A method for rapid evaluation of coalbed methane mineability

Abstract

The present invention discloses a rapid on-site evaluation method for the recoverability of coalbed methane. The steps are as follows: Step S1, obtain cuttings during the logging of the target coal seam; Step S2, screen several cuttings from the obtained cuttings with three-dimensional dimensions all greater than 1 cm and less than 3 cm; Step S3, measure the volume of each cutting; Step S4, seal each cutting with a separate sealed bag and completely remove the air in the sealed bag; Step S5, let the sealed bag containing the cuttings stand for 8 h, and then measure the volume of the gas desorbed by the cuttings in the sealed bag; Step S6, calculate the ratio λ of the desorbed gas volume of each cutting to the volume of the cutting itself. i , and then calculate all λ i values of the average value #imgabs0# S7. Evaluate the recoverability of the gas well according to the average value #imgabs1#. Based on the treatment method of logging cuttings during the drilling process, the present invention takes the desorption rate as the core parameter for evaluation and rapidly evaluates the recoverability of coalbed methane wells, overcoming the problems of poor timeliness and poor effectiveness in the existing evaluation technologies.

Description

Technical Field

[0001] The invention relates to the technical field of oil and natural gas development, in particular to a mine rapid evaluation method for the recoverability of coal bed gas. Background Art

[0002] The evaluation of coalbed methane recoverability is a core issue in the field of coalbed methane mining. For the evaluation of coalbed methane recoverability, a large number of academic studies currently focus on the gas content, coal rank, coal pore structure, coal permeability, water content, etc. of coal seams, which require a large amount of experimental test data.

[0003] At present, the evaluation of coalbed methane recoverability faces two problems. First, the evaluation requires a large amount of geological data and core test data. It is impossible to quickly evaluate the gas well production capacity in the early stage of block evaluation, and the timeliness of the evaluation research is not good. Second, the current focus of recoverability evaluation is on the evaluation of geological sweet spots for reserve abundance and engineering sweet spots for compressibility. However, since theoretical research has not clarified the core issues of coalbed methane mass transfer, the evaluation effect is not ideal, and reliable coalbed methane blocks cannot be screened.

[0004] At the same time, the existing coalbed methane recoverability evaluation technology focuses on the extent of coal rock gas content (total gas content), but does not consider the speed of coalbed methane desorption from coal rock (the intensity of desorption). The desorption rate is the core issue that restricts coalbed methane production. Therefore, the existing technology cannot effectively evaluate the recoverability of coalbed methane, and a rapid mine recoverability evaluation method based on coalbed methane desorption rate is urgently needed. Summary of the invention

[0005] In order to solve the problem of poor timeliness and effectiveness of the current coalbed methane recoverability evaluation methods, the present invention provides a mine rapid evaluation method for coalbed methane recoverability based on desorption rate.

[0006] The method for rapid evaluation of coalbed methane mineability provided by the present invention comprises the following steps:

[0007] S1. Obtain rock cuttings during logging of target coal seam:

[0008] During the gas well drilling process, it is usually necessary to conduct rock cuttings logging near the target coal seam. According to the logging return time, several rock cuttings samples from the middle of the coal seam just out of the well are selected as candidates. The subsequent steps are carried out quickly in the shortest possible time.

[0009] S2. Select several rock cuttings with three-dimensional dimensions greater than 1 cm and less than 3 cm from the obtained rock cuttings, assuming that the number of rock cuttings is n, preferably, n≥10. Number each rock cutting 1, 1, 2, ... n in sequence to facilitate the subsequent steps S3-S6.

[0010] S3. Test the volume of each rock cutting.

[0011] The volume of each cutting is quickly measured using the water displacement method and recorded as V ci , i = 1, 2, …n, representing the rock cuttings numbered 1, 2, …n respectively.

[0012] S4. Seal each rock cutting in a sealed bag with a maximum volume of 50 ml, quickly remove the air in the sealed bag, and then seal the sealed bag with a heat press.

[0013] S5. Let the sealed bag containing the rock cuttings stand for 8 hours, and then test the volume of gas desorbed by the rock cuttings in the sealed bag.

[0014] After being sealed for 8 hours, some desorbed gas will appear in each coal sample sealed bag. The volume of desorbed gas in each sealed bag is tested by drainage method and recorded as V. gi , i = 1, 2, …n, representing the rock cuttings numbered 1, 2, …n respectively.

[0015] S6. Calculate the ratio of the desorbed gas volume of each cutting to the cutting volume itself, λ i , then calculate all λ i The average value λ represents the volume of gas desorbed from a unit volume of cuttings in the first 8 hours, which corresponds to the desorption rate.

[0016] S7, based on the average The evaluation criteria for assessing the recoverability of gas wells are as follows:

[0017] like The coalbed methane well has a high degree of recoverability, and the daily output of a vertical well corresponding to a 5m thick coal seam exceeds 1,500 cubic meters per day.

[0018] like The coalbed methane well has a medium degree of recoverability, and the daily production of a vertical well corresponding to a 5m thick coal seam is between 1,000 cubic meters per day and 1,500 cubic meters per day.

