A method for determining the content of coalbed methane loss gas

Abstract

This invention relates to a method for determining the gas loss content in coalbed methane, comprising the following steps: S1, using the USBM linear regression method to preliminarily calculate the gas loss (Q) of the sample. L0 S2. Preliminary calculation of the critical desorption pressure (P) using the Langmuir formula. cd0 S3, Correct for lost time; S4, Correct for lost gas volume; S5, Repeat steps S2-S4 until the last corrected lost gas volume (Q) is reached. LN ) and its previous corrected loss volume (Q) LN‑1 The difference is within a preset range, where N > i, i ≥ 1. By introducing a critical desorption pressure to correct for gas loss, the amount of gas lost in coalbed methane can be measured more accurately, reducing errors caused by overestimation of loss time. This method is simple to operate, applicable to various drilling circulation media conditions, and can significantly improve the accuracy of coalbed methane content measurement, providing more reliable data support for coalbed methane resource assessment and exploitation plan development.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane resource exploration technology, and in particular to a method for determining the content of lost gas in coalbed methane. Background Technology

[0002] Accurate determination of coalbed methane content is crucial for coalbed methane extraction and resource assessment. Coalbed methane content consists of three parts: lost gas content, desorbed gas content, and residual gas content. The national standard GB / T 19559-2021, "Methods for Determining Coalbed Methane Content," specifies the calculation methods for lost gas content, desorbed gas content, and residual gas content. However, the estimation error for lost gas content is relatively large, affecting the accuracy of coalbed methane content data. Summary of the Invention

[0003] The purpose of this invention is to provide a method for determining the content of coalbed methane loss gas, so as to at least solve one of the above-mentioned technical problems.

[0004] To achieve the above objectives, the present invention provides a method for determining the content of coalbed methane loss gas, comprising the following steps:

[0005] S1. The initial calculation of the sample's gas loss (Q) was performed using the USBM linear regression method. L0 ):

[0006] The USBM linear regression method includes: using the cumulative desorption amount under standard conditions as the ordinate, and the time of gas loss (T) as the coordinate. L The x-axis is plotted as the sum of the desorption time (t) and the square root of the desorption time (t). A straight line is drawn connecting the initial desorption points. This line is extended to intersect the y-axis, and the intercept on the y-axis is the gas loss (Q). L );

[0007] Preliminary calculation of the sample's gas loss (Q) L0 )hour,

[0008] Under drilling conditions where the circulating medium is water and mud, the gas loss time (T) L0 The calculation formula is:

[0009]

[0010] When the drilling circulation medium is foam or air, the time of gas loss (T) L0 The calculation formula is:

[0011] T L0 =t4-t2

[0012] In the formula:

[0013] t2—Core extraction time

[0014] t3—Time it takes for the coal core to reach the wellhead

[0015] t4—Completion time of filling;

[0016] S2. Preliminary calculation of critical desorption pressure (P) cd0 ):

[0017] The critical desorption pressure (P) was calculated using the Langmuir formula. cd ):

[0018]

[0019] In the formula:

[0020] P cd —Critical desorption pressure, unit: MPa;

[0021] V 实 —Measured gas content, unit: m³ 3 / t, V 实 =V L +V D +V R , where V L The loss gas content is the amount of gas lost (Q). L The ratio of the weight of the sample to the weight of the sample, V D V represents the measured content of naturally desorbed gas. R This refers to the residual gas content;

[0022] P L — Langmuir pressure, unit: MPa;

[0023] V L — Langmuir volume, unit: m 3 / t;

[0024] S3, Correction loss time:

[0025] According to the critical desorption pressure (P) cdi-1 Determine the time at which the coal core sample begins desorption (t). 5i-1 Based on the time when the coal core sample begins desorption (t) 5i-1 ), to obtain the corrected loss time (T) Li ):T Li =t4-t 5i-1 , where i≥1 and is the number of corrections for lost gas volume;

[0026] S4, Correction for lost gas volume

[0027] Using the USBM linear regression method in step S1, the corrected loss time (t) is utilized. 5i The corrected loss volume (Q) is obtained. Li ) and the corrected measured gas content (V实i );

[0028] S5. Repeat steps S2-S4 until the final corrected loss volume (Q) is reached. LN ) and its previous corrected loss volume (Q) LN-1 The difference between N and i is within a preset range, where N > i.

