Method for determining residual yield of shale gas based on methane carbon isotope change of analytic gas

Abstract

The invention provides a method for determining the residual yield of shale gas based on methane carbon isotope change in desorption gas. The method comprises the following steps: S1, putting a shale sample into a desorption tank filled with saturated salt water, and sealing; s2, placing the desorption tank in water, recording the volume of desorption gas according to the desorption time by using a drainage method, and collecting the desorption gas; s3, according to the analytic gas collected in each analytic time period, calculating accumulated total analytic gas, and calculating the proportion of the analytic gas in each analytic time period to the total analytic gas to obtain a stage accumulated analytic rate; s4, measuring the ratio of methane carbon isotope delta 13C in the desorption gas when the desorption rate is accumulated in each stage; and S5, establishing a change curve of the stage cumulative analysis rate of the shale gas and the methane carbon isotope delta13C so as to obtain a model for predicting the shale gas productivity stage. The invention provides a non-destructive and direct gas potential measurement means, which is high in accuracy and can be well adapted to shale gas prediction.

Description

Technical Field

[0001] The present invention belongs to the technical field of shale gas exploration and development, and particularly relates to a method for determining the remaining production of shale gas based on the change of methane carbon isotope of desorbed gas. Background Art

[0002] In the process of shale gas development, accurately predicting the production stage of production wells and estimating the remaining recoverable resources is crucial for formulating development strategies and planning development cycles. Since the production cycle of shale gas reservoirs is generally long, usually 5 to 10 years or even longer, relying on actual measurements to establish gas production curves for different production stages faces huge challenges. As an unconventional natural gas reservoir, shale gas has the characteristics of self-generation and self-storage, and the assessment of its resources is strictly restricted by preservation conditions. Even if a gas production curve model is established for a specific production well, these models often cannot accurately predict the single-well production curves of other regions or structures due to differences in regional and tectonic conditions. Therefore, developing geochemical tracer methods applicable to different regional and tectonic conditions is particularly important for improving shale gas development efficiency.

[0003] Currently, in the process of shale gas desorption, two common evaluation methods are the methane carbon isotope change model based on capping gas and the dry coefficient evolution method based on desorbed gas. Both of these methods aim to accurately judge and predict the production stage of shale gas, but each has certain limitations and problems and urgently needs to be further optimized.

[0004] Among them, the methane carbon isotope change model based on capping gas analyzes the methane carbon isotope data of different desorption time periods and uses simple mathematical models such as linear equations or quadratic equations to describe the change trend of isotopes. Although this method is simple and easy to implement, it has the following problems: (1) It overly simplifies the multi-scale pore structure and multi-mechanism coupling process of methane migration (such as the dynamic differences between free gas and adsorbed gas in double-dispersed pores); (2) It is difficult to capture the non-linear characteristics such as the exponential weight gain of isotope values and the dynamic fluctuations of fractionation coefficients in the late stage of desorption; (3) It ignores external factors such as krypton interference in mass spectrometry and the influence of temperature and pressure changes on diffusion coefficients, resulting in prediction deviations. Although complex models (such as machine learning or higher-order equations) can improve the fitting accuracy, they are easily affected by data noise amplification, parameter redundancy, etc., leading to overfitting. The above reasons make it impossible to accurately predict in practical applications.

[0005] On the other hand, the drying coefficient evolution method based on the analyzed gas determines the drying coefficient by calculating the proportion of hydrocarbons with different carbon numbers in the analyzed gas, and then judges the production stage. However, since methane has the highest content in natural gas, the calculation of the drying coefficient requires the full-spectrum data of C1-C5. If the content of C2-C5 gases in the collected gas is low or cannot be detected, the calculation result of the drying coefficient will have a large deviation, affecting the accuracy.

[0006] Due to the essential differences in data sources, analysis principles, and time scales between these two methods, it is difficult to directly combine them. The top gas samples are usually collected in the early stage of gas reservoir development, reflecting the lighter methane components in the gas reservoir, while the drying coefficient of the analyzed gas may change at any stage of development, resulting in possible temporal mismatch between the two methods. Summary of the Invention

[0007] In view of this, the present invention provides a method for determining the remaining production of shale gas based on the change of methane carbon isotope in the analyzed gas.

