A method for evaluating shale gas sweet spot based on methane carbon isotope in tank top gas

Abstract

The present application provides a kind of shale gas dessert evaluation method based on methane carbon isotope in tank top gas, the shale gas dessert evaluation method includes the following steps: (1) obtain the tank top gas sample of logging, measure the methane carbon isotope value in tank top gas sample by time period;(2) the data sequence of different time periods of methane carbon isotope value obtained in step (1) is curve fitting;(3) the curve obtained in step (2) is sequentially integrated and differentiated, form the model algorithm of the characteristics of shale reservoir storage and desorption gas volume;(4) according to the model algorithm obtained in step (3), the quality of shale gas reservoir is classified, and the dessert interval is divided out.The evaluation method provided by the present application solves the technical problem of shale gas reservoir site dessert identification under the condition of no geological coring and no geophysical logging, improves the shale gas dessert drilling rate and single well development efficiency, and clarifies the related technical principles and technical routes, reduces the exploration cost, and improves the customer acceptance.

Description

Technical Field

[0001] This invention belongs to the field of shale gas geological exploration technology for unconventional oil and gas reservoirs, and relates to a method for evaluating shale gas sweet spots, particularly a method for evaluating shale gas sweet spots based on the carbon isotopes of methane in tank top gas. Background Technology

[0002] Carbon isotope logging is an emerging geochemical logging technique with advantages such as real-time, online, and automated operation. It has become an essential high-end logging technology in unconventional oil and gas reservoir exploration and deep-sea drilling logging projects abroad. Currently, isotope logging is a standard component in many domestic and international tenders.

[0003] CN 111622751A discloses a method for evaluating shale gas sweet spots based on gas carbon isotopes. It uses carbon isotope logging technology to obtain alkane carbon isotope composition data of formation mud gas and cuttings tank top gas. By using the carbon isotope sweet spot evaluation parameters of each alkane component in the mud gas and cuttings tank top gas, combined with geological parameters such as the effective thickness of organic-rich shale, shale gas sweet spot intervals are delineated, providing technical guidance for horizontal well drilling. Compared to traditional methods, this method can be implemented immediately after vertical well completion, effectively shortening the sweet spot evaluation cycle and improving oil and gas exploration efficiency. However, the abundance of heavy hydrocarbons such as ethane and propane in mud gas is extremely low, making accurate instrument detection difficult, and the method of evaluating sweet spots by calculating differences has poor applicability. Furthermore, this invention requires the cooperation of total organic carbon detection, resulting in high application costs, and the evidence for field verification is insufficient; the technical principle still needs further improvement.

[0004] Therefore, it is evident that providing a method for evaluating shale gas sweet spots, further improving the drilling rate and single-well development efficiency of shale gas sweet spots based on carbon isotope detection technology, clarifying the relevant technical principles and routes, reducing exploration costs, and enhancing customer acceptance have become urgent problems that need to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a method for evaluating shale gas sweet spots based on carbon isotopes of methane in tank top gas. This method solves the technical challenge of identifying sweet spots in shale gas reservoirs in the absence of geological coring and geophysical logging. Based on carbon isotope detection technology, it further improves the drilling rate of shale gas sweet spots and the development efficiency of single wells. At the same time, it clarifies the relevant technical principles and technical routes, reduces exploration costs, and enhances customer acceptance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] This invention provides a method for evaluating shale gas sweet spots based on methane carbon isotopes in top-of-tank gas. The method includes the following steps:

[0008] (1) Obtain the tank top gas sample from the well logging and measure the methane carbon isotope value in the tank top gas sample at different time intervals.

[0009] (2) Perform curve fitting on the data sequence of methane carbon isotope values ​​obtained in step (1) for different time periods;

[0010] (3) Take the integral and differential of the curve obtained in step (2) in turn to form a model algorithm that characterizes the reservoir characteristics and desorbed gas volume of shale reservoir;

[0011] (4) Based on the model algorithm obtained in step (3), the shale gas reservoir is classified according to quality and the sweet spot layer is divided.