[0029] Optionally, the time (t5) at which the coal core sample begins desorption:

[0030] In the formula:

[0031] P0—Pressure at the point where the coal core is encountered during drilling.

[0032] Optionally, after step S5, step S6 is also included:

[0033] Calculate the loss gas content V for the last correction. CLN , Among them, Q CLN M0 represents the loss volume after the last correction, and M0 represents the weight of the sample.

[0034] Optionally, in step 5, steps S2-S4 are repeated once.

[0035] Optional, the loss volume (Q) after the last correction. LN ) and its previous corrected loss volume (Q) LN-1 The difference (ΔQ) is less than 5%.

[0036] As can be seen from the above, the technical solution provided by this invention, by introducing a critical desorption pressure to correct for gas loss, can more accurately measure the gas loss of coalbed methane and reduce errors caused by overestimation of loss time. This method is simple to operate, applicable to various drilling circulation media conditions, and can significantly improve the accuracy of coalbed methane content measurement, providing more reliable data support for coalbed methane resource assessment and exploitation plan development. Attached Figure Description

[0037] Figure 1 This is a schematic diagram illustrating the use of the USBM linear regression method to correct for lost gas volume in an embodiment of the present invention. Detailed Implementation

[0038] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the present invention are shown in the accompanying drawings, not all of them.

[0039] This invention defines certain directional terms. Unless otherwise stated, the directional terms used, such as "up," "down," "left," "right," "inner," and "outer," are used for ease of understanding and therefore do not constitute a limitation on the scope of protection of this invention.

[0040] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0041] In the description of this invention, unless otherwise explicitly specified and limited, the terms "connected," "linked," and "fixed" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; 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; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0042] This embodiment provides a method for determining the coalbed methane content, which is used to calculate the coalbed methane content and improve the accuracy of coalbed methane content determination.

[0043] like Figure 1 As shown, the method for determining the coalbed methane loss gas content provided in this embodiment includes the following steps:

[0044] S1. The initial calculation of the sample's gas loss (Q) was performed using the USBM linear regression method. L0 ):

[0045] The USBM linear regression method includes: using the cumulative desorption amount under standard conditions as the ordinate, and the time of gas loss (T) as the coordinate. L The x-axis is plotted as the sum of the desorption time (t) and the square root of the desorption time (t). A straight line is drawn connecting the initial desorption points. This line is extended to intersect the y-axis, and the intercept on the y-axis is the gas loss (Q). L It is understandable that by taking different resolution times and measuring the cumulative resolution corresponding to the time, the coordinate positions of the initial desorption points can be determined in the coordinate system.

[0046] Preliminary calculation of the sample's gas loss (Q) L0)hour,

[0047] Under drilling conditions where the circulating medium is water and mud, the gas loss time (T) L0 The calculation formula is:

[0048]

[0049] When the drilling circulation medium is foam or air, the time of gas loss (T) L0 The calculation formula is:

[0050] T L0 =t4-t2

[0051] In the formula:

[0052] t2—Core extraction time

[0053] t3—Time it takes for the coal core to reach the wellhead

[0054] t4—Canning end time; it can be understood that the time here refers to a point in time. In this embodiment, the pickling time is used as the starting time, that is, t2=0.

[0055] In step S1, the existing method for calculating gas loss time yields the gas loss time (T). L0 Then, the loss volume (Q) was initially obtained using the USBM linear regression method. L0 ).

