CN105717202A - Method for representing component and isotope fractionation effect in natural gas adsorption and desorption process of shale - Google Patents
Method for representing component and isotope fractionation effect in natural gas adsorption and desorption process of shale Download PDFInfo
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- 238000001179 sorption measurement Methods 0.000 title claims abstract description 230
- 238000000034 method Methods 0.000 title claims abstract description 107
- 238000003795 desorption Methods 0.000 title claims abstract description 68
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 title claims abstract description 66
- 230000008569 process Effects 0.000 title claims abstract description 54
- 238000005194 fractionation Methods 0.000 title claims abstract description 36
- 239000003345 natural gas Substances 0.000 title claims abstract description 19
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- 238000002336 sorption--desorption measurement Methods 0.000 claims description 5
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- 230000003111 delayed effect Effects 0.000 description 2
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- GUJOJGAPFQRJSV-UHFFFAOYSA-N dialuminum;dioxosilane;oxygen(2-);hydrate Chemical compound O.[O-2].[O-2].[O-2].[Al+3].[Al+3].O=[Si]=O.O=[Si]=O.O=[Si]=O.O=[Si]=O GUJOJGAPFQRJSV-UHFFFAOYSA-N 0.000 description 2
- 238000006073 displacement reaction Methods 0.000 description 2
- 229910052900 illite Inorganic materials 0.000 description 2
- NLYAJNPCOHFWQQ-UHFFFAOYSA-N kaolin Chemical compound O.O.O=[Al]O[Si](=O)O[Si](=O)O[Al]=O NLYAJNPCOHFWQQ-UHFFFAOYSA-N 0.000 description 2
- 229910052622 kaolinite Inorganic materials 0.000 description 2
- 229910052901 montmorillonite Inorganic materials 0.000 description 2
- VGIBGUSAECPPNB-UHFFFAOYSA-L nonaaluminum;magnesium;tripotassium;1,3-dioxido-2,4,5-trioxa-1,3-disilabicyclo[1.1.1]pentane;iron(2+);oxygen(2-);fluoride;hydroxide Chemical compound [OH-].[O-2].[O-2].[O-2].[O-2].[O-2].[F-].[Mg+2].[Al+3].[Al+3].[Al+3].[Al+3].[Al+3].[Al+3].[Al+3].[Al+3].[Al+3].[K+].[K+].[K+].[Fe+2].O1[Si]2([O-])O[Si]1([O-])O2.O1[Si]2([O-])O[Si]1([O-])O2.O1[Si]2([O-])O[Si]1([O-])O2.O1[Si]2([O-])O[Si]1([O-])O2.O1[Si]2([O-])O[Si]1([O-])O2.O1[Si]2([O-])O[Si]1([O-])O2.O1[Si]2([O-])O[Si]1([O-])O2 VGIBGUSAECPPNB-UHFFFAOYSA-L 0.000 description 2
- 238000000197 pyrolysis Methods 0.000 description 2
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- GMACPFCYCYJHOC-UHFFFAOYSA-N [C].C Chemical compound [C].C GMACPFCYCYJHOC-UHFFFAOYSA-N 0.000 description 1
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- 230000002349 favourable effect Effects 0.000 description 1
- 229930195733 hydrocarbon Natural products 0.000 description 1
- 150000002430 hydrocarbons Chemical class 0.000 description 1
- 229910052500 inorganic mineral Inorganic materials 0.000 description 1
- 239000011229 interlayer Substances 0.000 description 1
- 239000010410 layer Substances 0.000 description 1
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- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 239000004058 oil shale Substances 0.000 description 1
- 239000005416 organic matter Substances 0.000 description 1
- 229910052655 plagioclase feldspar Inorganic materials 0.000 description 1
- 229910052683 pyrite Inorganic materials 0.000 description 1
- 239000011028 pyrite Substances 0.000 description 1
- NIFIFKQPDTWWGU-UHFFFAOYSA-N pyrite Chemical compound [Fe+2].[S-][S-] NIFIFKQPDTWWGU-UHFFFAOYSA-N 0.000 description 1
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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Abstract
The invention relates to a method for representing the component and isotope fractionation effect in the natural gas adsorption and desorption process of shale and belongs to the technical field of instruments used for analyzing materials by aid of measuring physical properties of the materials. The problem that in the prior art, due to the fact that a large amount of gas is lost in the drilling and sampling process, the experiment result cannot completely reflect the desorption process of shale gas is solved. According to the method, a competitive adsorption experiment is carried out in the shale gas adsorption and desorption process under high temperature and high pressure conditions, qualitative analysis is conducted on component fractionation in the process, qualitative analysis is conducted on isotope fractionation, and meanwhile a Langmuir model and an adsorption model are adopted for conducting numerical simulation on the adsorption quantity and the isotope value. Thus, elaborate description of the competitive adsorption effect in the shale gas adsorption and desorption process is realized, and it is beneficial for guiding gas-driven exploitation of the shale gas. The method can be widely applied to mathematical simulation research, gas-driven exploitation of the shale gas and other occasions.
