S4, the quantitative model established is:

[0014] z=p 00 +p 10 ×x+p 01 ×y+p 20 × 2 +p 11 ×x×y+p 02 ×y 2 +p 30 × 3 +p 21 × 2 ×y+p 12 ×x×y2 +p 03 ×y 3 +p 40 × 4 +p 31 × 3 ×y+p 22 × 2 ×y 2 +p 13 ×x×y 3 +p 04 ×y 4 ;

[0015] Where x is the methane content in the mixed gas, %; y is the pressure, MPa; z is the position of the methane peak, cm -1 ;p 00 =3.0124e 3 ;p 10 =73.7498; p 01 =-0.0255; p 20 =-300.0512; p 11 =-0.0101; p 02 =1.6700e -5 ;p 30 =459.2178; p 21 =0.0492; p 12 =-6.6928e -5 ;p 03 =1.0748e -7 ;p 40 =-233.5214; p 31 =-0.0398; p 22 =3.6652e -5 ;p 13 =2.5018e -8 ;p 04 =-1.2255e -10 ; R 2 =0.9942.

[0016] Preferably, in step S5, when calculating the methane content in the CO2-CH4 mixed gas inclusions according to the peak area ratio, the calculation is performed according to the following formula: methane content = PAR CH4 / (PAR CO2 +PAR CH4 )×100%, where PAR CH4 is the peak area ratio of methane, PAR CO2 is the peak area ratio of carbon dioxide.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention obtains the Fourier transform infrared spectra of each group of samples under different pressure conditions at room temperature by artificially synthesizing CH4-CO2 fluid inclusions and calibrating them with gas chromatography. By studying the infrared response of CH4 and CO2 in each group of mixed gases, a Fourier transform infrared spectrum quantitative model of CO2-CH4 mixed gas under different components and pressures is established to determine the ancient pressure of the relevant fluid inclusions. In the present invention, Fourier transform infrared spectrum is another kind of focusing spectrum. The same as Raman spectrum, both have the characteristics of in-situ non-destructive, rapid detection and high resolution. Fourier transform infrared spectrum belongs to absorption spectrum, and its quantitative basic theory is Lambert-Beer law: A=εcl, A is the Fourier transform infrared spectrum absorbance of measured methane v3, ε is the molar absorption coefficient representing the properties of the substance, c is the concentration, and l is the path length. Under each experimental condition, c can be expressed by methane density, so this theoretical equation shows that Ac has a linear relationship. Lambert-Beer's law constitutes the theoretical basis for the quantification of Fourier transform infrared spectroscopy. Since the absorbance in the quantitative formula is only related to the concentration of the substance and the optical path length, and has no influence on the parameters of the instrument itself and the environment in which it is located, Fourier transform infrared spectroscopy, as an absorption spectrum, can establish an inclusion quantitative model that is applicable to all laboratory quantitative analyses, eliminating the errors caused by different laboratories. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the structure of the system used for preparing the mixed gas in Example 1 of the present invention.

[0020] Figure 2 It is the variation trend of the methane peak position of each mixed gas with the pressure change in Example 1 of the present invention.

[0021] Figure 3 This is a diagram showing the change in the position of the methane peak in the infrared spectra of the mixed gases fitted in Example 1 of the present invention as a function of pressure.

[0022] Figure 4 This is a Fourier transform infrared spectrum of the natural CO2-CH4 mixed gas inclusions collected in Example 1 of the present invention.

[0023] Figure 5 These are (a) a picture of natural CO2-CH4 mixed gas inclusions in Yinggehai Basin in Example 1 of the present invention; (b) a Raman spectrum of natural CO2-CH4 mixed gas inclusions in Yinggehai Basin; and (c) an infrared spectrum of natural CO2-CH4 mixed gas inclusions in Yinggehai Basin.

[0024] In the figure, 1. High-pressure methane gas storage cylinder; 2. High-pressure carbon dioxide gas storage cylinder; 3. Gas distribution cylinder; 4. Gas outlet pipe; 5. Gas distribution pipe; 6. Valve; 7. Flow meter; 8. Vacuum pipe; 9. Vacuum pump. DETAILED DESCRIPTION

[0025] In order to enable persons skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0026] Example 1

[0027] The method of obtaining the paleopressure of CO2-CH4 mixed gas inclusions by Fourier transform infrared spectroscopy in this embodiment includes the following steps:

[0028] S1 prepares CO2-CH4 mixed gas with different component contents

[0029] When preparing the mixed gas, use Figure 1 The system shown includes a high-pressure methane gas cylinder 1, a high-pressure carbon dioxide gas cylinder 2 and a gas distribution cylinder 3. The gas outlets of the high-pressure methane gas cylinder 1 and the high-pressure carbon dioxide gas cylinder 2 are respectively connected to gas outlet pipes 4, the two gas outlet pipes 4 are connected to a gas distribution pipe 5, the gas distribution pipe 5 is connected to the gas inlet of the gas distribution bottle 3, valves 6 are respectively provided on the two gas outlet pipes 4 and the gas distribution pipe 5 located at the gas inlet of the gas distribution bottle 3, and a flow meter 7 is provided on the gas distribution pipe 5; at the same time, the gas distribution pipe 5 is also connected to a vacuum pipe 8, and the vacuum pipe 8 is connected to a vacuum pump 9.

