Method for measuring lost gas content of coal bed gas

Abstract

The invention relates to a method for measuring the lost gas content of coal bed gas. The method comprises the following steps: S1, preliminarily calculating the lost gas volume (QL0) of a sample by adopting a USBM linear regression method; s2, calculating a preliminary calculation critical desorption pressure (Pcd0) through a Langmuir formula; s3, correcting loss time; S4, correcting lost gas quantity; and S5, the step S2 to the step S4 are repeated until the difference value between the lost gas flow (QLN) after the last correction and the lost gas flow (QLN-1) after the last correction is within a preset range, N is larger than i, and i is larger than or equal to 1). The lost gas volume is corrected by introducing the critical desorption pressure, the lost gas volume of the coal bed gas can be measured more accurately, and errors caused by loss time overestimation are reduced. The method is easy and convenient to operate and suitable for various drilling circulating medium conditions, the accuracy of coal bed gas content measurement can be remarkably improved, and more reliable data support is provided for coal bed gas resource evaluation and mining scheme formulation.

Description

Technical Field

[0001] The present invention relates to the technical field of coalbed methane resource exploration, in particular to a method for measuring the lost gas content of coalbed methane. Background Art

[0002] Accurately measuring coalbed methane (CBM) content is crucial for CBM extraction and resource assessment. CBM content consists of three components: loss gas content, desorbed gas content, and residual gas content. The national standard GB / T 19559-2021, Coalbed Methane Content Determination Method, specifies methods for calculating these three components. However, the estimation error for loss gas content is significant, impacting the accuracy of CBM data. Summary of the Invention

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

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

[0005] S1. Use USBM linear regression method to preliminarily calculate the gas loss of the sample (Q L0 ):

[0006] The USBM linear regression method includes: taking the cumulative desorption amount under standard conditions as the vertical coordinate, the gas loss time (T L ) and the square root of the sum of the desorption time (t) are plotted as the horizontal axis. Connect the initial desorption points into a straight line, extend the straight line to intersect the vertical axis, and the intercept of the straight line at the vertical axis is the gas loss (Q L );

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

[0008] When the drilling circulation medium is clean water and mud, the gas loss time (T L0 ) is calculated as:

[0009]

[0010] When the drilling circulation medium is foam or air, the gas loss time (T L0 ) is calculated as:

[0011] T L0 =t4-t2

[0012] Where:

[0013] t2—core extraction time

[0014] t3—time when coal core reaches wellhead

[0015] t4—canning end time;

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

[0017] The critical desorption pressure (P cd ):

[0018]

[0019] Where:

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

[0021] V 实 —Measured gas content, unit: m 3 / t,V 实 =V L +V D +V R , where V L is the loss gas content, the loss gas content is the loss gas volume (Q L ) to the weight of the sample, V D is the measured natural desorption gas content, V R is 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 ) to determine the time when the coal core sample begins to desorb (t 5i-1 ), according to the time when the coal core sample starts to desorb (t 5i-1 ), and 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 of gas loss

[0027] The USBM linear regression method in step S1 is used to calculate the corrected loss time (t 5i ) to obtain the corrected gas loss (Q Li ) and the corrected measured gas content (V实i );

[0028] S5, repeat steps S2-S4 until the last calibrated gas loss (Q LN ) and the gas loss after the last calibration (Q LN-1 ) is within a preset range, where N>i.

[0029] Optionally, the time when the coal core sample starts to desorb (t5):

[0030] Where:

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

[0032] Optionally, after step S5, the method further includes step S6:

[0033] Calculate the last corrected lost gas content V CLN , Among them, Q CLN is the gas loss after the last correction, and M0 is the weight of the sample.

