A method and system for calculating gas content in shale reservoirs

Abstract

This invention discloses a method and system for calculating the gas content of shale reservoirs, comprising: determining a sampling plan for the target well section to be drilled; collecting flowback cuttings according to the sampling plan, and encapsulating the flowback cuttings to form a gas release process within a sealed space; measuring the carbon isotope and component content of the flowback gas at different times for each encapsulated sample; and calculating the shale reservoir gas content of the current well section based on the carbon isotope and component content of the flowback gas at different time series. This invention improves the timeliness of obtaining shale reservoir gas content parameters, facilitating real-time evaluation of shale reservoirs during drilling.

Description

Technical Field

[0001] This invention relates to the field of shale oil and gas exploration technology, and in particular to a method and system for calculating the gas content of shale reservoirs. Background Technology

[0002] Shale gas content is a crucial evaluation parameter for shale reservoir assessment, favorable zone selection, and reserve calculation. The desorption method is the most direct and commonly used method for testing shale gas content, and its principles, methods, and procedures all adhere to the current Chinese national standard GB / T 19559-008, "Method of determining coalbed gas content." Gas content consists of three parts: desorbed gas content (Vd), lost gas content (Vl), and residual gas content (Vr). Experimental tests show that lost gas accounts for 40%-80% of the total gas content. Therefore, lost gas content is a significant component of gas content, further indicating that the accuracy of gas content calculation depends primarily on minimizing and accurately calculating lost gas content.

[0003] However, existing technologies use the least squares method to regress and fit the measured desorption points, which initially appear as a straight line. The suitability of using coalbed methane (CBM) methods to fit the gas loss in shale gas, particularly the USBM method, for testing shale gas content, requires further investigation. Firstly, shale gas and CBM differ in many aspects, including target layer depth, reservoir temperature, coring methods, and desorption processes. The differences are particularly significant in coring methods and natural gas storage methods. Shale samples are often encased in mud or exposed to the environment for extended periods during coring, resulting in longer loss times and greater temperature variations, which differ considerably from CBM desorption. Furthermore, CBM desorption is primarily adsorption-based, unlike the higher proportion of free gas in shale gas. Therefore, using desorbed gas regression fitting methods inevitably introduces significant errors. Secondly, most of the gas is released during the core extraction process in the wellbore, which is a process of gradual pressure change. However, the desorption process after core sampling is a gas release under constant pressure. Therefore, it is obviously inappropriate to extrapolate the pressure-variable gas release process based on the constant pressure gas release process on a time scale. Moreover, the core sampling process is time-consuming, labor-intensive, and relatively expensive. Summary of the Invention

[0004] The purpose of this invention is to provide a scheme for calculating the gas content of shale reservoirs, so as to solve the difficulties in the actual operation and application of existing methods for core desorption testing of gas content.

[0005] To address the aforementioned technical problems, this invention provides a method for calculating the gas content of shale reservoirs, comprising: determining a sampling plan for the target well section to be drilled; collecting runoff cuttings according to the sampling plan, and encapsulating the runoff cuttings to form a cuttings gas release process within a sealed space; measuring the carbon isotope and component content of the cuttings gas at different times for each encapsulated sample; and calculating the shale reservoir gas content of the current well section based on the carbon isotope and component content of the cuttings gas at different time series.

[0006] Preferably, the step of calculating the gas content of the shale reservoir in the current well section based on the carbon isotope and component content of the cuttings top gas under different time series includes: calculating the total gas volume released by each cuttings sample based on the carbon isotope and component content of the cuttings top gas under different time series of each filling sample; and inverting the gas content of the shale reservoir in the current well section based on the changes in the carbon isotope value of the top gas and the total gas volume released under different time series of each filling sample.