[0030] When preparing the mixed gas, first close the high-pressure methane gas cylinder 1 and the high-pressure carbon dioxide gas cylinder 2, open all valves 6, and evacuate each pipeline and the gas distribution bottle 3 through the vacuum pump 9. Then close the vacuum pump 9 and the valve 6 on the vacuum tube 8, stop evacuating, close the valve 6 on the gas outlet pipe 4 connected to the high-pressure methane gas cylinder 1, open the valve 6 on the gas outlet pipe 4 connected to the high-pressure carbon dioxide gas cylinder 2, and open the high-pressure carbon dioxide gas cylinder 2 to pass carbon dioxide gas into the gas distribution bottle 3. After the carbon dioxide is passed, close the valve 6 at the air inlet of the high-pressure carbon dioxide gas cylinder 2 and the gas distribution bottle 3, open the valve 6 on the vacuum tube 8, and evacuate the pipeline again through the vacuum pump 9. Then close the vacuum pump 9, the valve 6 on the vacuum tube 8, and the valve 6 on the gas outlet pipe 4 connected to the high-pressure carbon dioxide gas cylinder 2, open the valve 6 on the gas outlet pipe 4 connected to the high-pressure methane gas cylinder 1, the valve 6 at the gas outlet of the gas distribution bottle 3, and the high-pressure methane gas cylinder 1, and pass methane gas into the gas distribution bottle 3. By adjusting the flow rate and time of the carbon dioxide and methane, the carbon dioxide and methane in the gas cylinder 3 can reach different ratios, thereby preparing a CO2-CH4 mixed gas with different component contents. S2 performs gas chromatography calibration on the components of the prepared mixed gas to determine the component content of the prepared mixed gas

[0031] The prepared mixed gas is placed in a gas mixing bottle for a period of time. After it is fully mixed, a gas chromatograph is used to detect the specific component content of the prepared mixed gas.

[0032] In this embodiment, step S1 controls the gas flow rate and the introduction time to prepare mixed gases with a volume ratio of methane to carbon dioxide of 15:85, 30:70, 65:35 and 80:20, and purchases a standard mixed gas with a volume ratio of methane to carbon dioxide of 50:50. Step S2 detects the component contents of the prepared and purchased mixed gases, and the actual component contents of each mixed gas detected are shown in Table 1.

[0033] Table 1

[0034]

[0035]

[0036] As can be seen from Table 1, the component contents of each mixed gas in this embodiment are: 50% CH4-50% CO2, 15% CH4-85% CO2, 31% CH4-69% CO2, 66% CH4-34% CO2 and 82% CH4-18% CO2.

[0037] S3 collects Fourier transform infrared spectra of various mixed gases in the pressure range of 0-60MPa

[0038] The Fourier transform infrared spectrum of the mixed gas is collected within the pressure range of 0-60 MPa.

[0039] The peak position of methane in each mixed gas will change with the change of pressure. The change trend is as follows: Figure 2 As shown in the figure, when the pressure is low, the volume fraction of methane has little effect on its peak position, but as the pressure increases, the influence of the volume fraction of methane on the peak position gradually increases. It can be seen that there is a corresponding relationship between the methane content, pressure and methane peak position in the mixed gas.

[0040] S4 extracts the methane peak position data from the Fourier transform infrared spectrum and establishes a quantitative model of the methane peak position, pressure, and methane content in the mixed gas

[0041] Matlab was used to perform polynomial fitting to show the change of methane peak position with pressure in the infrared spectra of each mixed gas (the fitting results are shown in Figure 3 As shown), the specific infrared spectrum quantitative model formula is as follows:

[0042] z=p 00 +p 10 ×x+p 01 ×y+p 20 × 2 +p 11 ×x×y+p 02 ×y 2 +p 30 × 3 +p 21 × 2 ×y+p 12 ×x×y 2 +p 03 ×y 3 +p 40 × 4 +p 31 × 3 ×y+p 22 × 2 ×y 2 +p 13 ×x×y 3 +p 04 ×y 4 ;

[0043] Where x is the methane content in the mixed gas, %; y is the pressure, MPa; z is the position of the methane peak, cm -1 ; p00 = 3.0124e 3 ;p10=73.7498; p01=-0.0255; p20=-300.0512; p11=-0.0101; p02=1.6700e-5 ;p30=459.2178;p21=0.0492;p12=-6.6928e -5 ;p 03 =1.0748e -7 ;p 40 =-233.5214; p 31 =-0.0398; p 22 =3.6652e -5 ;p 13 =2.5018e -8 ;p 04 =-1.2255e -10 ; R 2 =0.9942.

