Method for quantitatively evaluating shale gas preservation conditions

Abstract

The present application provides a kind of quantitative evaluation shale gas preservation condition method, comprising steps 1. Collecting gas sample to be evaluated area and carrying out natural gas component and isotope characteristic analysis.Step 2. Based on the kinetic Rayleigh fractionation model of carbon isotope in kerogen hydrocarbon generation process, the reaction process F of system is determined.Step 3. Determine the isotope fractionation factor ε.Step 4. The prediction of methane generation amount.Step 5. Quantitative characterization of natural gas dissipation efficiency.Step 6. Comprehensive evaluation of shale system preservation condition.The present application perfects the quantitative evaluation method of shale gas preservation condition, helps the comprehensive evaluation of shale preservation condition in complex tectonic deformation area of Sichuan Basin periphery, provides basis for the search of beneficial target area in shale gas exploration, and speeds up the exploration and development process of shale gas reservoir.

Description

Technical Field

[0001] This invention belongs to the field of shale gas preservation technology, and in particular relates to a method for quantitatively evaluating shale gas preservation conditions. Background Technology

[0002] "A Quantitative Evaluation Method for Shale Gas Target Preservation Conditions" (Application No. 202010006082.6): This invention provides a quantitative evaluation method for shale gas target preservation conditions. It includes: identifying the main controlling factors influencing shale gas target preservation conditions through exploration and development of shale areas, combined with shale gas preservation mechanisms; establishing an evaluation index system for shale gas target preservation conditions based on the understanding of these main controlling factors; determining the weights of each factor according to hierarchical indicators; quantifying shale gas target preservation conditions; determining the shale gas target preservation level based on the evaluation value of the shale gas target preservation conditions; classifying and evaluating preservation conditions to guide the evaluation and selection of shale gas reservoir targets. The shale gas target preservation level and the evaluation and classification of preservation conditions provided by this invention guide the evaluation and selection of shale gas reservoir targets. The quantitative evaluation method for shale gas target preservation conditions provided by this invention is more conducive to the efficient development of shale gas reservoirs.

[0003] One quantitative evaluation method for shale gas target preservation conditions mainly starts from the mechanism of shale gas preservation conditions, selects key evaluation parameters related to tectonics, and uses geophysical methods to quantitatively evaluate the preservation conditions of shale gas targets, guiding the evaluation and selection of shale gas reservoir targets. However, because the contribution of each parameter varies in different regions and subjective factors are often involved in weighting, the calculation process for establishing the preservation condition evaluation system is overly complex and the results lack objectivity. Secondly, this technique focuses on the spatial configuration relationship between shale tomography and specific geological elements, neglecting the physical or chemical responses of shale reservoir fluids, including fluid pressure and variations in fluid (natural gas, formation water) component isotopes. Therefore, this technique cannot effectively and comprehensively evaluate shale preservation conditions. Summary of the Invention

[0004] The purpose of this invention is to overcome the deficiencies of the existing technology and provide a method for quantitatively evaluating shale gas preservation conditions.

[0005] Due to significant differences in geological backgrounds across different regions, the tectonic deformation intensity within the Sichuan Basin is weaker than outside the basin. Therefore, the empirical rules for assigning weights to preservation condition evaluation parameters within the basin are not universally applicable to areas outside the basin. For the study of shale gas preservation conditions, gas content is the most direct indicator of the quality of preservation conditions. This method is simple and easy to use, with a clear and straightforward process. It only requires obtaining natural gas component isotope data, then determining the system's reaction progress and predicting methane generation based on a carbon isotope fractionation model during kerogen hydrocarbon generation, and finally multiplying this by the system's openness to generate the in-situ methane content [CH4] under different openness levels. in-place With δ 13 The upper and lower limit curves of the C1 function relationship are used to complete the model establishment. By comparing the theoretical and measured values, the natural gas loss efficiency and loss amount can be obtained. The correctness of the conclusions can then be verified by combining the strength of tectonic deformation in different regions. The process is relatively quick and highly applicable, which is beneficial for later exploration and development and provides a reliable basis for well location deployment.

[0006] The present invention adopts the following technical solution:

[0007] A method for quantitatively evaluating shale gas preservation conditions includes the following steps:

[0008] Step 1. Collect gas samples and perform natural gas composition and isotopic characterization analysis.

[0009] Step 2. Based on the Rayleigh fractionation model of carbon isotope kinetics during kerogen hydrocarbon generation, determine the reaction process F of the system, as shown in the following equation:

[0010] δ 13 C o =δ 13 C k +ε[1+ln[1-F]] (1)

[0011] δ 13 C c =δ 13 C k -ε[(1-F)ln(1-F) / F] (2)

[0012] Where, δ 13 C k δ is the carbon isotope value of kerogen. 13 C O and δ 13 C c ε represents the carbon isotope values ​​of methane in open and closed systems, respectively, and ε is the isotope fractionation factor.