[0007] Preferably, the step of inverting the gas content of the shale reservoir in the current well section based on the changes in the carbon isotope value of the cap gas and the changes in the total amount of gas released for each filled sample includes: based on the mathematical model of shale natural gas desorption, using the numerical solution analysis method of partial differential equations, simulating and calculating the dynamic changes in the gas pressure distribution, free gas content, adsorbed gas content, and gas outflow flux of the core within the rock cuttings throughout the entire process from the original underground state to extraction to the surface and completion of the desorption and gas release experiment. This allows for the continuous adjustment of the gas permeability to obtain simulated cap gas content values ​​at different well depths and time series, and the continuous adjustment of the carbon isotope flow coefficient to obtain methane content at different well depths and time series. Isotope simulation values; constructing an objective function for the deviation between simulated and experimental values ​​of top gas content parameters under different time series, solving the minimum problem using the gradient descent method, and continuously fitting the simulated top gas content values ​​of the top gas under different time series obtained from the simulation experiment to the corresponding experimental values; and constructing an objective function for the deviation between simulated and experimental values ​​of methane isotope parameters under different time series, solving the minimum problem using the gradient descent method, and continuously fitting the simulated carbon isotope values ​​of cuttings top gas under different time series obtained from the simulation experiment to the corresponding experimental values. After the fitting of the two parameters is completed, the solved fitting parameters are substituted into the shale natural gas desorption mathematical model to obtain the gas content of the shale reservoir in the current well section.

[0008] Preferably, the overburden gas content during the entire dynamic change process is simulated based on the carbon isotope conservation condition, wherein the carbon isotope conservation condition is expressed by the following expression:

[0009]

[0010]

[0011] Where φ represents shale porosity, t represents time, r represents distance, m represents shape factor, c represents adsorption mass ratio, and p and p * express 12 CH4 and 13 Partial pressure of CH4 gas, θ and θ * express 12 CH4 and 13 CH4 monolayer adsorption coverage, D and D * express 12 CH4 and 13 The gas permeability of CH4, p0 represents the standard pressure, k a and k a * They represent 12 CH4 and 13 The adsorption rate constant of CH4 molecules, k d and k d * They represent 12 CH4 and 13 The desorption rate constant of CH4 molecules.

[0012] Preferably, the objective function is represented by the following expression:

[0013]

[0014] Where m represents the vector of fitting parameters to be solved, m pr The prior distribution threshold, C, of ​​the fitted parameters to be solved M F(m) represents the autocorrelation function of the fitting parameters to be determined, F(m) represents the simulated value of the top gas content obtained by using the parameter vector m, d represents the experimental data of the top gas content at different time series obtained by the gas release experiment or the carbon isotope experimental data of the top gas from rock cuttings, and C represents the simulated value of the top gas content obtained by the gas release experiment. D Let represent the autocorrelation matrix of the experimental data, and j represent the number of fits.

[0015] Preferably, the process of encapsulating the returned rock cuttings includes: filling the returned rock cuttings with an algaecide, then sealing and recording sample information, wherein the sample information includes at least the well number, the sampling well depth, and the sampling time.

[0016] Preferably, the step of determining the sampling plan for the target well section to be drilled includes: collecting the stratification data and well trajectory of the target well, and determining the sampling interval of the target well section to be drilled based on the well type, well depth range, geological characteristics and reservoir type of different strata.

[0017] Preferably, during drilling, the sampling interval for the remaining undrilled sections is adjusted based on the gas content of the shale reservoir at the drilled depth within the target well section.

[0018] On the other hand, a system for calculating the gas content of shale reservoirs is provided. This system performs the method described above, wherein the system includes: a scheme design module for determining the sampling scheme for the target well section to be drilled; a cuttings collection module for collecting runoff cuttings according to the sampling scheme and encapsulating the runoff cuttings to form a cuttings gas release process within a sealed space; an experimental measurement module for measuring the carbon isotope and component content of the cuttings gas at different times for each filled sample; and a gas content fitting module for calculating the shale reservoir gas content of the current well section based on the carbon isotope and component content of the cuttings gas at different time series.