[0019] like The recoverability of coalbed methane wells is relatively low, and the daily output of vertical wells corresponding to 5m thick coal seams is less than 1,000 cubic meters per day.

[0020] In this step, the cutoff value 10 in the evaluation standard is calculated based on the gas leakage radius of 100m in the first year of the coalbed methane well, the coal seam thickness of 5m, and the corresponding gas production of 1500 cubic meters / day for three years of stable production. The cutoff value 7 is calculated based on the gas leakage radius of 100m in the first year of the coalbed methane well, the coal seam thickness of 5m, and the corresponding gas production of 1000 cubic meters / day for three years of stable production. The calculation formula is as follows:

[0021] Demarcation value = 1 / [(3.1415×deflation radius2 × coal seam thickness) / (corresponding gas production × 365 × 3)].

[0022] The values ​​calculated according to the formula are 10.45 and 6.79 respectively, which are rounded off to the dividing values ​​of 10 and 7.

[0023] Compared with the prior art, the present invention is beneficial in that:

[0024] (1) The evaluation method of the present invention is based on the processing method of logging cuttings during the drilling process, takes the desorption rate as the core parameter of the evaluation, and quickly evaluates the recoverability of the coalbed methane well. Through this method, the recoverability evaluation of the coalbed methane well can be completed before the well is drilled and completed, with high processing efficiency and good timeliness. The problems of poor timeliness and effectiveness of the existing evaluation methods are solved.

[0025] (2) The essential core of the method of the present invention is the coalbed methane mass transfer chain theory, the evaluation core is the desorption-diffusion rate in the coal rock matrix, and the processing accuracy is high.

[0026] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 , a flow chart of the method for rapid evaluation of coalbed methane mine recoverability of the present invention.

[0028] Figure 2 , desorption test data chart of Yunlian area.

[0029] Figure 3 , distribution map of production wells in Yunlian area.

[0030] Figure 4 , the evaluation results of Yunlian area based on the existing gas content data method.

[0031] Figure 5 , the evaluation results of Junlian area according to the evaluation method of the present invention. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0033] like Figure 1-5 As shown, the method for rapid evaluation of coalbed methane mineability provided by the present invention comprises the following seven steps performed in sequence:

[0034] S1. Obtain rock cuttings during logging of target coal seam:

[0035] S2. Screen the size of logging cuttings and select cuttings that can meet the test size:

[0036] Several rock cuttings with three-dimensional dimensions greater than 1 cm and less than 3 cm are selected from the obtained rock cuttings, assuming that the rock cutting data is n, preferably, n≥10. Each rock cutting is numbered 1, 1, 2, ... n in sequence to facilitate the subsequent steps S3-S6.

[0037] S3. Test the volume of each rock cutting.

[0038] The volume of each cutting was quickly measured by the displacement method using a 20 ml graduated cylinder and recorded as V ci , i = 1, 2, …n, representing the rock cuttings numbered 1, 2, …n respectively.

[0039] S4. Seal each rock cutting in a sealed bag with a maximum volume of 50 ml, quickly remove the air in the sealed bag, and then seal the sealed bag with a heat press. The whole process is similar to the vacuum extraction method of cooked food.

[0040] S5. Let the sealed bag containing the rock cuttings stand for 8 hours, and then test the volume of gas desorbed by the rock cuttings in the sealed bag.

[0041] After being sealed for 8 hours, some desorbed gas will appear in each coal sample sealed bag. The volume of desorbed gas in each sealed bag is tested by drainage method and recorded as V. gi , i = 1, 2, …n, representing the rock cuttings numbered 1, 2, …n respectively.

[0042] At this time, the measured desorbed gas is not the total gas content of the coal rock cuttings, but the desorption amount within the first 8 hours, which is a macroscopic desorption gas volume considering the desorption speed. A large number of coal rock desorption tests were carried out according to the national standard GB / T 19559-2021. The test data are as follows Figure 2 As shown. It can be concluded that the first 8 hours are the main component of the desorption of adsorbed gas, and the desorbed gas in the first 8 hours usually accounts for about 40% to 60% of the total gas content. The desorption rate is fast in the first 8 hours, and the desorption rate is slow afterwards. Therefore, the method of the present invention determines that the standing time is 8 hours, and the cumulative desorbed gas volume in the first 8 hours is taken as the standard of the desorption rate parameter. This is the fastest desorption rate of coal rock. If the coal rock cuttings can desorb more than 50% of the gas content within 8 hours, it means that the desorption rate of the coal sample can fully meet the standard of high production of coalbed methane wells (1500 cubic meters / day).

[0043] S6. Calculate the ratio of the desorbed gas volume of each cutting to the cutting volume itself, λ i , i = 1, 2, ... n, respectively representing the number of each cutting; the calculation formula is:

[0044] λ i =V gi / V ci

[0045] Then calculate all λ i The average value

[0046] S7, based on the average The evaluation criteria for assessing the recoverability of gas wells are as follows:

[0047] like The coalbed methane well has a high degree of recoverability, and the daily output of a vertical well corresponding to a 5m thick coal seam exceeds 1,500 cubic meters per day.

