Determination method for deep coal bed gas reservoir recovery ratio

Abstract

The invention relates to a deep coal bed gas reservoir recovery ratio determination method, belongs to the technical field of coal bed gas field development, and aims to solve the problems that the existing coal bed gas recovery ratio determination method is relatively simple and is not systematic in understanding of influence factors of the recovery ratio. The method comprises the following steps: calculating the pressure drop sweep efficiency of coal bed gas, the desorption efficiency of adsorbed gas, the exhaustion efficiency of free gas and the residual rate of the free gas in pores; subtracting the residual rate of the free gas from the sum of the desorption efficiency of the adsorption gas and the exhaustion efficiency of the free gas under the pressure drop sweep efficiency to obtain the final recovery rate of the coal bed gas; according to the method, different output characteristics and influence factors of each output stage are specifically analyzed in the whole determination process, engineering factors, coalbed methane free gas and adsorbed gas are considered, the coalbed methane recovery ratio is calculated by comprehensively considering multiple factors, the problem that the factors considered in the existing coalbed methane recovery ratio calculation are not systematic is solved, and the coal bed methane recovery ratio calculation efficiency is improved. And the calculation reliability and accuracy are improved.

Description

Technical Field

[0001] The invention relates to a method for determining the recovery rate of a deep coalbed methane reservoir, and belongs to the technical field of coalbed methane field development. Background Art

[0002] At present, the calculation method of coalbed methane recovery rate is relatively simple, and the understanding of the factors affecting the recovery rate is not systematic. In the production process of gas wells, the coalbed methane recovery rate is affected by factors such as pressure drop sweep efficiency, adsorbed gas desorption efficiency, free gas exhaustion efficiency, and residual rate of free gas in pores. The pressure drop sweep efficiency is controlled by engineering factors such as well pattern, well spacing, well type and fracturing; the desorption efficiency is affected by factors such as gas adsorption characteristics and abandonment pressure. The extent of coalbed methane recovery in the early stage of development depends on the exhaustion efficiency of free gas and the pressure drop sweep efficiency. Improving the pressure drop sweep efficiency can increase the gas production rate of coalbed methane; while in the later stage of development, the extent of coalbed methane recovery is mainly controlled by the desorption efficiency. An effective way to improve the pressure drop sweep efficiency of coalbed methane is to use a well pattern and well spacing that matches the geological characteristic parameters of the coal seam, so that the well pattern can effectively control the coal seam and form volume desorption; negative pressure gas production can further reduce the abandonment pressure of the coalbed methane reservoir, thereby improving the desorption efficiency of coalbed methane.

[0003] At present, there are many methods for calculating coalbed methane recovery rate, such as analogy method, isothermal adsorption curve method, desorption method, production decline method, material balance method and gas reservoir numerical simulation method. The analogy method is a prediction method that estimates coalbed methane recovery rate by analogy with similar blocks, and its theoretical basis is not strong. The isothermal adsorption curve method requires less data, takes into account the influence of gas adsorption characteristics on recovery rate, and believes that all gas desorbed under abandoned pressure can be recovered; the desorption method is a method for predicting recovery rate by calculating the ratio of the sum of desorbed gas and lost gas to the gas content of coal sample; both methods ignore the influence of engineering factors on recovery rate calculation. The production decline method, material balance method and numerical simulation method are all dynamic calculation methods of recovery rate, which are suitable for research areas with high exploration degree. It is difficult to ensure the accuracy of their prediction for research areas with less parameter acquisition. At the same time, the dynamic method is difficult to directly reflect the influence of engineering factors on coalbed methane recovery rate calculation. Except for the numerical simulation method, the current recovery factor prediction methods all ignore the impact of engineering factors on the recovery factor and directly calculate the final recovery factor of coalbed methane. This calculation method is not conducive to the systematic analysis of the factors affecting the recovery factor.

[0004] The Chinese patent application document with application publication number CN115931949A discloses a technology for improving the recovery rate of coalbed methane, but does not involve a method for calculating the accuracy of the recovery rate. Therefore, the current methods for calculating the recovery rate of deep coalbed methane reservoirs either have weak theoretical basis or ignore the influence of engineering factors on the recovery rate, and cannot obtain an accurate calculation method for the recovery rate of deep coalbed methane reservoirs. Summary of the invention

[0005] The purpose of the present invention is to provide a method for determining the recovery rate of deep coalbed methane reservoirs, so as to solve the problems of weak theoretical basis for the current deep coalbed methane recovery rate and unsystematic calculation ignoring engineering factors.