[0008] To achieve the above object, the present invention adopts the following technical solutions: A method for determining the remaining production of shale gas based on the change of methane carbon isotope in the analyzed gas, comprising the following steps: S1. Place the shale sample into an analytical tank filled with saturated brine and seal it. S2. Place the analytical tank in water, and use the water displacement method to record the volume of the analyzed gas according to the analytical time and collect the analyzed gas. S3. Calculate the cumulative total analyzed gas based on the analyzed gas collected in each analytical time period, and calculate the proportion of the analyzed gas in each analytical time period to the total analyzed gas, that is, obtain the stage cumulative analytical rate. S4. Measure the ratio of the methane carbon isotope δ 13 C in the analyzed gas when measuring the stage cumulative analytical rate. S5. Establish a change curve of the stage cumulative analytical rate of shale gas and the change of methane carbon isotope δ 13 C, determine the change of methane carbon isotope δ 13 C in shale gas under different analytical rates, so as to establish a model for predicting the shale gas production capacity stage based on the change of methane carbon isotope δ 13 C.

[0009] In some specific embodiments, preferably, in step S1, the shale sample is taken out from the ground by pressure-retaining coring, and after being taken out, it is cryogenically treated with liquid nitrogen.

[0010] In some specific embodiments, preferably, in step S1, the analytical tank is made of a transparent material, which is conducive to observing the gas analysis situation.

[0011] Further, the specific process of desorption in step S2 is as follows: First, desorb at 9 - 40°C for 24 - 25 days, and then raise the desorption temperature to 85°C and continue desorbing for 24 hours.

[0012] In some specific embodiments, preferably, the temperature programming of the gas chromatograph for measuring carbon isotope δ 13 C in step S4 is as follows: First, keep the temperature constant at 40°C for 5 minutes, then raise the temperature to 200°C at a rate of 5°C / min, and keep the temperature constant at this temperature for 10 minutes.

[0013] In some specific embodiments, preferably, in step S5, the change curve is established with the cumulative desorption rate as the abscissa and the change of methane carbon isotope δ 13 C as the ordinate.

[0014] Further, in the model in step S5, the shale desorbed gas production is positively correlated with the desorption rate, and the remaining production capacity of shale desorbed gas is negatively correlated with the desorption rate.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention uses the change of methane carbon isotope composition during the desorption process to estimate the remaining recoverable gas resources in the shale reservoir, providing a non-destructive and direct gas potential measurement method. That is, it avoids the problem that the complexity in the data cannot be fully captured in the methane carbon isotope change model based on capping gas, which affects the accuracy; it also avoids the problem that the content of C2 - C5 gases is low or undetectable in the dry coefficient evolution method based on desorbed gas, which affects the accuracy.

[0016] (2) When estimating the shale gas production capacity through the technology of the present invention, the following conclusions can be obtained: When the desorption rate reaches 75 - 80%, the methane carbon isotope δ¹³C is distributed in -52.7‰ - -51.4‰, which corresponds to the middle and late stages of shale gas exploitation. At this time, the methane isotope begins to show a linear trend of becoming heavier, reflecting the decrease in reservoir pressure and the decrease in the proportion of adsorbed gas. At this time, when continuing to exploit, the following can be considered: (1) Pressure maintenance technology: Inject CO2 or nitrogen (using the adsorption competition effect) to delay the decrease in reservoir pressure, inhibit the isotope fractionation rate, and maintain δ¹³C in the lighter range, so as to extend the high-production stage; (2) Dynamic monitoring system: Real-time monitor the change of δ¹³C, and establish a prediction model in combination with production capacity data; for example, when δ¹³C increases by 1‰, it is necessary to evaluate whether it is necessary to adjust the gas injection intensity or well pattern density, and other methods to increase shale gas production. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a flow chart of the technical solution of this application.

[0018] Figure 2For the evolution curve of the change in methane carbon isotope δ 13 C and the shale gas desorption rate in the desorbed gas components of shale sample A.

[0019] Figure 3 For the evolution curve of the change in methane carbon isotope δ 13 C and the shale gas desorption rate in the desorbed gas components of shale sample B. Specific implementation manners

[0020] The following further elaborates the present invention in conjunction with specific embodiments, so that those skilled in the art can understand the present invention more clearly. Unless otherwise specified, the technical means used in the following embodiments are all conventional means well-known to those skilled in the art, and all reagent consumables are commercially available products.