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

[0035] Optional, the amount of gas lost after the last calibration (Q LN ) and the gas loss after the last calibration (Q LN-1 ) is less than 5%;

[0036] As can be seen from the above, the technical solution provided by the present invention, by introducing the critical desorption pressure to correct for gas loss, can more accurately measure coalbed methane loss and reduce errors caused by overestimation of loss time. This method is simple to operate and applicable to various drilling circulation medium conditions. It can significantly improve the accuracy of coalbed methane gas content measurement, providing more reliable data support for coalbed methane resource assessment and development of production plans. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of a correction of gas loss using the USBM linear regression method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific embodiments. It will be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention. It should also be noted that, for ease of description, the accompanying drawings only show portions relevant to the present invention, not all of them.

[0039] Some directional words are defined in the present invention. Unless otherwise specified, the directional words used, such as "up", "down", "left", "right", "inside" and "outside", are used for ease of understanding and therefore do not constitute a limitation on the scope of protection of the present invention.

[0040] In the present invention, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Furthermore, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0041] In the description of the present invention, unless otherwise expressly specified or limited, the terms "connected," "connected," and "fixed" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention in specific circumstances.

[0042] This embodiment provides a method for measuring the coalbed methane loss content, which is used to calculate the coalbed methane content to improve the accuracy of coalbed methane content measurement.

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

[0044] S1. Use USBM linear regression method to preliminarily calculate the gas loss of the sample (Q L0 ):

[0045] The USBM linear regression method includes: taking the cumulative desorption amount under standard conditions as the vertical coordinate, the gas loss time (T L ) and the square root of the sum of the desorption time (t) are plotted as the horizontal axis. Connect the initial desorption points into a straight line, extend the straight line to intersect the vertical axis, and the intercept of the straight line at the vertical axis is the gas loss (Q L ); It can be understood that, by taking different desorption times and measuring the cumulative desorption amount corresponding to the corresponding time, the coordinate positions of the initial desorption points are determined in the coordinate system.

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

[0047] When the drilling circulation medium is clean water and mud, the gas loss time (T L0 ) is calculated as:

[0048]

[0049] When the drilling circulation medium is foam or air, the gas loss time (T L0 ) is calculated as:

[0050] T L0 =t4-t2

[0051] Where:

[0052] t2—core extraction time

[0053] t3—time when coal core reaches wellhead

[0054] t4—canning completion time; it can be understood that the time here refers to a time point. In this embodiment, the heart-lifting time is used as the starting time, that is, t2=0.

[0055] In step S1, the existing gas loss time calculation method obtains the gas loss time (T L0 ), and then the USBM linear regression method was used to preliminarily obtain the gas loss volume (Q L0 ).

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

[0057] The critical desorption pressure (P cd ):

[0058]

[0059] Where:

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

[0061] V 实 —Measured gas content, unit: m 3 / t,V 实 =V L +V D +V R , where V L is the loss gas content, the loss gas content is the loss gas volume (Q L ) to the weight of the sample, V D is the measured natural desorption gas content, which can be obtained by existing technology detection, V Ris the residual gas content, which can be detected by existing technology;

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

[0063] V L — Langmuir volume, unit: m 3 / t;P L and V L is a constant that can be obtained by calculation. The calculation method is known in the art and will not be described in detail here. Wherein, t is the weight unit, ton.

[0064] In step S2, according to the loss gas volume (Q L0 ), and the natural desorption gas content and residual gas content measured by the existing measurement method, the critical desorption pressure (P cd0 ).

[0065] S3. Correction loss time:

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

[0067] Where:

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

[0069] Specifically, in step S3, the sample is lifted up in the well at a uniform speed, so Where H0 is the well depth, H cd is the actual desorption point depth, i.e. cd At this depth, the sample is at the critical desorption pressure.

[0070] According to the time when the coal core sample starts to desorb (t 5i-1 ), and 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 critical desorption pressure (P cd0 ), and obtain the initial desorption start time (t 50 ), and then the loss time after the first correction (T L1), prepare for the next step of correcting the lost gas volume.

