Shale gas adsorption density calculation method

Abstract

The invention discloses a shale gas adsorption density calculation method. A shale kerogen model is constructed by adopting a molecular simulation technology, a high-temperature and high-pressure shale gas isothermal adsorption curve is simulated and calculated, and rock gas absolute adsorption capacity is converted into relative adsorption capacity according to a function relationship between the relative adsorption capacity and the absolute adsorption capacity established according to molecular quantity characteristics of adsorbed gas and free gas; deriving and establishing a function relationship between relative adsorption capacity and absolute adsorption capacity of adsorption gas density correction based on an adsorption theory, and meanwhile, calculating to obtain a shale gas adsorption state density change curve chart; by introducing a correction coefficient, a method for representing the adsorption state density through the shale gas free state density is provided, and an adsorption gas density calculation model is established. A high-temperature and high-pressure shale gas adsorption state density change characteristic research method is innovatively provided, shale gas adsorption state density scale upgrading application combining microcosmic and macroscopic is achieved, change characteristics of shale gas adsorption density can be revealed more accurately, and accurate calculation is conducted through a formula model.

Description

Technical Field

[0001] The present invention belongs to the technical field of shale gas exploitation, and in particular relates to a method for calculating the adsorption density of shale gas. Background Art

[0002] Shale gas exists in reservoirs in various states: free, adsorbed, and dissolved. Adsorbed gas, a key form of shale gas, accumulates on the surfaces of kerogen and clay particles, accounting for approximately 20% to 85%. Because adsorbed gas influences reserve evaluation and gas flow patterns, such as surface diffusion, numerous researchers have conducted in-depth research on this key issue, achieving significant progress.

[0003] Theories of shale gas adsorption have been proposed, including two-dimensional adsorption film theory, single / multilayer adsorption theory, micropore filling theory, simplified local density (SLD) adsorption theory, and Ono-Kondo adsorption theory. Each theory is described by a corresponding mathematical model. The Langmuir model, representing single-molecule adsorption, is currently the most widely used due to its simplicity and clear physical meaning. In addition, there are combined adsorption models based on multiple adsorption processes, such as the DA-Langmuir model corresponding to single-layer adsorption and micropore filling adsorption. Other adsorption models without physical context, such as the spherical model of the variogram, have been proposed based on the variations of general mathematical models. However, these models are rarely used due to their complex structure or lack of physical meaning.

[0004] Methods commonly used to evaluate shale gas adsorption capacity include well logging interpretation, field desorption, shale gas isothermal adsorption experiments, nuclear magnetic resonance (NMR), and molecular simulation. Since methane is the primary component of shale gas, accounting for over 94%, methane is often used as a proxy for shale gas in related theoretical studies. Each method has its advantages and disadvantages. Currently, isothermal adsorption experiments, including volumetric and gravimetric methods, are the most widely used. However, numerous test results have shown that, regardless of the testing method used, the amount of adsorbed gas measured typically shows an initial upward trend followed by a downward trend, and some even show negative adsorption. This is widely believed to be due to the density difference between the adsorbed gas and the free gas. The experimentally measured adsorption amount is referred to as the excess adsorption amount (also known as relative adsorption amount, apparent adsorption amount, or excess adsorption amount). The adsorption amount in porous media is called the absolute adsorption amount, which is positively correlated with pressure. The key to converting excess adsorption amount to absolute adsorption amount is determining the adsorbed gas density.

[0005] Although adsorbed gas density is crucial for data processing of isothermal adsorption test results, experimental investigation of its changing characteristics is difficult due to the typically thin adsorption layer and its dynamic transition with the free gas. Typically, the adsorbed gas density is simply treated as an undetermined coefficient, and fitted with the adsorption experimental data to obtain an average value, which fails to clearly define how the adsorbed gas density varies with temperature and pressure. Existing methods employ the van der Waals method, the atmospheric boiling point liquid density approximation, the critical density approximation, and the superheated liquid density approximation to approximate adsorbed gas density, but the resulting adsorbed gas density values ​​vary significantly across these methods.