[0019] Preferably, the rock cuttings collection module is used to fill the returned rock cuttings with an algaecide, then seal and record the sample information, which includes at least the well number, the sampling well depth, and the sampling time.

[0020] Compared with the prior art, one or more embodiments of the above solutions may have the following advantages or beneficial effects:

[0021] This invention proposes a method and system for calculating the gas content of shale reservoirs. Based on carbon isotope analysis of gas samples from the top of a single-well cuttings tank, this method and system calculate the gas content of the shale reservoir using isotope fractionation principles. This invention solves the problems of high sample requirements, high acquisition costs, and low accuracy in calculating lost gas in desorption methods, improving the timeliness of obtaining shale reservoir gas content parameters. It facilitates real-time evaluation of shale reservoirs during drilling and can provide a basis for selecting sweet spots in shale oil and gas wells and optimizing the stratigraphic positions of horizontal wells. It has guiding significance for the exploration and development of shale oil and gas and has broad prospects for widespread application.

[0022] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims, and drawings. Attached Figure Description

[0023] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0024] Figure 1 This is a step diagram of a method for calculating the gas content of shale reservoirs according to an embodiment of this application.

[0025] Figure 2 This is a schematic diagram of carbon isotope changes during the rock cuttings release process in the method for calculating the gas content of shale reservoirs according to an embodiment of this application.

[0026] Figure 3 This is a schematic diagram of carbon isotope changes in a fractionation simulation experiment in a method for calculating gas content in shale reservoirs according to an embodiment of this application.

[0027] Figure 4 This is a schematic diagram of the sampling design in the method for calculating the gas content of shale reservoirs according to an embodiment of this application.

[0028] Figure 5 This is an example diagram showing the calculation results of shale reservoir gas content in the method for calculating shale reservoir gas content according to an embodiment of this application.

[0029] Figure 6 This is a system block diagram for calculating the gas content of shale reservoirs according to an embodiment of the present invention. Detailed Implementation

[0030] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples, so that the process of how the present invention uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly. It should be noted that, as long as there is no conflict, the various embodiments and features in the various embodiments of the present invention can be combined with each other, and the resulting technical solutions are all within the protection scope of the present invention.

[0031] Furthermore, the steps illustrated in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Also, although a logical order is shown in the flowcharts, in some cases the steps shown or described may be performed in a different order than that shown here.

[0032] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments. Unless the context clearly indicates otherwise, the singular forms “a” and “an” as used herein are also intended to include the plural. It should also be understood that the terms “comprising” and / or “including” as used herein specify the presence of the stated features, integers, steps, operations, units, and / or components, without excluding the presence or addition of one or more other features, integers, steps, operations, units, components, and / or combinations thereof.

[0033] Shale gas content is a crucial evaluation parameter for shale reservoir assessment, favorable zone selection, and reserve calculation. The desorption method is the most direct and commonly used method for testing shale gas content, and its principles, methods, and procedures all adhere to the current Chinese national standard GB / T 19559-008, "Method of determining coalbed gas content." Gas content consists of three parts: desorbed gas content (Vd), lost gas content (Vl), and residual gas content (Vr). Experimental tests show that lost gas accounts for 40%-80% of the total gas content. Therefore, lost gas content is a significant component of gas content, further indicating that the accuracy of gas content calculation depends primarily on minimizing and accurately calculating lost gas content.

[0034] However, existing technologies use the least squares method to regress and fit the measured desorption points, which initially appear as a straight line. The suitability of using coalbed methane (CBM) methods to fit the gas loss in shale gas, particularly the USBM method, for testing shale gas content, requires further investigation. Firstly, shale gas and CBM differ in many aspects, including target layer depth, reservoir temperature, coring methods, and desorption processes. The differences are particularly significant in coring methods and natural gas storage methods. Shale samples are often encased in mud or exposed to the environment for extended periods during coring, resulting in longer loss times and greater temperature variations, which differ considerably from CBM desorption. Furthermore, CBM desorption is primarily adsorption-based, unlike the higher proportion of free gas in shale gas. Therefore, using desorbed gas regression fitting methods inevitably introduces significant errors. Secondly, most of the gas is released during the core extraction process in the wellbore, which is a process of gradual pressure change. However, the desorption process after core sampling is a gas release under constant pressure. Therefore, it is obviously inappropriate to extrapolate the pressure-variable gas release process based on the constant pressure gas release process on a time scale. Moreover, the core sampling process is time-consuming, labor-intensive, and relatively expensive.