[0044] S5 collects Fourier transform infrared spectra of natural CO2-CH4 mixed gas inclusions (such as Figure 4 As shown in step S4, the methane peak position data in the Fourier transform infrared spectrum is extracted, and the methane content in the CO2-CH4 mixed gas inclusions is calculated according to the peak area ratio. The methane content and methane peak position data are substituted into the quantitative model established in step S4 to calculate the paleopressure of the CO2-CH4 mixed gas inclusions.

[0045] Among them, when calculating the methane content in the CO2-CH4 mixed gas inclusion body based on the peak area ratio, it is calculated according to the following formula: Methane content = PAR CH4 / (PAR CO2 +PAR CH4 )×100%, where PAR CH4 is the peak area ratio of methane, PAR CO2 is the peak area ratio of carbon dioxide.

[0046] Substitute the methane content and methane peak position data into the quantitative model established in step S4 to calculate the paleopressure of the natural CO2-CH4 mixed gas inclusion.

[0047] The natural CO2-CH4 mixed gas inclusions in the Yinggehai Basin were selected as the research object. The infrared spectrum and laser Raman spectrum were used for quantitative analysis of the same CO2-CH4 mixed gas inclusions. Laser Raman spectrum can accurately obtain the paleopressure under the CO2-CH4 system. The data calculated by Raman were compared with the paleopressure results calculated by the infrared spectrum quantitative model established above. The comparison results are as follows: Figure 5As shown in Table 2-3. The comparison results show that the paleopressure of CO2-CH4 mixed gas inclusions calculated by the infrared spectrum quantitative model constructed by the present invention is 60.1MPa, and the Raman spectrum quantitative result is 60.3MPa, with a difference of only 0.2MPa between the two, indicating that the calculation results of the infrared spectrum quantitative model constructed by the present invention are reliable.

[0048] Table 2 Raman spectroscopy data

[0049]

[0050] Table 3 Infrared spectroscopy quantitative model measurement data

[0051] Methane peak area ratio <![CDATA[CO2%:CH4%]]> <![CDATA[v CH4 / cm -1 ]]> Homogenization temperature of brine inclusions during the same period / ℃ Capture pressure / MPa 0.385 61.5:38.5 3011.33 181.3 60.1

Claims

1. A method for obtaining the paleopressure of CO2-CH4 mixed gas inclusions by Fourier transform infrared spectroscopy, characterized in that: The following steps are involved: S1 prepares CO2-CH4 mixed gas with different component contents; S2 performs gas chromatography calibration on the prepared mixed gas components to determine the component contents of the prepared mixed gas; S3 collects Fourier transform infrared spectra of various mixed gases under different pressures; S4 extracts the methane peak position data in the Fourier transform infrared spectrum and establishes a quantitative model of the methane peak position and pressure, and the methane content in the mixed gas; S5 collects the Fourier transform infrared spectrum of the natural CO2-CH4 mixed gas inclusion, extracts the methane peak position data in the Fourier transform infrared spectrum, and calculates the methane content in the CO2-CH4 mixed gas inclusion according to the peak area ratio, substitutes the methane content and methane peak position data into the quantitative model established in step S4, and calculates the paleopressure of the CO2-CH4 mixed gas inclusion.

2. The method for obtaining the paleopressure of CO2-CH4 mixed gas inclusions by Fourier transform infrared spectroscopy according to claim 1, characterized in that: In step S3, when collecting the Fourier transform infrared spectra of each mixed gas, the pressure range is 0-60MPa.

3. The method for obtaining paleopressure of CO2-CH4 mixed gas inclusions by Fourier transform infrared spectroscopy according to claim 1, characterized in that: In step S4, the quantitative model established is: z=p 00 +p 10 ×x+p 01 ×y+p 20 ×x 2 +p 11 ×x×y+p 02 ×y 2 +p 30 ×x 3 +p 21 ×x 2 ×y+p 12 ×x×y 2 +p 03 ×y 3 +p 40 ×x 4 +p 31 ×x 3 ×y+p 22 ×x 2 ×y 2 +p 13 ×x×y 3 +p 04 ×y 4 ; Where x is the methane content in the mixed gas, %; y is the pressure, MPa; z is the position of the methane peak, cm -1 ;p 00 =3.0124e 3 ;p 10 =73.7498; p 01 =-0.0255; p 20 =-300.0512; p 11 =-0.0101; p 02 =1.6700e -5 ;p 30 =459.2178; p 21 =0.0492; p 12 =-6.6928e -5 ;p 03 =1.0748e -7 ;p 40 =-233.5214; p 31 =-0.0398; p 22 =3.6652e -5 ;p 13 =2.5018e -8 ;p 04 =-1.2255e -10 ; R 2 =0.9942.

4. The method for obtaining paleopressure of CO2-CH4 mixed gas inclusions by Fourier transform infrared spectroscopy according to claim 1, characterized in that: In step S5, when calculating the methane content in the CO2-CH4 mixed gas inclusions according to the peak area ratio, the calculation is performed according to the following formula: methane content = PAR CH4 / (PAR CO2 +PAR CH4 )×100%, where PAR CH4 is the peak area ratio of methane, PAR CO2 is the peak area ratio of carbon dioxide.

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