[0006] In recent years, molecular simulation techniques have been widely used to study the microscopic mechanisms of shale gas adsorption. Compared to testing methods, molecular simulation can simulate shale gas adsorption behavior at higher temperatures and pressures, clarifying the micromechanical mechanisms of gas-solid interactions from a molecular dynamics perspective, thereby revealing the microscopic mechanisms of shale gas adsorption. Although molecular simulation has been extensively studied on the characteristics of shale gas adsorption density, revealing the microscopic distribution of gas density within pore spaces, determining whether adsorption occurs based on density, and revealing adsorption mechanisms such as layer adsorption or micropore filling, there is currently no research on the macroscopic variation of adsorbed gas density with temperature and pressure.

[0007] In summary, although rich achievements have been made in shale gas adsorption theory, adsorption models and evaluation methods, the change of adsorbed gas density, one of the key adsorption parameters, has only remained at the stage of microscopic change law research, and the macroscopic change characteristics are still lacking. As a result, large errors may occur when processing experimental test results, and the excess adsorption amount cannot be accurately converted into absolute adsorption amount, which in turn reduces the reliability of adsorption parameters in the adsorption model. Of course, shale gas reserves cannot be accurately evaluated, which is not conducive to the formulation and optimization of production systems during the mining process. Summary of the Invention

[0008] In view of the above-mentioned deficiencies in the prior art, the technical problem to be solved by the present invention is: how to provide a method for calculating the adsorption density of shale gas to solve the problem that the prior art cannot accurately obtain the adsorption density of shale gas.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] A method for calculating shale gas adsorption density includes the following steps:

[0011] S1, based on adsorption theory, establish the relative adsorption capacity V corrected by adsorption gas density ex and the absolute adsorption capacity V ab Based on molecular simulation technology, the absolute adsorption capacity V is corrected by the number characteristics of adsorbed gas and free gas molecules.ab and relative adsorption capacity V ex The conversion relationship function between them;

[0012] S2, constructing a shale kerogen model;

[0013] S3, based on the shale kerogen model established in step S2, simulate and calculate the absolute adsorption amount V at different temperatures ab , obtain the corresponding shale gas isothermal adsorption curve, and use the absolute adsorption capacity V established in step S1 ab and relative adsorption capacity V ex The conversion relationship function between the absolute adsorption amount V ab Converted to relative adsorption capacity V ex , and presented in the form of a chart;

[0014] S4, based on the relative adsorption amount V established in step S1 ex and the absolute adsorption capacity V ab The relationship function between the two, and the absolute adsorption amount V obtained in step S3 ab and relative adsorption capacity V ex , the corresponding shale gas adsorption density change curve is calculated;

[0015] Using the NIST database, we searched for the corresponding shale gas free-state density change values ​​under simulated temperature and pressure, and drew a shale gas free-state density change curve.

[0016] S5, using the gas state equation, proposes a method for characterizing the adsorbed state density based on the free state density of shale gas. By introducing a correction coefficient α and combining it with the curve of changes in the free state and adsorbed state densities of shale gas in step S4, the density correction coefficient α at different temperatures is obtained. Based on the density correction coefficient α, data processing is performed to obtain a relationship function between the density correction coefficient α and temperature and pressure.

[0017] S6, based on step S5, deriving a calculation formula for adsorbed gas density taking temperature and pressure into consideration.

[0018] By adopting the above scheme, based on molecular simulation theory, through the adsorption microscopic simulation results, combined with adsorption theory and gas PVT properties, an innovative research method for the change characteristics of macroscopic adsorption state density is proposed, realizing the scale upgrade application combining micro and macro, which can more accurately reveal the change characteristics of shale gas adsorption density and accurately calculate it through formula models, which has important positive significance for shale gas reserve evaluation and production measures.