[0056] S2. Preliminary calculation of critical desorption pressure (P) cd0 )

[0057] The critical desorption pressure (P) was calculated using the Langmuir formula. cd ):

[0058]

[0059] In the formula:

[0060] P cd —Critical desorption pressure, unit: MPa;

[0061] V 实 —Measured gas content, unit: m³ 3 / t, V 实 =V L +V D +V R , where V L The loss gas content is the amount of gas lost (Q). L The ratio of the weight of the sample to the weight of the sample, V D The measured content of naturally desorbed gas can be obtained using existing technologies. RThe residual gas content can be detected using existing technologies.

[0062] P L — Langmuir pressure, unit: MPa;

[0063] V L — Langmuir volume, unit: m 3 / t;P L and V L The constant is t, which can be calculated using existing technology and will not be elaborated here. In the formula, t is the weight unit, ton.

[0064] In step S2, based on the preliminary calculation of the gas loss (Q) in step S1... L0 The critical desorption pressure (P) was initially calculated using the Langmuir formula, based on the content of naturally desorbed gas and residual gas obtained by existing measurement methods. cd0 ).

[0065] S3, Correction loss time:

[0066] According to the critical desorption pressure (P) cdi-1 Determine the time at which the coal core sample begins desorption (t). 5i-1 It is understandable that the sample begins to release gas when it reaches the critical desorption pressure. Optionally, the time (t5) at which the coal core sample begins desorption is:

[0067] In the formula:

[0068] P0—The pressure at the point where the coal core is encountered during drilling, that is, the pressure at the bottom of the well.

[0069] Specifically, in step S3, the sample is pulled up into the well at a constant speed, therefore... Where H0 is the well depth, H cd It is the actual desorption point depth, that is, at H cd At this depth, the sample is under critical desorption pressure.

[0070] Based on the time when the coal core sample begins desorption (t) 5i-1 ), to obtain the corrected loss time (T) Li ):T Li =t4-t 5i-1 , where i≥1 and is the number of corrections for lost gas volume;

[0071] In step S3, when i = 1, according to the intermediate critical desorption pressure (P) in step S2 cd0 ), thus obtaining the initial desorption start time (t) 50 This allows us to obtain the loss time (T) after the first correction. L1This prepares for the next step of correcting the lost gas volume.

[0072] S4, Correction for lost gas volume

[0073] Using the USBM linear regression method in step S1, the corrected loss time (t) is utilized. 5i The corrected loss volume (Q) is obtained. Li ) and the corrected measured gas content (V 实i );

[0074] In step S4, when i = 1, the desorption start time (t) is initially obtained from step S3. 50 The USBM linear regression method in step S1 is used to obtain the first corrected loss volume (Q). L1 ) and corrected V 实1 This is to prepare for the second correction of lost gas volume.

[0075] S5. Repeat steps S2-S4 until the final corrected loss volume (Q) is reached. LN ) and its previous corrected loss volume (Q) LN-1 The difference between N and i is within a preset range, where N > i. For example, the loss volume (Q) after the last correction... LN ) and its previous corrected loss volume (Q) LN-1 The difference (ΔQ) is less than 5%.

[0076] For example, N=2, then, based on the first corrected loss gas volume (Q) obtained in step S4... L1 ) and corrected V 实1 The USBM linear regression method in step S1 is used to obtain the second corrected loss volume (Q). L2 ).

[0077] After step S5, step S6 may also be included:

[0078] Calculate the loss gas content V for the last correction. CLN , Among them, Q CLN M0 represents the loss volume after the last correction, and M0 represents the weight of the sample.

[0079] In step S1, the cumulative desorption amount under standard conditions refers to the volume of naturally desorbed gas converted to the volume at a temperature of 0℃ and a pressure of 101.325kPa. The specific method for obtaining the cumulative desorption amount under standard conditions is existing technology and will not be described in detail here.