[0006] To achieve the above object, the solution of the present invention includes:

[0007] A method for determining the recovery rate of a deep coalbed methane reservoir of the present invention comprises the following steps:

[0008] 1) Calculate the pressure drop sweep efficiency of coalbed methane, the desorption efficiency of adsorbed gas, the exhaustion efficiency of free gas and the residual rate of free gas in the pores;

[0009] 2) The final recovery rate of coalbed methane is obtained by subtracting the residual rate of free gas from the sum of the desorption efficiency of adsorbed gas and the exhaustion efficiency of free gas under the pressure drop sweep efficiency.

[0010] The present invention specifically analyzes the different output characteristics and influencing factors of each output stage, takes into account the pressure drop sweep efficiency, the desorption efficiency of the adsorbed gas, the exhaustion efficiency of the free gas and the residual rate of the free gas in the pores, and calculates the coalbed methane recovery rate based on multiple factors, thereby solving the problem of unsystematic consideration of factors in the current calculation of coalbed recovery rate and improving the reliability and accuracy of the calculation.

[0011] Furthermore, the free gas depletion efficiency is calculated by the free gas binding rate.

[0012] Furthermore, the free gas exhaust efficiency is calculated by the following formula:

[0013] E f =Per f ×(1-E gcr )

[0014] Where: E f is the exhaustion efficiency of free gas, in %; Per f is the ratio of free gas in coalbed methane, in percentage; E gcr It is the free gas binding rate of coalbed methane, expressed in percentage.

[0015] Furthermore, the desorption efficiency of the adsorbed gas was calculated using the Langmuir formula.

[0016] Furthermore, the expression of the desorption efficiency of the adsorbed gas is:

[0017]

[0018] Where: E d V is the proportion of coalbed methane in the coal seam where desorption occurs; pi is the initial gas content of the coal seam, in m 3 / t; V p is the amount of coalbed methane adsorption, unit is m 3 / t; V c is the volume of the coal seam, in m 3 ; ρ is the density of coalbed methane, unit is t / m 3 .

[0019] Furthermore, in step 1), the pressure drop and sweep efficiency are calculated by the following formula:

[0020]

[0021] Where: E V is the pressure drop and efficiency; V d is the volume of coal seam where desorption occurs, in m 3 ; V c is the volume of the coal seam, in m 3 .

[0022] Furthermore, the residual rate of free gas is calculated by the following formula:

[0023]

[0024] Where: E np is the residual rate of free gas; is the average gas saturation; φ is the porosity of the coal seam; B g is the volume coefficient of coalbed methane, in m 3 / m 3 ; V pi is the initial gas content of the coal seam, in m 3 / t.

[0025] Furthermore, the ultimate recovery factor is calculated by the following formula:

[0026] E R =E V ×(E d +E f )-E np

[0027] Where: E R is the coalbed methane recovery rate, E V is the sweep efficiency of coalbed methane mining, E d Desorption efficiency of adsorbed gas, E f is the exhaustion efficiency of free gas, E np is the residual rate of free gas in the pores.

[0028] Furthermore, after a certain period of coalbed methane exploitation, the entire coal seam begins to desorb, and the pressure drop sweep efficiency is set to 1.

[0029] By setting the pressure drop and sweep efficiency at the beginning of desorption of the entire coal seam to 1, the subsequent formula calculations are simplified.

[0030] Beneficial effects: The present invention specifically analyzes the different output characteristics and influencing factors of each output stage, takes into account engineering factors, coalbed methane free gas and adsorbed gas, and can comprehensively consider multiple factors to calculate the coalbed methane recovery rate, thereby solving the problem of unsystematic consideration of factors in the current calculation of coalbed recovery rate and improving the reliability and accuracy of the calculation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a flow chart of a method for determining the recovery rate of a deep coalbed methane reservoir in this embodiment;

[0032] Figure 2 is a comparison chart of the recovery factor calculated during the verification process of this embodiment and the EUR reverse recovery factor result;

[0033] Figure 3 The production curve diagram in this embodiment. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0035] like Figure 1 As shown in the figure, by studying the influencing factors of coalbed methane recovery rate and the influence of various factors on the degree of coalbed methane recovery at different stages, the following understandings were formed: the coalbed methane recovery rate is controlled by the pressure drop sweep efficiency of the coal seam, the depletion effect of free gas, the adsorbed gas desorption efficiency and the free gas residual rate, among which the pressure drop sweep efficiency is affected by engineering factors, the free gas depletion efficiency is affected by the abandonment pressure, and the desorption efficiency is affected by gas adsorption characteristics and production-increasing measures such as negative pressure gas production.