[0019] Preferably, in step S2, a corresponding type of kerogen molecular model is first selected based on the shale sample to be calculated. Secondly, the number of kerogen molecules loaded is considered during the overall model construction process, and the kerogen model is optimized based on at least three schemes: geometry optimization, energy optimization, and simulated annealing. Due to the randomness of kerogen molecules within the unit cell, kerogen models constructed by different researchers often differ, thus requiring verification of the kerogen model. However, a commonly used kerogen model verification method is to compare kerogen density, resulting in significant differences in the morphology and adsorption amount of methane isothermal adsorption curves constructed by different researchers using the same kerogen molecular model under the same simulation conditions. Therefore, the prior art proposes a new scheme for verifying kerogen models using isothermal adsorption experiments. However, this scheme ignores the influence of the number of kerogen molecules on the kerogen model construction process, resulting in poor reliability of the kerogen model. In contrast, the present application addresses the influence of the number of kerogen molecules and optimizes it, thereby improving the reliability of the kerogen model.

[0020] As a preference, the number of kerogen molecules loaded is 10-13. Using the above scheme, when the molecular loading is within this range, the reliability of the kerogen model is relatively the best, and the results are more accurate.

[0021] Preferably, the number of kerogen molecules loaded is 11. Relatively speaking, the model is more accurate and reliable at this time.

[0022] Preferably, in step S5, the calculated density correction coefficient α is fitted using a hyperbolic function to obtain a relationship function between the density correction coefficient α and the pressure P at different temperatures.

[0023]

[0024] According to the relationship function between the density correction coefficient α and the pressure P, a scatter diagram between the parameter a, parameter b and the temperature is drawn, and the relationship function between the density correction coefficient α and the pressure P and temperature T is obtained.

[0025] in

[0026]

[0027] By adopting the above scheme, the changing trend of the correction coefficient α can be fitted more accurately and the accuracy of the calculation results can be improved.

[0028] As an example, the calculation formula for the adsorbed gas density is:

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The shale gas adsorption density calculation method provided by the present invention is based on molecular simulation theory. Through the adsorption microscopic simulation results, combined with adsorption theory and gas PVT properties, an innovative research method for the change characteristics of macroscopic adsorption state density is proposed, which realizes the scale upgrade application combining micro and macro. It can more accurately reveal the change characteristics of shale gas adsorption density and accurately calculate it through formula models. It is particularly suitable for the calculation of high-temperature and high-pressure shale gas adsorption density, and has important positive significance for shale gas and production measures. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the steps of the present invention;

[0032] Figure 2 This is the molecular model diagram of shale kerogen in the target layer;

[0033] Figure 3 Schematic diagram comparing the adsorption values ​​simulated by the kerogen model and the experimental values ​​for the number of molecules loaded;

[0034] Figure 4 This is a schematic diagram of the kerogen model structure for this application;

[0035] Figure 5 This is a schematic diagram of the pore space distribution of the kerogen model structure in this application;

[0036] Figure 6 isothermal adsorption curves of methane at different temperatures;

[0037] Figure 7 The isosteric adsorption heat-pressure relationship diagram of methane at different temperatures;

[0038] Figure 8 The relationship between the isosteric adsorption heat and adsorption amount of methane at different temperatures;

[0039] Figure 9 The curves of methane free state and adsorption state density change at different temperatures;

[0040] Figure 10 The curve diagram of the change of correction coefficient α at different temperatures;

[0041] Figure 11 Schematic diagram of the fitting results of the correction coefficient α at different temperatures;

[0042] Figure 12 is a scatter plot between parameter a and temperature;

[0043] Figure 13 is a scatter plot between parameter b and temperature;

[0044] Figure 14 Schematic diagram of methane adsorption isotherm curve at 150℃;

[0045] Figure 15 Schematic diagram of the comparison between the simulated density and the calculated density of the methane adsorption phase at a temperature of 150°C;

[0046] Figure 16 This is a comparison chart of the absolute adsorption amount calculation results under different adsorption phase density treatment methods;

[0047] Figure 17 This is a graph showing the relationship between fugacity and fugacity coefficient as a function of pressure. DETAILED DESCRIPTION

[0048] The present invention will be described in further detail below with reference to the accompanying drawings.