Description
Technical Field
The invention relates to a method for characterizing the fractionation of components and isotopes in the process of adsorbing and desorbing natural gas by shale, belonging to the technical field of instruments for analyzing materials by means of determining the physical properties of the materials.
Background
The adsorption phase is one of the important phase states of shale gas occurrence, and has important significance for the exploration and development of shale gas. In the past, researches on the shale gas in the adsorption phase mainly focus on influencing factors of shale gas adsorption quantity and corresponding quantitative simulation, and few researches on the fractionation of components and isotopes caused by competitive adsorption in the process of shale adsorption and desorption are carried out. The difference of the adsorption performance of each component of the natural gas determines that the adsorption capacity of some gases is strong (weak), the gases are easy (difficult) to be adsorbed and difficult (easy) to be desorbed, and therefore, the components can be fractionated in the adsorption and desorption processes of the natural gas. The previous competitive adsorption research on gas mainly focuses on the field of coal bed gas, and a coal sample tank desorption experiment shows that the difference of adsorption performance among different isotope components exists, and is the extension of component fractionation. Some researchers have conducted exploratory studies on competitive adsorption of shale gas to hydrocarbon gas (C) under low temperature (26 ℃ and 80 ℃) and low pressure (1atm, 2atm, 3atm) conditions of clay minerals (kaolinite, montmorillonite) and oil shale (green river shale)1-C6) Selective adsorption experiments, and shale tank desorption experiments.
As can be seen, the research on the competitive adsorption of shale gas is limited at present, and the research is carried out under the condition of low temperature and pressure. In addition, a large amount of gas is lost in the drilling and sampling process of samples used in the tank desorption experiment and the vacuum ball milling desorption experiment, and the experiment result cannot completely reflect the shale gas desorption process. Therefore, it is necessary to develop competitive adsorption experiments in the shale gas adsorption and desorption process under high temperature and high pressure and corresponding mathematical simulation research, and to finely describe the competitive adsorption in the shale gas adsorption and desorption process so as to guide shale gas drive exploitation.
Disclosure of Invention
The invention aims to overcome the defects in the prior art, and provides a method for representing the components and isotope fractionation in the process of shale gas adsorption and desorption of natural gas, wherein competitive adsorption experiments in the process of shale gas adsorption and desorption under the conditions of high temperature and high pressure are carried out, qualitative analysis is carried out on the component fractionation in the process, and numerical simulation is carried out on the adsorption quantity and the isotope value by adopting a Langmuir model and an adsorption potential model while the isotope fractionation is carried out, so that the fine description of the competitive adsorption in the process of shale gas adsorption and desorption is realized, and the method is favorable for guiding the gas drive exploitation of shale gas.
The invention is realized by adopting the following technical scheme: the method comprises the following steps: carrying out competitive adsorption experiments in the process of shale gas adsorption and desorption under the conditions of high temperature and high pressure: carrying out a multi-component adsorption and desorption experiment on the extracted sample, carrying out free gas sampling at each pressure balance point, analyzing the component composition, and carrying out qualitative analysis on the component fractionation; carrying out single-component adsorption and desorption experiments with different carbon isotope compositions on the extracted samples, sampling free gas at each pressure balance point, analyzing the isotope compositions, carrying out qualitative analysis on isotope fractionation, and carrying out numerical simulation on the adsorption quantity and the isotope value by adopting a Langmuir model and an adsorption potential model.
Step two: the qualitative analysis of the component fractions in the competitive adsorption experiments was performed: the competitive adsorption capacity of each component is determined by analyzing the change of the percentage content of each component of the free gas along with the pressure and the adsorption quantity in the adsorption and desorption process of the mixed gas.