[0012] Compared to costly core desorption and logging methods, the shale gas sweet spot evaluation method provided by this invention does not rely on geological coring or require geophysical logging conditions. This allows for a faster and simpler analysis of shale gas reservoir sweet spot distribution, making it particularly suitable for reservoir evaluation during the logging stage of horizontal wells. This invention solves the technical challenge of identifying sweet spots in shale gas reservoirs in the field and can be applied to well completion decisions, assisting in well completion and fracturing selection, thereby improving the shale gas sweet spot encounter rate and single-well development efficiency, and providing technical support for fracturing and layer selection. The shale gas sweet spot evaluation method has high customer acceptance, low exploration costs, few logging interference factors, and high measurement accuracy, showing promising market application prospects.

[0013] Preferably, the process of obtaining the tank top gas sample in step (1) is as follows: during logging, shale cuttings returned to the surface from the well are collected and cleaned, the cuttings are placed in a tank and water is added for standing. After sealing, the hydrocarbon gases adsorbed by the cuttings are gradually desorbed and released to the top of the sealed tank, thus obtaining the tank top gas sample.

[0014] Preferably, the depth at which the rock cuttings are retrieved is the same as the depth of the mud gas.

[0015] Preferably, the volume of the rock chips in the container is 1 / 2 to 5 / 6 of the volume of the sealed container, for example, it can be 1 / 2, 2 / 3 or 5 / 6, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0016] Preferably, 2-3 drops of algaecide are added between filling the can and adding water.

[0017] In this invention, the main component of the algaecide can be dodecyl dimethyl benzyl ammonium chloride, as long as it can remove algae. Therefore, the specific components of the algaecide are not specifically limited here.

[0018] Preferably, the time interval of the time period in step (1) is 1-5 days, for example, it can be 1 day, 2 days, 3 days, 4 days or 5 days, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0019] Preferably, the measurement of the methane carbon isotope value in step (1) is performed in an isotope analyzer.

[0020] Preferably, the curve in step (2) is fitted using a linear equation, a quadratic equation, or a cubic equation.

[0021] Preferably, the result of the integration in step (3) is used as the desorption gas volume parameter.

[0022] Preferably, the maximum value of the interval of the differential in step (3) is used as a parameter for the degree of nanopore development.

[0023] Preferably, a model algorithm is constructed using the desorption gas volume parameter and the nanopore development degree parameter to characterize the reservoir properties and desorption gas volume of shale reservoirs.

[0024] Preferably, the quality classification in step (4) is performed on the isotope logging composite map.

[0025] Preferably, the parameters for drawing the isotope logging composite map include tank top gas measurement data, desorbed gas volume parameters, and nanopore development degree parameters.

[0026] As a preferred technical solution of the present invention, the shale gas sweet spot evaluation method includes the following steps:

[0027] (1) During logging, shale cuttings returned to the surface from the drilling at the depth of the mud gas are collected and cleaned. The cuttings are then placed in a container, with the container volume occupying 1 / 2 to 5 / 6 of the sealed container volume. 2 to 3 drops of algaecide are added and water is added for standing. After sealing, the hydrocarbon gases adsorbed by the cuttings are gradually desorbed and released to the top of the sealed container, thus obtaining the top gas sample. The methane carbon isotope value in the top gas sample is measured in an isotope analyzer at different time intervals of 1 to 5 days.

[0028] (2) A polynomial is used to perform curve fitting on the data sequence of methane carbon isotope values ​​obtained in step (1) for different time periods; the polynomial includes a linear equation, a quadratic equation, or a cubic equation.

[0029] (3) Take the integral and differential of the curve obtained in step (2) in sequence, use the result of the integral as the desorbed gas volume parameter, and use the maximum value of the differential interval as the nanopore development degree parameter. Use the desorbed gas volume parameter and the nanopore development degree parameter to construct a model algorithm that characterizes the reservoir characteristics and desorbed gas volume of shale reservoir.