[0049] refer to Figures 1 to 17 The calculation method of shale gas adsorption density shown in the figure mainly includes the following steps: the first step is to establish the relative adsorption amount V corrected by adsorption gas density based on adsorption theory. ex and the absolute adsorption capacity V ab Based on molecular simulation technology, the absolute adsorption capacity V is corrected by the number characteristics of adsorbed gas and free gas molecules. ab and relative adsorption capacity V ex The conversion relationship function between them;

[0050] The second step is to build a shale kerogen model;

[0051] The third step is to simulate and calculate the absolute adsorption capacity V at different temperatures based on the shale kerogen model established in the second step. ab , the corresponding shale gas isotherm adsorption curve is obtained, and the absolute adsorption capacity V established in the first step is used ab and relative adsorption capacity V ex The conversion relationship function between the absolute adsorption amount V ab Converted to relative adsorption capacity V ex , and presented in the form of a chart;

[0052] The fourth step is based on the relative adsorption capacity V established in the first step. ex and the absolute adsorption capacity V ab The relationship function between the two, and the absolute adsorption amount V obtained in the third step ab and relative adsorption capacity V ex , the corresponding adsorption density change curve of shale gas (methane) is calculated;

[0053] Using the NIST database, we searched for the corresponding shale gas (methane) free state density change values ​​under simulated temperature and pressure, and drew a shale gas (methane) free state density change curve.

[0054] In the fifth step, a method for characterizing the adsorbed state density based on the free state density of shale gas was proposed using the gas state equation. By introducing the correction coefficient α and combining it with the curves of the free and adsorbed state densities of shale gas in the fourth step, the density correction coefficient α at different temperatures was obtained. Based on the density correction coefficient α, data processing was performed to obtain the relationship function between the density correction coefficient α and temperature and pressure.

[0055] Step 6: Derive the calculation formula for adsorbed gas density based on step 5.

[0056] Specifically, the results of the shale gas isothermal adsorption test show that the measured isothermal adsorption curve shows a change pattern of first rising and then falling, that is, there is a "negative adsorption" phenomenon, and even the adsorption amount is negative. At the same time, a large number of molecular simulation studies have shown that the density of adsorbed gas is related to the distance from the pore wall. Therefore, the absolute adsorption amount V ab It can be expressed as:

[0057]

[0058] Where V ab ——Absolute adsorption capacity, mmol / g;

[0059] V a ——adsorption space volume, cm 3 ;

[0060] ρ a,i (z)——adsorbed gas density at a distance z above the pore wall, g / cm 3 ;

[0061] ρ a ——Average adsorption gas density in the adsorption space at a certain temperature and pressure, g / cm 3 ;

[0062] ρ g ——Free gas density, g / cm 3 ;

[0063] m s ——mass of adsorbent, g;

[0064] M——molar mass of gas, kg / mol.

[0065] The relative adsorption capacity is caused by the density difference between the adsorbed gas and the free gas. Therefore, the relative adsorption capacity can be expressed as:

[0066]

[0067] Where V ex ——Relative adsorption capacity, mmol / g;

[0068] Derivation (2) yields:

[0069]

[0070] Formula (3) is the relative adsorption capacity V ex and the absolute adsorption capacity V ab In the calculation process, the free gas density ρ g It is usually obtained by consulting existing data or calculating the state equation, which is relatively easy to obtain. However, in the prior art, the adsorbed gas density ρ a It is basically regarded as an undetermined constant coefficient and then fitted or assigned according to Table 1.

[0071] Table 1 Summary of methane adsorption state density assignments

[0072]

[0073] The applicant believes that in actual production, the adsorption density should be a function of temperature and pressure. If the adsorption density is only regarded as a constant, it will inevitably lead to large errors. The molecular simulation method can use formula (4) to convert the absolute adsorption amount obtained by simulation into relative adsorption amount without determining the adsorption density, thereby avoiding the error caused by using the adsorption density. Formula (4) is the absolute adsorption amount V ab The relative adsorption capacity V ex The conversion relationship function.