[0072] S4. Correction of gas loss

[0073] The USBM linear regression method in step S1 is used to calculate the corrected loss time (t 5i ) to obtain the corrected gas loss (Q Li ) and the corrected measured gas content (V 实i );

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

[0075] S5, repeat steps S2-S4 until the last calibrated gas loss (Q LN ) and the gas loss after the last calibration (Q LN-1 ) is within the preset range, where N>i. For example, the gas loss after the last calibration (Q LN ) and the gas loss after the last calibration (Q LN-1 ) is less than 5%;

[0076] For example, N=2, at this time, according to the first corrected gas loss volume (Q L1 ) and the corrected V 实1 , using the USBM linear regression method in step S1, and then obtaining the second corrected gas loss volume (Q L2 ).

[0077] After step S5, the following step may be further included:

[0078] Calculate the last corrected lost gas content V CLN , Among them, Q CLN is the gas loss after the last correction, and M0 is the weight of the sample.

[0079] In step S1, the cumulative desorption amount under standard conditions refers to the volume of the naturally desorbed gas converted to the volume at a temperature of 0°C and a pressure of 101.325 kPa. The specific method for obtaining the cumulative desorption amount under standard conditions is prior art and will not be repeated here.

[0080] The coalbed methane content consists of three parts: loss gas content, desorbed gas content and residual gas content. The desorbed gas content and residual gas content data can be obtained through actual testing. The national standard GB / T19559-2021 Coalbed Methane Content Determination Method stipulates the test methods for desorbed gas content and residual gas content, and the loss gas content is obtained by estimation. Therefore, how to obtain accurate data on the loss gas content is the key to obtaining accurate gas content data.

[0081] The lost gas volume is also called the escaped gas volume, which refers to the gas volume released after the drill bit encounters the coal seam and the coal sample is taken out from the bottom of the borehole to the ground and placed into the desorption tank.

[0082] When the sample is lifted from the bottom of the well to the wellhead, the coalbed methane will not be desorbed before the sample pressure is higher than the critical desorption pressure. In the existing technology, in the USBM linear regression method calculation, half of the core lifting time plus the canning time is used as the loss time (drilling circulating medium is clean water and mud) or the time from the core lifting time to the canning time is used as the loss time (drilling circulating 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. At the same time, when the coal core reaches half the depth of the coal seam in half the drilling time, the pressure on the coal core is greater or less than the critical desorption pressure. At this time, no gas will be desorbed or the gas has begun to desorb. Therefore, the loss time of the existing calculation method is inaccurate, and the calculated loss gas volume is also inaccurate. After many practical experiences, it has been verified that the time from the beginning of gas desorption (that is, when the sample is at the critical desorption pressure in the borehole) to the completion of sample canning is used as the loss time, and the loss gas volume calculated using the USBM linear regression method is more accurate.

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

[0084] The method for determining the coalbed methane loss content in this embodiment is to calculate the initial gas loss time according to the existing calculation method, and to obtain the initial gas loss volume (Q L0 ). Using the initial loss of gas volume (Q L0 ) to obtain the initial critical desorption pressure (P cd0 ), and then use the initial critical desorption pressure (P cd0 ) to obtain the first corrected lost gas time (T L1 ). Then the USBM linear regression method is used again to obtain the first corrected gas loss volume (Q L0 ), using the first corrected loss volume (QL0 ) to obtain the first corrected critical desorption pressure (P cd1 ), and so on, we can finally get the accurate loss volume (Q LN ). The calculation method is simple and has high accuracy.

[0085] In an optional embodiment, in step 5, steps S2 to S4 are repeated once, that is, correction is performed on both sides to meet the requirements.

[0086] It can be understood that, in this embodiment, Q Li Indicates the amount of gas lost after correction i times, Q Li-1 Indicates the amount of gas lost after correction i-1 times. Similarly, Q L0 It represents the initially obtained gas loss volume, that is, the gas loss volume obtained without correction, that is, the gas loss volume obtained in step S1.