[0030] (4) Based on the model algorithm obtained in step (3), the shale gas reservoir is classified in quality in the isotope logging composite map and sweet spot sections are divided; the drawing parameters of the isotope logging composite map include tank top gas measurement data, desorbed gas volume parameters and nanopore development degree parameters.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) Compared with the high-cost core desorption method and electrical logging method, the shale gas sweet spot evaluation method provided by the present invention does not rely on geological core sampling and does not require geophysical logging conditions, thus achieving the purpose of analyzing the distribution of shale gas reservoir sweet spots more quickly and easily. It is particularly suitable for reservoir evaluation in the logging stage of horizontal wells.

[0033] (2) This invention solves the technical problem of identifying sweet spots in shale gas reservoirs on-site. It can be applied to well completion decision-making and assist in well completion fracturing and layer selection, thereby improving the shale gas sweet spot drilling rate and single-well development efficiency, and providing technical support for fracturing and layer selection. The shale gas sweet spot evaluation method has high customer acceptance, low exploration cost, few logging interference factors, and high measurement accuracy, and has good market promotion and application prospects. Attached Figure Description

[0034] Figure 1 The isotopic values ​​δ under different nanopore sizes in this invention 13 The pattern of C changing over time;

[0035] Figure 2 This is a diagram of the calculation model for the desorbed gas volume parameter in the evaluation method provided by this invention;

[0036] Figure 3 This is a diagram illustrating the calculation model for the nanopore development degree parameter in the evaluation method provided by this invention.

[0037] Figure 4 This is a schematic diagram of tank top gas sampling in the evaluation method provided in Example 1;

[0038] Figure 5 This is a photograph of the sampling needle with the gas-side opening at the top of the tank in the evaluation method provided in Example 1;

[0039] Figure 6 This is a curve fitting diagram of the tank top gas measurement data in the evaluation method provided in Example 1;

[0040] Figure 7 This is an explanatory chart of desorption volume and nanopore development degree in the evaluation method provided in Example 1;

[0041] Figure 8 This is the isotope logging composite map in the evaluation method provided in Example 1. Detailed Implementation

[0042] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0043] This invention provides a method for evaluating shale gas sweet spots based on methane carbon isotopes in top-of-tank gas. The method includes the following steps:

[0044] (1) During logging, shale cuttings returned to the surface from the drilling at the depth of the mud gas are collected and cleaned. The cuttings are then placed in a container, with the container volume occupying 1 / 2 to 5 / 6 of the sealed container volume. 2 to 3 drops of algaecide are added and water is added for standing. After sealing, the hydrocarbon gases adsorbed by the cuttings are gradually desorbed and released to the top of the sealed container, thus obtaining the top gas sample. The methane carbon isotope value in the top gas sample is measured in an isotope analyzer at different time intervals of 1 to 5 days.

[0045] (2) A polynomial is used to perform curve fitting on the data sequence of methane carbon isotope values ​​obtained in step (1) for different time periods; the polynomial includes a linear equation, a quadratic equation, or a cubic equation.

[0046] (3) Take the integral and differential of the curve obtained in step (2) in sequence, use the result of the integral as the desorbed gas volume parameter, and use the maximum value of the differential interval as the nanopore development degree parameter. Use the desorbed gas volume parameter and the nanopore development degree parameter to construct a model algorithm that characterizes the reservoir characteristics and desorbed gas volume of shale reservoir.

[0047] (4) Based on the model algorithm obtained in step (3), the shale gas reservoir is classified in quality in the isotope logging composite map and sweet spot sections are divided; the drawing parameters of the isotope logging composite map include tank top gas measurement data, desorbed gas volume parameters and nanopore development degree parameters.

[0048] This invention obtains isotopic values ​​δ under different nanopore sizes through experiments. 13 The pattern of C changing over time, such as Figure 1 As shown, with the development of nanopores, the slope of the curve gradually increases, and the area enclosed by the curve with respect to time, i.e., the integral area, also increases. Based on this, the present invention performs differential integration on the isotope fractionation curve to obtain parameters characterizing the desorption gas volume and the degree of nanopore development in shale reservoirs.