Step three: qualitative analysis of isotope fractionation in competitive adsorption experiments: the competitive adsorption capacity of each isotope is determined by analyzing the change of the composition of a free phase and an adsorbed isotope in the single-component gas adsorption and desorption process along with the pressure and the adsorption quantity. Wherein the carbon isotope value of the adsorption phase is obtained by the formula (1) according to the principle of material balance.
13Cadsorption＝(ninjection·13Cinjection-nfree·13Cfree-ndischarged·13Cdischarged)/nadsorption(1)
Wherein n isinjection、nfree、nadsorption、ndischargedRespectively the amount of injected gas, the amount of free gas in the system, the amount of adsorbed gas and the amount of system pressure relief exhaust gas,13Cinjection、13Cfree、13Cadsorption、13Cdischargedrespectively, the corresponding carbon isotope values.
Step four: the isotope fractionation in the competitive adsorption experiment adopts a Langmuir model and an adsorption potential model to carry out numerical simulation on the adsorption quantity and the isotope value: decomposing the single-component gas adsorption and desorption curve into adsorption and desorption curves of different isotopes, and then respectively optimizing the adsorption quantity calculation parameters of the different isotopes to fit the adsorption and desorption curves to obtain the adsorption quantity calculation parameters of the different isotopes, so that the corresponding carbon isotope values can be calculated (method 1). After the adsorption and desorption curves of different isotopes are obtained, the first pair of isotopes can be selected12CH4Fitting adsorption desorption curves followed by optimization13CH4The calculated adsorption amount parameters of (1) are directly fitted to the carbon isotope values during adsorption/desorption (method 2). The adsorption amount and the isotope value are simulated by respectively adopting a Langmuir model and an adsorption potential model by using the two methods.
Wherein, the langmuir model:
the Langmuir (Langmuir) isothermal adsorption model is the most widely used equilibrium state monolayer adsorption model at present, and the expression of the model is shown as the formula (2). Wherein, ViThe adsorption capacity in cm at the i-th experimental point3/g；VLIs the Langmuir volume, representing the maximum adsorption of the sample in m3/t；pLThe Lane pressure is the pressure corresponding to the adsorption capacity equal to half of the Lane volume, and the unit is MPa; p is a radical ofiIs the pressure at the ith experimental point in MPa. The formula (2) can be transformed into the formula (3), a scatter diagram with p/V as an abscissa and p as an ordinate can be drawn according to experimental data, linear fitting is carried out on the scatter diagram, and the slope of a straight line equation is VLAnd the inverse of the intercept isIs pL。
pi＝VL·pi/Vi-pL(3)
CH may be fractionated when studying isotopes4Is divided into12CH4、13CH4Two components, namely a Langmuir model expression (4). Wherein12CViAnd13CViis the ith experimental point12CH4 and13adsorption amount of CH4 in cm3/g；12CVL、13CVLIs composed of12CH4And13CH4lane volume of (D) in m3/t；12CpL、13CpLIs composed of12CH4And13CH4lane pressure of (D), in MPa;12Cpiand13Cpiis the ith experimental point12CH4And13CH4the partial pressure of (b) is in MPa.
Fitting CH using Langmuir model4The carbon isotope curve of adsorption and desorption is actually obtained by optimization12CH4And13CH4the Langmuir volume and the Langmuir pressure. The first method is to fit separately12CH4And13CH4p to p/V test points of (1), obtaining12CH4、13CH4The corresponding isotope value can be calculated according to the Langmuir volume and the Langmuir pressure.
The second method is to obtain the product by the method 112CH4Is obtained by fitting the values of the adsorbed phase methane isotope13CH4The Langmuir volume and the Langmuir pressure. The i-th experimental point isotope value can be obtained from the formula (4), as shown in the formula (5), wherein RstIs a labelQuasi-sample carbon isotope ratio, i.e. 1123.72 × 10-5。
① an objective function is constructed with an experimental isotope value of13Ci-expUnder the same conditions, assume that13CVL、13CpLThe isotope value calculated from the model (5) is then13Ci-cal. If there is a certain group13CVL、13CpLMake all the experimental points have13Ci-calIs equal to13Ci-expThen the group13CVL、13CpLThe result is obtained. But this is practically impossible due to experimental errors, model imperfections, etc. Therefore, it is only necessary to13Ci-calAnd13Ci-expthe difference is as small as possible13CVL、13CpL. To this end, an objective function is constructed, as shown in equation (6).