[0013] F can be obtained by reversing equations (1) and (2), that is:

[0014] F o=1-exp[(δ 13 C o -δ 13 C k ) / ε-1] (3)

[0015] F c =1-exp[ω(y / exp(-y))-y](4)

[0016] Where y = (δ 13 C c -δ 13 C k ) / ε, where ω is the product logarithm function, and the subscripts 'o' and 'c' are used to distinguish between open and closed systems.

[0017] Step 3. Determine the isotopic fractionation factor ε

[0018] In the hydrocarbon generation process of kerogen 36 The concentration of Ar gradually decreases as the reaction progresses, and is inversely proportional to the concentration of methane, as shown in the following formula:

[0019] ([ 36 Ar] / [CH4])×F=c (5)

[0020] Combining δ¹⁷ gas samples from 17 shale gas samples in the area to be evaluated 13 C1, 36 Ar and CH4 are calculated by giving an ε value in equations (1) and (2) respectively using the inverse estimation method. The corresponding F is then substituted into equation (5). The above operation is repeated until the c value is constant. The fit of ε is reflected by the variance of the c value.

[0021] Step 4. Prediction of methane production

[0022] Determine the total organic carbon (TOC) content and density (ρ) of the shale in the area to be evaluated. s The content of hydrocarbons S2 from kerogen pyrolysis was used to calculate the gas generation potential G of kerogen using equation (7), and then substituted into equation (6) to obtain the amount of methane generated in closed and open systems under different reaction processes [CH4]. closed-gen [CH4] open-gen In the formula V M , These represent the standard molar volume and molar mass of methane, respectively.

[0023]

[0024] G = 100 × S² / TOC (7)

[0025] Step 5. Quantitative characterization of natural gas loss efficiency

[0026] As shown in the following formula: [CH4]in-place =([CH4]) closed-gen -θ([CH4) closed-gen -[CH4] open-gen (8) Taking the system openness θ as 0%, 20%, 40%, 60%, 80%, and 100%, respectively, the in-situ methane content [CH4] is generated under different openness levels. in-place With δ 13 The upper and lower limit curves of the C1 function relationship were obtained, and then the measured shale gas data of the area to be evaluated were input into the model for observation and comparison to obtain the upper and lower limits of the openness θ of the shale system and the amount of methane loss [CH4]. expelled ;

[0027] [CH4] expelled =[CH4] in-place *θ / (1-θ) (9)

[0028] Step 6. Based on the spatial configuration relationship between shale strata and specific geological elements, combined with the current pressure coefficient of the shale system and the timing of tectonic uplift in geological history, verify the correctness of the model and comprehensively evaluate the preservation conditions of the shale system.

[0029] Furthermore, the spatial configuration relationship between shale formations and specific geological elements includes: top and bottom plate conditions, erosion zone spacing, fracture scale and spacing, and stratigraphic dip.

[0030] The beneficial effects of this invention are:

[0031] This invention enables a simple and rapid establishment of in-situ methane [CH4] content under different degrees of openness based on the Rayleigh fractionation model of isotope kinetics in kerogen hydrocarbon generation. in-place With δ 13 The C1 function relationship, through lateral comparison of fluid component isotopes within the shale system, yields the natural gas loss efficiency and amount, quantifying the preservation conditions of the shale system. This is combined with tectonic deformation intensity to comprehensively evaluate the preservation conditions. This improves the quantitative evaluation method for shale gas preservation conditions, contributing to the comprehensive evaluation of shale preservation conditions in complex tectonic deformation zones surrounding the Sichuan Basin. It provides a basis for finding favorable target areas in shale gas exploration, accelerating the exploration and development process of shale gas reservoirs. Attached Figure Description

[0032] Figure 1 The ε values ​​are for the open and closed systems of this invention;

[0033] Figure 2 For the system reaction process F and methane isotope δ 13 C1 intersection chart;

[0034] Figure 3 For δ13 Cross-sectional diagram of C1 and methane content in situ;

[0035] Figure 4 This is a geological profile.

[0036] Figure 5 This is a flowchart of the steps of the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0038] Example

[0039] This method mainly establishes a quantitative evaluation method for shale gas preservation conditions in the Wufeng-Longmaxi Formation of the Yangtze Plate by lateral comparison of fluid component isotopes within the shale system.