[0049] Because carbon has C 12 C 13 C 14 Three isotopes, of which C 12 and C 13 To stabilize isotopes, C 14 It is a radioactive isotope, and C13 Its abundance in nature is only C 12 Approximately 1% of the C in the tank top gas sample described in step (1) 13 The relative abundance, i.e., δ 13 The minute changes in C are the information to be collected in this invention, δ 13 The specific formula for calculating C is as follows:

[0050]

[0051] in, To test the stable isotope C in the top gas sample of the tank 13 With C 12 The ratio; For the stable isotope C in the standard sample 13 With C 12 The ratio of .

[0052] In this invention, isotope C is contained 12 Hydrocarbon gases such as methane, due to their higher carbon content than C... 13 The absence of one neutron results in a lighter mass, causing it to be preferentially released from rock cuttings—a phenomenon known to those skilled in the art as physical fractionation. The applicant conducted time-series testing of the δ¹⁸O₂ of the tank top gas. 13 The C value revealed the detected δ 13 The C value increases because of the gas C released in the later stages. 12 Because of the fractionation process, most of the product has accumulated at the top of the sealed container.

[0053] In this invention, the calculation model for the desorption gas volume parameter in step (3) is shown below. Figure 2 The area enclosed by f(x) and g(x) (the integral at time ab) is the desorption gas quantity parameter Q, and the specific calculation formula is as follows:

[0054]

[0055] In this invention, the calculation model for the nanopore development degree parameter in step (3) is shown below. Figure 3 The nanopore development parameter K for shale gas desorption can be calculated by differentiating the isotopic fractionation curve f(x). A larger K value indicates denser shale, more developed nanopores, and lower matrix permeability. The specific calculation formula is as follows:

[0056] K = d(f(X))

[0057] Example 1

[0058] This embodiment provides a method for evaluating shale gas sweet spots based on methane carbon isotopes in tank top gas. The shale gas sweet spot evaluation method includes the following steps:

[0059] (1) Gas sampling from the top of the tank:

[0060] like Figure 4 As shown, during logging, shale cuttings returned to the surface are retrieved and quickly washed. The cuttings are then placed in a container, water is added, and the container is sealed. The hydrocarbon gases (mainly methane) adsorbed by the cuttings gradually desorb and are released to the top of the sealed container, thus obtaining the top gas sample. A sampling needle with an opening on the top gas side (such as...) is used. Figure 5 (As shown) Gas is extracted from the top of the tank and injected into an isotope analyzer for detection, yielding isotope detection data value δ. 13 C.

[0061] In this embodiment, rock cuttings at the same depth as the mud gas are collected from the outlet of the wellhead vibrating screen. After cleaning, the rock cuttings sample is immediately placed in a sealed container. The rock cuttings sample is measured to 2 / 3 of the container's volume, between the max and min graduations. Two drops of algaecide are added, along with an appropriate amount of water, raising the liquid level in the sealed container to the lower edge of the threaded part of the container opening, while ensuring a certain amount of non-solid-liquid space inside the container. The sampling container is then tightly closed, ensuring the inside of the lid is clean and free of foreign objects, and that the container opening fits tightly against the rubber gasket to ensure a sealed container.

[0062] (2) Carbon isotope fractionation data processing:

[0063] Taking a shale gas well in Weiyuan as an example, a top gas sample was obtained at a well depth of 3496m. The isotopic values ​​of the top gas were measured sequentially at four times: time 0 (sealing time), day 1, day 3, and day 7. The four sets of measurements are shown in Table 1.

[0064] Table 1

[0065]

[0066]

[0067] (2.1) Algorithm processing:

[0068] Since the obtained data is scattered data, curve fitting is required first (see...). Figure 6 Then, the fitted line is integrated. In this embodiment, four sets of formulas from Excel software are used to calculate the polynomial coefficients of the fitted curve:

[0069] 1) INDEX(LINEST(Table1 data range^{1,2,3},TRUE,TRUE),1,1)=3.017;

[0070] 2) INDEX(LINEST(Table1 data range^{1,2,3},TRUE,TRUE),1,2)=-15.833;

[0071] 3) INDEX(LINEST(Table1 data range^{1,2,3},TRUE,TRUE),1,3)=22.62;

[0072] 4)INDEX(LINEST(Table1 data range^{1,2,3},TRUE,TRUE),1,4)=-34.8.