[0021] Embodiment 1 This embodiment provides a method for determining the remaining production of shale gas based on the change in methane carbon isotope of desorbed gas. Two collected shale samples A and B (both samples A and B are from Well Y27 in the Yanchang Formation of the Triassic) are analyzed respectively, and the specific steps are as follows: S1. Collect the shale samples taken with pressure preservation at the drilling platform site, freeze the shale core samples with liquid nitrogen, and quickly put them into a special transparent sealed desorption tank filled with saturated brine.

[0022] S2: Invert the desorption tank and immerse it in water. During the desorption process of shale gas, it can be clearly observed how the gas gradually releases from the rock and enters the container. The gas desorption will cause the pressure in the tank to rise, and then open the bottom of the desorption tank to drain part of the water from the gaps at the bottom of the tank. The whole desorption process is first carried out at 9 - 40°C for 25 days of desorption, and then the desorption temperature is raised to 85°C for continuous desorption for 24 h. At regular intervals, record the volume occupied by the desorbed gas in the desorption tank, and collect the gas in this stage by the traditional water displacement method.

[0023] S3: Calculate the cumulative gas volume in the current stage according to the gas volume collected in each desorption stage, and calculate the proportion of the cumulative gas volume in each stage to the total desorbed gas volume, that is, the stage cumulative desorption rate.

[0024] S4: Analyze the ratio of methane carbon isotope δ 13 C in the desorbed gas at each stage of cumulative desorption rate by a gas chromatograph. The temperature programming of the gas chromatograph is as follows: First, keep the temperature constant at 40°C for 5 minutes, then raise the temperature at a rate of 5°C / min to 200°C, and keep the temperature constant at this temperature for 10 minutes. According to the characteristics of the gas components, calculate the carbon isotope δ of methane in different desorption stages 13The variation of the δ 13 C ratio, specifically to determine the variation of the methane carbon isotope in the gas released during the shale gas reservoir desorption process.

[0025] S5: Establish the stage cumulative desorption rate and the methane carbon isotope variation curve of shale gas, and determine the variation of the methane carbon isotope δ 13 C in shale gas under different desorption rates, so as to establish a model for predicting the shale gas production capacity stage based on the variation of methane carbon isotope. Among them, the variation curve takes the cumulative desorption rate as the abscissa and the methane carbon isotope δ 13 C variation as the ordinate; while the remaining gas volume in the shale is negatively correlated with the desorption rate.

[0026] The specific results are shown in Figure 2 、 3 as follows: Among them, Figure 2 is the evolution curve of the methane carbon isotope δ 13 C variation of the shale desorption gas component of Sample A and the shale gas desorption rate; Figure 3 is the evolution curve of the methane carbon isotope δ 13 C variation of the shale desorption gas component of Sample B and the shale gas desorption rate.

[0027] It can be seen from Figure 2 that: The variation of the methane carbon isotope δ 13 C is negatively correlated with the desorption rate: When the cumulative desorption rate is relatively low (0 - 60%), the methane carbon isotope value is relatively stable, distributed between -52.2‰ and -50.9‰. When the cumulative desorption rate exceeds 78%, the methane carbon isotope value begins to become significantly heavier (i.e., the value becomes smaller), showing a linear variation trend. In the initial stage (0 - 60%), the methane carbon isotope value is stable, which may indicate that the gas desorbed at this time mainly comes from the same source or the same type of organic matter. When the cumulative desorption rate exceeds 78%, the methane carbon isotope value becomes heavier, which may indicate that different sources or types of organic matter begin to contribute more gas, or isotope fractionation occurs during the desorption process.

[0028] Then the development of shale gas can be carried out in stages: Initial stage (0 - 60%): Adopt conventional mining methods to maintain high mining efficiency and low cost; Middle stage (60% - 78%): Monitor the variation of methane carbon isotope, evaluate the variation of gas sources, and adjust the mining methods in a timely manner; Late stage (>78%): Adopt more efficient mining technologies, such as hydraulic fracturing, pyrolysis, etc., to improve the desorption rate of the remaining gas.