[0036] The extent of coalbed methane recovery in the early stage mainly depends on the pressure drop sweep efficiency; in the later stage of development, the extent of coalbed methane recovery mainly depends on the depletion efficiency and desorption efficiency of coalbed methane, while the free gas residual rate has little effect on the overall recovery rate.

[0037] Using a well pattern that matches the geological characteristic parameters of the coal seam and using well-to-well interference to achieve volume desorption can improve the early pressure drop sweep efficiency and achieve rapid coalbed methane recovery; reducing the coalbed methane waste pressure can improve the free gas exhaustion efficiency and adsorbed gas desorption efficiency, thereby affecting the coalbed methane recovery rate. The above analysis of the production process and influencing factors can be used to calculate the recovery rate of the coal seam and optimize the production method and production parameters to improve the final recovery rate.

[0038] A method for determining the recovery rate of a deep coalbed methane reservoir:

[0039] The specific implementation process of this method is as follows:

[0040] S1 calculates the exhaustion efficiency of free gas under the abandoned pressure. Free gas is the free gas existing in the initial state of coalbed methane. A part of it exists in the pore volume that cannot flow. The proportion of its volume to the total volume of free gas is the binding rate of coalbed methane free gas. This part of gas cannot be produced. The exhaustion efficiency calculation expression of free gas is:

[0041] E f =Per f (1-E gcr )

[0042] Where: Per f is the ratio of free gas in coalbed methane, expressed in percentage, E f is the exhaustion efficiency of free gas, expressed in percentage; E grc It is the free gas binding rate of coalbed methane, expressed in percentage.

[0043] S2 calculates the desorption efficiency of coalbed methane adsorbent. The physical meaning of desorption efficiency is the degree of movement of coalbed methane adsorption equilibrium in the coal seam where desorption occurs. It is a physical quantity that characterizes the effect of gas adsorption characteristics on recovery rate. The desorption efficiency is obtained by calculating the ratio of the desorption amount of the desorbed coal layer to the adsorption amount of coalbed methane under the original pressure. The calculation formula is:

[0044]

[0045] Where: E d V is the desorption efficiency within the pressure drop wave volume; pi is the initial gas content of the coal seam, in m 3 / t; V p is the amount of coalbed methane adsorption, unit: m 3 / t; V c is the volume of the coal seam, in m 3 ; ρ is the density of coalbed methane, unit t / m 3 ; p is the gas pressure, unit is MPa.

[0046] S3 calculates the residual rate of free gas in the air volume of the coal seam, that is, the ratio of the free gas volume in the coal seam that is desorbed but cannot be produced to the gas content of the coal seam under the original pressure. The residual rate of free gas in the pore volume of coal seam gas is the ratio of the free gas volume in the coal seam that is desorbed but cannot be produced to the gas content of the coal seam under the original pressure. Its physical meaning is the residual amount of free gas desorbed by the coal seam, which is a physical quantity that characterizes the volume of gas that cannot be produced. Its calculation formula is:

[0047]

[0048] Where: E npis the free gas retention rate; is the average gas saturation; φ is the porosity of the coal seam; B g is the volume coefficient of coalbed methane, unit: m 3 / m 3 ; V pi is the initial gas content of the coal seam, in m 3 / t.

[0049] S4 calculates the pressure drop sweep efficiency of the coal seam. In the initial state, after the reverse liquid production has been produced for a period of time (only liquid is produced but no gas is produced), the second stage is mainly the production of adsorbed gas, which is mainly manifested in the rapid increase of liquid production, the increase of gas production, the gradual increase of gas-liquid ratio, and the gradual production of adsorbed gas. The production in this stage comes from the adsorbed and desorbed gas in the pressure drop sweep volume of the coal seam, and its calculation formula is:

[0050]

[0051] Where: E V V is the pressure drop sweep efficiency of the coal seam; d is the volume of coal seam where desorption occurs, m 3 ; V c is the volume of the coal seam, m 3 .

[0052] After a certain period of mining, the entire coal seam begins to desorb, the pressure drop sweep efficiency is 1, and the coalbed methane recovery rate is numerically equal to the difference between the coalbed methane free gas exhaustion efficiency plus the desorption efficiency of the coalbed adsorbed gas and the free gas residual rate in the pore volume.

[0053] S5 calculates the free gas exhaustion efficiency, adsorbed gas desorption efficiency, and residual rate of free gas in the pore volume of the coal seam through S1, S2, S3, and S4. It is assumed that the final pressure drop sweep efficiency is 1, and the final recovery rate of coalbed methane is calculated by the following formula:

[0054] E R =E V ×(E d +E f )-E np .