[0035] To address the aforementioned technical problems, this application proposes a method and system for calculating the gas content of shale reservoirs. This method and system utilizes drilling cuttings returned during the drilling process, measures the carbon isotope and component content of the gas at the top of the cuttings container, and directly calculates the gas content of the shale reservoir. This solves the difficulties and problems encountered in the practical operation and application of existing methods for core desorption testing of gas content.

[0036] Figure 1 This is a step diagram of a method for calculating the gas content of shale reservoirs according to an embodiment of this application.

[0037] like Figure 1As shown, step S110 determines the sampling plan for the target well section to be drilled. The sampling interval for the shale reservoir section of the target well is determined. Specifically, in step S110, during drilling, the layered data and well trajectory of the target well are continuously collected. Based on the well type, depth range, geological characteristics, and reservoir type (e.g., oil-bearing, gas-bearing, and formation) of different layers, the sampling interval for the target well section to be drilled is determined. Furthermore, in determining the sampling interval for the target well section to be drilled, in addition to referring to the well type, depth range, geological characteristics, and reservoir type of different layers, this embodiment of the invention can also adjust the sampling interval for the remaining un-drilled sections based on the gas content of the shale reservoir at the already drilled depth within the target well section.

[0038] Furthermore, target well stratification data and well trajectory are collected, and appropriate sampling intervals are determined based on the target and non-target strata and the well trajectory. Figure 4 This is a schematic diagram of the sampling design in the method for calculating the gas content of shale reservoirs according to an embodiment of this application. Figure 4 This demonstrates how to determine the sampling density for different well depths based on characteristics such as well depth range, well type (e.g., vertical well, build-up section, horizontal section), geological features, reservoir type, and gas content in the drilled section. Here, Pack represents the sampling density of carbon isotopes in mud gas, and Jar represents the sampling density of carbon isotopes in the top gas tank.

[0039] Step S120 involves collecting runoff cuttings according to the sampling plan designed in step S110, and encapsulating each sample of runoff cuttings to allow the encapsulated sample to undergo a degassing process within a sealed space. Specifically, each collected runoff cuttings sample is filled with an algaecide and then sealed, thereby recording sample information. This sample information includes, but is not limited to, the well number, the sampling well depth, and the sampling time.

[0040] During the drilling process, rock cuttings samples were retrieved from the vibrating screen at preset sampling intervals for the current target well section. The retrieved samples were then placed in containers, algaecide was added, and the containers were sealed and labeled with the well number, sampling depth, and sampling time.

[0041] Next, in step S130, the carbon isotope and component content of the top gas of each filled sample are measured at different times. In step S130, for each returned cutter top gas sample, the tank is opened for measurement at different detection time points, that is, detection and analysis are carried out according to different detection time series (e.g., 1h, 12h, 48h, 72h), to obtain the carbon isotope value and component content of the top gas of the cutter tank under different detection time series. This clarifies the changes in the carbon isotope value and the changes in the carbon isotope component content of the top gas of the cutter tank during the gas release process of the current well section. Then, based on the changes in the carbon isotope value and the changes in the carbon isotope component content of the top gas of the cutter tank, the total amount of gas released in the tank for each filled sample is calculated.

[0042] Figure 2 This is a schematic diagram illustrating the carbon isotope changes during the cuttings release process in the method for calculating the gas content of shale reservoirs, as described in an embodiment of this application. Figure 2 As shown, the carbon isotope values ​​of the filled samples changed significantly after the sealing point.