[0074]

[0075] Where N t ——The sum of free gas content and adsorbed gas content, mmol / g;

[0076] V p ——pore volume, cm 3 .

[0077] Kerogen can be generally divided into three types: Type I, Type II, and Type III. In the actual modeling process, the selection is mainly based on the conditions of the target formation. In this embodiment, the target shale layer mainly has Type I and Type II kerogen, with Type II being the main type. Therefore, the Type II kerogen molecular model (C 205 H 158 O 19 N4S4) Figure 2 shown.

[0078] Due to the randomness of kerogen molecules in the unit cell, the kerogen models established by different scholars are usually different, so the kerogen models must be verified. However, the commonly used kerogen model verification method is to compare with the kerogen density, which leads to obvious differences in the morphology of the methane isothermal adsorption curve and the adsorption amount of the kerogen models constructed by different scholars using the same kerogen molecular model under the same simulation conditions. The prior art "Molecular simulation of methane adsorption performance of kerogen in shale gas layers of Longmaxi Formation" proposed a new scheme for verifying the kerogen model using isothermal adsorption experiments, but there was a large difference between the simulation results and the experimental results. The applicant found in further research that the number of kerogen molecules loaded plays a large role in the model. Therefore, in the modeling process of this application, the number of kerogen molecules loaded is fully considered, and the kerogen model is optimized based on three schemes: geometry optimization, energy optimization, and simulated annealing.

[0079] Key References Figures 3 to 5 The GCMC method was used to simulate the methane isothermal adsorption of the kerogen model with different molecular loadings at 60°C at pressures of 1MPa, 2MPa, 4MPa, 8MPa, 11MPa, 15MPa, and 19MPa. The absolute adsorption amount obtained by simulation was converted into relative adsorption amount according to formula (4). The results were compared with the experimental values. Figure 5 As shown in the figure, it can be seen that the number of kerogen molecules has a significant effect on methane adsorption, and the simulated adsorption results are closest to the experimental results only when the loading is 11. Therefore, it is unreasonable to ignore the influence of the number of kerogen molecules in the construction of existing kerogen models. The number of kerogen molecules must be verified in combination with isothermal adsorption experimental data. Only when the simulated data are close to the experimental data can the reliability of the constructed kerogen model be guaranteed.

[0080] According to the established kerogen model, the absolute adsorption capacity V at different temperatures can be simulated and calculated. ab Based on formula (4), the relative adsorption capacity V can be calculated simultaneously ex In this embodiment, four groups of isothermal adsorption curves at 60℃, 80℃, 100℃ and 120℃ are simulated respectively, and each group of isothermal adsorption curves is composed of 12 groups of pressure measurement points, where the pressure step length of the low pressure zone is 4MPa and that of the high pressure zone is 15MPa. The absolute adsorption amount of the simulation is converted into the relative adsorption amount using formula (4). The results are as follows: Figure 6 As shown in the figure, for the convenience of observation, the relative adsorption amount and absolute adsorption amount are represented by a graph. Further analysis shows that: at the same temperature, the absolute adsorption amount of methane adsorbed by kerogen increases first quickly and then slowly with the increase of pressure, and decreases with the increase of temperature, which is consistent with the type I isotherm adsorption curve.

[0081] To further demonstrate the reliability of the kerogen model in this application, the adsorption mechanism is further verified in the specific examples. Generally, a thermal effect will occur during the adsorption process, and the thermal effect of adsorption can be characterized by the isosteric heat of adsorption. The isosteric heat of adsorption refers to the heat released by the adsorption of the adsorbate on the adsorbent surface when the pressure, temperature and adsorbent surface area are constant. The adsorption heat of chemical adsorption is usually between 40 and 600 kJ / mol. Figure 7 It can be seen that the isosteric adsorption heat of methane on kerogen at different temperatures is lower than 40 kJ / mol, indicating that the adsorption on kerogen is physical adsorption, and the isosteric adsorption heat of each methane in the high-pressure stage has tended to be stable at about 20.7 kJ / mol; at the same time, the isosteric adsorption heat of methane decreases with increasing temperature. This is because as the temperature increases, the activation energy required for gas adsorption on kerogen increases, and the adsorption heat generated decreases.