[0073] Therefore, Figure 6 The fitting curve of the cubic polynomial is:

[0074] y = 3.017x 3 -15.833x 2 +22.626x-34.8

[0075] The desorption rate parameter Q is obtained by calculating the integral area enclosed by the baseline in the above equation; the nanopore development degree parameter K is obtained by taking the derivative of the above equation and taking the maximum value in the interval.

[0076] (2.2) Whole-well data processing:

[0077] Based on the above algorithm, Excel and Origin software were used to perform full-well batch processing calculations on the tank top gas sample of a shale gas exploration well, resulting in a parameter calculation table (see Table 2):

[0078] Table 2

[0079]

[0080]

[0081]

[0082] (3) Dessert review:

[0083] This embodiment establishes a diagrammatic interpretation of desorption volume and nanopore development degree (see...). Figure 7 The total hydrocarbon value on the chart is used to interpret and evaluate the sweet spot of shale gas reservoirs. The total hydrocarbon value is used as an auxiliary parameter for evaluation, that is, the mixed hydrocarbon value of the mud gas extracted from the well obtained from the logging.

[0084] like Figure 7 As shown, the specific evaluation method is explained below:

[0085] I: This region has well-developed nanopores, high desorption gas volume, and high total hydrocarbon value. After development, it can provide mid-to-late stage shale gas production capacity, and the reservoir quality is the best.

[0086] II: This region has poor nanopore development, loose lithology, and high total hydrocarbon value. The free gas of the shale reservoir is mainly stored here, which can provide initial production capacity in the early stage of development. The reservoir quality is relatively good.

[0087] III: This region has well-developed nanopores, low desorption gas volume, dense lithology, and low total hydrocarbon value, which is generally interpreted as a poor gas layer with poor reservoir quality.

[0088] IV: This region has poor nanopore development, low desorption gas volume, loose lithology, and low total hydrocarbon value. It is usually a fracture development zone, and the reservoir quality is the worst.

[0089] (4) Evaluation results:

[0090] The obtained tank top gas measurement data, desorbed gas volume parameters, and nanopore development parameters, combined with parameters obtained from other logging operations, were used to create a map using commonly used petroleum geological mapping software, resulting in a comprehensive isotope logging map (see...). Figure 8 Based on the evaluation method, shale gas reservoirs are classified according to quality, providing a basis for well completion decisions and reservoir fracturing selection.

[0091] exist Figure 8 In the diagram: GR (API) represents the logging gamma curve; Stra represents the formation; MD represents the drilling depth (m); Si represents the silicon content (%) in the elemental logging; Ca represents the calcium content (%) in the elemental logging; Total hydrocarbons represent the mixed hydrocarbon value obtained from mud gas logging (%); C1 represents the percentage of methane in the mixed hydrocarbons (%); Class I, Class II, Class III, and Class IV represent the results of reservoir sweet spot evaluation, respectively.

[0092] Therefore, compared to high-cost core desorption and logging methods, the shale gas sweet spot evaluation method provided by this invention does not rely on geological coring or require geophysical logging conditions, achieving a faster and simpler analysis of shale gas reservoir sweet spot distribution. It is particularly suitable for reservoir evaluation during the logging stage of horizontal wells. Furthermore, this invention solves the technical challenge of on-site sweet spot identification in shale gas reservoirs and can be applied to well completion decisions, assisting in well completion and fracturing selection, thereby improving the shale gas sweet spot encounter rate and single-well development efficiency. It provides technical support for fracturing and layer selection. The shale gas sweet spot evaluation method has high customer acceptance, low exploration costs, few logging interference factors, and high measurement accuracy, showing promising market application prospects.