[0043] Thus, in this embodiment of the invention, the variation characteristics of the carbon isotope value of the cuttings tank top gas and the total amount of gas released in the tank of each filled sample obtained in step S130 can be used to further estimate the shale gas reservoir content of the current well section, thereby proceeding to step S140.

[0044] Step S140 calculates the gas content of the shale reservoir in the current well section based on the carbon isotope and component content of the cuttings top gas under different time series. Further, based on the changes in carbon isotope values ​​and component content of the cuttings top gas under different time series for each filled sample, the total gas released by each cuttings sample is calculated. Then, based on the changes in carbon isotope values ​​of the cuttings top gas under different time series for each filled sample and the total gas released by the cuttings sample, the gas loss during the gas release process is estimated, thereby retrieving the gas content of the shale reservoir in the current well section.

[0045] During the rock cuttings venting process, carbon isotopes undergo fractionation, resulting in a gradual change in the carbon isotopes of the released natural gas. However, the total carbon isotope value of all released natural gas remains constant, representing the carbon isotope value of the "broken gas" or "mud gas." When the rock cuttings are encapsulated, the gas that can be released is divided into two parts: inside and outside the tank. The carbon isotope values ​​inside and outside the tank maintain a balance with the total gas volume. Because carbon isotope fractionation is weak in the early stages of rock cuttings venting, the total carbon isotope value of the gas outside the tank can be considered a constant. Therefore, the lost gas volume can be estimated by using the total gas volume released from the rock cuttings inside the tank and the carbon isotope change value. This allows for the reconstruction of the total gas content of the shale through an inversion process (see [link to relevant documentation]). Figure 1 ).

[0046] Specifically, in the inversion process, firstly, based on the mathematical model of shale natural gas desorption, the dynamic changes in gas pressure distribution, free gas content, adsorbed gas content, and gas flux out of the core are simulated and calculated throughout the entire fractionation process from the original underground state to extraction to the surface and completion of the desorption and gas release experiment. This simulation involves continuously adjusting the gas permeability to obtain simulated values ​​of the top gas content at different well depths and time series, and continuously adjusting the carbon isotope mobility coefficient to obtain simulated values ​​of methane isotopes at different well depths and time series. The initial conditions are determined by the bottom hole pressure and temperature during drilling and coring; the boundary conditions (which vary with time) are determined by the dynamic pressure at the corresponding depth in the well during coring and the desorption experiment conditions; and the dynamic adsorption amount is determined by the Langmuir adsorption characteristics and parameters obtained from the adsorption experiment. Key parameters such as the core permeability coefficient and creep coefficient are obtained through historical fitting of the numerical model.

[0047] Next, an objective function is constructed to describe the deviation between simulated and experimental values ​​of the top gas content parameter under different time series. The minimum value is then solved using the gradient descent method. The simulated values ​​of the top gas content under different time series obtained from the simulation experiment are continuously fitted to the corresponding experimental values. Then, an objective function is constructed to describe the deviation between simulated and experimental values ​​of the methane isotope parameter under different time series. The minimum value is then solved using the gradient descent method. The simulated values ​​of the methane isotope parameter under different time series obtained from the simulation experiment are continuously fitted to the corresponding simulated values. After fitting the two parameters, the obtained fitting parameters are substituted into the above shale natural gas desorption mathematical model to obtain the shale reservoir gas content of the current target well section.

[0048] Figure 3 This is a schematic diagram of carbon isotope variations in a fractionation simulation experiment used in a method for calculating gas content in shale reservoirs according to an embodiment of this application. Figure 3 As shown, the fractionation simulation calculations and the indoor measurement experimental results (step S130) also indicate that when the internal fluid pressure and content of shale are high, the final value of carbon isotope fractionation is often also high (e.g., Figure 2 (As shown). Therefore, by comparing the numerical calculation results under different gas-bearing conditions with the actual measured two-dimensional variation curves of carbon isotopes and gas content, the gas content of shale can be inverted.