[0048] like The coalbed methane well has a medium degree of recoverability, and the daily production of a vertical well corresponding to a 5m thick coal seam is between 1,000 cubic meters per day and 1,500 cubic meters per day.

[0049] like The recoverability of coalbed methane wells is relatively low, and the daily output of vertical wells corresponding to 5m thick coal seams is less than 1,000 cubic meters per day.

[0050] Here The situation not only takes into account the total gas content, but also focuses on the fastest desorption rate that the coal and rock cuttings can achieve within the first 8 hours.

[0051] Application Cases

[0052] The Yunlian block in southern Sichuan mainly develops the Permian Leping Formation coal seams, which are rich in coalbed methane resources. Since 2013, the Yunlian block has implemented 260 million m 3 Coalbed methane production capacity construction, with the continuous deepening of drainage and production, has shown different gas production capacities and production characteristics in different areas of the plane.

[0053] The distribution of production wells in the Junlian area is as follows: Figure 3 The southern part of the block is a coal mine outcrop denudation area. There is some methane gas escaping near the denudation area, resulting in a gas content of less than 8m 3 / t. The measured gas content of coalbed methane wells more than 2.5 km away from the denudation area is greater than 8m 3 / t, of which the upslope part of the slope is 8 to 12 m 3 / t. The gas content in the middle of the slope zone is 12-15m 3 / t; The core of the syncline is a low desorption zone, which is located in the basin belly and deep in the slope zone, with a high gas content of more than 15m 3 / t.

[0054] The results of the recoverability evaluation and the number of gas wells with the highest production in the Yunlian area based on the existing recoverability evaluation method of gas content data are shown in Figure 4 The results of the evaluation of recoverability and the number of wells with the highest gas production in the Yunlian area according to the method of the present invention are shown in Figure 5 .according to Figure 4 and Figure 5 By comparison, it can be seen that the high and low yields of wells cannot be accurately judged based on the gas content, and there are also high-yield wells with lower gas content. Figure 5 It can be seen that according to the evaluation parameters of the method of the present invention, if the evaluation parameter is less than 7, most wells are low-yield wells, and if the evaluation parameter is greater than 10, most wells are high-yield wells. It can be seen that the evaluation method of the present invention can better and more accurately solve the evaluation of gas well recoverability. According to the geological model, different wells have gas content data, but the desorption test speed is only completed for some wells, so Figure 4 and Figure 5 The total number of wells is not equal. In short, the comparison results show that the results of the recoverability evaluation of coalbed methane based on the gas content of the prior art are not ideal. However, the results obtained by the evaluation method of the present invention have better matching.

[0055] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for rapid evaluation of coalbed methane mineability, characterized in that: Here are the steps: S1. Obtain rock cuttings during logging of target coal seam; S2. Selecting several rock cuttings whose three-dimensional dimensions are all larger than 1 cm and smaller than 3 cm from the obtained rock cuttings; S3, test the volume of each rock cutting using the water displacement method; S4, sealing each rock cutting individually with a sealed bag and removing the air from the sealed bag; S5, leaving the sealed bag containing the rock cuttings to stand for 8 hours, and then testing the volume of gas desorbed by the rock cuttings in the sealed bag by the water displacement method; S6. Calculate the ratio of the desorbed gas volume of each cutting to the cutting volume itself, λ i , i = 1, 2, ... n, representing the number of each cutting; then calculate all λ i The average value S7, based on the average The evaluation criteria for assessing the recoverability of gas wells are as follows: like The coalbed methane well has a high degree of recoverability, and the daily output of a vertical well corresponding to a 5m thick coal seam exceeds 1,500 cubic meters per day; like The coalbed methane well has a medium degree of recoverability, and the daily output of the corresponding vertical well for a 5m thick coal seam is between 1000 cubic meters per day and 1500 cubic meters per day; like The recoverability of coalbed methane wells is low, and the daily output of vertical wells corresponding to 5m thick coal seams is less than 1,000 cubic meters per day; The cutoff value of 10 in the evaluation standard is calculated based on the gas leakage radius of 100m in the first year of the coalbed methane well, the coal seam thickness of 5m, and the corresponding gas production of 1500 cubic meters per day for three years of stable production. The demarcation value of 7 is calculated based on the gas leakage radius of 100m in the first year of the coalbed methane well, the coal seam thickness of 5m, and the corresponding gas production of 1000 cubic meters per day for three years of stable production. The calculation formula is as follows: Demarcation value = 1 / [(3.1415×deflation radius 2 × coal seam thickness) / (corresponding gas production × 365 × 3)].

2. The method for rapid evaluation of coalbed methane mineability according to claim 1, characterized in that: Step S1 specifically includes: during the gas well drilling process, when passing near the target coal seam, cuttings logging is required, and according to the logging return time, several cuttings samples in the middle of the coal seam just out of the well are selected as candidates.

3. The method for rapid evaluation of coalbed methane mineability according to claim 1, characterized in that: In step S4, each rock cutting is sealed in a sealed bag with a maximum volume of 50 ml, the air in the sealed bag is quickly removed, and then the sealed bag is sealed using a heat press.

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