[0082] like Figure 8 As shown, the relationship between the isosteric adsorption heat and the adsorption amount of methane at different temperatures was further established. According to the variation characteristics of the isosteric adsorption heat with the adsorption amount, it can be seen that the isosteric adsorption heat of methane at each temperature increases with the increase of the adsorption amount. The interaction force between methane molecules in the surface kerogen is dominant, which confirms that the molecular weight of kerogen plays a key role in the adsorption process.

[0083] Based on the absolute adsorption amount calculated by kerogen model simulation, the relative adsorption amount is converted. Since the adsorption density is not used as an intermediate variable in this process, the variation characteristics of the adsorption density can be obtained by combining the variation formula (5) of formula (3), as shown in the following example: Figure 9 shown.

[0084]

[0085] Depend on Figure 9 It shows that the density of the adsorbed phase is a function of temperature and pressure, and the variation characteristics of the adsorbed gas density are similar to those of the free gas density (the variation of the free gas density is obtained by searching the NTST database for the corresponding values ​​under simulated temperature and pressure, and placing it in the same chart with the adsorbed gas density makes it easier to conduct comparative analysis). Both increase with increasing pressure, and the increase rate is first fast and then slow, and decreases with increasing temperature. The biggest difference between the two is that the density increases rapidly in different intervals. In the early stage of adsorption, due to the large number of adsorption vacancies near the pore wall, the adsorbed gas is affected by the pore temperature and pressure as well as the pore wall gravity, while the free gas is only affected by the pore temperature and pressure. Therefore, with the increase of pressure, in the low-pressure stage ( Figure 9The increase in the density of adsorbed gas is greater than that of free gas. As the amount of adsorbed gas increases, the adsorbed gas in the adsorption space gradually approaches saturation. However, since the free space is much larger than the adsorption space, that is, the free gas in the free space is still far below the saturation of the free space, the increase in the density of free gas will begin to exceed that of adsorbed gas. This is the transition stage ( Figure 9 After that, as the pressure continues to increase, the adsorbed gas always maintains a small increase, and the free gas in the free space also tends to be saturated, and the increase in the free gas density also begins to decrease, that is, it enters the Figure 9 Zone III stage. In the transition stage, there will be an intersection between the density of adsorbed gas and the density of free gas. The pressure and density values ​​corresponding to the intersection will increase as the temperature decreases. This is because the energy of the gas is different at different temperatures, resulting in different gas expansion degrees and adsorption speeds of the adsorbed gas. Through analysis, we can know Figure 9 The performance results are consistent with the specific conditions of the reservoir and theoretical deduction, and the performance is relatively accurate.

[0086] According to the gas state equation, the expression of gas density can be obtained as follows:

[0087]

[0088] Where Z is the compression factor;

[0089] Since density is an intrinsic property of gas and a physical parameter unrelated to the amount of gas, the density of adsorbed methane is similar to that of free methane. Since adsorbed methane is affected not only by pressure but also by the forces exerted by the pore walls on methane molecules, the difference between the density of adsorbed and free methane is primarily due to the compression factor. The expression for the density of adsorbed methane can be expressed as:

[0090]

[0091] Introducing the correction coefficient α, the relationship between the compression factors corresponding to the methane adsorption state and free state density is established:

[0092] Z * =αZ (8)

[0093] Then, by substituting formula (8) into formula (7), we can obtain:

[0094]

[0095] Right now:

[0096]

[0097] based on Figure 9The trend of methane free state and adsorbed state density changes can be used to calculate the correction coefficient α at different temperatures and draw the corresponding change curve Figure 10 In this embodiment, the calculated density correction coefficient α is fitted using a hyperbolic function. The actual fitting process further predicts the change trend to 200 MPa. The fitting results are shown in Table 2 and Figure 11 shown.