[0080] Coalbed methane content consists of three parts: desorbed gas content, desorbed gas content, and residual gas content. The data on desorbed gas volume and residual gas volume can be obtained through actual testing. The national standard GB / T19559-2021 specifies the test methods for desorbed gas content and residual gas content. However, the desorbed gas volume is estimated. Therefore, obtaining accurate data on the desorbed gas volume is fundamental to obtaining accurate gas content data.

[0081] Lost gas volume, also known as escaping gas volume, refers to the amount of gas released after the drill bit encounters the coal seam and before the coal sample is taken out from the bottom of the borehole and placed into the desorption tank on the surface.

[0082] When a sample is pulled from the bottom of the well to the wellhead, coalbed methane will not desorb before the pressure exceeds the critical desorption pressure. However, in existing technologies, the USBM linear regression method calculates the loss time using half the core-lifting time plus the canning time (drilling circulation medium is water and mud) or the time from core-lifting to canning as the loss time (drilling circulation medium is foam or air). In actual production, the pressure on the coal core at the bottom of the well is often greater than the critical desorption pressure. Simultaneously, the pressure on the coal core at half the lifting time (i.e., when the coal core reaches half the coal seam depth) is either greater than or less than the critical desorption pressure. At this point, no gas will desorb, or gas will have already begun to desorb. Therefore, the existing calculation methods for loss time and the calculated gas loss are inaccurate. After repeated practical experience verification, using the time from the start of gas desorption (i.e., when the sample is at the critical desorption pressure in the borehole) to the completion of sample canning as the loss time, and then using the USBM linear regression method to calculate the gas loss, is more accurate.

[0083] This embodiment corrects for gas loss by introducing a critical desorption pressure, enabling more accurate measurement of coalbed methane gas loss and reducing errors caused by overestimation of loss time. This method is simple to operate, applicable to various drilling circulation media conditions, and can significantly improve the accuracy of coalbed methane content measurement, providing more reliable data support for coalbed methane resource assessment and exploitation planning.

[0084] The method for determining the coalbed methane loss gas content in this embodiment calculates the initial loss gas time using existing calculation methods and obtains the initial loss gas volume (Q) using the USBM linear regression method. L0 Using the initial loss gas volume (Q) L0 The initial critical desorption pressure (P) was obtained. cd0 Then, using the initial critical desorption pressure (P) cd0 The first correction loss time (T) was obtained. L1 Then, the loss volume (Q) after the first correction was obtained again using the USBM linear regression method. L0 ), utilizing the loss volume (Q) from the first correctionL0 The critical desorption pressure (P) for the first correction was obtained. cd1 ), and so on, until the accurate gas loss (Q) is obtained. LN The calculation method is simple and highly accurate.

[0085] In an optional embodiment, in step 5, steps S2-S4 are repeated once, that is, both sides can be corrected to meet the requirements.

[0086] It is understandable that in this embodiment, Q Li Q represents the amount of gas lost during the i-th correction. Li-1 This represents the amount of gas lost during the i-1th correction. Similarly, Q... L0 This represents the initial gas loss, which is the gas loss before correction, and is also the gas loss obtained in step S1.

[0087] P cdi P represents the critical desorption pressure after correction i. cdi-1 P represents the critical desorption pressure for the i-1th correction. cd0 The initial critical desorption pressure is the critical desorption pressure obtained without correction, which is the critical desorption pressure obtained based on the data in step S1 and the method in step S2.

[0088] t 5i t represents the time at which desorption begins after the i-th correction. 5i-1 This indicates the time at which desorption begins after the (i-1)th correction. When i = 1, the critical desorption pressure P obtained initially is used. cd0 Obtain uncorrected t 50 .

[0089] V 实i This represents the measured gas content after correction i.

[0090] In a specific embodiment, such as Figure 1 As shown in Tables 1-4, by comparing the total gas content and critical desorption pressure after the first, second, and third corrections (Table 4), it can be seen that the results of the second and third corrections are the same and not much different from the results after the first correction. However, the difference between the results and the uncorrected measured gas content is large. Therefore, it is only necessary to correct the critical desorption pressure once and then recalculate the total gas content.