[0087] P cdi represents the critical desorption pressure after correction i times, P cdi-1 represents the critical desorption pressure after correction i-1 times, P cd0 The critical desorption pressure obtained initially, that is, the critical desorption pressure obtained without correction, is the critical desorption pressure obtained based on the data in step S1 and by the method in step S2.

[0088] t 5i Indicates the time of desorption start after correction i, t 5i-1 It represents the time of desorption start after correction i-1 times. When i=1, the critical desorption pressure P obtained for the first time is cd0 Get the uncorrected t 50 .

[0089] V 实i Indicates the measured gas content after correction i times.

[0090] In a specific embodiment, Figure 1 As shown in Tables 1 to 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 are not much different from the results after the first correction. However, the difference is large compared with the uncorrected measured gas content. 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] This embodiment redefines lost gas as the gas released from the start of coal core sample desorption to the completion of canning. When testing 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. The lost gas volume is then corrected twice using data such as the critical desorption pressure and coal seam depth, ultimately yielding a more accurate total gas content.

[0100] Although the present invention has been described in detail above using general explanations, specific embodiments, and experiments, it will be apparent to those skilled in the art that modifications and improvements may be made based on the present invention. Therefore, such modifications and improvements, which do not depart from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A method for measuring coalbed methane loss content, characterized in that: The steps include: S1. Use USBM linear regression method to preliminarily calculate the gas loss of the sample (Q L0 ): The USBM linear regression method includes: taking the cumulative desorption amount under standard conditions as the vertical coordinate, the gas loss time (T L ) and the square root of the sum of the desorption time (t) are plotted as the horizontal axis. Connect the initial desorption points into a straight line, extend the straight line to intersect the vertical axis, and the intercept of the straight line at the vertical axis is the gas loss (Q L ); Preliminary calculation of the sample's gas loss (Q L0 )hour, When the drilling circulation medium is clean water and mud, the gas loss time (T L0 ) is calculated as: When the drilling circulation medium is foam or air, the gas loss time (T L0 ) is calculated as: T L0 =t4-t2 Where: t2—core extraction time t3—time when coal core reaches wellhead t4—canning end time; S2. Preliminary calculation of critical desorption pressure (P cd0 ): The critical desorption pressure (P cd ): Where: P cd —critical desorption pressure, unit: MPa; V 实 —Measured gas content, unit: m 3 / t,V 实 =V L +V D +V R , where V L is the loss gas content, the loss gas content is the loss gas volume (Q L ) to the weight of the sample, V D is the measured natural desorption gas content, V R is the residual gas content; P L —Langmuir pressure, unit: MPa; V L — Langmuir volume, unit: m 3 / t; S3. Correction loss time: According to the critical desorption pressure (P cdi-1 ) to determine the time when the coal core sample begins to desorb (t 5i-1 ), according to the time when the coal core sample starts to desorb (t 5i-1 ), and 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; S4. Correction of gas loss The USBM linear regression method in step S1 is used to calculate the corrected loss time (t 5i ) to obtain the corrected gas loss (Q Li ) and the corrected measured gas content (V 实i ); S5, repeat steps S2-S4 until the last calibrated gas loss (Q LN ) and the gas loss after the last calibration (Q LN-1 ) is within a preset range, where N>i.

2. The method for measuring the coalbed methane loss content according to claim 1, wherein: in, The time when the coal core sample starts to desorb (t5): Where: P0—Pressure at the coal core encountered during drilling.

3. The method for measuring the coalbed methane loss content according to claim 1, characterized in that: After step S5, the method further includes step S6: Calculate the last corrected lost gas content V CLN , Among them, Q CLN is the gas loss after the last correction, and M0 is the weight of the sample.

4. The method for measuring the coalbed methane loss content according to claim 3, characterized in that: In step 5, steps S2 to S4 are repeated once.

5. The method for measuring the coalbed methane loss content according to claim 1, characterized in that: The gas loss after the last calibration (Q LN ) and the gas loss after the last calibration (Q LN-1 ) is less than 5%;

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