Where n is the number of sampling points on the adsorption or desorption experimental curve. In this way it is possible to obtain,13CVL、13CpLbecomes the minimum point problem of the non-negative objective function (6) and needs to meet the requirement13CVL、13CpLIs a non-negative number.
② penalty function is constructed, the above solution problem of minimal value with constraint condition is complex, because it needs to make target be not onlyBesides the gradual decrease of the function value, the reasonability of the solution is also noticed, namely whether the solution meets the constraint condition of non-negative number. And the problem of the constrained extreme value is changed into the problem of the unconstrained extreme value by adopting a penalty function method. For any constraint condition, a function may be constructed, and when the obtained extreme point satisfies the condition, the function value is 0, otherwise, it is a positive number. To pair13CVLAnd (4) a constraint condition of being larger than zero, and constructing a function as the formula (7).
Namely, it is
The same principle is as follows:
the formulas (8) and (9) can be combined as follows:
G(13CVL,13CpL)＝G1+G2
＝[min(0,13CVL)]2+[min(0,13CpL)]2(10)
taking a sufficiently large positive integer R, a penalty function can be constructed from the equations (6) and (10):
F(13CVL,13CpL)＝Q(13CVL,13CpL)+R·G(13CVL,13CpL)(11)
if the minimum point is beyond the constraint condition, gradually increasing R, and when R is sufficiently large, the minimum solution of the formula (11) is the minimum solution of the formula (6) of the objective function, so that the constrained extreme value problem is converted into the unconstrained extreme value problem which is relatively easy to solve. The solution to the unconstrained value problem mathematically provides a variety of optimization algorithms. The second derivative matrix and the variable-scale method of the inverse matrix are selected to perform programming calculation, wherein the second derivative matrix has high convergence speed and does not need complicated calculation.
The Langmuir model is applied to obtain an isothermal adsorption formula applicable to a certain temperature condition from isothermal adsorption experimental data at the temperature, and the formula cannot predict adsorption amounts at other different temperature conditions. In actual geological work, the adsorption capacity under different temperature conditions is often required to be predicted. If the Langmuir model is applied to achieve the purpose, isothermal adsorption experiments at different temperatures need to be carried out respectively, the experiment workload is large, and the adsorption potential model can better solve the problem.
Wherein, the adsorption potential model is as follows:
the adsorption potential theory considers that the adsorption acting force between gas and solid is mainly dispersion force, and under the condition that an adsorbent and an adsorbate are fixed, a relation curve of the adsorption potential and the volume of an adsorption phase is unique and does not change along with the temperature, and the curve is called an adsorption characteristic curve. Therefore, if the adsorption isotherm data at a certain temperature is known and the adsorption characteristic curve is obtained, the adsorption data at any temperature can be calculated.
The key of establishing the adsorption potential model is to obtain an adsorption characteristic curve by calculating the adsorption potential and the volume of an adsorption phase. According to the adsorption potential theory, the adsorption potential corresponds to the pressure and temperature relationship of equation (12). Since the formation temperature is typically above the critical temperature of methane and the like, the saturated vapor pressure at critical conditions is not present. For this purpose Amankwah (1995) establishes a virtual saturated vapor pressure P under supercritical conditionss(formula (13)). The corresponding adsorption potential can be obtained by bringing the formula (13) into the formula (12). The adsorption potential is obtained, and an adsorption characteristic curve is established by combining the adsorption phase volume calculation formula (14)). When applied to geology, the adsorption potential is determined by the temperature and pressure conditions, and then the corresponding adsorption capacity can be calculated by utilizing the established adsorption characteristic curve (liatal, 2015).
Wherein P is equilibrium pressure (MPa); is the adsorption potential (J/mol); p0Is the gas saturation vapor pressure (MPa); piThe equilibrium pressure (MPa) of the gas at a certain temperature; r is a gas constant (8.314J-1.K-1) (ii) a T is the absolute temperature (K).