[0091] Table 1 Desorption data of coal seam sample 1

[0092]

[0093] Table 2 Data after the second correction

[0094]

[0095] Table 3 Data after the third correction

[0096]

[0097] Table 4 Comparison of data before and after correction

[0098]

[0099] In this embodiment, the lost gas is redefined as the gas released from the start of desorption of the coal core sample to the end of canning. When testing the gas content, the original natural desorption method is still used to measure the desorbed gas volume, lost gas volume, and residual gas volume. The critical desorption pressure is calculated based on the test results, and the lost gas volume is corrected twice using data such as the critical desorption pressure and coal seam depth to finally obtain a more accurate total gas content.

[0100] Although the present invention has been described in detail above with general descriptions, specific embodiments, and experiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for determining the content of coalbed methane loss gas, characterized in that, Includes the following steps: S1. The initial calculation of the sample's gas loss Q was performed using the USBM linear regression method. L0 : The USBM linear regression method includes: using the cumulative desorption amount under standard conditions as the ordinate, and the time T of gas loss. L The x-axis is plotted as the sum of the square roots of the desorption time t and the x-axis. A straight line is drawn connecting the initial desorption points. This line is extended to intersect the y-axis, and the intercept on the y-axis represents the gas loss Q. L ; Preliminary calculation of the sample's gas loss Q L0 hour, Under drilling conditions where the circulating medium is water and mud, the gas loss time T L0 The calculation formula is: When the drilling circulation medium is foam or air, the time T for gas loss is... L0 The calculation formula is: T L0 =t4-t2 In the formula: t2—Core extraction time t3—Time it takes for the coal core to reach the wellhead t4—Completion time of filling; S2. Preliminary calculation of critical desorption pressure P cd0 : The critical desorption pressure P was calculated using the Langmuir formula. cd : In the formula: P cd —Critical desorption pressure, unit: MPa; V 实 —Measured gas content, unit: m³ 3 / t, V 实 =V L +V D +V R , where V L The loss gas content is represented by the loss gas volume Q. L The ratio of the weight of the sample to the weight of V D V represents the measured content of naturally desorbed gas. R This refers to the residual gas content; P L — Langmuir pressure, unit: MPa; V L — Langmuir volume, unit: m 3 / t; S3, Correction loss time: Based on the critical desorption pressure P cdi-1 Determine the time t when the coal core sample begins to desorb. 5i-1 Based on the time t when the coal core sample begins desorption 5i-1 The corrected loss time T is obtained. Li :T Li =t4-t 5i-1 , where i≥1 and is the number of corrections for lost gas volume; S4, Correction for lost gas volume Using the USBM linear regression method in step S1, the corrected loss time t is utilized. 5i The corrected loss volume Q is obtained Li and the corrected measured gas content V 实i ; S5. Repeat steps S2-S4 until the final corrected loss volume Q is reached. LN Compared with the previous correction, the loss volume Q LN-1 The difference is within a preset range, where N > i.

2. The method for determining the coalbed methane loss gas content according to claim 1, characterized in that, in, The time t5 at which the coal core sample begins desorption: In the formula: P0—Pressure at the point where the coal core is encountered during drilling.

3. The method for determining the coalbed methane loss gas content according to claim 1, characterized in that, Following step S5, step S6 is also included: Calculate the loss gas content V for the last correction. CLN , Among them, Q CLN M0 represents the loss volume after the last correction, and M0 represents the weight of the sample.

4. The method for determining the coalbed methane loss gas content according to claim 3, characterized in that, In step 5, repeat step S2-S4 once.

5. The method for determining the coalbed methane loss gas content according to claim 1, characterized in that, Loss Q after the last correction LN Compared with the previous correction, the loss volume Q LN-1 The difference ΔQ is less than 5%.

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