[0055] Furthermore, considering various factors and steps, the coalbed methane recovery rate at abandonment is:

[0056]

[0057] Where: p abn is the abandoned pressure of gas reservoir, MPa.

[0058] Take the D1 well as an example. It was put into production on December 5, 2021. The initial test showed a daily gas production of more than 100,000 cubic meters and a casing pressure of 4.3-4.5 MPa. The well adopted the bare casing decompression production mode for trial production in the first three months, with an average gas production of 68,000 cubic meters / day, showing the characteristics of high production, high casing pressure, and high liquid carrying. The production curve is shown in Figure 3 According to the production dynamic fitting, the D1 well EUR is predicted to be 63.93 million cubic meters (the final cumulative production when the pressure is abandoned), the geological reserves are 129.41 million cubic meters, and the recovery rate is 49.9%. The recovery rate of the D1 well is calculated using the new invention method, where P i The value is 29.4MPa, p abn The value is 5.0MPa, and the value of ρ is 1.45t / m 3 , Per f Take the value of 20%, E gcr Take 5% as the value, V pabn Value: 6.9m 3 / t,B g The value is 0.0048m 3 / m 3 , φ is taken as 10.6%, and the final recovery rate calculated by the new method formula is 50.1%, which is 96% consistent with the recovery rate predicted by production dynamics. The same method is used to calculate the recovery rates of 6 wells in the test area, and the ratio of single well EUR to geological reserves is calculated to verify the recovery rate. The comparison results of the two methods are shown in Table 1.

[0059] Table 1

[0060]

[0061]

[0062] EUR: The final cumulative production of a single well under abandoned pressure, in ten thousand cubic meters.

[0063] The comparison results of the two are Figure 2 To show, from Table 1 and Figure 2 It can be seen that the relative errors of the calculated results of the present invention compared with the measured values ​​are all within 1%, indicating that the accuracy of the coalbed methane recovery rate calculated by the present invention by comprehensively considering multiple factors is guaranteed.

Claims

1. A method for determining the recovery rate of a deep coalbed methane reservoir, characterized in that: The steps include: 1) Calculate the pressure drop sweep efficiency of coalbed methane, the desorption efficiency of adsorbed gas, the exhaustion efficiency of free gas and the residual rate of free gas in the pores; 2) The final recovery rate of the coalbed methane is obtained by subtracting the residual rate of the free gas from the sum of the desorption efficiency of the adsorbed gas and the exhaustion efficiency of the free gas under the pressure drop sweep efficiency.

2. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 1, characterized in that: In step 1), the depletion efficiency of the free gas is calculated by the free gas binding rate.

3. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 2, characterized in that: The free gas exhaust efficiency is calculated by the following formula: AND f =For f ×(1-E gcr ) Where: E f is the exhaustion efficiency of free gas, in %; Per f is the ratio of free gas in coalbed methane, in percentage; E gcr It is the free gas binding rate of coalbed methane, expressed in percentage.

4. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 1, characterized in that: In step 1), the desorption efficiency of the adsorbed gas is calculated by Langmuir's formula.

5. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 4, characterized in that: In step 1), the expression for calculating the desorption efficiency of the adsorbed gas is: Where: E d V is the proportion of coalbed methane in the coal seam where desorption occurs; pi is the initial gas content of the coal seam, in m 3 / t; V p is the amount of coalbed methane adsorption, unit is m 3 / t; V c is the volume of the coal seam, in m 3 ; ρ is the density of coalbed methane, unit is t / m 3 .

6. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 1, characterized in that: In step 1), the pressure drop efficiency is calculated by the following formula: Where: E V is the pressure drop and efficiency; V d is the volume of coal seam where desorption occurs, in m 3 ; V c is the volume of the coal seam, in m 3 .

7. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 1, characterized in that: In step 1), the residual rate of the free gas is calculated by the following formula: Where: E np is the residual rate of free gas; is the average gas saturation; φ is the porosity of the coal seam; B g is the volume coefficient of coalbed methane, in m 3 / m 3 ; V pi is the initial gas content of the coal seam, in m 3 / t.

8. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 1, characterized in that: In step 2), the final recovery factor is calculated by the following formula: AND R =And V ×(And d +E f )-AND np Where: E R is the coalbed methane recovery rate, E V is the sweep efficiency of coalbed methane mining, E d Desorption efficiency of adsorbed gas, E f is the exhaustion efficiency of free gas, E np is the residual rate of free gas in the pores.

9. The method for determining the recovery rate of deep coalbed methane reservoir according to claim 1, characterized in that: After coalbed methane is mined for a certain period of time, the entire coal seam begins to desorb, and the pressure drop sweep efficiency is set to 1.

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