Ps＝Pc(T/Tc)k(13)
Wherein, PcAnd TcThe critical pressure and the temperature are respectively, k is a constant, the optimization can be carried out, and the value of the research is 2.
w＝VadM/ρad(14)
Wherein w is the volume of the adsorption phase (cm)3/g)；VadMeasured adsorption capacity (mol/g); m is the gas molar mass (g/mol); rhoadAs a gas adsorption phase density (g/cm)3) In the study, the density of the methane adsorption phase is taken to be 0.375g/cm3。
The following calculation procedure was followed with the Langmuir model, using the adsorption potential model to fit CH4The carbon isotope curve of the adsorption phase in the adsorption and desorption process is actually obtained by optimization12CH4And13CH4adsorption characteristic curve of (1). The first method is to fit separately12CH4And13CH4the relationship between the volume of the adsorption phase and the adsorption potential in the adsorption and desorption process is obtained12CH4、13CH4The corresponding isotope value can be calculated according to the adsorption characteristic curve.
The second method is to obtain the product by the method 112CH4After the adsorption characteristic curve is obtained by fitting the adsorption phase methane isotope value13CH4Adsorption characteristic curve of (1).12CH4And13CH4adsorption and desorptionIn the process, the relationship between the volume of the adsorption phase and the adsorption potential can be well fitted by using a unitary cubic polynomial, so that an adsorption characteristic curve equation is established as shown in a formula (15).
Wherein,12Cwi、13Cwiare respectively the ith experiment point12CH4And13CH4the volume of the adsorption phase of (a),12C i、13C iare respectively the ith experiment point12CH4And13CH4a, b, c, d and a ', b', c ', d' are constant.
The i-th experimental point isotope value can be obtained from the formula (15), and is shown in the formula (16). Thus, as in the langmuir model, the objective function is constructed as shown in equation (17), and a ', b', c ', and d' are optimized so that the Q values are as small as possible.
Compared with the prior art, the invention has the beneficial effects that: the method for representing the component and isotope fractionation effect in the natural gas adsorbing and desorbing process by shale carries out competitive adsorption experiments in the process of shale gas adsorbing and desorbing under the conditions of high temperature and high pressure, carries out qualitative analysis on the component fractionation in the process, and carries out numerical simulation on the adsorption quantity and the isotope value by adopting a Langmuir model and an adsorption potential model while carrying out qualitative analysis on the isotope fractionation. The method realizes the fine description of competitive adsorption in the adsorption and desorption process of the shale gas and is beneficial to guiding the gas drive exploitation of the shale gas.
Drawings
FIG. 1 shows CH in free gas during adsorption and desorption of mixed gas4The percentage is plotted as a function of pressure (a) (b) and adsorbed amount (c) (d).
FIG. 2 is CH4The composition of the free-phase and adsorption-phase carbon isotopes during adsorption/desorption is plotted as a function of pressure (a) (b) and adsorption amount (c).
FIG. 3 is a Langmuir model and adsorption potential model pair12CH4(a)、13CH4(b) Fitting graph of adsorption amount in adsorption process.
FIG. 4 is a Langmuir model and adsorption potential model pair12CH4(a)、13CH4(b) Fitting graph of adsorption amount during desorption.
FIG. 5 shows the Langmuir model and adsorption potential model for CH4And (c) fitting graphs of adsorption isotope values in the processes of adsorption (a) and desorption (b).
Detailed Description
In order to make the objects, research methods and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the following examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The technical method of the present invention will be further described with reference to the accompanying drawings and specific embodiments.
Three sub-segments of sand, namely 3336.79m in depth and II in organic matter type, of the mountain Jiyang depression river 172 well in Bohai Bay basin in China are selected in the research1And (4) molding. The original total organic carbon content of the sample was 2.98%, and the pyrolysis parameters were as follows: s1＝2.15mg/g，S216.74mg/g, and 443 ℃ in Tmax; total organic carbon content after extractionThe amount was 2.75%, and the pyrolysis parameters were as follows: s1＝0.14mg/g，S214.47mg/g, and 441 ℃ Tmax. X-ray diffraction analysis showed that the sample mineral composition was: 39% of clay, 22% of quartz, 2% of plagioclase, 29% of calcite, 5% of pyrite and 3% of siderite; the relative content of clay minerals is as follows: 4% of kaolinite, 2% of chlorite, 19% of illite, 75% of illite/montmorillonite layer and 20% of interlayer ratio.
This time, CH was developed for the extracted samples4、50％CH4+50％CO2、50％CH4+50％N2And (3) performing adsorption and desorption experiments, sampling free gas at each pressure balance point, and analyzing the components and/or the isotope composition.