[0098] Table 2 Parameter fitting results of the correction coefficient α model at different temperatures

[0099] Temperature, °C a, decimal b, decimal 60 0.58278942 14.59490330 80 0.58072008 14.30930366 100 0.57391462 14.12802477 120 0.57226086 13.75248915

[0100] The fitting results show that the fitting errors from 60°C to 120°C range from 2.69% to 4.01%, with an average error of 3.34%. Therefore, the hyperbolic function can better fit the changing trend of the correction coefficient α.

[0101]

[0102] According to the fitting results of the correction coefficient α, scatter plots between parameter a, parameter b and temperature are drawn respectively, as shown in Figure 12 and Figure 13 As shown, it can be seen that both parameter a and parameter b have a good linear correlation with temperature. Therefore, formula (10) can be specifically expressed as:

[0103]

[0104] in,

[0105]

[0106] Substituting formula (12) into formula (9), we get:

[0107]

[0108] Based on formula (14), the density of the adsorbed gas can be quickly calculated according to the density of the free gas, as well as the pressure P (MPa) and temperature T (K) parameters.

[0109] Molecular simulation technology is used to simulate the isothermal adsorption curve of the target shale layer at a temperature of 150°C and a pressure of 3 to 130 MPa. The simulation results are as follows: Figure 14 As shown, the adsorption phase density is calculated using formula (14) as follows Figure 15 As shown in the figure, it can be seen that except for the first two low-pressure points, the adsorption phase densities corresponding to the remaining pressure points can be accurately fitted, and the average fitting error is 6.87%.

[0110] Because the results of shale gas isothermal adsorption experiments are usually relative adsorption amounts, they must be converted into absolute adsorption amounts before they can be used. Figure 14 The relative adsorption amount in the sample was converted and compared with the known absolute adsorption amount to further prove the reliability of the conversion method.

[0111] According to formula (3) and Table 1, the adsorption state density is calculated using the van der Waals method, the atmospheric boiling point liquid density method and the calculation method proposed in this paper to calculate the absolute adsorption amount. The calculation results are as follows: Figure 16 The results show that the method proposed in this paper can calculate the absolute adsorption amount well, with an average error of 7.24%. The error mainly comes from the place where the relative adsorption amount changes from positive to negative, at the two pressure points before and after 30 MPa. The formula (15) is obtained by transforming formula (3):

[0112]

[0113] Combine Figure 14 It can be seen that the point near 30 MPa is exactly where the density of the adsorbed phase is equal to the density of the free gas. At this time, the denominator of formula (15) is 1-ρ g / ρ a The pressure approaches 0, which makes the error easily magnified. If the calculated value near this pressure is eliminated, the overall calculation error will be greatly reduced, which means that the calculation result is more accurate. The other two methods can no longer calculate the positive absolute adsorption amount in the high pressure section. This is because the free state density at 150℃ and 130MPa is 0.306g / cm 3 , which is always less than the adsorption density of 0.372 g / cm based on the van der Waals method and the atmospheric boiling point liquid density method. 3 and 0.424 g / cm 3 , so that the denominator of formula (15) is always greater than 0. Therefore, when the relative adsorption amount turns from positive to negative, the calculated absolute adsorption amount is also negative. In addition, the adsorption state density determined by the critical density method is 0.163 g / cm 3 , because it is very close to the free state density of 0.16315g / cm under the conditions of 150℃ and 40MPa. 3 , causing the denominator of formula (15) to almost approach 0, and then calculating a very large absolute adsorption amount, so its calculation is also inaccurate. Through the above verification, it is fully proved that the calculation method of this application is reliable and accurate, and has good applicability, and can well meet the temperature and pressure conditions of existing shale reservoirs, especially suitable for the calculation of high-temperature and high-pressure shale gas.

[0114] In addition, it should be noted that the fugacity f needs to be used instead of pressure during molecular simulation calculations. In this embodiment, by calculating the fugacity and fugacity coefficient under different temperature and pressure ranges, it can be seen that there is a nonlinear relationship between fugacity and pressure. Therefore, when fugacity is needed to represent pressure, under low conditions, that is, when the pressure is less than 30 MPa, the pressure and fugacity can be approximated. However, for high pressure (greater than 30 MPa), the pressure cannot be simply approximated. Formula (16) must be used for calculation, otherwise a large error will occur.