[0049] The gas content in the shale matrix conforms to the law of conservation of mass, meaning that the decrease in gas concentration in the matrix at any given time is equal to the mass of gas flowing out of the matrix at that time. Therefore, for carbon isotope methane gas, whether it is 12CH4 or 13CH4, the law of conservation of mass applies, and the following set of equations can be established (i.e., the carbon isotope mass conservation condition can be expressed using the following expression):

[0050]

[0051]

[0052] Where φ represents shale porosity; t represents time; r represents distance; m represents shape factor (0 represents planar plate-like pores, 1 represents cylindrical pores, 2 represents spherical pores), c represents adsorption mass ratio, and p and p * They represent 12 CH4 and 13 The partial pressure of CH4; θ and θ * They represent 12 CH4 and 13 Monolayer adsorption coverage of CH4; D and D * They represent 12 CH4 and 13 The gas permeability of CH4; p0 represents the standard pressure (equal to 1 bar); k a and k a * They represent 12 CH4 and 13 The adsorption rate constant of CH4 molecules; k d and k d * They represent 12 CH4 and 13 The desorption rate constant of CH4 molecules.

[0053] The objective function for the deviation between the simulated values ​​of methane isotope parameters under different time series and the experimental values ​​of carbon isotopes in the cuttings top gas measured in step S130 under different time series is expressed by the following expression:

[0054]

[0055] Where j represents the sequence number of the fitting iteration; m represents the vector of fitting parameters to be solved (gas permeability or carbon isotope flow coefficient); m pr C represents the prior distribution threshold of the fitted parameters to be solved; MThe autocorrelation function of the fitting parameters to be determined is represented; F(m) represents the simulated value of the top gas content obtained by using the parameter vector m; d represents the experimental data of the top gas content at different time series obtained by the gas release experiment or the experimental data of the carbon isotope of the top gas from the cuttings; C D This represents the autocorrelation matrix of the measured data.

[0056] Furthermore, embodiments of the present invention also utilize the stochastic gradient descent method, characterized by the following expression, to iteratively fit the changes in experimental data of methane isotope parameters over different time series with the sampling time of the tank top gas:

[0057]

[0058] Where η represents the adaptive optimization step size; Si represents the new objective function after the parameter vector is randomly changed; i represents the iteration number, and i+1 represents the next iteration.

[0059] The following describes the application of the method for calculating the gas content of shale reservoirs described in this invention to a specific shale gas well. Horizontal well drilling was carried out on this well group to expand into new shale gas exploration areas and strata, obtaining production and resource evaluation parameters for the Doushantuo Formation. The key tasks included optimizing the stratigraphic position traversed by the horizontal wells and evaluating the gas content of the shale reservoirs.

[0060] (1) According to Figure 4 The sampling design plan included on-site sampling, testing, and calculation of the gas content of the shale reservoir;

[0061] (2) The Doushantuo shale was the horizontal well section designed for the example well. Based on the TOC and gas logging data obtained from the pilot well, the horizontal well was assigned to section 3. However, after entering the horizontal section, the formation gas logging showed poor performance during the actual test drilling. Therefore, it was decided to select another horizontal well section. According to the carbon isotope logging results, the areas where the carbon isotope values ​​of the cuttings gas released from the Doushantuo section were significantly heavier than those of the mud gas appeared in sections 5-7 and the middle of section 3, but the subsequent changes over time were generally small. Furthermore, sweet spot index analysis showed that the permeability of the Doushantuo shale was generally lower than that of the lower part of the Niutitang section. Although there were some areas with higher gas volume in section 3, the overall thickness was thin and the continuity was poor. In contrast, the gas at the top of the cuttings tank in section 5-7 above it showed a larger increase in weight and a relatively better gas volume, which is consistent with the gas logging data during this drilling process. Therefore, revisiting the 5-7 segment as the drilling zone for the horizontal well is a better option. Figure 5 ).