FIG. 1 shows CH in free gas during adsorption and desorption of mixed gas4The percentage is plotted as a function of pressure and adsorption. As can be seen from FIG. 1a, 50% CH4+50％CO2CH in competitive adsorption process4Adsorption capacity weaker than that of CO2CH in free-phase gas mixture4The content is always greater than 50% and increases gradually with increasing pressure. As can be seen from FIG. 1b, 50% CH4+50％N2CH in competitive adsorption process4Adsorption capacity weaker than N2CH in free-phase gas mixture4The content is likewise always greater than 50%. However, the change law with increasing pressure is not obvious and shows a weak inverse parabolic shape. As can be seen from FIGS. 1a to 1b, 50% CH is present during desorption4+50％CO2、50％CH4+50％N2Two systems CH4The percentage content is steadily increased. Wherein, 50% CH4+50％CO2The system rise amplitude is 8.35%, and 50% CH4+50％N2The system has a small rise of 5.74%. The above phenomenon indicates that CO2、N2Relative CH4Has the characteristics of preferential adsorption and delayed desorption, can be used for displacement exploitation of shale gas, but CO2The displacement effect is better than N2，CO2The methane can be well stored in the shale while being displaced. As can be seen from FIGS. 1c to 1d, CH is present in the free gas4The percentage content curve has obvious changes in the later period of adsorption and the early period of desorption, which shows that the competitive adsorption effect is most obvious when the gas amount in the system is maximum.
FIG. 2 depicts CH4The composition of the carbon isotopes in the free phase and the adsorption phase in the adsorption and desorption process changes with the pressure and the adsorption amount, wherein the value of the carbon isotopes in the adsorption phase is obtained by the formula (1) according to the substance balance principle. As can be seen from FIG. 2a, during the methane adsorption-desorption experiment13CH4Preferential adsorption and delayed desorption are carried out, so that the carbon isotope value of the free methane is gradually reduced in the adsorption process and gradually increased in the desorption process. In addition, from CH4The composition of the carbon isotope in the adsorption phase during the adsorption and desorption process is changed with the pressure (figure 2b) and the adsorption amount (figure 2c), and it can be seen that the carbon isotope fractionation is obviously stronger in the later period of the adsorption process and the early period of the desorption process, which is similar to the component fractionation.
Numerical simulation of natural gas isotope values is essentially for each isotope component (e.g., for each isotope component)12CH4And13CH4) The research simulates the carbon isotope fractionation in the methane adsorption and desorption process.
FIGS. 3 and 4 depict the Langmuir model and adsorption potential model pair12CH4、13CH4The fitting condition of the adsorption quantity in the adsorption and desorption processes can be seen, and better fitting effects can be achieved by different models and different modeling methods on the whole. However, since method 1 is to directly fit the adsorption amount, method 2 is to fit the isotope value, which is to the adsorption amount13CH4The simulation of the amount of adsorption is indirect. Thus, method 1 is on13CH4The fitting accuracy of the adsorption amount was slightly higher than that of method 2, and the ratios of the square sum of errors (Q) were 0.80 and 0.71 for the langmuir model method 1 and method 2, respectively, and 0.99 and 1 for the adsorption potential model, respectively. However, the accuracy of the fit of method 2 to the isotope values was significantly higher than that of method 1 (FIG. 5), the ratios of the sum of squared errors (Q) were 0.43 and 0.30 for Langmuir model method 2 and method 1, respectively, and the adsorption potentials were 0.43 and 0.30The models were 0.93 and 0.88, respectively. Overall the fitting effect of method 2 is better than method 1 because of the smallness13CH4The change in the amount of adsorption of (a) may cause a larger change in the value of the isotope of methane carbon, so method 2 suffers little from the sacrifice of fitting the amount of adsorption when fitting the isotope value. The Langmuir model cannot effectively fit isotope values and can only be applied to fitting of the adsorption capacity, while the adsorption potential model can well fit the adsorption capacity and the isotope values, and the adsorption potential model can well fit the adsorption capacity and the isotope values12CH4、13CH4The fitting accuracy of the adsorption amount is higher than that of the Langmuir model, which shows that the fitting of the adsorption phase isotope value is difficult to fit the adsorption amount, and the fitting capability of the adsorption potential model is higher than that of the Langmuir model.