[0115]

[0116] in,

[0117] a=a c α(T r ,ω) (17)

[0118]

[0119] α(T r ,ω)=[1+k(ω)(1-T r 0.5 )] 2 (20)

[0120] k(ω)=0.3476+1.54226ω-0.26992ω 2 (twenty one)

[0121] Calculate the fugacity coefficient according to the definition

[0122]

[0123] Where T r ——Comparative temperature, T r =T / T c , dimensionless;

[0124] ω——eccentricity factor, dimensionless;

[0125] T c ——critical temperature, K;

[0126] P c ——Critical pressure, Pa.

[0127] R——molar gas constant, J·(mol·K) -1

[0128] ρ c ——critical density, g·cm -3 ;

[0129] T——temperature, K.

[0130] The above are only preferred embodiments of the present invention. It should be pointed out that various modifications and improvements made by those skilled in the art without departing from the present technical solution should also be deemed to fall within the scope of protection required by the claims.

Claims

1. A method for calculating shale gas adsorption density, characterized in that: The steps include: S1, based on adsorption theory, establish the relative adsorption capacity V corrected by adsorption gas density ex and the absolute adsorption capacity V ab Based on molecular simulation technology, the absolute adsorption capacity V is corrected by the number characteristics of adsorbed gas and free gas molecules. ab and relative adsorption capacity V ex The conversion relationship function between them; S2, constructing a shale kerogen model; S3, based on the shale kerogen model established in step S2, simulate and calculate the absolute adsorption amount V at different temperatures ab , obtain the corresponding shale gas isothermal adsorption curve, and use the absolute adsorption capacity V established in step S1 ab and relative adsorption capacity V ex The conversion relationship function between the absolute adsorption amount V ab Converted to relative adsorption capacity V ex , and presented in the form of a chart; S4, based on the relative adsorption amount V in step S1 ex and the absolute adsorption capacity V ab The relationship function between the two, and the absolute adsorption amount V obtained in step S3 ab and relative adsorption capacity V ex , the corresponding shale gas adsorption density change curve is calculated; Using the NIST database, we searched for the corresponding shale gas free-state density change values ​​under simulated temperature and pressure, and drew a shale gas free-state density change curve. S5, using the gas state equation, proposes a method for characterizing the adsorbed state density based on the free state density of shale gas. By introducing a correction coefficient α and combining it with the curve of changes in the free state and adsorbed state densities of shale gas in step S4, the density correction coefficient α at different temperatures is obtained. Based on the density correction coefficient α, data processing is performed to obtain a relationship function between the density correction coefficient α and temperature and pressure. S6, deriving a calculation formula for adsorbed gas density based on step S5.

2. The shale gas adsorption density calculation method according to claim 1, characterized in that: In step S2, a kerogen model containing multiple kerogen molecules is first constructed based on the shale kerogen molecular model. Secondly, the number of kerogen molecules loaded is considered in the process of constructing the kerogen model, and the kerogen model is optimized using three schemes: geometric optimization, energy optimization, and simulated annealing.

3. The shale gas adsorption density calculation method according to claim 2, characterized in that: The number of kerogen molecules loaded is 10-13.

4. The method for calculating shale gas adsorption density according to claim 3, characterized in that: The number of kerogen molecules loaded is 11.

5. The method for calculating shale gas adsorption density according to claim 1, wherein: In step S5, the calculated density correction coefficient α is fitted using a hyperbolic function to obtain a relationship function between the density correction coefficient α and the pressure P at different temperatures. According to the relationship function between the density correction coefficient α and the pressure P, a scatter diagram between the parameter a, parameter b and the temperature is drawn, and the relationship function between the density correction coefficient α and the pressure P and temperature T is obtained. in 6. The method for calculating shale gas adsorption density according to claim 5, characterized in that: The calculation formula for the adsorbed gas density is:

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