[0062] Figure 5 This is an example diagram showing the calculation results of shale reservoir gas content in the method for calculating shale reservoir gas content according to an embodiment of this application. Figure 5The data show the characteristics of carbon isotope value variation, relative gas content variation, pore development degree variation, and gas logging curves at different well depths.

[0063] On the other hand, based on the above-described method for calculating the gas content of shale reservoirs, this embodiment of the invention also provides a system for calculating the gas content of shale reservoirs. This system for calculating the gas content of shale reservoirs executes the aforementioned method for calculating the gas content of shale reservoirs. Figure 6 This is a system block diagram for calculating the gas content of shale reservoirs according to an embodiment of the present invention. Figure 6 As shown, the system described in this embodiment of the invention includes: a scheme design module 61, a rock cuttings collection module 62, an experimental measurement module 63, and a gas content fitting module 64.

[0064] Specifically, the scheme design module 61 is implemented according to the method described in step S110 above, and is configured to determine the sampling scheme for the target well section to be drilled; the cuttings collection module 62 is implemented according to the method described in step S120 above, and is configured to collect the returned cuttings according to the above sampling scheme, and encapsulate the returned cuttings to form a cuttings gas release process in a sealed space; the experimental measurement module 63 is implemented according to the method described in step S130 above, and is configured to measure the carbon isotope and component content of the cuttings top gas at different times for each filled sample; the gas content fitting module 64 is implemented according to the method described in step S140 above, and is configured to calculate the shale reservoir gas content of the current well section based on the carbon isotope and component content of the cuttings top gas under different time series.

[0065] The cuttings collection module is also used to fill the returned cuttings with an algaecide, then seal and record the sample information. This sample information includes at least the well number, well depth, and sampling time.

[0066] This invention discloses a method and system for calculating the gas content of shale reservoirs. Based on carbon isotope analysis of gas samples from the top of a single-well cuttings tank, the method and system use isotope fractionation principles to fit and calculate the gas content of the shale reservoir. This invention solves the problems of high sample requirements, high acquisition costs, and low accuracy in calculating lost gas in desorption methods. It improves the timeliness of obtaining shale reservoir gas content parameters, facilitates real-time evaluation of shale reservoirs during drilling, and can provide a basis for selecting sweet spots in shale oil and gas wells and optimizing the stratigraphic sequence of horizontal wells. It has guiding significance for the exploration and development of shale oil and gas, and its application prospects are broad.

[0067] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

[0068] In the description of this invention, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," "tail," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0069] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0070] It should be understood that the embodiments disclosed herein are not limited to the specific structures, processing steps, or materials disclosed herein, but should be extended to equivalent substitutions of these features as understood by those skilled in the art. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0071] The phrase "an embodiment" or "an embodiment" used in this specification means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Therefore, the phrase "an embodiment" or "an embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0072] While the embodiments disclosed in this invention are as described above, the content is merely for the purpose of facilitating understanding of the invention and is not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and changes in form and detail of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection of this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A method for calculating the gas content of shale reservoirs, characterized in that, include: Determine the sampling plan for the target well section to be drilled; Collect the returned rock fragments according to the sampling plan, and encapsulate the returned rock fragments to form a rock fragment venting process in a sealed space; For each filled sample, the carbon isotope content and component content of the cuttings top gas were measured at different times; as well as Based on the carbon isotope and component content of the cuttings top gas under different time series, the gas content of the shale reservoir in the current well section is calculated, including: Based on the carbon isotope and component content of the cuttings top gas at different time series for each filling sample, the total gas volume released by each cuttings sample was calculated; and Based on the changes in the carbon isotope values ​​of the top gas and the total gas released at different time series for each filled sample, the gas content of the shale reservoir in the current well section is inverted, including: Based on the mathematical model of shale natural gas desorption, the dynamic changes of gas pressure distribution, free gas content, adsorbed gas content, and gas outflow flux from the rock core are simulated and calculated throughout the entire process from the original underground state to extraction to the surface and completion of the desorption and release experiment. By continuously adjusting the gas permeability, the simulated values ​​of the top gas content at different well depths and time series are obtained, and the simulated values ​​of methane isotopes at different well depths and time series are obtained by continuously adjusting the carbon isotope flow coefficient. An objective function is constructed to describe the deviation between simulated and experimental values ​​of top gas content parameters under different time series. The minimum value is then solved using the gradient descent method. The simulated top gas content values ​​obtained from the simulation experiments under different time series are continuously fitted to the corresponding experimental values. Similarly, an objective function is constructed to describe the deviation between simulated and experimental values ​​of methane isotope parameters under different time series. The minimum value is then solved using the gradient descent method. The simulated carbon isotope values ​​of cuttings top gas under different time series are continuously fitted to the corresponding experimental values. After fitting both parameters, the obtained fitting parameters are substituted into the shale natural gas desorption mathematical model to obtain the shale reservoir gas content of the current well section.