The above-described embodiments are merely illustrative of the preferred embodiments of the present invention and do not limit the spirit and scope of the present invention. Various modifications and improvements of the technical solutions of the present invention may be made by those skilled in the art without departing from the design concept of the present invention, and the technical contents of the present invention are all described in the claims.
Claims (10)
1. A method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas is characterized in that: the method comprises the following steps:
the method comprises the following steps: carrying out a competitive adsorption experiment in the shale gas adsorption and desorption process under the high-temperature and high-pressure condition;
step two: carrying out qualitative analysis on component fractionation in a competitive adsorption experiment;
step three: performing qualitative analysis on isotope fractionation in a competitive adsorption experiment;
step four: for isotope fractionation in competitive adsorption experiments, a Langmuir model and an adsorption potential model are adopted to carry out numerical simulation on the adsorption quantity and the isotope value.
2. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 1, wherein: in the first step, carrying out a multi-component adsorption and desorption experiment on an extraction sample, carrying out free gas sampling at each pressure balance point, analyzing component composition, and carrying out qualitative analysis on component fractionation; carrying out single-component adsorption and desorption experiments with different carbon isotope compositions on the extracted samples, sampling free gas at each pressure balance point, analyzing the isotope compositions, carrying out qualitative analysis on isotope fractionation, and carrying out numerical simulation on the adsorption quantity and the isotope value by adopting a Langmuir model and an adsorption potential model.
3. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 1, wherein: in the second step, the percentage content of each component of the free gas is analyzed to change along with the pressure and the adsorption quantity in the process of adsorbing and desorbing the mixed gas, so that the competitive adsorption capacity of each component is determined.
4. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 1, wherein: in the third step, the competitive adsorption capacity of each isotope is determined by analyzing the change of the composition of a free phase and adsorbed isotope in the process of single-component gas adsorption and desorption along with the pressure and the adsorption quantity;
wherein the carbon isotope value of the adsorption phase is obtained by the following formula (1) according to the principle of material balance:
13Cadsorption＝(ninjection·13Cinjection-nfree·13Cfree-ndischarged·13Cdischarged)/nadsorption(1)
wherein n isinjection、nfree、nadsorption、ndischargedRespectively the amount of injected gas, the amount of free gas in the system, the amount of adsorbed gas and the amount of system pressure relief exhaust gas,13Cinjection、13Cfree、13Cadsorption、13Cdischargedrespectively, the corresponding carbon isotope values.
5. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 1, wherein: in the fourth step, the single-component gas adsorption and desorption curve is decomposed into adsorption and desorption curves of different isotopes, then the adsorption and desorption curves are fitted by respectively optimizing the adsorption quantity calculation parameters of the different isotopes, and the corresponding carbon isotope values can be calculated by obtaining the adsorption quantity calculation parameters of the different isotopes.
6. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 5, wherein: adopting a Langmuir model to simulate the adsorption amount and the isotope value, and respectively fitting12CH4And13CH4p to p/V test points of (1), obtaining12CH4、13CH4The corresponding isotope value can be calculated according to the Langmuir volume and the Langmuir pressure.
7. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 5, wherein: adopting an adsorption potential model to simulate the adsorption quantity and the isotope value, and respectively fitting12CH4And13CH4the relationship between the volume of the adsorption phase and the adsorption potential in the adsorption and desorption process is obtained12CH4、13CH4The corresponding isotope value can be calculated according to the adsorption characteristic curve.
8.The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 1, wherein: in the fourth step, after the adsorption and desorption curves of different isotopes are obtained, the first step is to carry out the adsorption and desorption on different isotopes12CH4Fitting adsorption desorption curves followed by optimization13CH4The calculated parameters of the adsorption quantity are directly fitted with the carbon isotope value in the adsorption and desorption process.
9. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 8, wherein: adopting a Langmuir model to simulate the adsorption quantity and the isotope value to obtain12CH4Is obtained by fitting the values of the adsorbed phase methane isotope13CH4The Langmuir volume and the Langmuir pressure.
10. The method for characterizing the fractionation of components and isotopes in the process of shale adsorption and desorption of natural gas according to claim 8, wherein: adopting an adsorption potential model to simulate the adsorption quantity and the isotope value to obtain12CH4After the adsorption characteristic curve is obtained by fitting the adsorption phase methane isotope value13CH4Adsorption characteristic curve of (1).
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