[0029] It can be seen from Figure 3 that: The methane carbon isotope δ 13The change of C is negatively correlated with the resolution rate: when the cumulative resolution rate is relatively low (0 - 60%), the methane carbon isotope value is relatively stable, distributed between -52.7‰ and -51.4‰. When the cumulative resolution rate exceeds 75%, the methane carbon isotope value begins to become significantly heavier (i.e., the value becomes smaller), showing a linear change trend. In the initial stage (0 - 60%), the methane carbon isotope value is stable, which may indicate that the gas desorbed at this time mainly comes from the same source or the same type of organic matter. When the cumulative resolution rate exceeds 75%, the methane carbon isotope value becomes heavier, which may indicate that different sources or types of organic matter begin to contribute more gas, or isotope fractionation occurs during the desorption process.

[0030] Then the development of shale gas can be carried out in stages: Initial stage (0 - 60%): Use conventional mining methods to maintain high mining efficiency and low costs; Middle stage (60% - 75%): Monitor the change of methane carbon isotope, evaluate the change of gas source, and adjust the mining method in a timely manner; Late stage (>75%): Use more efficient mining technologies, such as hydraulic fracturing, pyrolysis, etc., to improve the desorption rate of the remaining gas.

[0031] Figure 1 , 2 The results maintained a high degree of similarity, indicating that the technical solution has good repeatability and can be well used in actual development. According to the desorption kinetic model, the lighter ¹²CH4 is preferentially desorbed, resulting in a lighter isotope value (-52.7‰ ~ -50.9‰) in the early stage (desorption rate 0 - 60%). As the desorption rate increases (above 75%), the proportion of ¹³CH4 in the remaining methane increases, and the isotope value becomes linearly heavier. This phenomenon is related to the competitive adsorption and diffusion ability differences during the migration of methane in porous media. Accordingly, different mining strategies are adopted according to the change of methane carbon isotope, so as to improve the mining rate and save costs.

[0032] In the present invention, the specific raw materials not described are all existing substances and can be directly purchased from the market.

[0033] The above are only the preferred embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for determining the remaining production of shale gas based on the change of methane carbon isotope in the analytical gas, characterized in that It includes the following steps: S1. Put the shale sample into a desorption tank filled with saturated brine and seal it; S2. Place the desorption tank in water, and use the water displacement method to record the volume of desorbed gas according to the desorption time and collect the desorbed gas; S3. Calculate the cumulative total desorbed gas based on the desorbed gas collected in each desorption time period, and calculate the proportion of the desorbed gas in each desorption time period to the total desorbed gas, that is, obtain the stage cumulative desorption rate; S4. Measure the ratio of the methane carbon isotope δ 13 C in the desorbed gas when measuring the cumulative desorption rate at each stage; S5. Establish the variation curve of the stage cumulative analytical rate of shale gas and the methane carbon isotope δ 13 C, determine the variation of the methane carbon isotope δ 13 C in shale gas at different analytical rates, and thus establish a model for predicting the shale gas production capacity stage based on the variation of the methane carbon isotope δ 13 C.

2. The method according to claim 1, characterized in that, In step S1, the shale sample is taken out from the ground by pressure-retaining coring, and after being taken out, it is frozen with liquid nitrogen.

3. The method according to claim 1, characterized in that In step S1, the desorption tank is made of transparent material, which is conducive to observing the gas desorption situation.

4. The method according to claim 1, wherein The specific process of desorbing the desorbed gas in step S2 is as follows: first desorb at 9-40°C for 24-25 days, and then raise the desorption temperature to 85°C and continue desorbing for 24h.

5. The method according to claim 1, characterized in that In step S4, the temperature program of the gas chromatograph for measuring carbon isotope δ 13 C is as follows: First, hold the temperature at 40°C for 5 minutes, then increase the temperature to 200°C at a rate of 5°C / min, and hold the temperature at this temperature for 10 minutes.

6. The method according to claim 1, wherein In step S5, the variation curve is established with the cumulative analysis rate as the abscissa and the methane carbon isotope δ 13 C variation as the ordinate.

7. The method according to claim 1, characterized in that, In step S5, in the said model, the shale desorbed gas production is positively correlated with the desorption rate, and the remaining production capacity of the shale desorbed gas is negatively correlated with the desorption rate.

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