[0040] like Figure 5 As shown, the present invention provides a method for quantitatively evaluating shale gas preservation conditions, comprising the following steps:

[0041] Step 1. Collect gas samples and perform natural gas composition and isotopic characterization analysis;

[0042] Step 2. Based on the Rayleigh fractionation model of carbon isotope kinetics during kerogen hydrocarbon generation, determine the reaction process F of the system, as shown in the following equation:

[0043] δ 13 C o =δ 13 C K +ε[1+ln(1-F)] (1)

[0044] δ 13 C c =δ 13 C k -ε[(1-F)ln(1-F) / F] (2)

[0045] Where, δ 13 C k The carbon isotope values ​​of kerogen and the δ¹⁸O values ​​of the Wufeng-Longmaxi Formation shale are given. 13 C k It is -30‰. δ 13 C o and δ 13 C cε represents the carbon isotope values ​​of methane in open and closed systems, respectively, and ε is the isotope fractionation factor.

[0046] F can be obtained by reversing equations (1) and (2), that is:

[0047] F o =1-exp[(δ 13 C o -δ 13 C k ) / ε-1] (3)

[0048] F c =1-exp[ω(y / exp(-y))-y](4)

[0049] Where y = (δ 13 C c -δ 13 C k ) / ε, where ω is the product logarithm function, and the subscripts 'o' and 'c' are used to distinguish between open and closed systems.

[0050] Step 3. Determine the isotopic fractionation factor ε.

[0051] In the hydrocarbon generation process of kerogen 36 The concentration of Ar gradually decreases as the reaction progresses, and is inversely proportional to the concentration of methane (CH4), as shown in the following equation:

[0052] ([ 36 Ar] / [CH4])×F=c (5)

[0053] Combining δ¹⁷ gas samples from the Wufeng-Longmaxi Formation shale 13 C1, 36 Ar, CH4, and c are arbitrary constants, as shown in Table 1:

[0054] Table 1 Gas samples from the Wufeng-Longmaxi Formation 36 Ar content, methane content, and methane carbon isotope value

[0055]

[0056] By using the inverse estimation method, given an ε value in Equations 1 and 2, the corresponding F is calculated and substituted into Equation (5). This process is repeated until the c value is constant. The goodness of fit of ε is reflected by the variance of the c value. The ε values ​​of the Wufeng-Longmaxi Formation shale are -19.32‰ and -19.00‰ in open and closed systems, respectively. (e.g.) Figure 1 As shown), δ 13 The evolution of C1 as the system reaction progresses, such as Figure 2 As shown.

[0057] Step 4: Predictive equations (6) and (7) for methane production are shown below:

[0058]

[0059] G = 100 × S² / TOC (7)

[0060] Among them, V M Molar volume of methane under standard conditions Molar mass of methane, total organic carbon (TOC) content and density (ρ) of the Wufeng-Longmaxi Formation shale. s The hydrocarbon content of S2 kerogen pyrolysis was 4wt%-6wt% and 2.56 g / cm³, respectively. 3、 0.3 mg HC / g Rock. The gas generation potential G of kerogen was calculated using equation (7), and then substituted into equation (6) to obtain the methane production [CH4] in closed and open systems under different reaction processes. closed-gen [CH4] open-gen In the formula, V M , These represent the standard molar volume of methane and the molar mass of methane, respectively.

[0061] Step 5. Quantitative characterization of natural gas loss efficiency, as shown in Equation 8 below:

[0062] [CH4] in-place =([CH4]) closed-gen -θ([CH4) closed-gen -[CH4] open-gen ))×(1-θ) (8)

[0063] The system openness (θ) was set to 0%, 20%, 40%, 60%, 80%, and 100%, respectively, to determine the in-situ methane [CH4] content at different openness levels. in-place With δ 13 The upper and lower limit curves of the C1 function relationship, such as Figure 3 Then, the measured data of shale gas from the Wufeng-Longmaxi Formation were input into the model for observation and comparison, resulting in the upper and lower limits of the openness θ of the shale system and the amount of methane loss [CH4]. expelled As shown in Table 2.

[0064] [CH4] expelled =[CH4] in-place *θ / (1-θ)(9)

[0065] Table 2 Natural Gas Loss Efficiency in the Wufeng-Longmaxi Formation Shale

[0066]

[0067]

[0068] Step 6. Based on the spatial configuration relationship between the shale system and specific geological elements, such as top and bottom plate conditions, erosion zone spacing, fracture scale and spacing, and stratigraphic dip, and combined with the current pressure coefficient of the shale system and the timing of tectonic uplift in geological history, verify the correctness of the model and comprehensively evaluate the preservation conditions of the shale system.