2. The method according to claim 1, characterized in that, The carbon isotope conservation condition is used to simulate the overburden gas content during the entire dynamic process. This condition is expressed by the following formula: in, Indicates shale porosity. t Indicates time, r Indicates distance, m Represents the shape factor. c Indicates the adsorption mass ratio. p and p * express 12 CH4 and 13 partial pressure of CH4 gas θ and θ * express 12 CH4 and 13 CH4 monolayer adsorption coverage D and D * express 12 CH4 and 13 CH4 gas permeability p 0 indicates standard pressure. k a and k a * They represent 12 CH4 and 13 The adsorption rate constant of CH4 molecules k d and k d * They represent 12 CH4 and 13 The desorption rate constant of CH4 molecules.

3. The method according to claim 1 or 2, characterized in that, The objective function is expressed by the following expression: in, m This represents the vector of fitting parameters to be solved. m pr The prior distribution threshold of the fitting parameters to be solved. C M This represents the autocorrelation function of the parameters to be fitted. F ( m ) indicates the use of parameter vectors m The solution yields the simulated value of the top gas content or the simulated value of the top gas content. d This represents experimental data on the content of top gas or carbon isotope data of top gas from rock cuttings measured at different time series during the gas release experiment. C D Let represent the autocorrelation matrix of the experimental data, and j represent the number of fits.

4. The method according to claim 1 or 2, characterized in that, The process of encapsulating the returned rock cuttings includes: The returned rock cuttings were filled with algaecide and then sealed and the sample information was recorded. The sample information included at least the well number, the sampling well depth, and the sampling time.

5. The method according to claim 1 or 2, characterized in that, The steps for determining the sampling plan for the target well section to be drilled include: Collect stratified data and well trajectory of the target well, and determine the sampling interval of the target well section to be drilled based on the well type, depth range, geological characteristics and reservoir type of different strata.

6. The method according to claim 5, characterized in that, During drilling, the sampling interval for the remaining undrilled sections is adjusted based on the gas content of the shale reservoir at the drilled depth within the target well section.

7. A system for calculating the gas content of shale reservoirs, characterized in that, The system performs the method as described in any one of claims 1 to 6, wherein the system comprises: The scheme design module is used to determine the sampling scheme for the target well section to be drilled; A rock cuttings collection module is used to collect returned rock cuttings according to the sampling plan and encapsulate the returned rock cuttings to form a rock cuttings degassing process in a sealed space; The experimental measurement module is used to measure the carbon isotope and component content of the cuttings top gas at different times for each filled sample. The gas content fitting module is used to calculate the gas content of the shale reservoir in the current well section based on the carbon isotope and component content of the cuttings top gas under different time series.

8. The system according to claim 7, characterized in that, The rock cuttings collection module is used to fill the returned rock cuttings with an algaecide, then seal and record the sample information, which includes at least the well number, sampling well depth, and sampling time.

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