[0093] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for evaluating shale gas sweet spots based on methane carbon isotopes in tank top gas, characterized in that, The method for evaluating shale gas sweet spots includes the following steps: (1) Obtain the tank top gas sample from the well logging and measure the methane carbon isotope value in the tank top gas sample at different time intervals; (2) Perform curve fitting on the data sequence of methane carbon isotope values ​​obtained in step (1) for different time periods; (3) Take the integral and differential of the curve obtained in step (2) in sequence to form a model algorithm that characterizes the reservoir characteristics and desorbed gas volume of shale reservoir; the result of the integral in step (3) is used as the desorbed gas volume parameter; the maximum value of the interval of the differential in step (3) is used as the nanopore development degree parameter. (4) Based on the model algorithm obtained in step (3), the shale gas reservoir is classified according to quality and the sweet spot layer is divided.

2. The shale gas sweet spot evaluation method according to claim 1, characterized in that, The process of obtaining the tank top gas sample in step (1) is as follows: during logging, shale cuttings returned to the surface from the well are collected and cleaned. The cuttings are then placed in a tank, water is added, and the tank is left to stand. After sealing, the hydrocarbon gases adsorbed by the cuttings are gradually desorbed and released to the top of the sealed tank, thus obtaining the tank top gas sample.

3. The shale gas sweet spot evaluation method according to claim 2, characterized in that, The rock cuttings were retrieved at the same depth as the mud gas.

4. The shale gas sweet spot evaluation method according to claim 2, characterized in that, The volume of the rock cuttings in the container occupies 1 / 2 to 5 / 6 of the volume of the sealed container.

5. The shale gas sweet spot evaluation method according to claim 2, characterized in that, Two to three drops of algaecide are added to the rock chips between filling the container and adding water.

6. The shale gas sweet spot evaluation method according to claim 1, characterized in that, The time interval for the segmented periods in step (1) is 1-5 days.

7. The shale gas sweet spot evaluation method according to claim 1, characterized in that, The measurement of the methane carbon isotope value in step (1) is performed in an isotope analyzer.

8. The shale gas sweet spot evaluation method according to claim 1, characterized in that, The curve in step (2) is fitted using a linear equation, a quadratic equation, or a cubic equation.

9. The shale gas sweet spot evaluation method according to claim 1, characterized in that, A model algorithm is constructed to characterize the reservoir properties and desorbed gas volume of shale reservoirs using the desorbed gas volume parameter and the nanopore development degree parameter.

10. The method for evaluating shale gas sweet spots according to claim 1, characterized in that, The quality classification described in step (4) is performed on the isotope logging composite map.

11. The shale gas sweet spot evaluation method according to claim 10, characterized in that, The parameters for drawing the isotope logging composite map include tank top gas measurement data, desorbed gas volume parameters, and nanopore development degree parameters.

12. The shale gas sweet spot evaluation method according to claim 1, characterized in that, The method for evaluating shale gas sweet spots includes the following steps: (1) During logging, shale cuttings returned to the surface from the drilling at the depth of the mud gas are collected and cleaned. The cuttings are then placed in a container, with the container volume occupying 1 / 2 to 5 / 6 of the sealed container volume. 2 to 3 drops of algaecide are added and water is added for standing. After sealing, the hydrocarbon gases adsorbed by the cuttings are gradually desorbed and released to the top of the sealed container, thus obtaining the top gas sample. The methane carbon isotope value in the top gas sample is measured in an isotope analyzer at intervals of 1 to 5 days. (2) A polynomial is used to perform curve fitting on the data sequence of methane carbon isotope values ​​obtained in step (1) at different time periods; the polynomial includes a linear equation, a quadratic equation, or a cubic equation. (3) Take the integral and differential of the curve obtained in step (2) in sequence, use the result of the integral as the desorbed gas volume parameter, and use the maximum value of the differential interval as the nanopore development degree parameter. Use the desorbed gas volume parameter and the nanopore development degree parameter to construct a model algorithm that characterizes the reservoir characteristics and desorbed gas volume of shale reservoir. (4) Based on the model algorithm obtained in step (3), the shale gas reservoir is classified in quality in the isotope logging composite map and sweet spot sections are divided; the drawing parameters of the isotope logging composite map include tank top gas measurement data, desorbed gas volume parameters and nanopore development degree parameters.

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