[0069] Quantitative characterization results show that the θ of the Wufeng-Longmaxi Formation shale is approximately 65%-70% in the Changning area, approximately 70%-77% in the Fuling area, and approximately 73%-82% in the Zheng'an area (e.g., Figure 3 As shown in Table 2, the Eagle Ford Shale in North America has relatively good preservation conditions, with an θ of approximately 20%-40%, which is generally lower than that of the Wufeng-Longmaxi Formation shale in the Yangtze Plate. The Yanshanian-Himalayan orogeny has a significant impact on the preservation of shale gas in the Wufeng-Longmaxi Formation. Stratigraphic uplift and erosion will lead to the termination of hydrocarbon generation, resulting in continuous shale gas loss without replenishment. The timing of the uplift determines the amount of shale gas loss. The Zheng'an area had the earliest tectonic uplift (140 Ma), with Jurassic-Triassic strata subjected to erosion, resulting in numerous internal faults with long extensions; the current pressure coefficient of the system is approximately 1.0-1.2. The Fuling and Changning areas had a relatively later tectonic uplift (80 Ma), with decreasing tectonic deformation intensity, a broad and gentle anticline structure, and small-scale faults; the current pressure coefficients are all greater than 1.2. Figure 4 As shown in the figure. The qualitative evaluation results indicate that the preservation conditions improve sequentially from outside the pot to inside the pot, which is consistent with the quantitative characterization results, verifying the correctness of the model.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for quantitatively evaluating shale gas preservation conditions, characterized by, Comprising the following steps: Step 1. Collecting gas samples in the area to be evaluated and analyzing the natural gas components and isotopic characteristics; Step 2. Based on the kinetic Rayleigh fractionation model of carbon isotopes in the process of kerogen hydrocarbon generation, the reaction progress F of the system is determined, and the formula is as follows: delta 13 C o equals delta 13 C k plus epsilon times one plus the natural log of one minus F (1) delta 13 C c equals delta 13 C k - epsilon times [(1 - F) ln(1 - F) / F] (2) where δ 13 C k is the carbon isotope value of the kerogen, δ 13 C O and δ 13 C c are the carbon isotope values of the methane under open and closed systems, respectively, and ε is the isotope fractionation factor; Through formula (1), formula (2) is solved to F, that is: F o = 1 - exp[(δ 13 C o - δ 13 C k ) / ε - 1] (3) F c = 1 - exp[ω(y / exp(-y)) - y] (4) where y = (δ 13 C c - δ 13 C k ) / ε, ω is the product log function, and the subscripts 'o' and 'c' distinguish open and closed systems; Step 3. Determining the isotopic fractionation factor ε In the process of hydrocarbon generation of dry kerogen 36 The concentration of Ar gradually decreases with the deepening of the reaction process and is inversely proportional to the concentration of methane, and the formula is as follows: ([ 36 Ar] / [CH4]) x F = c (5) The δ of 17 shale gas samples in the area to be evaluated is combined 13 C1、 36 Ar, CH4, an ε value is given in formula (1) and formula (2) by inverse estimation method, the corresponding F is calculated and brought into formula (5), the above operation is repeated until the value of c is constant, the fitting degree of ε is embodied by the variance of the value of c, and c is an arbitrary constant; Step 4. Prediction of methane generation V M Molar volume of methane at standard conditions, Molar mass of methane, determining the total organic carbon content TOC of the shale to be evaluated, density p s , the content of kerogen pyrolysis hydrocarbon is S2, the gas generation potential G of kerogen is calculated by formula (7), and then formula (6) is substituted to obtain the generation amount of methane [CH4] closed-gen , [CH4] open-gen : G=100×S2 / TOC (7) [CH4] gen The amount of methane produced, including [CH4] closed-gen - the amount of methane produced under closed system and [CH4] open-gen - the amount of methane produced under open system, where F o and F c In the unity represented by F: Step 5. Quantitative characterization of natural gas dissipation efficiency [CH4] in-place =([CH4] closed-gen -θ([CH4] closed-gen -[CH4] open-gen ))×(1-θ) (8), respectively, the system is open degree θ equal to 0%, 20%, 40%, 60%, 80%, 100%, generate different opening degree under the methane content [CH4] in-place And δ 13 C1 function relationship between the upper and lower limit curve, then the shale gas measured data to be evaluated into the model (8) for observation and comparison, the shale system open degree θ upper limit and lower limit and the amount of methane loss [CH4] expelled ; [CH4] expelled = [CH4] in-place * θ / (1 - θ) (9) Step 6. According to the spatial configuration relationship between the shale series and the specific geological elements, combined with the present pressure coefficient of the shale system and the early or late tectonic uplift time in the geological history period, the correctness of the model (8) is verified, and the preservation condition of the shale system is comprehensively evaluated.

2. The method for quantitatively evaluating shale gas preservation conditions according to claim 1, characterized by, The spatial configuration relationship between the shale series and the specific geological elements in step 6 includes: top and bottom conditions, denudation interval distance, fault scale and interval, and stratum inclination amplitude.

Images